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Electrochimica Acta 80 (2012) 213– 218

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

emplated bi-metallic non-PGM catalysts for oxygen reduction

lexey Serov, Michael H. Robson, Mayat Smolnik, Plamen Atanassov ∗

epartment of Chemical and Nuclear Engineering, 1 University of New Mexico, University of New Mexico, Albuquerque, NM 87131, USA

r t i c l e i n f o

rticle history:eceived 12 April 2012eceived in revised form 22 June 2012ccepted 2 July 2012vailable online 14 July 2012

a b s t r a c t

A series of bi-metallic oxygen reduction catalysts derived from pyrolyzed Fe–M (where M = Co, Cu, Niand Mn) and 4-aminoantipyrine (Fe–M–AAPyr) compounds were prepared using a sacrificial supportmethod (SSM). The influence of the iron interacting with the second metal on the catalytic activity ofthe oxygen reduction reaction (ORR) was investigated by adjusting the ratio (Fe–M) from 1:3, to 1:1and 3:1. This series of materials was analyzed and characterized by scanning electron microscopy (SEM)

eywords:on-PGM catalystsuel cellRRathode

and the BET method (BET) in order to establish structural morphology, and rotating ring disk electrode(RRDE) experiments were performed to evaluate catalytic activity. The results were then correlated toone another, thereby establishing a composition to function relationship. Data from the ring current wasused to execute a mechanistic study of the materials for the ORR, and it was found that supplementationof iron with a second transition metal significantly improved catalytic activity.

ixed metal

. Introduction

Increasing demand for energy generation derived from sourcesther than petroleum has spurred extensive research globally, andas renewed interest in fuel cell technology to achieve this end. Ifuccessfully deployed, fuel cells can provide power to a broad rangef applications; from automotive propulsion, to combined heat andower, to portable electric devices. Cost of manufacturing, and byxtension the cost to the consumer, is the greatest impedimento successful commercialization of fuel cell systems. Sufficientlyowerful and efficient fuel cells rely on high platinum loadingsn both sides of the membrane electrode assembly (MEA), whichonstitutes the greatest cost contribution to the final device.

Maximizing the efficiency of platinum by using ultra-low load-ng and/or alloying can mitigate some of the cost pitfalls, however,nly by completely substituting the precious metal with a less nobleaterial can manufacturers divest themselves of the volatility of

he commodity.Recently, the number of organizations and institutions invested

n researching non-platinum group metal (non-PGM) catalysts hasncreased dramatically. The use of non-PGM anode catalysts [1],s well as cathode materials was extensively studied [2]. Literatureeviews of the current state of the art of non-PGM cathode catalysts

eveals that the activity and durability of non-PGM catalysts arepproaching that of the industry standard platinum based catalysts.

∗ Corresponding author. Tel.: +1 505 277 2640; fax: +1 505 277 5433.E-mail address: plamen@unm.edu (P. Atanassov).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.07.008

© 2012 Elsevier Ltd. All rights reserved.

Non-PGM catalysts can be categorized into three separategroups based upon their synthetic precursors: chalcogenides(mainly ruthenium or rhodium selenides) [3–10], heteroatomicpolymers [11–13], and small molecules [14–66]. The later cate-gory can be further bisected into coordinated metal macrocycliccompounds as the sole source of the M–N–C network formation[13–27], and soluble metal salt with nitrogen and carbon rich smallmolecule precursors [28–66].

Recent reports from several research groups have demonstratedthat bi-metallic Fe–M–N–C catalysts (where M = Co, Cu) are promis-ing materials for facilitating the ORR [11,67–70]. However, theinteraction of iron with the second metal at different ratios oncatalytic activity was not systematically studied.

The present work is devoted to synthesis, characterization, andevaluation of several Fe–M–N–C materials (where M = Co, Cu, Niand Mn) comprised of iron to metal ratios of 1:3, 1:1 and 3:3. Fur-ther differentiation of this synthetic approach from those reportedelsewhere involves templating the carbon support using the SSM,prior developed by our group [71,72]. Morphological characteriza-tion was then performed on the synthesized catalyst materials bySEM and the BET method, and the reaction kinetics and mechanismof ORR on these materials was then analyzed using the RRDE.

2. Experimental

2.1. Catalysts preparation

Fe–M–AAPyr catalysts were prepared by wet impregnationof iron, the second metal, and 4-aminoantipyrine precursorsonto the surface of fumed silica (Cab-O-SilTM EH-5, surface area:

214 A. Serov et al. / Electrochimica Acta 80 (2012) 213– 218

f the

∼FuSat(SaatiTsUrwwta

aa

2

m

2

faaiatRtwf

FaTcaf

Fig. 1. Schematic representation o

400 m2 g−1). The schematic representation of SSM is shown inig. 1. First, a known mass of silica was dispersed in water byltrasonication. Then, the aminoantipyrine (4-aminoantipyrine,igma–Aldrich) was dissolved in acetone, and the solution wasdded to silica and ultrasonicated for 20 min. Finally, a solu-ion of iron nitrate (Fe(NO3)3·9H2O, Sigma–Aldrich), cobalt nitrateCo(NO3)2·6H2O, Sigma–Aldrich), nickel nitrate (Ni(NO3)2·4H2O,igma–Aldrich), copper nitrate (Cu(NO3)2·2H2O, Sigma–Aldrich)nd/or manganese nitrate (Mn(NO3)2·H2O, Sigma–Aldrich) wasdded to SiO2–AAPyr solution and ultrasonicated for 8 h, wherehe total metal loading to silica was ∼15% by mass. After ultrason-cation, solution of silica and Fe–M–AAPyr was dried overnight at

= 85 ◦C. The resulting solid was ground to a fine powder, and thenubjected heat treatment (HT). The general conditions of HT wereHP nitrogen (flow rate 100 cm3 min−1), 10◦ min−1 temperature

amp rate, and a 3 h pyrolyzation time at 800 ◦C. Finally, the silicaas leached out by means of 20 wt.% of HF and resulting powderas washed with DI water until neutral reaction. Final metal con-

ent after washing with HF was found to be ∼2–3 at.% for mono-nd bi-metallic catalysts.

The iron to secondary metal ratios were selected to be 1:3, 1:1nd 3:1, while the mass ratio of Fe–M to AAPyr was held constantt 1–8.

.2. Characterization

All SEM images were generated using the Hitachi S-800 instru-ent.

.3. Electrochemical analysis

Electrochemical analysis of the synthesized catalysts was per-ormed using the Pine Instrument Company electrochemicalnalysis system. The rotational speed reported was 1200 rpm, with

scan rate of 5 mV s−1. The electrolyte was 0.5 M H2SO4 saturatedn O2 at room temperature. A platinum wire counter electrode and

Ag/AgCl reference electrode were used. The potential of ring elec-rode was hold at 1.4 V vs. RHE. The conversion between Ag/AgCl toHE was done by measuring the voltage between reference elec-rode and Pt-wire in hydrogen saturated electrolyte and the valueas found to be 250 mV. The reported disk current was corrected

or capacitive current.The working electrodes were prepared by mixing 5 mg of the

e–M–AAPyr electrocatalyst with 850 �L of a water and isopropyllcohol (4:1) mixture, and 150 �L of Nafion® (0.5 wt.%, DuPont).

he mixture was sonicated before 30 �L was applied onto a glassyarbon disk with a sectional area of 0.2474 cm2. The loading of cat-lyst on the electrode was 0.6 mg cm−2. The error bar for E1/2 wasound to be ±5 mV.

Sacrificial Support Method (SSM).

3. Results and discussion

Analysis of morphological data for the selected samples pre-pared by varying the Fe–M ratio revealed the highly porous natureof the catalysts with a bi-modal pore distribution (Fig. 2). Thesmaller pores have a diameter of 25–40 nm, and they likely orig-inate from the leaching of individual silica substrate. The largerpores have diameters of 150–170 nm, which stem from the disso-lution of silica agglomerates, thus resulting in high surface area,and all synthesized materials were in the range of 600 m2 g−1.The porous structure and high surface area have been positivelyshown to affect the mass transfer of gaseous reagents to the activesites of the catalyst, while simultaneously providing channels forthe removal of water, preventing flooding of catalyst. SEM imagesrevealed a continuity of morphology across all synthesized mate-rials, suggesting that the surface area and porosity is primarilyaffected by sacrificial support, which was the same in all experi-ments.

The comparison of catalytic activity of Fe3M–AAPyr with corre-sponded Fe–AAPyr and M–AAPyr is represented in Fig. 3. Considerthe example of the Fe–Co (Fig. 3(A)) and Fe–Cu (Fig. 3(B)) com-pounds, where the ORR activity peaked at the ratio of 1:1, andfurther increase of iron content had no affect on E1/2. By con-trast, the catalysts based on bi-metallic Fe–Ni (Fig. 3(C)) and Fe–Mn(Fig. 3(D)) exhibited a constant increase in activity with increasingiron content. The trend in all 4 classes of catalyst was that the addi-tion of iron to a second metal improved performance, but catalystswith Fe to Ni or Mn ratios of 3:1 possessed the highest level of activ-ity. A head-to-head comparison of catalytic activities for oxygenreduction of Fe3M–AAPyr catalysts is shown on Fig. 4. Materialsderived from Fe–Co, Fe–Cu and Fe–Ni all possess similar activity,while Fe3Mn–AAPyr was found to be significantly more active.

In order to determine the synergetic effect of the metals in bi-metallic catalysts, and to confirm that iron is the primary promoterof high oxygen reduction activity, Fe3M materials were comparedwith mono-metallic M–AAPyr and Fe–AAPyr, respectively (Fig. 5).It is clear that mono-metallic Co–AAPyr, Cu–AAPyr, Ni–AAPyr andMn–AAPyr are poor catalysts for oxygen reduction. Through theaddition of iron, a dramatic increase in catalytic activity is observed.Conversely, a comparison of Fe3M for Co, Cu and Ni to the mono-metallic Fe–AAPyr did not reveal any significant synergetic effect.In other words, the catalytic activity of Fe3M–AAPyr (when M = Co,Cu and Ni) was similar to that of Fe–AAPyr. However, the additionof Mn (Fe3Mn–AAPyr) confirms the synergetic effect in conjunctionwith Fe in bi-metallic catalysts (Fig. 5(D)). Fig. 5(D) depicts how the

material with Fe:Mn ratio of 3:1 is significantly more active thanboth mono-metalic Mn–AAPyr and Fe–AAPyr. Further investigationof the nature of the contribution of Fe and Mn to the ORR active site,and their role in the transfer of electrons from the analyte is needed.

A. Serov et al. / Electrochimica Acta 80 (2012) 213– 218 215

Fig. 2. SEM images for Fe–Co–AAPyr catalysts: (A) FeCo3–AAPyr, (B) FeCo–AAPyr and (C) Fe3Co–AAPyr.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-5

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0

0.0

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0.5

FeCo3-AA Pyr

FeCo-AA Pyr

Fe3 Co-AA Pyr

Cu

rre

nt

de

nsit

y,

mA

cm

-2

E, V (vs . RHE )

A

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-6

-5

-4

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0

0.05

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0.15

FeCu3-AA Pyr

FeCu-AA Pyr

Fe3 Cu-AA Pyr

Cu

rre

nt

de

nsit

y,

mA

cm

-2

E, V (vs. RHE)

B

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-5

-4

-3

-2

-1

0

0.00

0.03

0.06

0.09

0.12

0.15

FeNi3-AAPyr

FeNi-AAPyr

Fe3Ni-AAPyr

Cu

rren

t d

en

sit

y,

mA

cm

-2

E, V (vs. RH E)

C

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-5

-4

-3

-2

-1

0

0.02

0.04

0.06

0.08

0.10

FeMn3-AAPyr

FeMn-AA Pyr

Fe3 Mn-AAPyrCu

rren

t d

en

sit

y,

mA

cm

-2

E, V (vs. RHE)

D

Fig. 3. RRDE data for Fe–M–AAPyr catalysts: (A) FeCo3–AAPyr (-), FeCo–AAPyr (- - -) and Fe3Co–AAPyr (· · ·); (B) FeCu3–AAPyr (-), FeCu–AAPyr (- - -) and Fe3Cu–AAPyr (· · ·);(C) FeNi3–AAPyr (-), FeNi–AAPyr (- - -) and Fe3Ni–AAPyr (· · ·); and (D) FeMn3–AAPyr (-), FeMn–AAPyr (- - -) and Fe3Mn–AAPyr (· · ·). Conditions: 0.5 M H2SO4 saturated withO2, 1200 rpm, 5 mV s−1, catalyst loading 0.6 mg cm−2.

216 A. Serov et al. / Electrochimica

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-5

-4

-3

-2

-1

0

0.00

0.05

0.10

0.15

0.20

0.25

Fe3Co- AAPyr

Fe3Ni-AAPyr

Fe3Cu- AAPyr

Fe3Mn- AAPyr

Cu

rre

nt

den

sit

y,

mA

cm

-2

E, V (vs. RHE)

Fig. 4. RRDE data for Fe M–AAPyr catalysts: Fe Co–AAPyr (-), Fe Ni–AAPyr (- - -),FO

Pcfinwu

FCs

3 3 3

e3Cu–AAPyr (· · ·) and Fe3Mn–AAPyr (- · -). Conditions: 0.5 M H2SO4 saturated with2, 1200 rpm, 5 mV s−1, catalyst loading 0.6 mg cm−2.

When one sets out to intelligently engineer a successful non-GM catalyst, there are two key performance parameters toonsider: activity comparable to the platinum benchmark, and suf-

ciently high durability. One of the problems typically plaguingon-PGM cathode catalysts is high hydrogen peroxide generation,hich is the result of the (2 × 2)e− mechanism. The H2O2 byprod-ct is chemically active, and can corrode the carbonaceous catalyst

0.90.80.70.60.50.40.30.20.1

-5

-4

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-1

0

0.05

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0.25

Fe3 Co-AA Pyr

Co-AAPyr

Fe-AAPyr

Cu

rren

t d

en

sit

y,

mA

cm

-2

E, V (vs. RHE )

A

Cu

rre

nt

den

sit

y,

mA

cm

-2

0.90.80.70.60.50.40.30.20.1

-5

-4

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-2

-1

0

0.0

0.1

0.2

0.3

Fe3 Ni-AAPyr

Ni-AA Pyr

Fe-AA Pyr

Cu

rren

t d

en

sit

y,

mA

cm

-2

E, V (vs. RHE)

C

-2

ig. 5. RRDE data for Fe3M–AAPyr catalyst compared to mono-metallic M–AAPyr and Fe–u–AAPyr (- - -), Fe–AAPyr (· · ·); (C) Fe3Ni–AAPyr (-), Ni–AAPyr (- - -), Fe–AAPyr (· · ·); anaturated with O2, 1200 rpm, 5 mV s−1, catalyst loading 0.6 mg cm−2.

Acta 80 (2012) 213– 218

support, membrane, and ionomer. This results in poor stability ofMEA, and ultimately a catastrophic failure. The US Department ofEnergy has established a performance goal that sets the hydrogenperoxide yield at less than 2%.

In order to investigate H2O2 yield, a series of RRDE experimentswere performed on mono-metallic and bi-metallic catalysts withvarying iron contents (Fig. 6), and Eq. (1) was used to evaluate theyield,

%H2O2 = 100 × 2 × (IR/N)ID + IR/N

(1)

where IR, ID and N are the ring current, disk current and ring col-lection efficiency (0.37), respectively.

It was found in all cases that Co–AAPyr, Ni–AAPyr, andMn–AAPyr are prolific hydrogen peroxide generators, yieldingH2O2 in excess of 20%. Surprisingly, in all Fe–M–AAPyr materials,the synergetic effect on the mitigation of hydrogen peroxide yieldis observed. It can be seen in Fig. 6(C) and (D) that addition of nickeland manganese to the iron–aminoantipyrine complex results inperoxide yield that is on par with the goal established by the DoE(∼3%). Taking into account that activity of Fe–M–AAPyr (M = Co,

Cu and Ni) was similar to Fe–AAPyr, but that H2O2 production waslower in case of bi-metallic catalysts, employing bi-metallic materi-als in MEA may be a viable avenue to pursue if they can demonstrateacceptable durability in acid.

0.90.80.70.60.50.40.30.20.1-5

-4

-3

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-1

0

0.05

0.10

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0.20

0.25

Fe3C u-AAPyr

Cu-AAPyr

Fe-AAPyr

E, V (vs. RHE)

B

0.90.80.70.60.50.40.30.20.1

-5

-4

-3

-2

-1

0

0.0

0.1

0.2

0.3

Fe 3Mn-AA Pyr

Mn- AAPyr

Fe-AAPyr

Cu

rre

nt

den

sit

y,

mA

cm

E, V (vs. RH E)

D

AAPyr: (A) Fe3Co–AAPyr (-), Co–AAPyr (- - -), Fe–AAPyr (· · ·); (B) Fe3Cu–AAPyr (-),d (D) Fe3Mn–AAPyr (-), Mn–AAPyr (- - -), Fe–AAPyr (· · ·). Conditions: 0.5 M H2SO4

A. Serov et al. / Electrochimica Acta 80 (2012) 213– 218 217

0.1 0.2 0.3 0.4 0.5 0.60

5

10

15

20

25

30

35

40

45

50

Co-AA Pyr

Fe3 Co-AAPyr

Fe-AAPyrH

2O

2 Y

ield

, %

E, V (vs. RH E)

A

0.1 0.2 0.3 0.4 0.5 0.60

5

10

15

20

25

30

35

40

45

50

Cu-AAPyr

Fe3C u-AA Pyr

Fe-AA Pyr

H2O

2 Y

ield

, %

E, V (vs. RHE)

B

0.1 0.2 0.3 0.4 0.5 0.60

5

10

15

20

25

30

35

40

45

50 Ni-AAPyr

Fe3N i-AAPyr

Fe-AAPyr

H2O

2 Y

ield

, %

E, V (vs. RHE )

C

0.1 0.2 0.3 0.4 0.5 0.60

5

10

15

20

25

30

35

40

45

50

H2

O2

Yie

ld,

%

E, V (vs. RH E)

Mn-AAPyr

Fe3Mn-AAPyr

Fe-AAPyr

D

F mono( AAPyC mg c

4

taoeop

snefiit

itmeofheio

ig. 6. RRDE data of hydrogen peroxide yield for Fe3M–AAPyr catalyst compared to· · ·); (B) Cu–AAPyr (-), Fe3Cu–AAPyr (- - -), Fe–AAPyr (· · ·); (C) Ni–AAPyr (-), Fe3Ni–onditions: 0.5 M H2SO4 saturated with O2, 1200 rpm, 5 mV s−1, catalyst loading 0.6

. Conclusions

Bi-metallic non-PGM catalysts for oxygen reduction were syn-hesized using the SSM, derived from Fe–M (where M = Co, Cu, Ni,nd Mn) and aminoantipyrine precursors. The influence of the ratiof iron to the secondary metal on ORR activity was systematicallyxamined. An attempt to find synergetic effect between Fe and M inxygen reduction reaction and hydrogen peroxide generation waserformed.

The continuity of morphology between the prepared materialsuggests that variation of ratios and the transition metal compo-ent has no effect on nanostructural features, and all catalystsxhibited a similarly high surface area and bi-modal porosity. Sur-ace areas of ∼600 m2 g−1 is directly attributable to the SSM, andn combination with highly developed pore structure, it positivelympacts the density of active sites while promoting beneficial massransfer properties.

It was found that addition of a secondary metal to complementron significantly increases the catalytic activity of oxygen reduc-ion. In all 4 of the bi-metallic AAPyr derived of material sets, the

ost active catalysts were those with a 3:1 Fe–M ratio. The syn-rgetic effect between Fe and M was most pronounced in the casef manganese, while in all other cases activity of Fe3M–AAPyr wasound to be similar to that of Fe–AAPyr. However, the analysis of

ydrogen peroxide yields revealed the presence of a synergeticffect for all prepared materials, and addition of second metal toron significantly decreases H2O2 production, which is in the realmf Department of Energy requirements.

[[[[

-metallic M–AAPyr and Fe–AAPyr: (A) Co–AAPyr (-), Fe3Co–AAPyr (- - -), Fe–AAPyrr (- - -), Fe–AAPyr (· · ·); and (D) Mn–AAPyr (-), Fe3Mn–AAPyr (- - -), Fe–AAPyr (· · ·).m−2.

A summary of information concludes that bi-metallic iron basednon-PGM catalysts are promising materials to supplant platinumas the electrocatalyst for cathodic side of the MEA.

Acknowledgement

This work was supported by the DOE-EERE Fuel Cell TechnologyProgram: “Development of Novel Non Pt Group Metal Electrocata-lysts for PEM Fuel Cell Applications”.

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