Renewable hydrogen production

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. (2007)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/er.1372

Renewable hydrogen production

John Turner, George Sverdrup*,y, Margaret K. Mann, Pin-Ching Maness, Ben Kroposki,Maria Ghirardi, Robert J. Evans and Dan Blake

National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393, U.S.A.

SUMMARY

The U.S. Department of Energy and the National Renewable Energy Laboratory are developing technologies toproduce hydrogen from renewable, sustainable sources. A cost goal of $2.00–$3:00 kg�1 of hydrogen has been identifiedas the range at which delivered hydrogen becomes cost competitive with gasoline for passenger vehicles. Electrolysis ofwater is a standard commercial technology for producing hydrogen. Using wind and solar resources to produce theelectricity for the process creates a renewable system. Biomass-to-hydrogen processes, including gasification, pyrolysis,and fermentation, are less well-developed technologies. These processes offer the possibility of producing hydrogenfrom energy crops and from biomass materials such as forest residue and municipal sewage. Solar energy can be used toproduce hydrogen from water and biomass by several conversion pathways. Concentrated solar energy can generatehigh temperatures at which thermochemical reactions can be used to split water. Photoelectrochemical water splittingand photobiology are long-term options for producing hydrogen from water using solar energy. All these technologiesare in the development stage. Copyright # 2007 John Wiley & Sons, Ltd.

KEY WORDS: hydrogen production; renewable energy; electrolysis; solar; wind energy; photobiology; biomass

1. INTRODUCTION

Providing affordable, reliable, environmentallysustainable energy to the world’s populationpresents a major challenge for the first half of thiscentury and beyond. Global population is pre-dicted to increase by a factor of 36% to 8.9 billionpeople by 2050, and global primary energyconsumption is projected to increase by 77% to837 quads during the same time period [1–3]. Inthe United States alone, energy consumption is

expected to increase from 102 to nearly 200 quadsbetween now and 2050 [4–6]. Hydrogen producedfrom renewable energy sources offers the promiseof a clean, sustainable energy carrier that can beproduced from domestic energy resources aroundthe globe. Realizing this promise will requiretechnological advances in producing, storing, andusing hydrogen.

The U.S. Department of Energy (U.S. DOE) hasset a cost goal for hydrogen at $2.00–$3:00 kg�1;including production, delivery, and dispensing [7].

*Correspondence to: George Sverdrup, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393,U.S.A.yE-mail: george [email protected]

Received 2 February 2007Revised 28 February 2007Accepted 21 March 2007Copyright # 2007 John Wiley & Sons, Ltd.

This is the level at which U.S. DOE estimatesthat hydrogen will be cost competitive withpetroleum fuels.

In this paper we summarize the status ofrenewable hydrogen production technologies (seeFigure 1), with an emphasis on their application inthe United States. Section 2 describes the status ofelectrolysis technology, Section 3 discusses bio-mass to hydrogen, Section 4 discusses fermenta-tion, Section 5 presents the status of thermolysisvia solar-driven thermochemistry, Section 6 dis-cusses photolysis via photoelectrochemistry, andSection 7 discusses photolysis via photobiologicalwater splitting.

2. ELECTROLYSIS

The electrolysis of water is an electrochemicalreaction that requires no moving parts and a directelectric current (DC), making it one of the simplestways to produce hydrogen. The electrochemicaldecomposition of water into its two constituentparts is reliable and clean and, when water vapor isremoved from the product, capable of producingultra-pure (> 99:999%) hydrogen. The electrolyticproduction of hydrogen with carbon-free electri-

city sources is currently the only way toproduce large quantities of hydrogen withoutemitting the traditional by-products associatedwith fossil fuels.

2.1. Description

At 100% efficiency, 39 kilowatt-hours (kWh) ofelectricity and 8.9 liters (l) of water are required toproduce 1 kilogram (kg) of hydrogen at 258C and1 atmosphere pressure. Typical commercial elec-trolyzer system efficiencies are 56–73%, whichcorresponds to 70.1–53:4 kWh kg�1 [8]. Manufac-turers currently produce two basic types of low-temperature electrolyzers, alkaline, and polymerelectrolyte membrane (PEM).

The alkaline electrolyzer is a well-establishedtechnology that typically employs an aqueoussolution of water and 25–30wt% potassiumhydroxide (KOH) as an electrolyte. However,sodium hydroxide (NaOH), sodium chloride(NaCl), and other electrolytes have also been used.The liquid electrolyte enables the conduction ofions between the electrodes. It is not consumed inthe reaction, but needs to be replenished periodi-cally because of other system losses. Typically,commercial alkaline electrolyzers are run with

Figure 1. Renewable pathways for hydrogen production.

J. TURNER ET AL.

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. (2007)

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current densities in the range of 100–300 mA cm�2:The reactions for the alkaline anode and cathodeare shown in Equations (1) and (2), respectively.The reactions show the hydroxyl ðOH�Þ iontransport:

4OH�1ðaqÞ ! O2ðgÞ þ 2H2Oð1Þ ð1Þ

2H2Oð1Þ þ 2e� ! H2ðgÞ þ 2OH�ðaqÞ ð2Þ

A second commercially available electrolyzertechnology is the solid PEM. PEM electrolysis isalso referred to as solid polymer electrolyte orproton exchange membrane (also PEM), but allrepresent a system that incorporates a solidproton-conducting membrane that is not electri-cally conductive. The membrane serves a dualpurpose, as a gas separation device and an ion(proton) conductor. High-purity deionized (DI)water is required in PEM-based electrolysis, andPEM electrolyzer manufacturers regularly recom-mend at least 1MO-cm resistive water to extendstack life.

DI water is introduced at the anode of the cells,and a potential is applied across the cell todissociate the water. The protons ðHþÞ are pulledthrough the membrane under the influence of anelectric field and rejoin with electrons beingsupplied by the power source at the cathode toform hydrogen gas. PEM electrolyzers are oper-ated at higher current densities ð> 1600 mA cm�2Þ;almost an order of magnitude higher than theiralkaline counterparts. Stack efficiency decreases ascurrent density increases, but high current densityis necessary to increase hydrogen production tooffset the higher capital costs of PEM cells. PEMadvantages over alkaline electrolyzers include theability to maintain a significant differential pres-sure across the anode and cathode and avoidingthe risk of high-pressure oxygen. In addition, PEMelectrolysis requires DI water but avoids thehazards surrounding KOH. The PEM anode andcathode reactions are described in Equations (3)and (4), respectively:

2H2O! 4Hþ þ 4e� þO2 ð3Þ

4Hþ þ 4e� ! 2H2 ð4Þ

2.2. Potential

Coupling renewable energy systems withhydrogen-generating electrolyzers has the poten-tial to provide low-cost, environmentally friendlyelectricity and hydrogen. Using available windand solar energy offers a large potential forhydrogen production via electrolysis. To meetthe DOE cost goals of $2.00–$3:00 kg�1; hydrogenproduction via electrolysis needs installa-tions where electrolyzer capital costs are low,less than $800 kW�1; and to electricity that costsless than $0:055 kWh�1:

The United States has great potential forboth wind and solar electricity and thus forhydrogen produced from these renewable electri-city sources. Figure 2 shows the total of hydrogenproduction resource potential across theUnited States (in thousands of kilograms persquare kilometer per year) from wind. Forreference, in the United States, the total landarea of onshore Class 4 and higher wind areas is568 944 square kilometers (km2) after standardexclusions (such as the U.S. National ParkService areas, Fish and Wildlife Service areas,federal lands with special designations, ForestService lands, and Department of Defenselands) are applied. Assuming 5 megawatts ðMWÞkm�2 installed nameplate capacity, wecalculate a potential of 2 845 000MW for thisarea. The amount of hydrogen that can beproduced from this area from wind can becalculated by multiplying the amount ofenergy produced by the electrolyzer energyneed. Using 58 kWh kg�1 for the electrolyzerenergy (a system electrolyzer efficiency of67%) leads to a hydrogen production rateof 154 billion kg yr�1 from Class 4 and higherwind in the United States. In comparison,the United States consumed 140 billion gallonsof gasoline in 2004. One kilogram of hydrogen isapproximately equivalent to one gallon of gasolineon an energy basis; hence, all the current U.S.gasoline needs could potentially be suppliedwith wind-generated hydrogen. Of course, thisillustrates resource (wind) potential, not therequired economic production, delivery, andstorage of hydrogen.

RENEWABLE HYDROGEN PRODUCTION


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2.3. Status

Electrolysis was discovered in the 1800s. Theelectrolyzer industry grew substantially duringthe 1920s and 1930s. Multimegawatt hydrogenproduction facilities that incorporated alkalineelectrolyzers were developed during this time andwere installed near hydroelectric plants thatsupplied an inexpensive source of electricity.PEM electrolyzers were originally developed inthe United States as part of the space program inthe 1960s. Alkaline and PEM electrolyzers arecommercially available in a variety of sizes fromsmall laboratory models to high-production sys-tems rated higher than 2MW. Advanced concepts

and research into electrolysis include the examina-tion of higher pressure operation, with reducedcompression in hydrogen systems, and operationat elevated temperatures to improve efficiencies.

Integrating electrolyzers with renewable energysystems can present challenges as well as uniquebenefits. Currently most renewable energy systemsproduce power and interconnect with the electricalgrid via some form of power electronics. To useelectrical grid power, today’s commercial electro-lyzers include a power electronics interface thatcan represent a significant portion of the overallsystem cost. Several potential applications forelectrolysis use solar- and wind-produced elec-tricity. Since photovoltaics (PV) produce DC

Figure 2. Potential for hydrogen production from wind resources [9] (a solar map appears as Figure 10 in Section 5).

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electricity, it can be directly connected to anelectrolyzer stack. PV can also be tied to theelectric power system through an inverter and cansupply energy to the grid, which is then used toproduce hydrogen via electrolysis. This approachallows for the solar electricity production to belocated separately from hydrogen production.Two similar pathways can use wind energy as anelectricity source. Wind energy can be put onto theelectric power system and then transferred to thehydrogen generation point via the grid;or wind electricity can be used to coproducehydrogen and grid electricity at the wind site.Figure 3 shows two scenarios: central hydrogenproduction versus distributed hydrogen produc-tion via electrolysis.

2.4. Barriers to achieving potential

There are opportunities for reducing the capitaland operating costs of electrolyzers; however,electricity prices are key to hydrogen cost viaelectrolysis. Analyses of production, operation,and maintenance costs demonstrate that to meet

the U.S. DOE target of $2.00–$3:00 kg�1 hydro-gen, electrolyzers with today’s efficiencies wouldneed to have access to electricity prices lower than$0.045–$0:055 kWh�1:

Another important driver of the cost of hydro-gen is whether it is produced centrally anddelivered or produced in a distributed fashion atthe point of use. Recent studies have shown thathydrogen produced from wind-generated electrici-ty}either at the wind farm or at the point ofuse}has the potential to meet the U.S. DOE costgoals of $2.00–$3:00 kg�1; including production,delivery, and dispensing [7]. Hydrogen productionat the wind site makes fiscal sense if the wind/hydrogen system can be optimized in such a waythat cost reductions of the combined system offsetdelivery costs. The central and distributed systemswill require increases in electrolyzer efficiencies anddecreases in capital costs from today’s numbers.

2.5. Research approaches

Several current projects in the United States aredesigned to demonstrate the potential of using

Figure 3. Central hydrogen production versus distributed hydrogen production via electrolysis.



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renewables, especially wind energy, to producehydrogen. Details on these projects can be found inKroposki et al. [10]. Research in the United Statesfocuses on exploring synergies from coproductionof electricity and hydrogen to address the inter-mittent nature of wind power [11]. This creates aready source of electricity for times when the winddoes not blow or the demand for electricity is high,consistent support of the electricity grid from off-peak storage of hydrogen, and the potential use ofhydrogen for vehicle use. Research includes com-paring multiple electrolyzer technologies (alkalineelectrolyzers and PEMs) to gauge their efficienciesand abilities to be brought on- and off-line quicklyas well as AC–DC and DC–DC converters todirectly couple the wind turbine to the electrolyzerto achieve efficiency gains.

3. BIOMASS TO HYDROGEN

Biomass has long been considered a leading near-term source of renewable hydrogen [7]. Theresources of lignocellulosic biomass in the UnitedStates amount to between 400 000 tons and 1.3billion dry tons yr�1: The lower value is theeconomic potential at a cost of less than$50 dry ton�1 [7, 12]. The higher value is the latestassessment of the technical potential assumingimprovements in crop science and harvestingsystems, but it does not address economics [13].The more conservative estimate represents thenear-term economic potential for the annualproduction of 40 million tons of hydrogen, enoughto fuel more than 150 million fuel cell vehicles [7].The distribution of biomass resource in the UnitedStates is shown in Figure 4.

A recent analysis of biomass gasification optionshas shown that production of hydrogen is the mostpromising economic route for the conversion ofsyngas to transportation fuels [14]. Furthermore, arecent technoeconomic assessment of hydrogenfrom biomass shows plant gate cost to beapproximately $1.38 per gasoline gallon equivalent(essentially per kilogram hydrogen) at the plantgate for large-scale gasification [15].

The major technical barriers facing biomassthermochemical conversion include the effect of

variable feedstock composition on downstreamprocessing, efficient and durable catalysts for gasconditioning, and efficient heat integration[7, 14, 16]. This is of particular importance forsmall-scale modular systems for distributed pro-duction of hydrogen while costs for distributionand storage remain high and work againstcentralized production plants. Pyrolysis vaporand bio-oil reforming are a better fit for distrib-uted production because of potentially lower costsat smaller scale.

The major biomass-to-hydrogen pathways areshown in Figure 5. There are three majorthermochemical technology routes: gasification,pyrolysis/reforming, and high-pressure water/steam treatment. Centralized and distributedproduction scenarios based on biomass are possi-ble, and feedstock logistics and target marketcharacteristics will determine the best technologyoptions and scale of operation. Milne et al. [16]previously reviewed research in the area ofproduction of hydrogen from biomass, includingconversion of storable intermediates, such asmethanol and ethanol, from the 1980s through2001. Czernik et al. recently updated the work ingasification, pyrolysis, and supercritical waterconversion from 2001 to early 2004 [17]. Thesestudies have shown that thermochemical conver-sion of biomass is a viable near-term option forrenewable production of hydrogen and has thepotential to provide a significant fraction of futuretransportation fuels.

3.1. Gasification

Gasification systems have potential at large scalewhere feedstock is abundant and they can beinterfaced with hydrogen distribution systems suchas pipelines. Hydrogen via gasification may becoproduced with other fuels, such as ethanol, in anintegrated biomass refinery. Theoretically, thehighest yield of hydrogen from lignocellulosicbiomass is from steam gasification and is approxi-mately 17% based on biomass weight; thiscorresponds to a stoichiometric reaction of bio-mass with water when carbon is totally convertedto carbon dioxide (CO2) and two-thirds of thehydrogen comes from water. Practical yields are

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lower because of the need for process energy,although some of that can be recovered from by-products (char and hydrocarbons) as well as

unconverted carbon monoxide (CO) and methaneðCH4Þ that result from the chemical equilibrium ofthe reforming and water/gas shift reactions [18].

Figure 4. Potential for hydrogen production from biomass resources in the United States by county [9].

Figure 5. Biomass-to-hydrogen pathways.



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Oxygen or air gasification followed by water–gas shift is a simple way of producing hydrogenfrom biomass. On the basis of the followingreactions and typical composition for biomass,the stoichiometric yield of hydrogen produced bypartial oxidation is 14.3wt%:

CH1:46O:67 þ 0:16O2 ! COþ 0:73H2

Biomass Syngas

COþH2O$ CO2 þH2

ð5Þ

CH1:46O:67 þ 0:16O2 þH2O! CO2 þ 1:73H2 ð6Þ

Practical yields are lower because in reality asmall percentage of biomass carbon is converted inthe first phase to char, tar, and CO2 resultingin less CO available for the production ofhydrogen by the second-phase water–gas shift.For example, the Renugas Process developed bythe Gas Technology Institute (formerly the In-stitute of Gas Technology) [19] generates 0.31normal cubic meters (Nm3) of hydrogen from1 kg of wood. Assuming that other gas compo-nents such as CO, CH4; and other hydro-carbons could be converted to CO2 and hydrogen,the hydrogen production would increase to 1:19Nm3; which corresponds to a total yield of10.7wt%. An advantage of oxygen or air gasifica-tion is that it requires no external source of energybecause the partial oxidation reactions areexothermic.

A third type of gasification is essentially highseverity pyrolysis, often referred to as indirectgasification (see Figure 6). In this approach thecontinued cracking of the reactive volatiles ther-

mally cracks the vapors to permanent gasesdominated by CO and hydrogen. CH4 and higherhydrocarbons are then removed by catalyticconversion or wet scrubbing. The char is removedfrom the product gas stream and used for processenergy. The Battelle gasifier does this with dualcirculating fluid beds [20]. Although steam gasifi-cation may contribute to the gasification process,the rates are too low at the practical temperaturesof 750–8508C: If steam is used as a carrier gas thena medium Btu gas that is free of nitrogen can beproduced.

Novel approaches to gasification have beendescribed in the literature in the context ofhydrogen production. Hydrogen from biomass ina thermally ballasted gasifier was studied byBrown et al. [21]. Zhang et al. [22] used thesequential processes of thermal gasification in afluid bed, followed by catalytic steam reforming oftars and water–gas shift, to present results for thegeneration of hydrogen from switchgrass. Degra-dation of the catalysts resulted probably becauseof deposition of coke, sulfur, and chlorine on thecatalyst.

Catalytic gasification has high potentialfor hydrogen production. Asadullah and Miyaza-wa [23] used Rh=CeO2=SiO2 catalysts to develop alow-temperature fluid-bed gasifier. Cellulosewas completely converted to gas products at773K. Rapagna et al. [24] reported on tri-metallicand ternary oxide structures for conditioningthe gas produced by biomass steam gasificationin a fluidized bed of olivine particles. One of thefew reported studies of hydrogen from steamgasification of biomass-derived chars showspromising results (up to 75mol% hydrogen inthe gas) [25].

Recent studies of hydrogen-rich gas productionfrom biomass are given in Lv et al. [26] for steam–air gasification in a fluidized bed. The highest yieldwas 71 g hydrogen per kg biomass, which wasproduced at 9008C; equivalence ratio (oxygenused/oxygen for complete combustion) 0.22, andsteam/biomass (S/B) 2.70. The use of dolomite inthe fluidized bed reactor and nickel-based catalystsin a fixed-bed, downdraft gasifier are described forhot gas conditioning, which is the major technicalchallenge for biomass gasification.Figure 6. Indirect gasification concept [20].

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3.2. High-pressure water conversion

As in steam gasification, the reaction of biomasswith liquid water according to this equation couldproduce the highest hydrogen yield, stoichiome-trically 17wt%, and at higher efficiency than steamgasification, since the heat of vaporization of waterwould not have to be supplied:

CH1:46O0:67 þ 1:33H2O! CO2 þ 2:06H2 ð7Þ

Significant amounts of char and tar, in addition tothe desired gas, are produced when high-tempera-ture steam is used to realize this reaction. Char andtar formation might be avoided when the reactionis carried out in liquid water with an appropriatecatalyst. However, the product gas may includeCO and CH4 from competing equilibrium reac-tions, which would reduce the hydrogen yield.

Early work in high-pressure aqueous systemsused supercritical water. The most extensive workin this area has been carried out at the HawaiiNatural Energy Institute by Antal et al. [27] andhas been reviewed by Milne et al. [16]. In a bench-scale fixed-bed reactor operating at 700–8008C; at28MPa, and using activated carbon as catalyst,they reported gasification carbon conversion forsawdust of 84–100%. Gas composition stronglydepended on the temperature and was close to thatpredicted by equilibrium calculations. Under theseconditions, serious corrosion of the Hastelloyreactor was observed.

Low-temperature catalytic aqueous process wasfirst demonstrated at the University of Wisconsin[28], where researchers used biomass modelcompounds with water at 225–2658C and 27–54 bar using a platinum catalyst. The mostcomplex biomass tested was glucose. Using abench-scale reactor fed with 1% solution of modelcompounds (molar water to carbon ratio ¼ 150),they achieved 84% gasification efficiency (energybalance around the gasifier). Later work used atin-promoted Raney-nickel catalyst [29] and morepractical concentrations (10%) and H2O=C[17, 30].

3.3. Pyrolysis and reforming

A near-term option for the hydrogen productionfrom biomass is a two-step process that includes

pyrolysis of biomass followed by catalyticsteam reforming of pyrolysis vapors or liquids(bio-oil). Pyrolysis is a thermal decompositionprocess that occurs in an inert atmosphere. Highheat transfer systems maximize the productionof volatile intermediate compounds. Such ap-proaches are known as ‘fast pyrolysis.’ Severaltechnologies at the pilot and demonstrationlevels can achieve high yields (70–75wt%, includ-ing water) of bio-oil at 500–6008C and residencetimes of approximately 1 s, with high heat transferrates. Fluid beds [31] and entrained flow reactors[32] are the major reactor types now in commercialoperation.

The composition of pyrolysis oil depends onraw material, the reactor, severity (temperature,residence time, and heating rate), and efficiencyof the condensation system. Bio-oil is a mixture ofcarboxylic acids (mainly acetic and formic),aldehydes and alcohols, and lignin-derivedmethoxy phenolics (denoted as pyrolyticlignin), present as a low- to medium-molecular-weight material that precipitates by addingwater.

Pyrolysis has several advantages at small scaleover traditional gasification approaches:

* Bio-oil may be more economical to transportthan solid biomass because of higher volumetricenergy density and its ability to be pumped andstored.

* Pyrolysis and reforming can be carried out atdifferent locations to improve the economics.For instance, pyrolysis units could be con-structed at sites where low-cost feedstocks areavailable. The bio-oil could be transported to acentral reforming plant located at a site withhydrogen storage and distribution infrastruc-ture.

* Bio-oil or its fractions can also be used fordistributed production of hydrogen at fuelingstations, which is a major deployment focus ofbiomass-to-hydrogen work [33].

Considering the following stoichiometry, thepotential yield of hydrogen produced by pyrolysis/reforming could reach 13% (based on the weightof biomass), which is comparable with that



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obtained by gasification [34]:

Biomass! Bio-oilþ CharþGas ð8Þ

CH1:46O0:67! 0:71CH1:98O0:76

þ 0:21CH0:1O0:15 þ 0:08CH0:44O1:23

CH1:98O0:76 þ 1:24H2O! CO2 þ 2:23H2 ð9Þ

National Renewable Energy Laboratory (NREL)began to develop a biomass-to-hydrogenprocess in 1993 [34, 35]. Catalytic steam reformingof bio-oil at 750–8508C over a nickel-basedcatalyst is a two-step process that includes theshift reaction:

Bio-oilþH2O! COþH2 ð10Þ

COþH2O! CO2 þH2 ð11Þ

The overall stoichiometry gives a maximum yieldof 17:2 g H 100 g�1 bio-oil (11.2 wt% based onwood). In reality, this yield will always be lowerbecause both the steam reforming and water–gasshift reactions are reversible, resulting in thepresence of some CO and CH4 in the productgas. In addition, thermal cracking that occursparallel to reforming produces carbonaceousdeposits.

The basic assumption is that the mechanismof metal-catalyzed reforming of oxygenatedorganic molecules ought to be similar to thatproposed for hydrocarbons. Biomass-derivedliquids are more reactive than hydrocarbonsbecause they already have some carbon–oxygenbonds. They also show greater tendency to formcarbon deposits because of the large size andthermal instability of their constitutive molecules(furans, phenols).

Catalytic conversion of biomass pyrolysis gasand vapors to produce hydrogen is easier thanprocessing bio-oil because it avoids unwantedprocesses during heating and evaporation of thisthermally unstable liquid. Evans et al. [36]reported on reforming the whole vapor streamfrom pyrolysis of peanut shells. Initially, a 2-in(3.1-cm) diameter fluidized bed reformer operatedon a slipstream of pyrolysis vapor coming from a10 kg h�1 fluidized bed reactor. Later, the processwas scaled up and a 10-in (25.4-cm) diameter

fluidized bed reformer accepted the wholethroughput from the pyrolyzer. In both cases acommercial nickel-based catalyst performed verysatisfactorily, although attrition and entrainmentof the fine particles were observed.

Bio-oil can be catalytically steam reformed tohydrogen and CO2 in a distributed manner (e.g. atrefueling stations). NREL is developing a systemfor partial oxidation and catalytic reforming ofvolatilized/atomized whole bio-oil and is develop-ing a low-temperature oxidative cracking step thatwill be used to define process parameters forsyngas generation as shown in Figure 7.

A fine mist of oil is generated at ambientconditions and heated to the target temperature,typically 6508C where a residence time of 0.5 s isachieved. The primary and secondary products areessentially removed, and CO is the dominantproduct. The CO from bio-oil/methanol is highlycorrelated with the oxygen to carbon (O/C) ratio.At a ratio of 1.6, the carbon conversion to CO was60%. This corresponds to a CO2 carbon conver-sion of only 10%. The flat CO2 response to O/C isencouraging, since it means that the downstreamwater–gas shift reaction can be used to recover thehydrogen that may have been consumed inoxidative cracking.

In summary, biomass is an important renewableresource for producing hydrogen. More than 50million tons of hydrogen could be producedannually in the United States in the near term fromavailable biomass resources. Two main techno-logy pathways are being explored: gasification to

PYROLYSIS Bio-oil

REFORMING

H2

Biomass

Methanol

VOLATILIZAT ION

O2Low Temperature

Oxidative Cracking

SHIFT

CO2SEPARATION

H2O

Figure 7. Schematic of bio-oil distributed reformingapproach.

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synthesis gas followed by gas conditioning andwater–gas shift and pyrolysis to bio-oil followedby catalytic reforming. The gasification approachhas the potential to produce hydrogen for less than$1:50 kg�1 at a scale of 2000 tons per day. The bio-oil approach is a potential low-cost process fordistributed reforming.

4. FERMENTATION

4.1. Description

Biological hydrogen production is a viable alter-native to sustained hydrogen production if severaltechnical barriers are overcome. Many anaerobicmicroorganisms carry out the dark fermentationreaction during which the metabolism of sugars,amino acids, and fatty acids, for example, resultsin the production of hydrogen, CO2; and otherreduced end products. Hydrogen production iscatalyzed via the hydrogenase enzyme according tothe following equation:

2Hþ þ 2e�1 $ H2 ð12Þ

Hydrogenase is present in phylogenetically diversemicrobes [37]; consequently, exploring the variousmicrobes for their hydrogen production potentialis an active research subject. The hydrogen yieldvaries based on the pathways that variousmicrobes use. Using glucose as the model sub-strate, Figure 8 outlines a simplified fermentationpathway via glycolysis leading to hydrogen pro-duction in two types of microbes: (1) enteric

bacteria such as Escherichia coli mainly using thepyruvate–formate hydrogenlyase (PFHL) enzymecomplex [38] and (2) strict anaerobes such as themany clostridial species via the pyruvate–ferredox-in oxidoreductase (PFOR) coupling with a hydro-genase [39]. In each example, breakdown ofpyruvate via different routes dictates the finaltheoretical hydrogen yield as defined by its sugarmetabolic network. On the basis of Figure 8, theyield of hydrogen per mol of glucose (hydrogenmolar yield) is 2 in enteric bacteria. The yield ishigher in clostridial microbes due to the presenceof the nicotinamide adenine dinucleotide(NADH): ferredoxin oxidoreductase (NFOR)activity, by which two additional mol of hydrogencould be produced coupling with a hydro-genase [39].

Fermentation has several attributes that make itan attractive technology: (1) it has a simple reactordesign and operation (darkness); (2) fermentativemicrobes are readily available in sewage sludge,garden soils, and anaerobic compost [40–42]; (3)diverse waste materials can be used; and (4) it hashigh rates of hydrogen production unsurpassed byother biological processes, with values rangingfrom 184 to 2710 ml hydrogen l�1 h�1 reported[40, 42]. Fermentation thus has significant poten-tial provided that several technical barriers can beovercome.

4.2. Potential

To fully realize the potential of fermentation, twomajor barriers must be addressed: the high cost ofglucose feedstock and the relatively low hydrogenmolar yield. Glucose is the ideal substrate, yet it istoo costly at present. Many agricultural residues andfood wastes are rich in carbohydrates that couldserve as feedstock. Lignocellulosic biomass is asustainable feedstock for hydrogen production.Approximately 70% of biomass consists of hemi-cellulose (mainly xylose) and cellulose (glucosepolymer) [43], the bulk of which is fermentable ifmonomeric sugars can be readily released. The U.S.DOE Biomass Program estimated that 512 milliondry tons of biomass}equivalent to 8.09 quads ofprimary energy}could initially be available for$50 dry ton�1 delivered [44].

Glucose

2 Pyruvate

2 Formate

2 H2

2 Ferredoxin

2 H2

(1) PFHL (2) PFOR

2 NADH 2H2

2 Acetyl CoA

2 Acetate

NFOR2 ATP

(2)

Figure 8. Pathways of hydrogen production duringglucose fermentation.



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Nevertheless, the challenge of using biomass liesin its crystallinity and heterogeneity, which pre-vents its direct utilization by most microbes. Evenafter chemical pretreatment, the cellulose consti-tuent still has to be further hydrolyzed via a suiteof cellulase enzymes to produce the more fermen-table glucose [45]. The U.S. DOE BiomassProgram is developing technologies to lowerthe cost of biomass-derived sugar for the bioetha-nol refinery, the advancement of which will benefitthe development of fermentative hydrogen pro-duction.

Perhaps the more challenging barrier of fermen-tation is its low hydrogen molar yield. Thaueret al. [46] predicted that 4 hydrogen per molglucose is the biological maximum in clostridialmicrobes if acetate is the only waste by-product(Equation (13)). When the more reduced endproduct such as butyrate is the sole end product,however, only 2mol of hydrogen is expected(Equation (14)). Considering the energy contentin 1mol of glucose (678.2 kcal), hydrogen molaryield of 2–4 thus recovers approximately only 17–33% of the chemical energy in glucose:

Glucoseþ 2H2O! 2acetateþ 2CO2 þ 4H2

DG0 ¼ �182:4 kJ ð13Þ

Glucose! butyrateþ 2CO2 þ 2H2

DG0 ¼ �257:1 kJ ð14Þ

Most microbes opt to produce an array of wasteproducts (acetic, formic, butyric, and lactic acids,and alcohols) that provide multiple pathways toconsume NADH and regenerate NADþ at theexpense of hydrogen [47]. This metabolic diversi-fication lowers the hydrogen molar yield, yet itensures the further breakdown of sugars to yieldmore cell mass. Moreover, the decrease in pH(below 4.5) of the medium from acid accumulationcauses a metabolic shift of the microbe duringwhich acids are re-assimilated toward solventproduction. This further lowers the hydrogenyield. Controlling medium pH is therefore im-portant to improving hydrogen yield. To compen-sate for the low hydrogen yield, the cost offeedstock has to be decreased significantly forfermentation to be cost competitive. A technoeco-

nomic analysis conducted by NREL indicatedthat if glucose can be purchased at $0:05 lb�1

($0:11 kg�1), and assuming a hydrogen molaryield of 4, a minimum hydrogen selling price of$2:47 kg�1 hydrogen could be achieved [48]. Thisminimum hydrogen selling price is based on onlyfeedstock cost, which normally accounts for 75%of the overall cost. Nevertheless, it is near thetarget of the hydrogen cost goal of $2.00–3:00 kg�1

set out by U.S. DOE in 2005 [49]. This encoura-ging study guides new research approaches toovercoming the two barriers.

4.3. Status

Theoretically, hydrogen molar yield could ap-proach 4, but most laboratories thus far havereported values of 2–3 by using either pure cultureor mixed microbial consortium and glucose,sucrose, molasses, starch, and food wastes as thesubstrates [49–51]. This range of molar yield iscalculated to be 50–75% efficient biologically,although only reaching 17–25% efficiency if basedon the energy content in glucose. To improvefeasibility of fermentation, less expensive and moreabundant alternative feedstock has to be explored(see ‘Research Approaches’). Taguchi et al. [52]have isolated from the termite Clostridium beijer-inckii strain AM21B, which produced hydrogenfrom arabinose, galactose, and xylose, all of whichare constituents of hemicellulose. They reported ahydrogen molar yield of 2.6 with xylose. Whencultured on xylan, Clostridium sp. strain X53simultaneously produced the hemicellulose-hydro-lyzing enzyme xylanase along with hydrogen, afirst report for such conversion [53]. Datar et al.[40] also reported the conversion of steam-ex-ploded corn stover hemicellulose (mainly xylose)to hydrogen with a molar yield of 2.8–3.2 withnatural inoculants. Collectively, these findingssuggest that converting sugars, food waste, andhemicellulose to hydrogen is a feasible process.Although hydrogen production had been reportedfrom hydrolysate of paper sludge [54] and cellulose[55], both substrates were predigested with cellu-lase enzymes before being fermented.

Hydrogen production from delignified woodfibers and a-cellulose was recently reported by

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Levin et al. [55, 56], catalyzed by batch culture ofC. thermocellum with a molar yield around 2.Similarly, hydrogen production from corn stoverlignocellulose was also measured in C. thermo-cellum with a hydrogen molar yield approaching2.4 [57]. Consequently, both cellulose and hemi-cellulose can be utilized directly if suitable microbesare used. This demonstrates that lignocellulosicbiomass is a feasible substrate for hydrogenproduction. Improvement of molar yield was firstdemonstrated in Enterobacter aerogenes in whichthe redirection of glucose metabolism via geneinactivation improved its hydrogen molar yield byup to twofold, from 0.56 to 1.17 [58]. Although theenteric bacteria give rise to lower yield of hydro-gen, this work provides the proof of concept thatgenetic engineering is a viable approach toimproving hydrogen molar yield. A genetic systemis being developed in C. thermocellum [59] andpathway optimization toward hydrogen produc-tion is now technically possible in this microbe. Theoutcome is a consolidated bioprocessing of ligno-cellulosic biomass to hydrogen.

Progress has also been made in photofermenta-tion research in which hydrogen production fromthe more dominant fermentation end productsacetic and butyric acids approaches 75% for theformer, and 8.4 to 75% for the latter, of theirrespective theoretical yield [60, 61]. However,conversion yield via photofermentation could besignificantly lower when real fermentation effluentis used instead of the pure chemicals tested above,because inhibitors and other unknown factors maybe present in the fermentation effluent. Moreresearch is needed to overcome barriers associatedwith the stand-alone system and the integratedprocess as a whole.


One viable option to lowering the feedstock cost isto identify microbes that can utilize hemicelluloseand cellulose directly. This approach eliminates theneed for the expensive cellulase enzymes andsimplifies biomass pretreatment. Many microbesare known to produce hydrogen from the 5-carbonsugar xylose, a dominant sugar in hemicellulose (seeStatus) [62]. Bio-prospecting additional microbes

for high rates of hydrogen production from 5-carbon sugars will ensure better utilization of planthemicellulose. The more crystalline cellulose pre-sents a unique challenge to most microbes. Never-theless, many cellulolytic microbes, includingClostridium thermocellum, can hydrolyze cellulosevia the action of a suite of cellulase enzymesorganized within a cellulosome structure on thecell surface [41]. The mining of various cellulolyticbacteria thereby offers a promising solution toconverting cellulose to hydrogen in a one-stepconsolidated process. There are several potentialsolutions to circumvent the molar yield barrier.The advent of genomics and molecular biology hasprovided effective tools to redirect metabolicpathways toward maximal hydrogen productionin lieu of waste by-products accumulation, espe-cially when genetic engineering is conducted incellulolytic microbes. A more in-depth under-standing of the underlying biochemical metabo-lism is needed to target pathways that yield thegreatest improvements in hydrogen molar yield.Moreover, an integrated approach gaining accep-tance recently is to photoferment the waste organicacids of dark fermentation to generate additionalhydrogen, catalyzed by the nitrogenase enzyme ofthe photosynthetic bacteria [60]. Energy contentssuch as acetate, formic, lactic, and butyric acids inthe waste acids are thus converted into additionalhydrogen according to the following equation:

CxHyOz þ ð2x� zÞH2O

! ðy=2þ 2x� 2ÞH2 þ xCO2 ð15Þ

Theoretically, 1mol acetate could yield 4molhydrogen while the more reduced butyrate wouldyield 10mol hydrogen. The total sum of the twoprocesses could approach 12mol hydrogen ðn1 þn2Þ; the equivalent of the energy content in glucose(Figure 9). In this case, the photobioreactor mustbe optimized to ensure the success of the morecomplex integrated process.

5. SOLAR-DRIVEN THERMOCHEMICALREACTIONS

Water represents a virtually limitless sourceof hydrogen. However, water is a very stable



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substance that requires a large energy input torelease the elements. If the process is to be carbonneutral and strictly renewable, solar energy mustbe used. The energy can be supplied in the form ofheat (thermochemical), light (photochemical), orelectricity (electrolysis) [10]. Solar energy collectedas heat has the potential to be the most efficientsolar path to hydrogen from water since it doesnot have the inefficiencies associated with photo-chemical transformations or the conversion ofsolar energy to electricity followed by electrolysis.However, water does not thermally decomposeinto elemental hydrogen and oxygen to a signifi-cant extent at temperatures below about 2500K[63–65]. The temperature can be reduced bycarrying the reaction out using a series of chemicaltransformations. Two parallel projects in theUnited States are working to develop high-temperature water splitting cycles}the NuclearHydrogen Initiative (NHI) [66] and the SolarHydrogen Generation Research (SHGR) project[67]. The NHI is the larger effort and targetsprocesses with a maximum temperature of about1200K in anticipation of a new generation of

nuclear reactors that can supply heat in that range.The solar project is looking primarily at highertemperature cycles that are within the range ofconcentrating solar systems.

5.1. Description

The basis for using multistep closed-cycle chemicalprocesses to reduce the temperature required forthermal water splitting has been the subject ofnumerous articles [68,69]. Since the 1960s, nearly400 multistep chemical processes have been putforward. These are summarized in a number ofreviews [70,71]. The most up-to-date compilationcan be found in a database assembled for theSHGR Project [67]. A very thorough coverage ofthe state of the art for thermal cycle work waspublished by the Institute of Gas Technology in1981 [72]. The reaction systems are assembled touse water as the only input and have hydrogen andoxygen as the only output. Ideally, all otherchemicals are recycled with minimal loss withinthe process. The following examples illustrate theprinciple:

Thermolysis TemperatureH2OðgÞ ¼ H2ðgÞ þ 1=2O2ðgÞ > 2500 K (16)

Zinc-zinc oxideZnO ¼ Znþ 1=2O2ðgÞ 2473 (17)ZnþH2OðgÞ ¼ ZnOþH2ðgÞ 900 (18)

Nickel-manganese ferriteNiMnFe4O6 þ 2H2OðgÞ ¼ NiMnFe4O8 þ 2H2ðgÞ 1073 (19)NiMnFe4O8 þNiMnFe4O6 þO2ðgÞ 1273 (20)

Sulfur-iodineH2SO4ðgÞ ¼ SO2ðgÞ þH2OðgÞ þ 1=2O2ðgÞ 1123 (21)2HIðgÞ ¼ H2ðgÞ þ I2ðgÞ 573 (22)I2ðaÞ þ SO2ðgÞ þ 2H2O ¼ 2HIðaÞ þH2SO4ðaÞ 373 (23)

a=aqueous; g=gas.

DarkFermentation

Sugar sandwastes

Wasteacids

Photo-Fermentation

n2 = 4-10H2Lightn1 = 2-4H2

Figure 9. An integrated scheme of dark fermentation followed by photofermentation.

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The temperatures given usually reflect the mostfavorable chemical conversions at equilibriumconditions. In practice lower temperatures maybe selected. Reaction kinetics or a recombinationof products can be limitations beyond thoseimposed by thermodynamics.

A number of groups have used criteria that areappropriate to selected temperature ranges toscreen processes [67, 70, 73–76]. Initial screeningis usually based on thermodynamic considerationsalone or in combination with limitations includingcost, the availability of chemicals involved, orenvironment, safety, and health factors. The nextlevel of screening includes expected equilibriumconversions for each reaction and separation ofproducts. The final level for screening purposes

requires evaluation of potential process flowdiagrams that include as much of the knownchemistry as possible. The baseline efficiency tobeat is that for conventional electrolysis. Manychemical cycles have the potential to be 40–50%efficient.

5.2. Potential

5.2.1. Energy resource. The temperature require-ments (> 800 K) of the thermochemical water-splitting processes dictate that concentrating solarsystems be used to collect solar energy and convertit to heat. Hydrogen production by such processesin the United States will be limited to the south-western United States as shown in Figure 10 [9].

Figure 10. Potential for hydrogen production from solar resources in the United States [9].



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Concentration ranges for solar collectors arenormally expressed in terms of suns, where onesun ¼ 1 kWm�2: Solar concentrating systems areshown in Figure 11. These include one axis-tracking parabolic trough, towers with a field oftwo axis-tracking heliostats, and dish systems.

The maximum efficiency for solar energy collec-tion and conversion to heat has been covered in anumber of review articles [64, 77]. Parabolictroughs can achieve concentration ratios of about100 suns and achieve temperatures of about 800Kwith a maximum efficiency of about 60%. Towerconfigurations achieve about 1000 suns and atemperature of about 1000K with an efficiency ofabout 75%. Dishes can go to 10 000 suns, amaximum temperature of about 2200K, and reachefficiencies of about 85%. The concentration andefficiency that are achievable for each configuration

can be increased by adding nonimaging secondaryconcentrators to the optical path [64]. The overallenergy efficiency will be a combination of the solarefficiency with that of the chemical cycle.

The current status of concept development isdirected to stand-alone operations. There may bepotential for producing electricity with waste heat.

5.3. Status

The proposed thermochemical cycles have beenscreened over the years with criteria that reflect theenergy source and achievable temperature ranges,considerations related to the nature and cost ofchemicals that are required for the process, andpotential health and environmental hazards.Representative criteria are given on the SHGRWeb site and in a number of reports [67, 70, 73, 78].

Figure 11. Pictures of parabolic trough (a), tower (b), and (c) dish solar collector systems (NREL PIX11070, 02186, and 08982).

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Some of the metal–metal oxide and ferrite cycleshave been investigated under solar conditions [79–82]. These have been selected for national pro-grams because they are chemically less compli-cated and involve gas–solid separations ratherthan separations involving gas mixtures. They alsoinvolve process temperatures that can only beachieved with concentrating solar systems. TheNHI has selected the sulfur–iodine process as thefirst to be built at an integrated laboratory scalethat will produce about 200 l h�1: The system is ajoint project of French and U.S. laboratories andis scheduled to be in operation in 2008.


Adapting chemical processes to the daily andannual solar cycles presents significant challengesin plant operations and to the efficient use of thecapital investment in the plant. Storing energy asheat for off-sun operation can extend the time forhydrogen production and allow some processoperations to occur at night. The current state ofthe art for high-temperature solar processes, atleast in the United States, favors tower configura-tions. The costs of the heliostat field and tower aremajor fractions of the total plant cost. If thetemperature requirement exceeds about 1300K,secondary concentrators are likely to be required(G. Kolb, Sandia National Laboratory, personalcommunication, November 21, 2006), whichfurther increases the plant cost. Dish configura-tions can achieve the high solar concentrationswithout secondary concentrators, but they wouldrequire a reactor unit for each dish. Parabolictrough systems can, in principle, achieve tempera-ture levels that would match the requirements ofthe lower temperature cycles. A lower temperaturemeans the chemical process would have more stepsthat would increase the complexity of the system.We need to determine if using a trough configura-tion instead of a tower would have a favorableimpact on cost.


Many national programs are addressing high-temperature solar hydrogen production [79–82].Fundamental work on engineering solar concen-

trators, solar receivers, reaction kinetics, gasseparation, and materials of construction is re-quired. The demands on performance in all theseareas increase with the temperature requirements.

6. PHOTOLYSIS–PHOTOELECTROCHEMISTRY

6.1. Description

The thermodynamic potential for splitting water intohydrogen and oxygen at 258C is 1.23V. Addingovervoltage losses and some energy to drive thereaction at a reasonable rate we calculate a voltageof about 1.6–1.8V for water decomposition. In fact,current commercial electrolyzers operate at 1.7–1.9V. Translating the energy of 1.9 eV into acorresponding wavelength of light, we come to650nm, red light, which is in the lowest energyportion of the visible spectra. This means that nearlythe entire visible spectrum has enough energy to splitwater into hydrogen and oxygen. The key is to findthe right combination of a light harvesting systemand a catalyst that can efficiently collect the energyand direct it toward the water-splitting reaction.

The direct photoelectrochemical (PEC) splittingof water is a one-step process for producinghydrogen with solar irradiation; water is splitdirectly upon illumination. This direct conversionsystem uses the process in which an illuminatedsemiconductor immersed in aqueous solution isused to decompose water directly. This type ofdirect conversion system combines a PV materialand an electrolyzer into a single monolithic device.Light is absorbed in the semiconductor and wateris split at the semiconductor surface. For thisprocess to be viable, two major criteria must bemet: the light harvesting system must generatesufficient voltage to decompose water, and thesystem must be stable in an aqueous environment.The simplest PEC-based direct water-splittingsystem would consist of an illuminated single gapsemiconductor with a bandgap greater than 1.6 eVcoupled to a surface catalyst immersed in anaqueous solution. Figure 12 details how the water-splitting reactions (1–4) occur at the two types ofsemiconductors, p-type and n-type.



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The primary alternative for PEC hydrogenproduction is of course PV/electrolysis. However,one major advantage of a direct conversion system(whether it is PEC or photobiological) is that thearea available for electrolysis approximates that ofthe solar cell, and depending on the type of solarcell, this current density is 10–20 mA cm�2 atstandard solar intensities. This compares withcurrent densities of several hundred mA cm�2 forcommercial electrolyzers. At these lower solar-driven current densities, the voltage required forelectrolysis is much lower; hence, the correspond-ing electrolysis efficiency is much higher. At acurrent density similar to short circuit photocur-rent from a solar cell, hydrogen and oxygen aregenerated at an applied voltage of approximately1.35V, giving rise to an electrolysis efficiency of91%. Coupling this to a 12% efficient PV arrayleads to an overall solar-to-hydrogen efficiency of10.9%. This then is one advantage of a directconversion hydrogen generation system: not onlydoes it eliminate most of the costs of theelectrolyzer, but it may also increase the overallefficiency of the process.

A successful PEC water-splitting semiconduct-ing material needs to meet five primary require-ments:

* It must be stable in an aqueous electrolyte.

* The band gap ðEgÞ should fall in the range 1:7eV5Eg52:2 eV; sufficient to achieve the ener-getics for electrolysis yet allow maximumabsorption of the solar spectrum.

* It must have a high quantum yield (>80%)across its absorption band to reach the effi-ciency necessary for a viable device.

* It must straddle the redox potentials of thehydrogen and oxygen half-reactions with itsconduction and valence band edges.

* It must have a pathway to low-cost, high-volume synthesis.

6.2. Potential

PEC production of hydrogen is based on solarillumination; hence, the energy resource is verylarge. In fact a 10% solar-to-hydrogen PECsystem would need an area of only about 4000square miles to provide hydrogen for the entireU.S. fleet (236 million vehicles). This shows theresource potential (sunlight) for PEC. Of course,economical production, distribution, and storageof hydrogen are necessary.

The current highest water-splitting efficiency fora PEC device is 12.4% with a tandem configura-tion [83]. Gerischer [84] has calculated for a two-layer semiconductor system (tandem cell) that themaximum efficiency for PEC water splitting would

p-type and n-type.

p-typeO2 2H2O + 2e- 2OH- + H2

n-typeH22H2O + 4h+ 4H+ + O2

SemiconductorAqueous solution

Figure 12. This diagram depicts the energetics, configuration, and reactions for a PEC water-splitting device. Theillumination direction is shown. The bent lines labeled n-type and p-type depict the electrical field for each that isspontaneously generated when the semiconductor is immersed in an aqueous solution. The generated electric fields run inopposite directions for the two types (* electrons, * positive holes). Upon illumination, this electrical field provides thevoltage between the front (illuminated) and back (dark) sides of the semiconductor that drives the water-splitting reaction.

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be 24%. Bolton et al. [85] estimated 16% asmaximum realizable chemical conversion efficiencyfor a tandem system of this type (a D4 scheme inhis nomenclature). Weber and Dignam [86, 87]calculated that for different tandem cell configura-tions, the maximum water-splitting efficiencywould be 10–18%. Their efficiency analysis tookinto account various solid state and electrochemi-cal losses. More recently Licht et al. [88] calculatedthat efficiencies as high as 30% are possible withsome systems.

6.3. Status

The groundbreaking work of Fujishima andHonda in 1972 [89] showed that hydrogen genera-tion via splitting of water with visible light waspossible at a semiconductor electrode. An explo-sion of scientific research soon followed in thisarea. However, 35 years later, a visible light-drivenwater-splitting system that is efficient and stablestill remains an elusive goal. The only efficientworking PEC device is the 12.4% efficient directwater-splitting system that incorporates a PECdevice, voltage-biased with a state-of-the-art in-tegrated PV device [90].

This system, shown in Figure 13, consists of ap-GaInP2 PEC electrode and a GaAs solar cell

combined into a single monolithic device.Although GaInP2 has sufficient bandgap energyto split water, its band edges are 0.1–0.4 eV, toonegative for water splitting. This configurationuses a two-electrode design. The bent lines showthe direction of the electric fields that separate thecharges. Electrons are driven to the surface wherethey reduce water-producing hydrogen. Oxygenevolution occurs some distance away on the metalanode that is connected to the illuminatedsemiconductor by a wire. In this way hydrogenand oxygen are generated spatially separated;hence a hydrogen/oxygen mixture is not produced.Whereas the electrons have sufficient energy toreduce water, the holes do not have sufficientenergy to oxidize water. The mismatch in the bandedges of GaInP2 has been compensated for byintegrating a solid-state PV device into the PECdevice. Operationally, under illumination, elec-trons flow toward the illuminated GaInP2 surface,reducing water and forming hydrogen (hydrogenbubbles off the illuminated surface). Holes flowtoward the back and their energy is boosted via thep/n junction GaAs cell such that they can nowoxidize water, forming oxygen (at a platinumcounter electrode). Integrating a PV cell with aPEC electrode adds complexity and cost to thesystem. Additionally, this system lasts only two to

Figure 13. Energy and spatial diagram for the tandem water-splitting system.



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three days. A preferred and more direct approachwould be to realize a system that uses a singlesemiconductor electrode of appropriate bandgapwhose band edge positions have been engineeredfor stability and optimal rate of fuel production.


The material properties for a working PEC deviceare well known, but the demands on these materialproperties for PEC water splitting are stringent.They require a combination of physical, chemical,structural, and economic properties that no knownmaterial satisfies.


Since this is principally a search for a specificmaterial, the focus is on the synthesis andcharacterization of materials, primarily mixedmetal oxides. However, to fully explore themixed-metal oxide parameter space (ternary,quaternary, or possibly higher) we need to con-sider a 40þ element composition space that leadsto more than 2 million possible combinations. Inthe past candidate materials were synthesized andcharacterized individually, which made progressvery slow. More recently, two approaches havebeen taken that use combinatorial techniques toexpedite the survey process [91–93]. No ‘winner’has been identified, but a novel iron-doped cobaltaluminate has been identified with a low bandgapð1:58 eVÞ: Additional, more detailed characteriza-tion is necessary to determine the viability of thismaterial as a PEC device. Possibilities include theuse of advanced first-principles theory, which hasreached the point where material properties can bepredicted [94]. This approach may speed this areatoward the ‘Holy Grail’ [95] of a viable directwater-splitting system.

7. PHOTOBIOLOGICAL WATER SPLITTING

7.1. Description

Photoproduction of hydrogen by microorganismsis linked to the light absorption and chargeseparation reactions of photosynthesis. This link-

age is mediated by electron carriers (such asferredoxin and NADPH) that transfer photo-synthetically generated reductants to hydrogen-producing hydrogenase or nitrogenase enzymes,instead of diverting them to CO2 fixation, theirnormal physiological fate. This review will focusonly on hydrogenase-containing organisms such ascyanobacteria and green algae that can also extractreductants from water. These organisms canachieve very high light conversion efficiencies[96–101], and they photoproduce hydrogen with-out the input or output of carbon-based molecules.

In oxygenic photosynthetic organisms, light isabsorbed by light-harvesting pigments (chloro-phylls, carotenoids, phycobilins) and transferredvery efficiently to membrane-bound reaction cen-ters. At the Photosystem II (PSII) reaction center,absorbed sunlight is converted into chemicalenergy, oxidizing water into protons, electrons,and O2: Through a series of electron transferreactions (known as the Z-scheme of photosynth-esis), the electrons are delivered through Photo-system I (PSI) to a soluble carrier protein,ferredoxin. In green algae, ferredoxin transfersphotosynthetic reductants to an [FeFe]-hydroge-nase (either HydA1 or HydA2 [102]), where theelectrons are recombined with protons that yieldhydrogen gas (see Figure 14(a)). In cyanobacteria,the electrons are initially used to reduce NADPþ toNADPH, which interacts with the bidirectional[NiFe]-hydrogenase that is a component of aprotein complex that resembles respiratory complexI [103, 104]. The role of complex I in respiration isto oxidize NADH and transfer electrons to aquinone molecule. In cyanobacteria, this complexseems to be able to oxidize NADPH as well, and totransfer electrons either to a respiration-linkedquinone or to a photosynthetic plastoquinone[105, 106], as indicated in Figure 14(b).

7.2. Potential

Cyanobacteria and green algae can absorb about40–45% of the energy of the sunlight spectrum[97]. Photosynthesis requires 4 photons of ab-sorbed light per molecule of hydrogen produced,which translates into a quantum yield of 25%.When converted into energy values (1 mole quanta

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of visible light ¼ 214� 103 J; 1 mol hydrogen ¼237� 103 J [107]), the overall sunlight conversionefficiency of hydrogen production is about 10–13%. These efficiencies are not yet attainable forprolonged periods (see below), and research needsto be done to understand the limiting factors andto develop mechanisms to circumvent theselimitations. The potential for commercial applica-tions will depend on optimizing the process byengineering (physiological, molecular, and materi-als) approaches.

7.3. Applications

Although oxygenic photosynthetic organismscould by themselves in an area of about 500 km�

500 km eventually produce enough energy to fulfillthe world’s transportation needs (J. Turner,National Renewable Energy Laboratory, personalcommunication), the magnitude of cell biomassrequired raises the issue of how to handle the spentalgae after they produce hydrogen. Researchgroups worldwide have been considering inte-grated processes that combine fermentative withphotobiological methods. U.S. DOE demon-strated the concept of integrated hydrogen-produ-cing systems [7]; it was described recently by Melisand Melnicki [108]. It consists of two maincomponents: (1) photobioreactors that combinegreen algae, cyanobacteria, and photosyntheticbacteria and allow for the use of an extendedspectrum of sunlight and (2) a fermentative

Figure 14. Photosynthetic electron transport pathways in green algae (a) and cyanobacteria (b). The orange-coloredcomponents in (b) represent pathways at which respiration and photosynthesis intercept in cyanobacteria.



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hydrogen-producing reactor where the spent bio-mass from the photobioreactor is fermented, withconcomitant production of organic acids. Theseorganic acids, in turn, serve as substrate for thephotobiological organisms. The integration be-tween photosynthetic bacteria and fermentativeorganisms for hydrogen production has beeninvestigated by many groups. Many systems areavailable for coupling biomass use [109, 110] orwastewater treatment to hydrogen production [60].Although no technoeconomic analysis is available,clearly integrated systems must provide a morecost-effective approach for large-scale hydrogenproduction [111].


The major biochemical barriers to achieving the10% maximum sunlight conversion efficiency withgreen algae and cyanobacteria are (1) the extremesensitivity of the hydrogenase to O2; a by-productof water oxidation, (2) the low rates of photo-synthetic electron transport observed under hy-drogen-producing conditions, (3) the competitivepathways with hydrogenase for photosyntheticreductants in green algae and cyanobacteria, and(4) the low-light saturation properties of theseorganisms caused by the large light-absorbingpigment antennae [100, 107, 108]. Moreover, majorengineering barriers contribute to high estimatedcosts of the photobiological processes [112, 113].These are mainly related to (1) the nature and costof reactor materials, (2) reactor design to optimizelight use, and (3) gas separation issues.

7.5. Research approaches and status

The solution to the biochemical barriers describedabove has been pursued mainly through molecularengineering efforts, although bioprospecting andphysiological manipulation efforts are also under-way. A few O2-tolerant [NiFe]-hydrogenases havealready been identified, such as those fromRalstonia eutropha [114], Thiocapsa roseopersicina[115], and Rubrivivax gelatinosus [116]. None ofthese organisms, however, can photosyntheticallysplit water. Current efforts are aimed at expressingthese O2-tolerant hydrogenases in cyanobacteriato link their activity of photosynthetic water

splitting. Earlier attempts met with limited success[117].

Because O2-tolerant [FeFe]-hydrogenases arescarce in nature, the research about green algalhydrogen production is directed toward identify-ing factors that are responsible for attributing O2-tolerance to these enzymes and then engineeringenzymes that function under aerobic conditions.Among these factors is the O2 accessibility to thecatalytic site of the enzyme [118]. The researchrelated to the generation of O2-tolerant [FeFe]-hydrogenases has used a rational and a randomapproach. The rational approach is based on theinvestigation of the presence of O2 gas channels inthe structure of hydrogenases, followed by muta-genesis efforts aimed at closing these channels[119, 120]. This approach requires detailed knowl-edge of the structure of the enzyme (which is onlyavailable for the Desulfovibrio desulfuricans Ddhand the Clostridium pasteurianum CpI hydroge-nases, at present), and the use of moleculardynamics simulations to identify pathways forgas diffusion through the structure. This processidentified two very well-defined pathways by whichO2-sized molecules move from the catalytic site tothe surface of the protein [119, 121], and it showedevidence that hydrogen-sized molecules coulddiffuse through much larger portions of thehydrogenase structure. The study also proposedthat substituting bulky amino acid residues forsmaller ones along the channel would lead to anincrease in the O2-tolerance of the enzyme. Thishypothesis has been validated by generating amutant with partial increase in O2-tolerance [122].

Industry has used gene shuffling, a randomgenetic approach, to create diversity of catalystsand to yield proteins with new function. Nagyet al. [123] describe the expression of activeshuffled [FeFe]-hydrogenases in the heterologousE. coli-expression system developed previously[124, 125]. This opens up the possibility of usinghigh-throughput methods to screen large popula-tions for improved hydrogenases, generated eitherby mutagenesis or found in nature.

A useful, low-cost, but potentially less efficientmethod to circumvent the O2-sensitivity propertiesof the algal hydrogenase is to physiologicallydecrease the rates of photosynthetic water oxidation

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to allow algal cultures to become anaerobic,induce hydrogenase activity, and photoproducehydrogen from water for significant periods oftime. Melis et al. originally developed this method[126] by temporarily depriving cultures of sulfate.Recent advances include (1) the optimization ofthe process with respect to light intensity[127, 128], (2) pH of the medium [129], (3) presenceor absence of a carbon source [130], (4) amount ofpre-accumulated starch [131], and (4) cell immo-bilization [132] at low light intensity. Processes toallow continuous hydrogen photoproduction inthe light have also been reported with either cellsuspension [133] or immobilized organisms (Laur-inavichene, submitted for publication).

Melis and Chen reported another promisingmethod to partially decrease the rate of photo-synthetic O2 evolution [134]. It is based on thegenetic attenuation of sulfate transport to thechloroplast. The sulP mutants, which were gener-ated by mRNA interference techniques [135], allshow different degrees of reduction in photosyn-thetic O2 evolution rates and they undergotransition to the hydrogen-production stage evenwith some sulfate in the growth medium [135, 136].This system has the potential for further improve-ments in yield, conversion efficiencies, and con-tinuity of algal hydrogen-production systems[137].

The limiting rates of electron transport observedwith green algae and cyanobacteria under hydro-gen-producing conditions have been attributed toa number of possible factors. In green algae,anaerobiosis [138] inactivates the CO2 fixationpathway that leads to downregulation of photo-synthetic electron transport from water to ferre-doxin (see Figure 14(a)). Consequently, anaerobicgreen algae show limited rates of hydrogenproduction, as reported under conditions ofanaerobiosis [139, 140]. The downregulation ofphotosynthesis under anaerobiosis may be a resultof both the nondissipation of a proton gradientthat is established during photosynthetic electrontransport [141], and of the establishment of cyclic,nonproductive electron transfer around Photosys-tem I, the so-called state 2 [142]. Recent resultswith a Chlamydomonas mutant that cannot transi-tion to state 2 and that over-accumulates starch

demonstrates significantly increased rates of hy-drogen photoproduction [143]. This confirms thatcyclic electron transport may also be a contributorfor the low rates of hydrogen photoproductionobserved with wild-type organisms.

In cyanobacteria, the main competitors withhydrogenase are the respiratory complex I, theuptake hydrogenase, and cyclic electron transportaround Photosystem I. Indeed, sustained hydro-gen photoproduction for 5min was recentlyreported in a Synechocystis mutant defective intype I NAD(P)H-dehydrogenase complex [106].Metabolic engineering of all competitive pathwaysmay be the solution to improve the overall rates ofhydrogen photoproduction in this organism.

Finally, the most promising approach to addressthe low light saturation levels of algal hydrogenphotoproduction consists of using mutagenesisapproaches to generate mutants with small light-harvesting antennae. Organisms with small anten-nae do show higher photosynthetic productivity[144], and researchers who used either cyanobac-teria or green algae confirm these results [145–151].They display increased photosynthetic productiv-ity, although hydrogen photoproduction rateswere not reported.

8. CONCLUSIONS

Hydrogen can be produced from the renewableenergy resources, water and biomass, by a varietyof processes (e.g. photolysis, electrolysis, thermo-chemical, and biochemical). Electrolysis of water isthe simplest technology for producing hydrogen.The electrolytic production of hydrogen is cur-rently the only way to produce large quantities ofhydrogen without emitting the traditional by-products associated with fossil fuels. Biomass-to-hydrogen processes, including gasification, pyro-lysis, and fermentation, are less well-developedtechnologies. These processes offer the possibilityof producing hydrogen from waste materials suchas cellulosic biomass and sewage. Hydrogenproduction may be the most promising economicroute for the conversion of syngas to transporta-tion fuels. Solar energy can be used to producehydrogen in the form of heat (thermochemical),



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light (photochemical), or electricity (electrolysis).Solar energy collected as heat may be the mostefficient solar path to hydrogen from water since itdoes not have the inefficiencies associated withphotochemical transformations or the conversionof solar energy to electricity followed by electro-lysis. Photoelectrochemical water splitting andphotobiology are also options for producinghydrogen with solar energy. Hydrogenase-contain-ing organisms such as cyanobacteria and greenalgae can extract reductants from water andachieve very high light conversion efficiencies,and they photoproduce hydrogen without theinput or output of carbon-based molecules. Allthese technologies are in the development stage.

In this paper the status of these technologies andtheir potential is summarized. Efficiencies ofenergy conversion are summarized for the indivi-dual technologies. To assess the contribution thatthese technologies could make to future energyeconomies, factors such as energy-conversionefficiency, greenhouse gas emissions, and costthrough the complete chain from primary energysource to delivered hydrogen at pressure arerequired [152, 153].

ACKNOWLEDGEMENTS

NREL’s work and preparation of this paper wassupported by the U.S. Department of Energy, Office ofEnergy Efficiency, and Renewable Energy’s Hydrogen,Fuel Cells and Infrastructure Technologies Program.Thanks are also due to Dean Armstrong, KevinHarrison, Johanna Levene, Julia Thomas, and StefanieWoodward for their support.

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DOI: 10.1002/er

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