PROGES REPORT ON SINGLE LAYER 2D CRYSTALS FOR ELECTRO-

CHEMICAL APPLICATIONS OF ION EXCHANGE MEMBRANES AND

HYDROGEN EVOLUTION CATALYSTS

Maria Perez-Page, Madhumita Sahoo, Stuart M. Holmes*

School of Chemical Engineering and Analytical Science

The University of Manchester

M13 9PL, United Kingdom*corresponding author: stuart.holmes@manchester.ac.uk

Keywords: 2D materials, single layer, polymer exchange membrane, hydrogen evolution

reaction, electrochemical applications.

Abstract:

The isolation of graphene is the beginning of a new generation of materials, 2D materials or

2D crystals. Because of their particular layered structure, with planar topology and a

thickness from single to few atomic layers, which provide them with unique properties, 2D

material are expected to have important applications in the next generation of energy systems.

Electrochemical energy storage devices such as batteries or supercapacitors have already

incorporated these materials as electrodes, electrocatalyst etc. However, most of the

applications reported so far use graphene based materials such as Graphene Oxide (GO) or

Reduced Graphene Oxide (rGO), or different 2D materials in a bulk feature. This review aims

to provide an overview of the recent progress of using single layer and few layer 2D materials

in electrochemical applications of ion exchange membranes and hydrogen evolution

catalysts, providing a better understanding of 2D materials features.

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1. Introduction

Energy is one of the most important research fields at present due to its necessity for modern

life. It is widely known that current global energy demand is mainly sustained by energy

sources based on fossil-fuels. However, the growing energy consumption will diminish

fossil-fuel reserves at high rates together with increased environmental concerns because

their emissions have caused the necessity to find a flexible, efficiency, environmental friendly

and low cost energy system and storages which does not depend on fossil fuel. [1, 2] Renewable

energies such as solar or wind were the first consideration for new model of energy, however

they are intermittent and depend on the weather which make them unreliable.[3] Finding this

energy system has become an important challenge over the last years.

Hydrogen is considered one of the most promising fuels, having the largest energy density

and its combustion generates only by-product of water and heat. However, hydrogen needs to

be generated in an environmentally benign manner.

Fuel cells are electrochemical device which convert the chemical energy produced by an

electrochemical reaction into electricity. Polymer Electrolyte Membrane Fuel Cells

(PEMFCs) are the most promising ones and they have been used for stationary and

automobile applications as well as for portable devices. Hydrogen as a fuel and oxygen as an

oxidant agent are the most common reactants used for these fuel cells. Other fuels such as

methanol and formic acid are also used in PEMFC. The heart of fuel cells is the Membrane

Electrode Assembly (MEA) which is composed of two electrodes, the anode and the cathode,

and the electrolyte. The electrolyte is a Proton Exchange Membrane (PEM) which allows

protons through it from anode to cathode side. The membrane needs to have high proton

conductivity, thermal and mechanical strength, be light weight, flexible, have low

permeability to reactant species, resistance to fuel transport, highly durable and facilitate

2

quick electrode kinetics, suitable for use with different fuels and low in cost. As electrolyte in

fuel cells, the three main functions of the membrane are to act as ion conductor, electronic

insulator and a separator for the reactant gases.[4] Nafion is the most widely used polymeric

membrane in PEMFC. It has a particular structure which requires hydration leading to high

proton conductivity. It cannot be used at temperatures beyond 100 oC since the proton

conductivity decrease when Nafion is not perfectly hydrated. Another drawback of Nafion is

fuel crossover, which is the permeability of the fuel from the anode side to the cathode side

leading a decrease of the efficiency of the fuel cell. This particular problem is more

significant in Direct Methanol Fuel Cells (DMFC) which use methanol as a fuel which can

easily permeate through Nafion. In this context, great efforts have been made to find a new

electrolyte membrane which can replace Nafion and 2D materials could play an important

role in this.

Graphene is the name given to a flat monolayer of carbon with a two-dimensional (2D)

honeycomb lattice and is the basic building block for graphitic materials of all other

dimensionalities.[5] It has particular properties such as notably high surface area around 2630

m2·g-1,[1, 6] exceptionally high electrical conductivity, 104.36 S·cm-1·,[1, 7] high flexural

strength, 44.28 MPa,[1, 8] and good chemical and thermal stability,[1, 9] which make it a

potential material for a large number of applications.

Following on from the development of graphene, a whole new family of 2D materials, have

been studied which exhibit an exciting range of novel properties for a multitude of new

potential applications. Transition Metal Chalcogenides (TMDs), Transition Metal Oxides

(TMOs)/Hydroxides (TMHs), Layered Double Hydroxides (LDHs), metal sulfides,

phosphorenes, MXenes, silicines, are some of this new family of 2D crystals.[2]

3

Graphene based materials such as Graphene Oxide (GO) and Reduced Graphene Oxide

(rGO) have been used widely in electrochemical energy devices such as batteries,

supercapacitors and fuel cells.[4, 10, 11-14] In the particular application of fuel cell, rGO has been

mostly reported as a support for catalysts.[15] The catalyst support has attractive properties

such as, high conductivity, high surface area to improve the dispersion of the metal, high

stability under oxidative and reductive environment and easy recovery of the metal catalyst.

Two different issues are being tackled by using rGO as a support material, firstly the

improvement of the electrocatalytic activity and secondly improvement of durability.[12, 13] A

different application has GO used in the membrane as a filler material for fuel cells,

electrodialyzers etc. By definition, a filler material is an inorganic material which has a good

compatibility with the polymer and improves its properties. Currently, they are in huge

demand for membrane modification for fuel cells. Different materials such as zeolites,[16, 17]

carbon nanotubes[4] have already been reported as filler materials. Recently, it has been

demonstrated that the incorporation of GO in PEM membranes could lead to an enhancement

on fuel cells performance. GO has excellent compatibility with Nafion and could provide

potential advantages in the PEM membrane. This excellent compatibility between Nafion and

GO is due to their strong interfacial attraction. GO can improve side chains by incorporating

new groups in the structure and the backbone of Nafion leading to improve thermal and

mechanical properties.[4] Abundant oxygen-containing functional groups of GO supports its

amphiphilic nature and allows convenient chemical functionalization, making this material

promising inorganic filler for composite membranes. GO-Nafion matrix membranes are the

main application of GO in fuel cells, particularly on DMFC where the methanol crossover is

important and GO can act as a methanol barrier.

Although hydrogen is a good option to replace fossil-fuels, the main production of hydrogen

still depends on the these hydrocarbons since it is produced by steam reforming of natural gas

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which releases carbon dioxide.[18] As it is well known, water could be the most environment

friendly source of hydrogen and it can be produced via electrolysis or photocatalytic water

splitting.[18] However, these reactions are not without cost since they need energy to take

place. Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) are

involved in the water splitting reaction. An electrocatalyst can help to accelerate the

electrochemical reactions providing alternative mechanisms and lower activation energy to

reduce overpotentials. 2D material such as Transition Metal Chalcogenides (TMDs) have an

important role on HER, since it has been found that can improve the catalytic activity for this

reaction. Molybdenum based dichalogenides are the most widely investigated materials

owing to their exciting prospects in HER catalyst. [19-21, 22-25]

Graphene oxide, reduced graphene oxide and 2D based materials as electrodes,

electrocatalysts and catalyst supports are already well reviewed for different electrochemical

applications such as electrochemical reduction of carbon di-oxide[26], electrochemical

reduction of nitrogen[27], electrochemical sensors for environmental analysis[28], gas sensors[29],

bio sensors[30], electrochemical reduction of oxygen[12, 31], redox flow batteries[32],

supercapacitors[33], and rechargeable metal ion/air batteries[11, 34]. Other reviews[1, 2, 4, 11-14, 24]

have described the application of graphene oxide, reduced graphene oxide and 2D based

materials in nanoparticle form as composite Nafion membranes, catalyst supports and

electrodes for fuel cells. However there is no existing review to address the advancement of

single and few layer graphene and other 2D materials as ion exchange membranes and

hydrogen evolution catalysts, to the best of our knowledge. This review is an overview of the

application of single and few layers 2D materials in electrochemical technologies, first as ion

exchange membranes membranes for fuel cells and electrodialyzers and as catalyst in the

water splitting reaction. A summary of the focussed 2D materials in this review and the

electrochemical applications can be observed in the Schematic 1.

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2. 2D materials for ion exchange membranes.

Ion exchange membranes (IEMs) are of two types, namely-anion exchange membrane

(AEM) and cation exchange membrane (CEM) and mostly used in fuel cells and for

electrodialysis. Polymer electrolyte membrane fuel cells use a proton conducting membrane

(PEM) or AEM, depending on the particular cell type, as their electrolyte. These solid

electrolytes together with the electrodes are the heart of the particular electrochemical device,

allowing the ions to flow and suppressing the unwanted crossover of the reactants. In an ideal

case it should be a very good ion conductor, complete insulator to electrons and impermeable

to the reactants. In case of proton exchange membrane fuel cells, Nafion, with its high proton

(H+) conductivity (0.1 S·cm-1), high chemical resistance and high electrochemical stability is

the dominant material so far. A hydrophobic polytetrafluoroethylene (PTFE) backbone and

hydrophilic perflourinated sulfonic acid chain provides a unique structure to the Nafion.

Properties such as water content and working temperature play a major role in its proton

conductivity; delivering maximum proton conductivity requires hydrated conditions. The

high cost of commercial Nafion together with fuel cross-over, swelling and temperature

limited usage demands further research for a better membrane. Alkaline membrane fuel cells

(AMFCs) are a different type of fuel cell based on alkaline electrolytes and compete with

PEMFC in terms of advantage of operating in alkaline environment and thus giving the scope

of employing non-precious metal catalysts. Anion Exchange Membranes (AEMs) are used as

a gas separator and ion conductor (OH- and carbonate). The shift to AMFC to operate in an

alkaline environment could be feasible if good hydroxyl ion conductors can be used and non-

precious metal catalysts are used to reduce significantly the cost. Unfortunately, transport of

OH- is relatively slow compared to H+. Typical ion conductivities for AEM are between 10-3 –

10-2 S·cm-2[25] which is significantly lower than that of the Nafion proton conductivity. In

addition, AEMs are based on polymeric quaternary ammonium groups which usually related

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with highly toxic preparation technique.[25, 35-37] Also, problems like poor thermal and

mechanical stability of these membranes led the researchers to search for a suitable AEM

with good ion conductivity.[25, 36-38]

These drawbacks for ion exchange membranes have forced the need to find a new generation

of ion conductors. Single layer 2D materials and graphene based materials, because of their

high proton conductivity and barrier properties,[39] could be a good candidate to be an

exceptional ion exchange membranes. Here, we have discussed the recent advances in

graphene and some other 2D materials based IEMs. Table 1 lists a brief summary of some of

the various important properties such as proton conductivity, mechanical properties, water

uptake etc. of 2D material based ion exchange membranes.

2.1. Single layer 2D materials as ion exchange membrane

Graphene has been studied as a possible technology to develop novel separation technologies.

A pristine single layer of graphene is impermeable to all atoms and molecules under ambient

conditions, even the smallest atom, hydrogen. Similar behaviour is expected from all of

atomically thin crystals.[40] This fact was proved by Hu et al.,[39] they investigated the proton

transport through different mono-crystalline membranes composed from single to few layers

of different 2D materials: graphene, hBN and MoS2. Results obtained by mass spectroscopy

and transport measurements established that, under ambient conditions, thermal protons can

permeate single layer graphene and hBN. However, no proton transport is detected for MoS2,

which is a thicker crystal since it consists of three atomic layers containing large atoms, or for

multilayer hBN or bilayer graphene. A specific experiment was prepared[39] to collect the

proton conductivity measurements. 2D crystal membranes, previously prepared and

supported on a particular substrate, were coated by Nafion on both side and sandwiched

between two proton-injecting PdHx electrodes. Similar device without 2D crystal

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membranes, which authors called “bare-hole”, was developed to use as a reference. They

proved that at room temperature, monolayer hBN provided higher proton conductivity than

graphene. The obtained resistivity of this material to proton flow is about 10 Ω·cm2 and the

activation energy about 0.3 eV. When the temperature increases, the single layer graphene

performs better than hBN with a resistivity below of 10-3 Ω·cm2 for temperatures above 250

oC. They also reported that proton transport can be improved by decorating the 2D crystals,

graphene or hBN, with catalytic metal nanoparticles[39] such as platinum, since it provides

high affinity to hydrogen. Catalytic layers added onto the surfaces of 2D crystals obtained a

substantial increase of the conductivity by unit area. The total conductivity value obtained for

a monolayer hBN is at the same level as the values obtained for “bare-hole” devices, which

prove that proton transport, even at temperatures around 21-23 oC, is limited by the resistance

of the Nafion rather than the single layer hBN. Figure 1 a) and b) show the proton

conductivity for 2D crystals and 2D crystals decorated with platinum respectively. Each bar

represents a different sample with 2 µm-diameter membranes. In Figure 1 b), a shaded area

can be observed which represents the conductivity range found for “bare-hole” devices,

reference experiment without single layer crystal. The inset images in Figure 1 a) represents

charge density (e·Å-2) integrated along the direction perpendicular to graphene at left and

monolayer hBN at right. The white areas are minima at the hexagon centres; the maxima

correspond to positions of C, B and N atoms. The inset plot in Figure 1 b) show the

Arrhenius-type behaviour for graphene decorated with platinum, yielding E ~ 0.24 eV. [39]

The main conclusion of this work is that for the first time, it was proved that protons can pass

through single layer graphene and hBN, reporting those materials as a new class of proton

conductors. Even though the mechanisms is not clear, the high proton conductivity, thermal

and chemical stability, and the fact they are impermeable to water, methanol even hydrogen

make these materials attractive for use as membranes in hydrogen technologies, particularly

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PEM fuel cells, since by definition they could be the perfect electrolyte membrane,

presenting a high proton conductivity and solving the problem of fuel cross-over.

A barrier layer for fuel-crossover has commonly been used to impede fuel permeation

helping in an effective utilization of fuel, thereby increasing the performance. [16, 41] This is a

simple approach, however, the proton conductivity should not be affected since both

mechanisms do not scale linearly[42] and a decreasing of proton conductivity will significantly

reduce the fuel cell performance, even with the enhanced fuel barriers properties.[43] Hence, a

balance between proton conductivity and fuel repulsion properties has to be found when a

barrier layer is incorporated on a fuel cell. Taken together the observations and conclusions

from Hu et al.’s work, a single layer graphene (SLG) or single layer hBN could be an ideal

candidate as an electrolyte for fuel cells.

Trying to prove this fact, Holmes et al.[44] have incorporated SLG and hBN in a membrane

electrode assembly (MEA) for a Direct Methanol Fuel Cell (DMFC), to prove that both 2D

single layer crystals, even with some defects in the area, can enhance the DMFC efficiency

by decreasing the methanol cross-over without any decrease in the proton conductivity as can

be observed in Figure 1 c). Both monolayers, graphene and hBN, were grown by Chemical

Vapour Deposition (CVD) on copper support and they were transferred onto the amorphous

carbon electrode, anode side, following the protocol reported by Reina et al..[45] To confirm

the presence of SLG and hBN onto the electrodes, Raman measurements were carried out and

the percentage of single layer coverage was evaluated. Results show that the DMFC with a

single layer hBN incorporated on the MEA has shown higher performance than a Nafion

membrane across the temperature range tested (30-90 oC). However, with SLG the

improvement was detected at temperatures above 60 oC, which is in agreement with the

temperature dependency reported by Hu et al.[39] previously, where they also demonstrated

that SLG only achieves proton conductivity higher than Nafion at temperatures above 50 oC,

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whereas hBN shows high proton conductivity even at room temperature (21-23 oC).[39]

Holmes and co-workers also showed the link between SLG coverage and DMFC

performance, demonstrating that methanol cross-over decreases with increase in SLG

coverage; hence DMFC performance enhancement. The dense lattice structure of highly

crystalline 2D graphene and hBN blocks molecules and atoms, improving the methanol

impermeability. This is due to the 2D materials decreasing the effective membrane area for

methanol transport across the membrane, at the same time protons can pass through the single

layer 2D crystals without any deviation, which is another important aspect of this study.[44]

No increments or variations on the proton resistance were found by adding single layers on

the MEA, obtaining similar values with and without the 2D materials at the same conditions.

The maximum power density reported with and without SLG on the MEA has achieved at 70

oC showing an improvement of 45 % by incorporating the SLG, as it can be observed in the

polarisation curves and power curves displayed in Figure 1 d).

Previous work carried out by Yan et al.[46] has applied the same concept to a passive DMFC.

They developed a sandwiched Nafion membrane by incorporating two layers of Nafion 212

and a monolayer graphene in the middle as can be shown in Figure 2 c). In this case,

monolayer graphene was also grown by CVD on a copper support and similar transfer

process was carried out, however, a difference with Holmes et al. who added the SLG on the

carbon electrode, Yan et al. transferred the SLG directly onto the Nafion layer.

After proper characterisation of the monolayer graphene, the electrolyte was tested in a

passive DMFC and the performance was evaluated. Characterisation results provide some

evidence of the presence of some defects at different dimensions and cracks that could

probably be formed during the transfer process. The methanol cross-over and proton

conductivity were also tested by using Electrochemical Impedance Spectroscopy. Results at

80 oC reported a decrease of methanol cross-over of 68.6% when the graphene was

10

incorporated on the MEA, whereas the proton conductivity decreases only by 7 % in

comparison with the pristine Nafion membrane. The presence of SLG onto the MEA

enhances the power density at high methanol concentrations, 10 M, due to the reduction of

methanol cross-over.

A similar configuration of sandwich nafion-graphene membrane was used by Bukola et al.[47]

who tried to demonstrate the high selective proton/deuteron transport through this

sandwiched graphene membrane at high current density using a hydrogen pump cell, Figure 2

b). The drawing (Figure 2 b) is not to scale; in reality, the Nafion/graphene/Nafion membrane

is thin relative to the electrode diameters, and the graphene layer extends several millimetres

beyond the electrodes edges, to avoid contributions from proton current around. Also based

on the previous mentioned work reported in 2014 from Hu et al.[39] and Lozada-Hidalgo et al.

work in 2016[48] where they also reported that monolayers of graphene and hBN can also be

used to separate hydrogen ion isotopes with high selectivity, Bukola et al. developed a proton

exchange membrane formed by a single layer graphene sandwiched between two Nafion 211

membranes. In this case, they used a different process to transfer the CVD graphene

monolayer from the copper support to the Nafion membrane which consists on a simple hot-

press process, as can be observed in the Figure 2 a).

The membrane is placed inside the cell between two platinum-coated carbon cloth electrodes,

and protons/deuteron are pumped from one side to the other side as can be shown in Figure

2b). This way, the proton/deuteron transfer should occur through the graphene monolayer.

Performance of the cell pumping deuterium and hydrogen gas using the membranes with and

without graphene were recorded to study the transfer of protons and deuterons through the

graphene membrane. The authors obtained an ion conductivity of 30 S·cm -2 for protons and

2.1 S·cm-2 for deuterons. The ratio of proton and deuteron conductance obtained is 14:1,

which is in agreement with previous results published by Lozada-Hidalgo et al.[49] where they

11

extend their previous work by demonstrating that electrochemical pumping with centimetre

square sized macroscopic cells could separate hydrogen/deuterium. The improvement

obtained of the ion conductance in comparison with the previous work can be due to the

improvement in the design, which improves the interfacial contact between the Nafion and

graphene.

Different 2D materials have been used to improve the ion conductivity for AEM membranes.

Recently, studies have reported the possibility of using 2D materials such as Layered Double

Hydroxides (LDHs) as inorganic solid electrolytes in alkaline fuel cells and water electrolysis

to improve the ion conductivity of this technology. LDHs present a high ionic conductivity

and stability in alkaline media.[25]

The general formula of LDHs is [M2+ 1-xM3+ x(OH)2]x+ [An- x/n]x- mH2O, where M represents

metal cations and A anions. LDHs are layered compounds which consist of alternate stacked

positively charged layers thinly bonded with charge-balancing interlayer anions. The host

layer is made of divalent/trivalent heterogeneous metal cations placed between two hydroxyl

blocs. To balance the positive host charge, the interlayer hydrated anions are situated

above/below the trivalent cationic centres, creating a hydrogen bond network among

hydroxyl groups, interlayer anions, and water molecules in the gallery.[25, 37]

A large possibility of layered materials with negatively charged host layers and cations in the

interlayer spaces makes LDHs unique. Anionic single-layer nanosheets can be obtained by

exfoliation of lamellar LDHs, which generate properties such as high surface area, as well as

a positive charge.[50] Sun et al.[25] studied the ion conductivity of LDH single-layer

nanosheets. Ion conductivity properties of LDHs laminar or bulk materials have been widely

study,[51] however, some aspect are still unknown such as the intrinsic ion conductivity and

the possible anisotropic transport behaviour. In their work, Sun et al. have studied the in-

12

plane and cross-plane hydroxyl ion conductivities for different single-layer and multilayers

LDHs, lamellar membranes composed of overlapped and stacked LDH nanosheets and for

pellet samples containing randomly orientated lamellar LDHs platelets. Micrometer-sized

hexagon-shaped lamellar LDH platelets were also assembled onto comb electrodes similar to

the single-layer nanosheets.[25, 37] Results reported show in-plane conductivities nearly of 10-1

S·cm-1 at a relative humidity of 80% and 60 oC for the single layer, and values of 10-2 S·cm-1

for multilayers LDHs nanosheets at same condition. Cross-plane conductivities obtained were

ultralow with values of 10-6 S·cm-1. At the same conditions, in-plane ion conductivity values

obtained for lamellar LDH platelets are between 10-5 and 10-3 S·cm-1, which are one to three

orders of magnitude smaller than those obtained for 2D LDHs. A summary of this work can

be found in Figure 4, where schematic diagrams for the measurements of hydroxyl ion

conductivities and the comparison of the ion conductivity measured for the different materials

are presented.

From the results obtained, the authors concluded that the highly anisotropic hydroxyl ion

conductivity was a consequence of the morphological and structural anisotropy of 2D

nanosheets obtained from the exfoliation of LDH crystals. By contrast, the bulk and average

conductivities values obtained from pellet samples containing randomly orientated LDH

platelets are in the same order of magnitude as the platelets measured along the plate plane

direction on comb electrodes. In summary, in-plane conductivity values of LDH nanosheets

are comparable to the proton conductivities of Nafion, showing promise for applications of

single-layer LDHs nanosheets as AEMs membranes.[25, 37]

Because of their particular structure as a lamellar compounds consist of positively charged

and hydrated exchangeable anions located in the interlayers for balance, LDH could build

composite materials with two oppositely charge building block, that can also be a promise

materials for supercapacitor applications.[25, 52] Cationic Co-Al LDH nanosheets and anionic

13

Graphene Oxide were tested showing a good performance without any significant

degradation after thousand charge-discharge cycles.

2.2. Reduced graphene oxide based composites as ion exchange membrane

Inorganic fillers in the polymer membrane have been reported to enhance the ionic

conductivity and separation of reactants due to a range of chemical and physical properties.[53]

Although the single layer 2D materials exhibit exciting performance in electrochemical

devices, achieving those as membranes have several drawbacks such as low yield, demand

for sophisticated technologies and complicated transfer processes. Multilayer graphene sheets

of reduced graphene oxides (often termed as graphene) are in comparison much easier to

produce with high yield. [9, 54, 55] One of the derivatives of graphene, graphene oxide (GO),

because of its properties; electrical insulator, gas impermeability, and hydrophilicity has been

used extensively as a filler material in composite membranes.[4] Recent studies have shown

that reduced graphene oxide improves the conductivity of GO membranes.[56] The high

electrical conductivity of graphene inhibits the use as a membrane material for fuel cells.

However the high surface area, mechanical strength and gas impermeability of the sheets

made researchers approach graphene as a nanofiller.

Quantity and quality of the nanofiller in the polymer matrix together with their interaction

and hence dispersion directly influences the efficacy of the composite membrane

permeability, ionic conductivity, thermal and mechanical properties. Graphene sheets, due to

the van der Waals force, are inclined to agglomerate causing the problem of reduced surface

area, and dispersion. Functionalization of the surface and edges is a way of reducing

agglomeration.[55] Sulfonation is desired for high proton conductivity of aromatic (such as

polymide, poly (ether ether ketone)) PEMs. However, a higher degree of sulfonation leads to

membrane swelling and eventual dissolution. Graphene as a filler material in the polymer

14

matrix has been investigated to achieve flexible and freestanding composite membranes.[35, 36,

38, 57-67] Work based on composite membranes of these multilayer graphene or reduced

graphene oxide as IEMs will be summarized in this section with focus on their components,

synthesis process and performance.

Ye et al. first reported the use of graphene in a high temperature composite electrolyte

membrane.[57] They used ionic liquid polymer modified graphene sheets with polyionic liquid

in a sulfonated polymide (SPI) polymer matrix to achieve a SPI/PIL (NTFSI)-G/PIL

composite membrane via simple solution casting method. The PIL (NTFSI)-G was

synthesised via hydrazine reduction of ionic polymer modified graphene oxide (Hummers

and Offeman’s method). A variation in the PIL (NTFSI)-G loading from 0.1% to 0.9 % gave

enhancement in ionic conductivity compared to the membrane without the ionic liquid

modified graphene (Figure 4a). 0.5% of graphene addition showed four times higher

enhancement in the ionic conductivity at 160 °C with a 20% PIL cost reduction compared to

SPI/PIL membrane without graphene (Figure 4b). Enhancement in mechanical properties was

also reported with the incorporation of PIL (NFTSI)-G (Figure 4c). Composite membranes

show 25 times more storage modulus than the pristine membrane at 150 °C, proving the

composite’s strong mechanical strength and thus expectedly less swelling at higher

temperatures. The improvement in the ionic conductivity was attributed to the property of

graphene sheets to transport ions on their surfaces and through the connected 3D channels

throughout the membrane (Figure 4d). This method of accomplishing bendable and self-

supporting membrane with the incorporation of 2D graphene nanofiller as a crossover barrier

and building block of organized conducting networks is a promising development for PEM.

Tseng et al.[58] reported the incorporation of graphene/poly(sodium-4-styrenesulfonate) (PSS-

G) to enhance the sulfonic acid content in their sulfonated polymide (SPI) membrane in order

to develop a self-humidifying graphene oxide based low cost membrane for direct methanol

15

fuel cells (DMFCs). Tseng and co-workers prepared hydrazine reduced GO/poly(sodium-4-

styrenesulfonate), PSS-G integrated SPI (SPI/PSS-G) membrane with different loading. They

found that optimized SPI/PSS-G 0.5 wt% shows 9.5 and 25 times more selectivity (proton

conductivity/methanol permeability) than Nafion 117 and pristine SPI respectively, at 80 °C

with ~22% tensile strength enhancement compared to pristine SPI. Ye et al. also reported a

graphene composite as an AEM for direct methanol alkaline fuel cells (DMAFCs). [35] They

dispersed GO (Hummers and Offeman’s method) with poly(vinyl alcohol) PVA in water,

reduced by hydrazine solution, cast and alkaline doped to get composite membranes of

PVA/graphene with a thickness of 100-120 µm. Fully exfoliated graphene nanosheets with

high aspect ratio were seen to be well dispersed into the PVA matrix. An optimised loading

of 0.7 % exhibited the lowest water diffusion coefficient of 1.49 10-7 cm-2 s-1 and the best

ionic conductivity (at 80 °C, 21.3 10-2 S cm-1) with lower activation energy (10.8 kJ mol-1)

than pristine PVA. Reduction of crystallinity in PVA-G composites was held to be

responsible for the enhancement of the ion transfer through the amorphous domains together

with the 3D interconnected graphene network (Figure 5). In a single DMAFC test with 2 M

methanol in 5M KOH at 60 °C, PVA-G 0.7 wt% showed peak power density enhancement of

147.6 % compared to pristine PVA, showing prospect as a good AEM. Fuel cell

performances based on 2D material membranes are summarized as a function of various

influencing parameters in Table 2.

Liu et al. prepared highly exfoliated quaternized graphenes (QGs) with transparency like silk

muslin (Figure 6a) and combined with quaternized polysulfone (QPSU) to investigate the

composite as AEMs for alkaline fuel cells (AFCs).[36] Introduction of the quaternary

ammonium groups in the aromatic backbone is an established route to prepare AEMs. The

authors measured bicarbonate conductivities for the testing of the QPSU-QGs composites.

GO prepared by the modified Hummers method was functionalized with 3-amino-

16

propyltriethoxysilane (APTES) to achieve highly dispersible APTES-functionalized graphene

(A-FGs) using a hydrothermal method followed by the opening of the epoxide rings. The

solution casted composite membranes of chloromethylated polysulfone (CMPSU) and QGs

were then quaternized using a trimethylamine aqueous solution, followed by alkalization

using KOH solution. Continuous increment in the loading of QGs in the composites resulted

enhancement in ion-exchange capacity (IEC) and water uptake. With optimized 0.5 wt%

loading, QPSU-0.5-QGs showed the best bicarbonate conductivity (18.7310-3 S cm-1) at 80

°C, four times greater than that of the pristine QPSU membrane (Figure 6b) with a low

swelling ratio, owing to well distributed hydrophilic domains of quaternary ammonium ion

clusters of uniform size in the hydrophobic region of the membrane. The interrelated ionic

conducting pathways (Figure 6c) of the quaternary ammonium ion clusters and their water

absorbing capability are reported to be the reason behind the increased conductivity. Young’s

modulus of QPSU-0.5-QGs was found to be 1.3 higher than the pristine QPSU, with slightly

higher tensile strength. However the QPSU-0.25-QGs composite shows three fold higher

Young’s modulus and tensile strength. In 1 M KOH solution at 60 °C, pristine QPSU showed

slow yet constant degradation, while QPSU-0.5-QGs membrane, although it decreased to

50% of its original conductivity in starting 48 h, showed 40-50% of initial conductivity even

after 10 days. Quaternery groups tend to be unstable over time in concentrated alkaline

solution,[68] the higher conductivity retention of QPSU-0.5-QGs proves the membranes better

stability in alkaline environment suggesting probable longer lifetime of the membrane.

Anion exchange membranes for electrodialysis with high permselectivity and separation

efficiency was prepared by Shen et al., with the modification of sulfonated reduced graphene

oxide (S-rGO) in a commercial AEM.[38] Graphene oxide prepared by modified Hummer’s

method was first treated with sulfanilic acid to decorate its surface with –SO 3- groups and

then coated onto the AEM surface by stir-coating followed by hydrazine reduction. On the

17

permselectivity between Cl- and SO42- (PSO 4

¿ ¿), the S-rGO modified AEM with slightly higher

surface area resistance compared to unmodified AEM showed very good monovalent

selectivity. Figure 6(d and e) shows the results of the monovalent selectivity measurement of

the unmodified and modified AEM after 40 min and 80 min respectively. Zone-I in the plots

with PSO 4¿ ¿ value above 1 and separation efficiency (SE) above zero defines strong

monovalent anion selectivity. Zone-II, with negative SE and PSO 4¿ ¿ value lower than 1,

represents no selectivity of the membrane. All the modified AEMs reported showed

monovalent selectivity compared to the unmodified AEM. Improved conductivity of the

hydrazine reduced S-rGO layer is reported to be the reason behind the enhanced monovalent

selectivity. S-rGO-1 with 10 mins reduction time showed improved separation efficiency than

60 min reduced S-rGO-2. Peeling off of the graphene sheets from the membrane surface

when exposed for longer time in the hydrazine hydrate steam affects the performance and

stability of the membrane and thus the life of the electrodialyzer. A hot press thermal

reduction technique was used by Aragaw et al. to achieve high electron and proton

conducting RGO-Nafion electrolyte membranes from the solution cast GO-Nafion

composites with thickness of 130-300 µm.[59] Ion exchange treatments were done before and

after in-situ thermal reduction to achieve the thermal stability and proton conductivity of the

membrane respectively. The thermal reduction increases the C/O ratio of RGO to 1.12 from

the 0.8 of GO, evidencing the reduction at 200 °C. With the optimal loading of 7 wt% RGO

in Nafion, the composite membrane exhibited 30 times higher proton conductivity than

pristine recast Nafion in wet conditions and 32 fold enhancement in case of ambient

conditions. A rearrangement of the composites proton channels parallel to the membrane

surface during hot pressing is reported to be responsible for the enhancement in the proton

conductivity over the water retention property (Figure 7).

18

To remove the extra weight and space of humidifiers, thereby reducing the complexity and

cost and to make it suitable for portable application, development of self-humidifying

membranes (SHM) has been researched. Proposed by Watanabe almost two decades ago,

combining the diffused H2 and O2 in the Nafion membrane by Pt catalysts, is expected to

reduce the reactant crossover and increase the water content in the membrane.[69] To achieve

the self-humidifying membrane with graphene, Lee et al. investigated microwave reduced,

average 1.8 nm Pt on graphene as the nanofiller in the Nafion matrix.[60] GO produced by the

modified Hummers method, was used as the support for Pt nanoparticles in this in-situ

reduction. The authors prepared Nafion/Pt-G composite with 38% Pt loading on the graphene

by solution casting method. The role of Pt nanoparticles in the membrane is to produce in-

situ water by combining the diffused H2 and O2 from anode and cathode respectively. Three

different loadings of the filler 0.5 wt%, 3 wt % and 4.5 wt% were used for preparing the

composites. Optimized Nafion/Pt-G 3 wt% showed eight fold tensile strength and 40%

enhancement in the proton conductivity than the pristine Nafion. With no humidification, the

composite was reported to retain ~ 50% of its 100% relative humidity (RH) performance

when tested in a 9 cm2 hydrogen fuel cell at 80 °C. In comparison to Nafion/GO composites,

Nafion/Pt-G composites exhibited obvious lower water uptake due to the absence of oxygen

functional groups.

Pt-graphene had also been reported with the incorporation of SiO2 to retain the water by Lee

et al..[61] Controlling the components proportion to get enhancement in water retention and

self-humidifying properties is needed to achieve the optimum performance.

Nafion/xPt-G/ySiO2 composites were prepared and tested under different humidity conditions

of 100% to 0% by the authors. Pt loading of 36.4 % was achieved. Pt-G and SiO 2 contents

were varied from 0.5-1.5 wt % and1-3 wt % respectively. Water uptake and proton

conductivity of the Nafion/xPt-G/ySiO2 composites increased with the SiO2 loading. With an

19

optimized 1.5 wt% content of Pt-G, the fuel cell performance enhances with the increment of

SiO2. However with further increment in Pt/G, the performance reduced owing to the possible

blockage by the fillers and the formation of electronic conducting network in the membrane,

suggested by the decreased impedance with increasing Pt content. Without any applied

humidity, Nafion/1.5 wt% Pt-G/1.5wt% SiO2 composite showed the best performance of

~280 mW cm-2 peak power density in comparison to the overall best with humidification of

~550 mW cm-2 of Nafion/1.5 wt% Pt-G/3 wt% SiO2. Future studies utilizing Pt-G/SiO2

compound instead of separate addition of the two in the composite membrane could be of

interest to look for the synergy due to uniform distribution.

Anantha-Iyengar et al. reported another kind of self-humidifying composite membrane made

of graphene, phosphotungstic acid (PWA) filler and sulfonated poly (ether ether ketone)

(SPEEK) matrix.[62] Similar to Nafion, SPEEK also requires water for its proton conduction.

PWA was used because of its super ionic (highly mobile ions) conductivity and thermal

stability. Commercial graphene was refluxed with concentrated nitric acid to functionalize it

with the carboxylated groups. Solution casted SPEEK/PWA/G(c) composite with the

optimized weight ratio of 75/1.5/0.25 showed maximum proton conductivity (5.19 10-2 S

cm-1), seven times higher than the pristine SPEEK membrane due to the good miscibility and

uniform dispersion. However the authors did not report the actual fuel cell performance.

Limitation of Nafion usage at low relative humidity is challenged in work by Sahu et al..[63, 64]

The MEA dehydrates at low humidification due to the limited availability of water. They

employed sulfonated graphene-Nafion composite membrane for hydrogen[64] and methanol[63]

fuelled PEMFCs at low humidity conditions. Commercial graphene was functionalized by

sulfonic acid and impregnated with Nafion by solution casting method. At 20% RH, the 50

µm thick, optimized 1wt% S-graphene/Nafion composite membrane showed five-fold

enhancement in proton conductivity and five-fold reduction in hydrogen crossover current

20

density than that of pristine recast Nafion membrane (Figure 8 a-c). Sulfonation of the

graphene sheets reported to further enhance the surface area through the incorporation of

sulfonic acid groups into the sheets providing distinct layers with exposed edges. Improved

effective area of reaction and reduced ohmic loss contributes to the higher performance of the

cell at low RH. Even above 80 °C, the composite membrane showed enhancement in proton

conductivity over pristine Nafion membrane and representing the existence of extra sites of

water retention. Higher ion exchange capacity of S-graphene/Nafion than the pristine recast

Nafion and Nafion-graphene infers higher density of -SO3H groups, to act as solid acid

proton conducting material. At 100% RH, the peak power density of S-graphene/Nafion was

only 7.4% and 5.6% higher than the pristine graphene and Nafion-graphene while at 20% RH

the authors reported enhancements of 26% and 18% respectively for hydrogen fuel cell. In

the case of the methanol fuel cell, the S-graphene/Nafion composite membrane with 1 wt%

also increased the performance with 118 mW cm-2 compared to 54 mW cm-2 of pristine

Nafion at 70 °C.[63] The authors claimed that due to the S-graphene integration, the membrane

surface becomes coarser, modifying the hydrophilic domains for high absorption of water and

thereby providing fast proton-transport across membrane with higher mechanical strength

(Figure 8d). Qiu et al. also investigated sulfonated reduced graphene oxide (SRGO) as a filler

for the SPEEK based electrolyte for hydrogen fuelled proton exchange membrane fuel cell. [65]

In order to achieve a working fuel cell at low humidity, the sulfonic acid groups of SRGO

was employed here as well to control the water retention of the composite membrane.

SPEEK/SRGO composite membranes and pristine SPEEK membranes are reported to exhibit

similar ion-exchange capacity (IEC) value; however the proton conductivity of the composite

membranes were higher than the control SPEEK over the whole (50% - 95%) range of RH.

Though, the proton conductivity values are almost same for all the membranes at 95% RH, at

reduced humidification of 50%, SPEEK/SRGO-1 shows three-fold higher proton conduction

21

than the SPEEK, owing to the synergistic effect of the interfacial interaction of the filler and

matrix material. In a single cell assembly, the composite SPEEK/SRGO-1 showed ~ 11 %

improvement over the SPEEK membrane at 70 °C and 80 % RH. However the target study of

lower humidification operation of the cell is not reported by the authors.

Recently, Jia et al. employed ‘slightly reduced’ graphene oxide modified Nafion membrane

via self-assembly as a proton exchange membrane.[66] Because both protons and methanol

have similar movement mechanism,[70] with the improvement of proton conductivity

methanol barrier properties tend to decrease in case of normal PEMs. Selectivity of GO

membranes towards protons over methanol due to the different transportation channels has

already been established by Paneri et al..[71] To increase the stability of GO based membranes

in water, the authors slightly reduced the surface of Nafion-GO membrane and found

remarkably high proton conductivity. Nafion-slightly reduced graphene oxide membrane (N-

srGOM) was prepared by microfiltration of aqueous GO solution with a Nafion binder,

followed by reduction of the filtered membrane in hydro iodic acid solution and acidification

in sulfuric acid. XPS results showed slight reduction in the O/C ratio for N-srGOM (0.96)

from N-GOM (1.0). With 58% content of srGOM, N-srGOM displayed 10 times greater

proton conductivity (0.58 S cm-1) than srGOM and almost 4 and 3 times higher than Nafion

117 and GOM respectively, at 95% RH and 80 °C. Due to the GO base skeleton of the N-

srGOM, even at low humidity conditions of 40%, it showed twice the proton conductivity of

Nafion 117. The high proton conductivity was attributed to the self-assembled long range

order of 3D ionic transportation and strong acidity of the sulfonic groups of Nafion.

Asmatulu and co-workers studied the transport properties of graphene based thin Nafion

membranes in comparison to carbon black (CB) and carbon nanotube (CNT) based

membranes to design a more effective graphene based cathode catalyst layer.[67] Carbon based

Nafion solution with different carbon loading was sprayed over both sides of a bare Nafion

22

membrane. The reported total thickness of carbon material coating over the membrane is

reported to be 12-29 µm. These carbon-Nafion coated membranes were used to study the role

of the different filler materials effect on the proton conductivity. Liquid equilibrated, bare

Nafion swollen membrane at ambient temperature and N2 atmosphere showed 0.21 mS cm-1

proton conductivity compared to reported 0.1 S cm-1. The authors claimed the non-pre-

treatment of the membranes behind this lower value. The work showed that the proton

conductivity value decreased with increased membrane thickness and 2 wt% graphene-

Nafion coated membrane exhibited higher proton conductivity than carbon black and carbon

nanotube based membranes and almost 5 times higher value than bare Nafion 212. The

authors showed that bare Nafion, carbon black and carbon nanotube based membranes show

hydrophilic nature while the graphene-Nafion coated membranes displays hydrophobic

nature enabling quicker water transport. Higher surface area of graphene compared to CB and

CNT facilitates better-quality hydrogen bond network this, together with its hydrophobic

nature, is claimed to be the reason behind the improved proton conductivity.

3. 2D crystal nanosheets for water-splitting applications

Splitting water into H2 and O2 by a photo electrochemical methods using solar energy as a

photon source has been reported as an ideal solution to produce H2 to serve future demand, as

H2.[20, 72] Several issues make the practical application difficult. The most important is the low

quantum efficiency in the visible light region. During the reaction, a recombination between

the electron and hole (electron-vacancy) lead to a decrease in the photocatalytic activity. Ida

et al.[72] in their report explain this particular mechanism. If a nanocrystal in a powder shape

is used as photocatalyst and is illuminated with an energy which is great than the band gap

energy, a pair of electron-holes are generated with a diameter between 500-300 nm. These

electrons created inside the nanocrystal particle must to move to the surface to react with the

water. During movement to the surface, they can recombine or become trapped at defect

23

sites. Semiconducting nanocrystals without internal surface area and defects, which provide

easy and short travel distance, could be the ideal material for this application. Moreover, to

get the four-electrode oxidation of water into oxygen in one nanocrystal particle with a

diameter of 1 nm, it should adsorb four photons with enough energy in a short time. Solar

irradiation has a photon flux density of 2000 µmol·s-1 m-2 with a diameter between 400-

700nm. If it assumed that the nanocrystal particle with a cross-section area of 1 nm2 absorbs

all of the photons, it takes minimum 4 ms for the particle to collide with four photons. On the

other hand, the photoexcited carriers have a lifetime of less than 1 µs. This may explain the

possibility of not obtaining sufficient photon flux density from solar energy to produce

hydrogen and oxygen. If the lifetime of the reaction intermediates is longer, 100 ms, the

reaction may take place on the nanoparticle. TiO2 was the first material used as photocatalyst

for the reaction of water-splitting making semidoncuting oxide nanoparticles one of the most

common photocatalyst for this particular reaction.[20, 73] However, the intrinsic limitations

aforementioned are still presents in these materials. This requirement could be satisfied by a

2D structure nanosheet obtained by exfoliation of a layered compound.[74] A simple

representation of this mechanism can be observed in Figure 9.b).

Theoretical studies have reported[20, 75, 76] the potential application of 2D crystal nanosheets as

photocatalysts for this particular reaction. Li et al.[75] reveal first-principle calculations where

they found that pristine single-layer MoS2 is a good candidate as a catalyst for hydrogen

production and the overall water splitting reaction. Recently, there have been many studies on

different nanosheet photocatalysts which reported[72] the poor photocatalystic activity for

water splitting of photocatlyst which doesn’t present co-catalyst loading. However, it has

been found[77] that specific 2D crystal nanosheets have high photocatalyst activity for

hydrogen evolution from water without co-catalyst loading. Those materials are layered

24

double hydroxide nanosheet-based materials (Zn-Cr-LDHs),[72, 78] MoS2 nanosheets[72, 79, 80] and

graphene-based materials.[72, 81]

MoS2 is in the group of Transition-Metal Dichalogenides (TMDCs) along with Molibdenum

diselenide MoSe2, Tungsten diselenide WSe2, Tungseten ditelluride, WTe2, Molymdenun

ditelluride MoTe2, etc. It have been reported that single-layer MoS2 is an insulator with a

direct bandgap about 1.75-1.9 eV. The bulk MoS2 is not a good photocatalyst, whereas

nanoscale structures such as nanoribbons and nanocluster, present unusual physical and

chemical properties due to quantum confinement[82-84] and possesses photocatalytic activity.[75,

82, 85] MoS2 similar to graphite has a 2D layered structure with interlayer Van der Waals (vdW)

forces in hexagonally packed structures. MoS2 single layer is three atoms thick, with S atoms

above and below and Mo in the middle, bonded by strong ionic-covalent bonds. The distance

between a Mo and its nearest S is 2.41 Å and the thickness of one S-Mo-S layer is ~3.49 Å

suggesting that the interlayer vdW interaction is weaker than interlayer covalent bonding.

Structural studies on the MoS2 show flat polygons of S-Mo-S trilayers. These three layers can

stack in a graphite-like manner or remain as single trilayers depending on the synthesis

process and conditions. When the single layer of S-Mo-S material is formed, two kinds of

surface sites exist, firstly the basal plane and the secondly the edge sites.[86, 87]

Computational[88] and experimental [87] works have demonstrated that the basal plane is

catalytic inert, while the catalytic activity is placed on the edge sites of the MoS2 layers.

Jaramillo et al.[87] reported an experimental study to determine the active sites on MoS2 for

the hydrogen evolution reaction by atomically resolving the surface before measuring the

electrochemical activity in solution. MoS2 samples were prepared in Ultra High Vacuum

(UHV) with specific nanoparticulate morphologies with fractions of the basal and edge sites

systematically varied. Scanning Tunneling Microscopy (STM) was used to characterise the

25

samples. Figure 9 a) shows STM images of MoS2 nanoparticles on Au (111) where it can be

observed the typical polygon morphology with the bright lines along the layer perimeter with

the conducting edge. Three different images can observed in Figure 9 a) which represent

different coverages surface and annealing temperature, a1) low coverage and 400 oC, a2) high

coverage and 550oC and finally a3) is a imaged atomically resolved of a MoS2 particle at

550oC. MoS2 samples were synthetised by CVD on Au(111) support followed by annealing

using the procedure described by Helveg et al..[89] The authors selected Au (111) as substrate

for the samples since it does not present activity for the HER.[84, 90] Larger particle sizes were

obtained by increasing the annealing temperatures. It has been found that sulfided Mo edge of

the structure of MoS2, is the dominant part in terms of catalytic activity and it is favoured by

the particle size.[84, 87, 91] Electrocatalytic activity measurements for HER correlate linearly

with the number of edge sites on the MoS2 catalyst. Thereby, Jaramillo and co-workers

concluded that the catalytic activity of MoS2 nanosheet is mainly present on their edge.

Ambient pressure XPS (AP-XPS) in-situ experiments were carried out by Bruix et al.[92]

allowed them to follow the formation of the catalytically MoS2 edge sites in their active state,

confirming, under electrocatalytic hydrogen evolution conditions, the presence of MoS2 as

the active catalyst. Amorphous MoS3 was used as precursor in this work. In parallel to this

experimental work Density Functional Theory (DFT) was also carried out to elucidate the

resulting edge structures.[92]

Unfortunately, the edge sites are low in concentration in layered materials due to their

inherently high surface energy. Because of this, to create structures where edges of the layers

are predominately exposed and exhibit high surface area is an important challenge.[93] This

led to a synthesis process to grow MoS2 and MoSe2 thin films with vertically aligned layers,

leading to maximum exposure of the edges on the film surface, the work by Kong et al..[94]

This method involves a rapid sulfurisation/selenisation to transform Mo thin films into

26

polycrystalline molybdenum dichalocogenide films giving the metastable molecular structure

described previously. Figure 9 c) represent different layered crystal structure of MoS2 and

MoSe2 obtained by Kong and co-workers. In Figure 9 c.1) it can be observed the layered

crystal structure of molybdenum chalcogenide with individual S-Mo-S or Se-Mo-Se layers

stacked along the c-axis by weak vdW interaction. Figure 9 c.2) represent different

schematics of MoS2 nanoparticles with platelet-like morphology, nanotubes and fullerene of

MoS2 and MoSe2. Finally, Figure 9 c.3) shows the idealised structure of edge-terminated

molybdenum chalcogenide films with the layer aligned perpendicular to the substrate.

Catalytic HER activity was evaluated for both materials MoS2 and MoSe2 which suggest the

Mo atoms are the main active centres on the edges. The exchange current densities obtained

for the edge-terminated films are about ten times larger than previous MoS2 nanoparticles

tested as electrodes[87] and notably better compared to most common metal catalysts. [95] This

increase of the HER catalyst activity is due to the larger number of edge sites. Long terms

experiments also confirm that edge-terminated films MoS2 and MoSe2 are stable in acid

environment and remain intact through repeated cycling. Other reactions such as oxygen

reduction[96] and methane conversion can be also potential applications for these edge-

terminated film materials.

Sulfurisation methodology was used by Jung et al.[97] to study the morphology of MoS2 and

WS2 films grown by sulfurising elemental metal, Mo and W respectively on silica substrates.

They demonstrated the transition of vertical-to-horizontal growth directions in MoS2 and WS2

and reported that the thickness of metal seed layer as a critical factor to control the growth

orientation of these 2D layers. Figure 9 d) shows these different orientations for the

nanosheets. Using the same methodology but with different substrate Wan et al.[98],

demonstrated that MoSe2 and tungsten diselenide, (WSe2) nanofilms with molecular layers

perpendicular to the surface can be grown on rough and curved surfaces, silica nanowires and

27

carbon fibre paper, preferentially exposing the active edge sites for HER, producing highly

active catalysts for HER. Figure 9 e.1) and e.2) show the molecular layers grown on different

substrates flat and curved respectively. Significant improvement of catalytic activity for HER

using MoSe2 and WSe2 on carbon fibre paper obtaining Tafel slopes of 59.8 and 77.4 mV/dec

respectively, compared with the flat films was reported with Tafel slope of 120 mV/dec on

the flat gassy carbon[94, 99] this indicates that the rate is determined by the discharge step with

a small surface coverage of adsorbed hydrogen.[94, 99, 100, 101] Authors concluded that the

roughness and surface curvature are able to expand or squeeze the molecular layers and

because of that, a change on the electrctronic properties of the nanofilms happens modifying

the reaction barriers. This way, the surface would be the responsible for the electrochemical

improvement observed on the Tafel slopes.[93]

A different method to grow the MoS2 thin films is used by Li et al.,[102] in this case an edge-

rich MoS2 thin film nanoplatelet was synthetised by CVD obtaining a material with a large

surface area. CVD parameters were optimised to get a specific MoS2 structure for the HER.

This specific structure consists of large MoS2 platelets with smaller layered MoS2 sheets

growing off it in a perpendicular direction, which increases the total ratio number of edge

sites within a given geometric area. Different substrates were used to grow the nanoplatelet

MoS2 including glassy carbon, graphite, silicon wafers and single-wall nanotubes. Those

support materials for MoS2 thin films can be easily integrated into different electrochemical

applications.[93] By controlling the growth conditions Li and co-workers were able to get

tuneable films thickness and surface morphology, which enables optimisation of

combinations of the active site quantity, interplanar charge transport and sufficient binding

with the substrate. Figure 10 a.1) shows a demonstration of the edge enrichment mechanism

which comprises two main steps, firstly, the primary crystal formation and secondly the edge

enrichment process. Spatial evolution of the as-grown MoS2 thin film in the direction

28

perpendicular to the substrate surface can be also observed in Figure 10 a.2). Electrochemical

characterisation was carried out to evaluate the catalytic activity of this material in HER. It

was concluded that the thickness of the MoS2 films is directly related to the performance of

the reaction, obtaining a lower current density and increase of the impedance by increasing

the thickness for the MoS2 film. The thinnest samples reached a current density of 60 mA·cm-

2 at an overpotential of 0.64V vs RHE with a Tafel slope of ~90 mV·dec -1 and exchange

current density of 23 µA·cm-2. Those results, low Tafel slope and large exchange current

density, demonstrate that high-porosity edge-exposed MoS2 network is a promising structure

for HER catalyst. The HER catalytic performance reported by Li et al. over a given

geometric area with long-term study was the best reported to date. Recent work[103] reported

the synthesis of a fractal-shaped single-layer MoS2 by CVD on fused silica with a large

tensile strain and abundant active edge sites, which also present a better catalytic activity on

HER in comparison with Pt.

In order to determine what is the effect of nanosheet length and thickness of the Transition-

Metal Dichalogenides (TMDs) on the electrocatalytic applications, Gholamvand et al.[104]

have used size-selected WS2 nanosheets with two different electrochemical surface areas for

HER and as electrochemical double layer capacitors electrodes. These particular applications

depend on the interaction of ions with the nanosheet edge, TMDs to catalyse HER, and basal

plane TMD nanosheets for supercapacitors electrodes, which are expected to be affected by

the nanosheet-size. Previous work reported an improvement of the catalytic activity for HER

by using different structured TMD nanosheets, but they did not quantitatively analyse the

dependence of either parameter on size. Supercapacitor electrodes which are fabricated by

TDM nanosheets involve interactions which are localised at the basal plane. A WS2

nanosheet was used as a model system to study this size dependency in both electrochemical

applications. An HER study shows the exchange current density to vary inversely with

29

nanosheet length, which is in agreement with previous theoretical studies and the Jaramillo et

al.[87] work, where they proved that catalytically active site reside on the nanosheet edge. In

the same way, WS2 has been used as an electrical double layer capacitor electrode finding the

volumetric capacitance to scale inversely with the mean nanosheet thickness.

A different approach was developed by Chatti et al. [105] where they grew vertically interlayer

space aligned thin nanosheets supported on rGO by microwave synthesis. A scheme about

how the process was carried is represented in Figure 10 d) After the optimization of the

microwave process and the morphologies, advantageous electrocatalytic properties were

found that were due to the higher concentration and easy accessibility of the edge sites of the

MoS2-rGO catalyst which improve the electrical conductivity.

The incorporation of transition metals such as Co, Ni or Fe into TMDs enhances their

catalytic activity for HER.[106, 107] Theoretical and experimental studies[88, 106, 107] reported that

the binding energy of hydrogen on Mo-edge is smaller than in S-edge, as Mo-edge is more

catalytic active for HER. However, if Co is added S-edge reduced the hydrogen binding

energy, making the S-edge also active for this reaction. Wang et al.,[98] performed a Ni-doped

MoSe2 nanofilm on Si nanowires with a vertically aligned MoSe2 obtaining an improvement

of the activity by increasing the exchange current density to 2.5 10-3 mA·cm-2 (in comparison

with the one obtained without Ni) and without any variation on the Tafel slope.

All the previous studies have concluded the fact to control the nanostructure and engineer the

surface of TDM 2D materials to maximize the number of active edge sites, can enhance the

catalyst activity. However, MoS2 synthesized is a semiconductor with an anisotropic

electrical transport and poor bulk conduction which could limit the catalytic activity. MoS2

and other 2D TMD can be found in different polymorphs which erase different structures and

different electric properties. Two S-Mo-S layers formed by edge-sharing MoS6 trigonal prims

30

lead to the semiconducting and thermodynamically favoured 2H phase, which is the most

common for MoS2. On the other side, a S-Mo-S monolayer built from edge-sharing MoS6

octahedral leads to the metallic phase 1T which cannot be found naturally in bulk [73, 108]. The

structure 1T-MX2 has been widely study and characterized but the catalytic properties for

HER were not clear explored. Chemical exfoliation by lithium intercalation with excess water

to generated H2 and 2D nanosheets separately, is the method used by Lukowski et al.[108] to

convert MoS2 nanostructures with a high density of exposed edges grew by CVD into the

metallic polymorph 1T-MoS2. They demonstrated that the electron transfer form intercalated

Li erases a destabilization of the original trigonal-prismatic phase, 2H-MoS2, and favors the

octahedral phase leading the metallic 1T phase. This approach erases a better metallic

conductivity, kinetics properties and an increment of the active sites in the exfoliated 1T-

MoS2. All these new properties make the material more stable catalytic activity and

competitive for HER. Evidence of enhancement in the catalytic activity was obtained by

Lukowski and co-workers, particularly in the Tafel slopes with values of 54 mV·dec-1 for the

exfoliated 1T-MoS2 nanosheets and 110 mV·dec-1 for the 2H-MoS2 grew by CVD. Lithium

borohydride (LiBH4) was used by Voiry et al. [109] as lithium source to intercalate and

exfoliate bulk MoS2 powder into layered 1T-MoS2. After remove the extra negatives charges

produced due to the electron transfer between LiBH4 and MoS2, they proved an excellent

catalytic activity of 1T-MoS2 for HER obtaining a Tafel slope of 40 mV·dec-1. A significant

reduce of the catalytic activity of the 2H-MoS2 was found after the partial oxidation of the

edge, while 1T-MoS2 remains unaltered after the same oxidation. The polarization curves

obtained are represented on Figure 10 b). This fact suggests that the edge of the metallic

nanosheets are not the main source of active sites[109]. Maitra et al.[19] also synthetized MoS2

by lithium intercalations followed by exfoliation to obtain the 1T form. Their studies reported

an extraordinary performance of 1T-MoS2 as catalyst in HER with a TOF of 6.25 h -1. Based

31

on these results, Gupta et al. [73] prepared the metallic 1T phase of MoSe2 from bulk MoSe2,

also prepared by lithium intercalation and exfoliation, in order to compare with their

analogous 1T-MoS2 and 2H-MoS2. .After the proper characterization of the material, they

was employed for the visible-light induced generation of hydrogen, obtaining the higher

hydrogen evolution activity for 1T-MoSe2 showing a better performance even than 1T-MoS2.

Similar results were obtained for 2H structure, obtaining also more hydrogen generation with

MoSe2 than MoS2. Theoretical analysis were carried out by the authors to explain this

particular behaviour, which revel that MoS2 present a lower work function in comparison to

MoS2 and 1T-structure has lower work function than the 2H structure than both MoX2 tested

in their work (MoS2 and MoSe2), supporting the experimental work.

A different approach was also studied by Maitra et al [19], who developed some investigations

on visible-light hydrogen generation using 2H-MoS2 layers and its composites with exfoliated

graphene (EG)-MoS2(2H) and nitrogen-doped graphene (EGN)-MoS2(2H), based on the idea

of graphene can act as a channel to transfer electrons to MoS2 in graphene-MoS2 composites,

as Min et al. demonstrated in a previous work. In this work, Min et al. [110] studied that

growing MoS2 layers on rGO sheets can be a catalyst for HER providing more accessible

catalytically edge sites, proving that MoS2/rGO can catalyse protons more efficiently than

aggregated pristine MoS2 under visible light irradiation. The union between nitrogen doped

graphene (n-type) with MoS2, p-type, gives the formation of a p-n junction composites,

which enhance the electrochemical activity.[80, 111] Results obtained supports the fact of N-

doped graphene favors the electron transport, improving the conductivity thereby an

enhancement of HER activity.

Pramoda et al.[23] based in the same idea demonstrated by Min et al.[110] and in the fact of the

poor electronic conductivity of MoS2 and C3N4, investigated the positive effect of covalent

bonding between those materials. C3N4-MoS2 and C3N4-NrGO nanosheets were covalently

32

cross-linked synthesized by carbodimde method. The composites obtained by this method

show better catalytic activity than the physical mixtures made with each composites or C3N4

alone. Results proved a notably enhance of the photochemical HER activity for C3N4-MoS2

composites which presents ~246 times more activity compared with the alone component

C3N4. Furthermore, an improvement on the HER activity of ~68 times and 11 times higher

was obtained for cross-linked C3N4-MoS2composites in comparison to the activity obtained

for the physical mixture and the solid state method respectively. Authors concluded, based on

the experimental and theoretical studies carried out that, the encahncedment obtained for

HER activity with cross-linked composites is due to the increased planarity, improved the

charge transfer and higher surface area. Electrochemical HER activity was also improved by

using the covalent cross-linked composites.

More recently, also Pramoda et al.[22] further investigate the idea of use borocarbonitrides

(BCN) covalently linked with MoS2 to get an improvement in HER activity since both

components have been reported such as metal free active catalyst for HER[112]. This study

supports their previous work[23] showing the advantages of cross-linking of catalytically

active heterolayers materials. Making use of the different functional groups present on the

surfaces of BN (-NH2) and graphene (-COOH) domains of the BCN, two nanocomposites

were synthetized having designated as CN/BCN-MoS2 and G/BCN-MoS2. The main

difference between each other is the location of the cross-linking, which depend on the

domine in the borocarbonitride participate in cross-linking. The schematic trategy that the

authors followed to synthesized the nanocomposites can be observed in Figure 10 c).

Electrochemcial HER activity obtained for G/BCN-MoS2 shows an important enhanced with

a Tafel slope of 33 mV·dec-1 which is similar that the Tafel slope obtained for Pt (29 mV·dec-

1) However, the performance obtained for CN/BCN-MoS2 is lower than G/BCN-MoS2 which

can be explained because the absence of conductive carbon. Authors concluded that the

33

enhanced obtained for HER activity by using cross-linked composites is due the increased of

the charge-transfer rates and from the more active sites exposed. Photochemical activity HER

also present an improvement using cross-linked composites, implying the covalent cross-

linking can be a valuable strategy for HER and similar reactions.

Table 3 summarized the most significant electrochemical parameters for the catalyst activity

on HER. Exchange current density (j0), Tafel slope, turn over frequency (TOF) and the

presence of long terms study are presented in this Table. The catalyst activity obtained for the

layered 2D crystals are compared with previous results obtained for similar material in a bulk

or nanoparticle features.

4. Conclusions

It is well known that the necessity is to find a viable and renewable energy system. The

massive consumption of fossil fuels and the large quantities of pollutant gases produced

during its combustion leads great efforts to find an alternative fuel. Hydrogen can produce

energy by combustion without any contaminate and has the largest energy density which

make it the best candidate to replace fossil fuels. The use of hydrogen in different

electrochemical applications to produce energy and its production by electrolysis or water

splitting is two important research fields to achieve this new energy system.

In PEM fuel cells, high proton conductivity with mechanical strength and low reactant

crossover are desired from an ideal polymer electrolyte membrane. A single layer graphene

or hBN improves significantly the performance of DMFC, however the difficulty of growth

and transfer of this single layer 2D material add an extra cost to the fuel cell technology.

Graphene or functionalized graphene as filler in the polymer matrix showed to enhance the

desired properties when integrated in optimum loading, being more economical and feasible

process than the single layer 2D materials. It is seen for all the instances that higher amount

34

of filler loading decreases the performance by blocking the ion pathways; hence an optimal

loading level is needed to be decided for individual study. An exact comparison of the

reviewed membranes to find the best one isn’t possible, as the conditions of testing were

different for most of them. All the mentioned parameters plays major role in the final

performance of the membrane, however, not all of them are focussed by the researchers when

investigated a membrane, even for the same targeted application. Some reports were not even

discreet with many of the influencing experimental conditions. A standard practice to use a

set of experimental conditions for the same application must be prepared and followed by the

scientific community in order to promote the significant growth made by each contribution.

It has been found that TMDs based nanomaterials show good catalytic activity in hydrogen

evolution reaction in comparison with the Pt-based catalyst and also, TMDs, particularly

molybdenum and tungsten sulphides, are made of more abundant and economical materials.

However, it has been proved that the catalyst activity of these materials is more intense in the

edge of the single layer MoS2. Thus, creating structures with edges to have predominated

exposed surface area has become an important challenge. This review provides a highlight of

literature for the preparation of the different structures of TMD single layer crystal to

improve the aforementioned edge exposed surface area. These materials show comparable

exchange current densities and Tafel slope and could be a promising catalyst for HER.

However, some drawbacks are still present. Even though the TMDs are economical materials

than Pt-based materials, the overall cost of the catalyst deepens on the preparation methods,

as can be seen when the single layer MoS2 is deposited on gold Au (111) which is quite

precious and expensive material.

The novel properties of graphene and other 2D materials have attracted huge attention to be

used in electrochemical applications. A lot of research is pouring in in the designing of 2D

material based membranes with guidelines on the optimization or controlling parameters to

35

improve the performance of the particular device. The current report aimed to briefly discuss

the role of 2D materials for ion-exchange membranes and water splitting electrocatalysts.

Even though we are still much behind in replacing the commercial materials, advances in 2D

material chemistry keeps opening stimulating pavements for future research. Various

explored challenges in those fields are summarized and reviewed in this article with the

discussion of still existing problems for the commercialization of the materials.

Acknowledgements

The authors would like to thank the EPSRC- Elucidation of membrane interface chemistry

for electro-chemical processes (EPSRC Gran No. EP/P009050/1) and 2D materials as the

next generation membranes in hydrogen generation and low temperature fuel cells (EPSRC

Grant No EP/N013670/1) for financial support.

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Bibliography

Maria Perez-Page is research associate in the School of Chemical Engineering and Analytical

Science. She obtained her BSc and PhD degree on Chemical Engineering from the University

Polytechnic of Valencia (Spain). During her PhD she started to work with PEM fuel cell. She

worked as postdoc at Department of Chemical Engineering and Material Science, University

of California, Davis during 2 years where her research was focused on synthesis and

characterization of nonporous materials. She joined the University of Manchester on 2016

where her work focuses on the application of 2D materials in electrochemical energy storage,

particularly fuel cells.

Madhumita Sahoo joined as a research associate in the

School of Chemical Engineering and Analytical Science on July, 2017. Her work in the

University of Manchester is on 2D materials for fuel cells. Prior to joining the university, she

worked as a project officer for Na ion battery anode materials at Indian Institute of

Technology Madras (IIT M, India) for one year. She obtained her Ph. D degree in 2015 in

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Engineering and Analytical Science, University of Manchester. He is Chemist by first degree

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43

Schematic 1: Schematic diagram of the focussed 2D materials and their electrochemical applications.

44

aa) b)

d)c)

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47

Figure 4: a) Ionic conductivities as a function of temperature of SPI/PIL(NTFSI)-G/PIL membranes in

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Figure 5: Illustration of the ion transport mechanism in the PVA and PVA/graphene composite membranes

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Figure 7: Schematic of proton and electron transport mechanism: a) electron withdrawing groups of Nafion

shifting electron density of RGO and functional groups of RGO contributing for proton transport, b) the change

in alignment of proton transport channels as a result of the annealing of Nafion during hot press thermal

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51

Figure 8: a) Proton conductivity as a function of relative humidity, b) proton conductivity as a function of

temperature for composite membranes; c) H2 crossover current and d) stress−strain curves for pristine Nafion

and Nafion−S-graphene composite membrane at ambient condition Reprinted with permission from A. K. Sahu,

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52

a) a1) a2) a3)(a) (a.1) (a.2) (a.3)

(b.1)

(b.1)

b)b1) b2)

b3)

(c)(c.1) (c.2)

(c.3)

c) c1)

c2)

(d) (d.1)

(d.2)

d)d1)

d2)

(e)

(e.1)

(e.2)

Figure 9: (a) Series of STM images of MoS2 nanoparticles on Au(111) at different coverages surface and

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13, 1341. Copyright (2013) American Chemical Society.(d) Schematics for the horizontal and vertical growth of

2D TMDCs[97] Reproduced with permission from Y. Jung, J. Shen, Y. Liu, J. M. Woods, Y. Sun, J. J. Cha, Nano

Letters 2014, 14, 6842. Copyright (2014) American Chemical Society. (e)Schematic of MoSe2 and WSe2

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Yan, N. Liu, Y. Cui, Nano Letters 2013, 13, 3426.Copyright (2013), American Chemical Society.

53

e2)

e)e1)

(a)(a.1)

(a.2)

(b)

(c) (d)

Figure 10: (a) Configuration and edge enrichment mechanism of the vertically aligned MoS2 network. (a.1)

Mechanism (a.2) Spatial evolution of the as-grown MoS2 thin film[102]. Reproduced with permission from S. Li,

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54

Table 1: 2D material based ion exchange membranes

Membrane material[thickness in µm]

Ionic conductivityActivation energ

y (kJ·

mol-1)

Tensile strength (MPa)

Water uptake (%)

IEC (mmol·

g-1)

Methanol permeability(10-7cm-2.s-1)

Application

Type, meas.

ConditionT (oC)/RH

(%)

Value (mS·cm-1)

T (

oC)/RH

(%)

Value (%)

[44]SLG + N 117 hBN + N117 N117

[183]

H+~1333~1333~1333

- - - -0.190.2750.292

Tested in DMFC

[46]N212 + SLG + N212 N 212 [50.8]

H+ 120~128.4 -

- - - 0.441.4

Tested in passive DMFC

[58]SPI/PSS-G 0.5 wt% Pristine SPI

N117

H+

90/60~0.12~0.02~0.06

-64.4551.61

38

82.5576.42

-

2.5942.604

-

40.0544.45

-

Possible PEM for PEMFC

[17] PVA-G 0.7 wt% PVA [100-120]

OH-

80 (30)/-213 (117)127 (52)

10.815.9

Dry (wet)

~18.5 (25)~41 (23)

* water diff.

coefficient (

10-7

cm-2 s

-1)

1.493.37

- 1.914.28

Tested in DMAFC

[18] QPSU-0.5-QGs Pristine QPSU

HCO3-,

25 (80) /-8.95 (18.73)2.24 (4.73)

- 7572

25 (60) 8.5 (15)6.8 (13)

1.21.11

Not applicablePossible AEM for AMFC

[66]N-srGOM N 117

H+

100 / 40(80 / 95)

43 (580)~20 (~145)

20.2627.98

- 25 /- 50-

1.50.9

40/80 v/v%

CH3OH/H2O

0.025.5

Possible PEM in DMFC

55

~values are extrapolated from the reported figures

Table 1: 2D material based ion exchange membranes (continued)

Membrane material[thickness in µm]

Ionic conductivity Activation

energy (kJ·mo

l-1)

Tensile strength (MPa)

Water uptake (%)IEC (mmol·

g-1)

Methanol permeability(10-7cm-2·s-1)

ApplicationType, meas.

ConditionT (oC)/RH

(%)

Value (mS·cm-

1) T(oC)/RH(%) Value (%)

[62]SPEEK/PWA/G(c)(75/1.5/0.10) Pristine SPEEK

H+

65 / 100 46.6

7.05- - 40(80)/

-

38.9 (73.2)

41.7 (55.3)-

40 (80)

/-

* uptake (%)30.9 (58.3)

36.6 (80.4)

Possible PEMs for

IT-DMFC

[63]Nafion-S-graphene 1wt% Pristine Nafion [50]

H+

80 / 100104±1

65.3±0.5

10-13

14.5

12.78

9.375

25

-

27.32±0.7

20.12±0.5

0.96 ± 0.01

0.88 ± 0.01

3.72 ± 0.1

5.53 ± 0.1(10-7 mol.cm-2 s-1)

Tested in CH3OH-

O2

DMFC

[64]Nafion-S-graphene (1%) Recast Nafion

H+

80 /100 (20)

104 (17)

65.3 (3.5)

Same as

earlier[48]

Same as earlier[48]

Same as earlier[48] Same as earlier[48]

Not applicable Tested in H2-O2

PEMFC

[65]SRGO/SPEEK-1.0

SPEEK control

H+

80 / 50 (95)

8.6 (146.5)

2.6 (131.1)-

25/70

40.1

28.6

80/50

(95)

16.8 (31.1)

13.4 (27.9)

1.69

1.64Not applicable

Tested in H2-Air

PEMFC

[60]Nafion/Pt-G 3 wt% Casting Nafion

H+

- / -

(*S cm-2)~0.1~0.07

- 76.719.41

- / - ~30~33

- Not applicableTested in H2 –O2

PEMFC

[61]Nafion/ 1.5 wt% Pt-G/ 3 wt% SiO2

Casting Nafion

H+

- / -~82.5

~63

- ~75

~7.5

- / - ~27~23

-Not applicable

Tested in H2 –O2

PEMFC

56

~values are extrapolated from the reported figures

57

Table 2: Comparison of fuel cell parameters and performances.Fuel cell

typeArea of the cell

(cm2)

Flow rates of the

reactants: fuel/oxida

nt(ml/min)

Catalyst loading:

anode/cathode

(mgPt cm-2)

Operating condition

Maximum power density(mW cm-2)

Investigated membranematerialT(°C) Humidity

(%)

CH3OH- dry air

DMFC[44]

2 50/1000(1M

aqueous CH3OH)

1/1 70 0 75

60

50

SLG + Nafion 117

hBN + Nafion 117

Nafion 117

Passive CH3OH[46]

1(5 M

aqueous CH3OH)

4/2 80 0 25.2

24.9

N212 + SLG + N212

Nafion 212

CH3OH- dry O2

DMFC[63]

4 2 / 250(2M

aqueous CH3OH)

2 / 2 70 0 118

54

Nafion-S-graphene 1wt%

Recast Nafion

CH3OH - humidified

O2

DMAFC[35]

3 5 / 100(2 M

CH3OH in 5M KOH)

5 / 5 60 Value not

available

45.8

18.5

PVA-G 0.7 wt%

PVA

H2 –air PEMFC[65]

6.25 200 / 500 0.5 / 0.5 70 80 705

636

SRGO/SPEEK-1.0

Pristine SPEEK

H2-O2

PEMFC[64]25 450 / 600 0.5 / 0.5 70 100 / 20 667 / 300

720 / 220

Nafion-S-graphene(1%)

Recast Nafion

H2-O2

PEMFC[61]9 100 / 100 0.2 / 0.3 80 100 / 0 ~550 / ~280

~350 / ~30

Nafion/1.5 wt% Pt-G/ 3 wt% SiO2

Casting Nafion

H2-O2

PEMFC[60]9 100 / 100 0.2 / 0.3 80 100 / 0 ~500 / ~230

~350 / ~30

Nafion/Pt-G 3 wt%

Casting Nafion

~ values are extrapolated from the reported figures

58

Table 3: Electrochemical parameters for water splitting catalystMaterial Nanostructure j0 (A·cm-2) Tafel slope (mV·dec-1) TOF (1·s-1) Long-term study References

MoS2 Thin films vertically aligned flat surface 2.2·10-6 105-120 0.013 at η = 0 V yes [94]

MoSe2 Thin films vertically aligned flat surface 2·10-6 105-120 0.0414 at η = 0 V yes [94]

MoS2 Single layer 1.26·10-7-1.73·10-7 73-56 0.02 at η = 0 V [87]

MoS2 Platelets 1.58·10-6 98 yes [102]

a)MoSe2 Thin films vertically aligned rouge and curve surface

3.8·10-4 59.8 yes [98]

WSe2 Thin films vertically aligned rouge and curve surface

77.4 yes [98]

b)MoSe2 Thin films vertically aligned rouge and curve surface

63.9 yes [98]

Ni-MoSe2 Thin films vertically aligned rouge and curve surface

2.8·10-3 62.1 [98]

MoS2 Thin films horizontally aligned flat surface

[97]

WS2 Thin films horizontally aligned flat surface

[97, 101]

MoS2 Nanoparticle 4.6·10-6 120 [107]

WS2 Nanoparticle 135 [107]

Co-MoS2 Nanoparticle 101 [107]

Co-WS2 Nanoparticle 132 [107]

MoS2/RGO Nanoparticle 41 yes [113]

MoS2-rGO(microvawe)

Vertical aligned interlayer nanosheets on rGO

63 yes [105]

a) and b) are similar material b) is thicker than a

59

Table 3: Electrochemical parameters for water splitting catalyst (cont.)Material Nanostructure j0 (A·cm-2) Tafel slope (mV·dec-1) TOF (1·s-1) Long-term study References

MoS2-carbon cloth Nanoparticles 9.2·10-3 50 [114]

1T-MoS2 Nanosheet 43 yes [108]

2H-MoS2 110 [108]

1T-MoS2 Nanosheet 40 0.00172 [109]

MoS2/N-rGO 7.2·10-4 41.3 [115]

NEG-MoS2(2H) 0.000125 [109]

NEG-MoS2(2H)c) 0.00319 yes [109]

EG-MoS2(2H) 0.0000583 [109]

rGO-MoS2 0.0008055 [109]

BCN 62 [22]

BCN-MoS2 (physical mixture 1:2)

71 [22]

CN/BCN-MoS2 2:1Covalent cross-linked composite

58 [22]

CN/BCN-MoS2 1:1Covalent cross-linked composite

46 [22]

CN/BCN-MoS2 1:2Covalent cross-linked composite

36 [22]

G/BCN-MoS2 1:2Covalent cross-linked composite

33 [22]

rGO-supported MoS2

Nanoparticles 41 [113]

MoS2-rGO Nano sheets 63 [105]

MoS2 quantum dots-rGO

101 [116]

MoS2-amorphous carbon

Nanoparticles 0.474·10-3 40 yes [117]

c)Higher content of Nitrogen

60

Table 3: Electrochemical parameters for water splitting catalyst (cont.)Material Nanostructure j0 (A·cm-2) Tafel slope (mV·dec-1) TOF (1·s-1) Long-term study References

MoS2-N-doped graphene aerogel

aerogel 230 yes [118]

MoS2-carbon aerogel

Aerogel 59 [119]

MoS2-C3N4 nanoparticle 95 [23]

Pt/C (40 wt%) Nanoparticle 29

61