Journal of Electroanalytical Chemistry 687 (2012) 102–110

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Journal of Electroanalytical Chemistry

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Dependence of the oxygen reduction reaction at sol–gel derived Co-basedcatalysts on acidic solution pH and temperature

A. Mani ⇑, V.I. BirssDepartment of Chemistry, University of Calgary, 2500 University Drive N.W. Calgary, Alberta, Canada T2N 1N4

a r t i c l e i n f o

Article history:Received 14 September 2011Received in revised form 12 August 2012Accepted 14 September 2012Available online 24 October 2012

Keywords:Proton exchange membrane fuel cells(PEMFCs)Phenylenediamine (pda)Ethylenediamine (en)Oxygen reductionKineticsMechanism

1572-6657/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jelechem.2012.09.041

⇑ Corresponding author. Present address: DepartmeUniversity, 8888 University Drive, Burnaby, BC, Cana2640.

E-mail address: amani@sfu.ca (A. Mani).

a b s t r a c t

This work is focused on the oxygen reduction reaction (ORR) at non-precious metal catalysts formedusing a modified sol–gel (SG) synthesis approach, in which carbon and nitrogen were added in the formof either ethylenediamine (en) or 1,2-phenylenediamine (pda) to a pre-formed Co oxide gel. It was con-firmed that the Co-pda-derived material, when heat-treated at 900 �C, is a much better ORR catalyst thanthe analogous en-derived material, heat-treated to its optimum of 700 �C, giving an activity about 10–11times higher and a lower H2O2 yield. The ORR was examined over a range of acidic solutions (all at roomtemperature and an oxygen pressure of 1 atmosphere). For both catalysts, the reaction rate was found tobe independent of the H+ concentration at pH < 2.5, with the Co-pda and -en catalysts giving a transfercoefficient of ca. 1 and 0.5–0.7, respectively. At pH > 2.5, an unusual pH response was seen, suggestiveof the development of local pH conditions inside the catalyst layer. While the catalysts were stable onlyto solution temperatures of ca. 55 �C, the ORR activation energy in the kinetic (Eact = 34 kJ/mol) and dif-fusion controlled (Eact = 8.8 kJ/mol) regions could still be determined. Based on all of these results, a pos-sible mechanism for the ORR at below and above pH 2.5, for both the Co-pda and Co-en catalysts, wasproposed.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

One of the key challenges of hydrogen-operated proton ex-change membrane fuel cells (PEMFCs) is the high cost and poten-tially short supply of Pt, especially for the cathode at which thesluggish oxygen reduction reaction (ORR) occurs. Two general ap-proaches to lowering catalyst cost are currently being actively pur-sued. The first involves lowering the Pt loading by using techniquessuch as sputter-deposition [1], employing higher surface area car-bons to better distribute and hence utilize the Pt nanoparticles [2]and by alloying Pt with other transition metals (e.g., Ni, Co, Fe, Cu).This latter approach includes the design and application of core–shell particles, which can lower cost and often further enhancethe ORR activity [3–5]. The second major approach involves theuse of non-noble metal ORR catalysts [6]. In the short-term, cata-lysts containing lower amounts of Pt are the priority and are alsopractical, but in the longer term, non-precious metal (NPM) cata-lysts would be the better solution [7]. While NPM catalysts arepromising, their activity towards the ORR must be increased fur-ther in order to generate the power density levels required formost practical applications.

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From previous research [8], it is known that N4-metal chelates,such as Co complexes adsorbed on a high specific area carbon sup-port and then pyrolyzed at high temperatures, produce active ORRcatalysts in the acidic environment (ca. 0.5 M H+) of PEMFCs. Theactivity, stability, and selectivity of these catalysts can all be im-proved by heat-treatment, resulting in the partial thermal decom-position of the metallated N4 macrocycle ring.

Dahn et al. [9] reported the ORR activity of CoxC1�x�yNy

(0 < x < 0.107, 0.003 < y < 0.389) thin films, prepared by combina-torial magnetron sputter deposition in an Ar/N2 gas mixture, fol-lowed by heat-treatment at 700, 800, or 1000 �C in a N2

atmosphere. The N content of the catalysts was controlled by theN2 partial pressure during sputtering, with the N content afterheat-treatment correlated with improved ORR activity of these cat-alysts. For Co–N–C and Fe–N–C thin film catalysts modified by con-trolled doping with boron, the catalytic activity was found todecrease with boron content, even though the N content increased[9c]. Moreover, when the ORR catalytic activity of these annealedFe–N–C materials, supported on CNTs, was examined using therotating ring disk electrode technique [9c], a similar activity wasobserved to those prepared by Dodelet et al. [10].

Dodelet et al. [10] also optimized the catalytic performance ofCo/Fe porphyrins (e.g., formed from tetra(4-methoxyphenyl)por-phyrin (TMPP) and metal acetates (Ac)) using fifteen differenttypes of carbon black powder in a pH 1 H2SO4 solution. They used

A. Mani, V.I. Birss / Journal of Electroanalytical Chemistry 687 (2012) 102–110 103

cyclic voltammetry and X-ray diffraction analysis to determine thecatalytic activity and the amount of H2O2 generated during theORR. The surface N concentration was obtained from X-ray photo-electron spectroscopy (XPS) measurements, showing that theactivity of the catalysts made with the same grade of carbon blackincreases with increasing nitrogen concentration. The ORR activityof catalysts formed from FeAc and Fe TMPP was almost the same,although a higher percentage of H2O2 was obtained for CoAc-de-rived catalysts vs. those made from FeAc and FeTMPP. At the sametime, the catalyst producing the larger amounts of H2O2 exhibited alower surface nitrogen content. These results suggest the presenceof at least two catalytic sites for the ORR.

It has also been suggested that [6d,11], regardless of which M–N–C (M = Fe, Co) precursors are used (e.g., metal acetate/NH3 or ametal macrocycle such as M-TMPP)), the same two catalytic sitesare formed. In this previous work [6d,11], a series of Fe- and Co-based ORR electrocatalysts were prepared using different metaland N-containing precursors. In addition to the two observed cat-alytic sites [10e,11a,11c,d], the existence of a third catalytic site,related to iron oxide, was proposed, displaying the lowest catalyticactivity [12].

In contrast, Gouerec et al. [12] found that CoTAA (tetraazaannu-lene)/C, heat-treated at 600 �C, showed the best performance interms of activity and durability. The fragments detected by ToF-SIMS (time of flight secondary ion mass spectrometry) showedthe existence of oxygen in addition to Co and N, suggesting thatstrong interactions between the metal in the CoNx moiety andthe oxygen surface groups on the support may play a role in cata-lyst activity enhancement.

In previous work by Birss et al. [13], an active sol–gel (SG)-de-rived Co-based catalyst, also containing nitrogen and carbon intro-duced by the addition of several simple ligands (ethylenediamine(en) and 1,2-phenylendiamine (pda)) during the synthesis, wasdeveloped. The combination of a transition metal and the –C–N–components, where N is bonded to Co early on in the catalyst syn-thesis, is of particular interest. The SG technique also tends to givevery high surface materials, thus facilitating the addition of the li-gands to the Co sites. The Co–N materials were then deposited onhigh surface area carbon powder, followed by heat-treatment. Birsset al. [13b] proposed two ORR active sites at the sol-derived Co–N–C catalyst in acidic solutions. The structure formed at low temper-atures (6500 �C), based on XPS data, was suggested to be a CoN4

unit, also containing oxygen [13b]. The oxygen atom was sug-gested to serve to hold the Co–NxCx dimer in an appropriate posi-tion to make it suitable for the ORR to occur. Consistent with this,dimers have been described in the literature [14] to be more activeORR catalysts than their monomeric form.

The ORR is known to be quite complex and several reactionpathways are possible, each containing numerous discrete steps.The preferred pathway involves the production of water througha four-electron pathway, while the other is the production ofhydrogen peroxide through a two-electron pathway. Very little isknown about the mechanism of the ORR at NPM M–N–C catalysts.

Therefore, the primary focus of this paper is on developing abetter understanding of the ORR mechanism at M–N–C catalyststhrough the determination of the impact of the acidic (H2SO4) solu-tion pH and temperature on reaction activity and selectivity. Thiswork is focused specifically on SG-derived Co-based materials,involving both the ethylenediamine (en) and phenylenediamine(pda) ligands in the synthesis. It is shown that the use of the pdaligand in the synthesis gives approximately 10 times (at 0.7 V vs.RHE) more activity than when en is used, both at their optimumheat-treatment temperatures. The two ligands also result in differ-ent H2O2 yields and transfer coefficients, suggesting a differentORR slow step for catalysts formed using these two ligands. Thiswork also focuses on the pH dependence of the ORR, as well as

on the dependence of the kinetic and stability parameters on solu-tion temperature, showing that the ORR is pH independent up to apH of �2.5. Finally, some possible ORR mechanisms are proposed.

2. Experimental methods

2.1. Catalyst and ink preparation

Co-based catalysts were synthesized using a novel SG syntheticapproach in which C and N were introduced through the use ofinexpensive compounds, such as ethylenediamine (en) and phen-ylenediamine (pda), into a Co oxide gel network [13]. These Cooxide sols were first formed in ethanol, followed by the addition(determined to have to occur in a drop-wise fashion) of the enand pda ligands. The optimum Co: ligand ratio is 1:2, correspond-ing to a Co:N ratio of 1:4, as expected [13].

The catalyst solution was applied by pipette (typically 0.01 g to0.38 g Co) to 0.25 g dry carbon powder (Vulcan XC-72R) in ratiosthat produced 14–60 wt.% Co catalyst. The mixtures were soni-cated for 30 min and allowed to evaporate to a dry powder beforeheat-treatment at 700 or 900 �C for 2 h in a N2 atmosphere for theCo-en and Co-pda based catalysts, respectively. The optimum cat-alyst concentration was nominally 22% at the start of the synthesis,corresponding to a final concentration of 5.7% Co for the Co-pdacatalyst and 4.5% Co for the Co-en material (determined by ICPanalysis) after heat treatment [13].

An 11% (w/w) Nafion solution (EW 1100, supplied by BallardAdvanced Materials Corporation, Canada), was diluted with abso-lute ethanol to give a 1% (w/w) Nafion mixture. The catalyst inkwas prepared by adding 0.02 g of heat-treated catalyst and 0.87 gof 1% Nafion solution. The ink was then sonicated for 15 min tomix the catalyst powder thoroughly. Then, 7 lL of the catalystink was pipetted onto the surface of a 5.1 mm diameter GC diskof the rotating disk electrode (RDE) or of the rotating ring disk elec-trode (RRDE). This resulted in a total catalyst loading (M–N–C/car-bon powder/Nafion) of ca. 37 lg/cm2, with a catalyst film thicknessof about 5 lm. A Thermo Jarrell Ash model AtomScan 16 CoupledPlasma Atomic Emission Spectrometer (ICP-AES) was used todetermine the true concentration of Co in some of the catalysts.The samples were prepared by dissolving them in aqua regia(HNO3:HCl; (1:3)) for 24 h and then filtering and diluting withdeionized water. The emission was analyzed at 228.6 nm.

2.2. Kinetic measurements

The kinetics, mechanism and mass transport parameters of theORR were examined by coating the catalyst ink on the surface of arotating glassy carbon (GC) disk (5.1 mm dia., giving a surface areaof 0.204 cm2) RDE or, in some cases, a RRDE (width of Pt ring was0.22 mm, with a disc–ring gap of 0.18 mm). Cyclic voltammograms(CVs) were run at 10 mV/s at different rotation rates (500–3000 rpm) using a Pine analytical rotor (model ASR-2) and employ-ing a reversible hydrogen electrode (RHE) as the reference elec-trode. A Pine AFCBP1 bipotentiostat was used to collect the CVsover a range of temperatures (12–72 �C) and solution pH. Prior tocell assembly, the surface of the GC working electrode was pol-ished and then pretreated in O2-saturated 0.5 M H2SO4 by potentialcycling at 100 mV/s. The initial introduction of O2 gas into the celltook about 30 min. A flow meter with a 40–70 ml/min range wasused for the control of reactant flow rate in the experiments. Elec-trochemical evaluation included slow sweep (10 mV/s) CVs, first inO2-saturated conditions, then in N2, and then again in O2-saturatedsulfuric acid solutions. After subtraction of the N2 data from the re-sults in the presence of O2, the data were analyzed using both Tafeland Koutecky–Levich methods.

-2.0x10-3

-1.0x10-3

0.0

0.72 0.76 0.80-5.0

-4.5

-4.0

-3.5

E vs. RHE (V)

K-L

evic

h Lo

g[(I lxI

)/(I l-I)

]

TS= 81 mV

TS= 60 mV

Co-pda/VC (900 oC)

Cur

rent

(A/c

m2 )

Co-en/VC (700 oC)

104 A. Mani, V.I. Birss / Journal of Electroanalytical Chemistry 687 (2012) 102–110

The effect of pH on the ORR rates was determined by using mix-tures of 0.5 M H2SO4 and 0.5 M Na2SO4 in order to keep the ionicstrength constant. The solution pH was varied over a range of0.5–5. Controlled temperature studies were carried out in 0.5 MH2SO4 by placing the electrochemical cell in a water bath. The tem-perature of the bath was raised in 10 �C increments, with measure-ments being made at room temperature before and after eachincrease of solution temperature. The time required to stabilizethe solution temperature (counter and working electrodes) wasabout 2 h, while the reference electrode was kept at room temper-ature. The catalyst films on the GC disk were coated with a 1% Naf-ion solution in order to physically stabilize the catalyst during CVmeasurements at different solution temperatures.

0.2 0.4 0.6 0.8

E vs. RHE (V)

Fig. 2. ORR CV response at Co-ligand/VC-derived catalyst, deposited on GC, in O2-saturated, RT, 0.5 M H2SO4 at 10 mV/s and 1000 rpm. Co-pda, HT at 900 �C and Co-en, HT at 700 �C). Inset: ORR Tafel plots in activation-controlled region obtainedfrom CV data.

3. Results and discussion

3.1. General ORR activity and selectivity as a function of nature ofligand employed

In earlier work [13a], it was shown that the addition of phenyl-enediamine (pda) vs. ethylenediamine (en) to a Co oxide gel,formed from Co chloride or nitrate, led to better catalysis of theoxygen reduction reaction (ORR) in 0.5 M H2SO4 solutions. It wasalso revealed [13b] that the ORR activity of Co-pda was highestand the H2O2 yield the lowest when the catalyst, supported on Vul-can carbon, was heat-treated in a N2 environment at 900 �C, whilefor Co-en, the optimum heat-treatment temperature was consis-tently 700 �C. It was also shown that the most active catalysts,formed using this method, involved a Co:ligand (either pda oren) ratio of 1:2, i.e., a ratio of 4 N atoms per Co. The best catalystalso had a nominal concentration of 22 wt.% Co (corresponding toa 5.7 wt.% Co loading after heat-treatment of Co-pda and 4.5% forCo-en), the same conditions used to examine the ORR mechanismin the present work.

Typical cyclic voltammetry (CV) data obtained for the Co-en vs.the Co-pda-based catalyst, showing the effect of the GC disk rota-tion rate, are presented in Fig. 1 in O2-saturated 0.5 M H2SO4 at22 �C. Fig. 2 directly compares the results for the two catalysts at1000 rpm. The CVs are well behaved and the Co-pda catalyst isshown to be significantly more active than the Co-en material, asseen from the higher potential at the onset of the ORR. The lowerlimiting currents seen for the Co-en catalyst may be due to somedifferences in catalyst layer morphology [15], or, more likely, a lar-ger extent of H2O2 generation at the en-based catalyst [16], as willbe discussed below in more detail. The inset of Fig. 2 shows a Kou-tecky–Levich plot of the data in the activation (kinetic) region, withthe Tafel slope being substantially lower for the pda-derived cata-lyst than for the analogous en-based catalyst.

0.2 0.4 0.6 0.8-4.0x10-3

-3.0x10-3

-2.0x10-3

-1.0x10-3

0.0(a)

3000 rpm

2000 rpm

1500 rpm

1000 rpm

500 rpm

0 rpm

E vs. RHE (V)

Cur

rent

(A/c

m2 )

Fig. 1. CVs (10 mV/s) for the ORR in O2-saturated, RT, 0.5 M H2SO4 at: (a) Co

Table 1 summarizes these data, showing quantitatively that thearomatic pda catalyst is significantly more active than the en-de-rived materials by a factor of about 11 (at 0.7 V vs. RHE, which isin the kinetic region) when using the optimum HT temperaturefor each material, consistent with previous work [13b]. Notably,the amount of H2O2 generated (Table 1) is also significantly lowerfor the pda (900 �C) vs. the en-derived (700 �C) catalyst measuredusing the RRDE technique [16].

Table 1 also shows that the room temperature ORR Tafel slopesare generally smaller for the pda (e.g., 60 mV per decade of currentfor the most active 900 �C pda catalyst, giving a transfer coefficient(a) close to 1) vs. the en produced catalysts. The a value can beused to help determine the nature of the rate-determining step(rds) in the ORR mechanism, utilizing Eq. (1), where c is the num-ber of steps preceding the rds, m the stoichiometric coefficient, zequals 0 when the rate determining step is a chemical step and 1if the rds is an electron transfer step, and b is the symmetry factor,which is 0.5 for most systems of interest [17].

a ¼ c#þ zb ð1Þ

For the most active en-derived catalyst (heat-treated at 700 �C),the Tafel slope is closer to 80 mV and thus the a value is corre-spondingly smaller. According to classical electrochemical kinetics,an observed a value of 1 can be interpreted as indicative of a slowchemical step following a rapid first electron transfer reaction [18],assuming that b = 0.5, as is normally the case.

(b)

-en/C, heat-treated at 700 �C, and (b) Co-pda/C, heat-treated at 900 �C.

Table 1Effect of ligand and heat-treatment (HT) temperature on ORR activity, selectivity, and Tafel slope data at [Co–N–C]-based (Co-pda/VC or Co-en/VC) catalysts.a

Catalyst HT (�C) I @ 0.7 V (lA) I @ 0.05 V (lA) H2O2b (%) T.S. (mV) a

Co-pda/VC 900 408 600 33 60 1.0Co-en/VC 900 – 370 48 152 0.4Co-pda/VC 700 20 550 37 100 0.6Co-en/VC 700 37 450 42 81 0.75

a In O2-saturated 0.5 M H2SO4 (room temperature) at 10 mV/s and 1000 rpm.b At 0.05 V vs. RHE.

A. Mani, V.I. Birss / Journal of Electroanalytical Chemistry 687 (2012) 102–110 105

In the case of the ORR at Pt cathodes, two linear Tafel regionsare normally seen [19], with slopes close to 60 and 120 mV/decseen at low and high overpotentials, respectively. However, thefirst electron transfer step is believed to be rate determining atall potentials. In the low current density regime, it has been sug-gested that the ORR occurs on a Pt surface at which anions or oxy-gen species are adsorbed according to a Temkin isotherm, giving a60 mV/dec Tafel slope [19]. The appearance of a 120 mV per decadeTafel slope at high current densities is consistent with Pt being freefrom any adsorbed surface species and thus with adsorption obey-ing a Langmuir adsorption isotherm [20]. A standard interpretationfor an 80 mV Tafel slope could reflect two parallel reaction path-ways, or the presence of adsorbed intermediates with a coveragedependent on the potential [21].

Wang and Balbuena [22] suggested a chemical step following afast first electron step as the rds for the ORR at CoPd3 alloy cata-lysts, giving a 60 mV Tafel slope, as is observed here for the Co-pda catalyst (Fig. 2 and Table 1). A 60 mV slope was also obtainedfor Fe–N–C catalysts produced from pyrolized polyaniline [23]. Inother work, an observed 120 mV Tafel slope was suggested to arisefrom the thickness of the catalytic layer, leading to a doubling of a60 mV slope [8d,15,24]. In the case of our catalyst materials, weuse the conventional approach (Eq. (1)) to explain the a values ob-tained, as we have no evidence for any significant catalyst mor-phology effects on the Tafel slope, or of the presence of anypotential-dependent adsorbed intermediates (e.g., OH, water, an-ions, etc.) on either our en- or pda-derived catalyst surfaces duringthe ORR. Related to this, the ORR activity was shown to be inde-pendent of the solution anion (HSO�4 vs. ClO�4 ) at Fe-polyanilinecatalysts [23].

4. Influence of solution pH on the ORR activity of Co-derivedcatalysts

In order to gain further insight into the ORR mechanism, the ef-fect of the solution pH, all in acidic H2SO4 environments of con-stant ionic strength (formed using the appropriate mixtures of0.5 M H2SO4 and 0.5 M Na2SO4) was examined in this work. Thesesolutions were not buffered, due to the unknown impact of thebuffer solution anions on the Co-containing catalyst (e.g., surfaceadsorption/blocking, dissolution of metallic species, etc.). Ourstudy of the effect of pH on the ORR activity has led to importantnew mechanistic information, as discussed below.

Fig. 3 shows the CV data for the ORR as a function of solutionpH, all at room temperature (RT) conditions, in this case for theCo-pda/VC materials. The peak seen in the cathodic CV sweepsmay be related to the retention of some O2 within the catalystlayer, as it was seen that these peaks usually disappeared withina few full cycles of the potential. The CVs are otherwise very stablein these RT studies and the potential could be cycled for manyhours without a change in the measured ORR current. As is shownin the activation-controlled region at >0.75 V in Fig. 3a, the rate ofthe ORR is unaffected by pH at pH < 2.5, while at pH’s greater than�4, the CVs change quite dramatically (see discussion below).

The independence of the kinetic currents on pH, which was alsoobserved (but not shown in a figure) for the Co-en-based materialsat pH < 2.5, indicates that the ORR pathway does not involve a pro-ton up to and including the slow step. Fig. 3b shows that the Tafelslope (in the activation-controlled potential range) for CV-pda isalso independent of solution pH (pH < 2.5), with a close to constanta value of �1. For the en-based catalyst, a Tafel slope of 80–90 mV/decade of current, independent of pH, was obtained in this pHrange.

It is of interest that the rate of the ORR at Pt cathodes in acidicsolutions is also pH independent, with the slow step assumed to bethe transfer of the first electron to an adsorbed O2 molecule [19a].A chemical step following the first electron transfer step was alsoproposed as the rate determining step at Co–Pd3 ORR catalysts[25]. Zagal et al. [26] showed that the ORR is pH independent atpH < 3 for Co and Fe tetrasulfonate phthalocyanines (M-TSPc).Zhang et al. [8a] reported that adsorbed 5,10,15,20-tetrakis (penta-fluorophenyl) porphine Fe(III) (FeTPFPP) on graphite has good ORRelectrocatalytic activity. Similar to our results (Fig. 3), at pH 0–2.7,the current was independent of pH, while at pH > 2.7, an increasein the overpotential was seen [8a].

At pH > 2.5, the CV response shown in Fig. 3a is rather peculiar,showing a substantially negatively shifted ORR response as well asa decreasing limiting current, especially for the CV in the pH 4.14and pH 5 solutions. In these cases, the rotation rate independentcurrents (normally activation controlled region) extend negativelyto at least 0.4 V, and have unusual characteristics. Although theORR at pH 4.14 does begin at a potential positive of the situationat pH 5, as it should, the onset of the main wave is actually morenegative at pH 4.1 than at pH 5, also seen for the Co-en-basedmaterial [16]. This may be reflective of a local pH, which is differ-ent from that of the bulk solution. Thus, as the ORR commences, itwill generate a locally alkaline environment, consistent with thenegative shifts seen in Fig. 3. In any case, as our primary interestin the ORR is in strongly acidic solutions, the results at pH < 2.5are the most critical to understanding the ORR reaction mechanismand kinetics under PEM fuel cell conditions and thus these data areinterpreted in more detail below.

It is notable in Fig. 3 that the limiting ORR current decreaseswith increasing solution pH, up to pH 4.14, increasing again atpH 5. This most likely reflects an increase and then decrease inH2O2 production, leading to a change in the number of electrons(n) passed. Limiting currents can also be affected significantly bycatalyst layer morphology, with the currents normally decreasingthe thicker the layer is (giving a more pronounced concentrationgradient within the layer, which would not be affected by electroderotation rates) [27]. However, as the same electrode was trans-ferred between solutions of different pH, the layer morphologywould have been unchanged and thus this cannot explain thechanges in the limiting currents in Fig. 3. These are therefore morelikely indicative of changes in the reaction pathway, leading tomore or less H2O2 generation.

Because a rotating ring-disc electrode (RRDE) was not employedin this part of the work, the disk limiting current densities wereused to construct Levich plots (Fig. 4) to then determine the num-

0.2 0.4 0.6 0.8

-3.0x10-3

-2.0x10-3

-1.0x10-3

0.0(a)

pH=5.0pH=4.14

pH=2.6

pH=1.5

pH=0.53

Cur

rent

(A/c

m2 )

E vs. RHE (V)0.75 0.80 0.85

-5

-4

-3(b)

TS=62 mVpH=2.6

TS=57 mVpH=1.5

TS=56 mVpH=0.53

E vs. RHE

K-Le

vich

Log

[(IlxI

)/(I l-I)

]

Fig. 3. (a) pH dependence of the ORR at GC/Co-pda/VC catalyst, heat-treated at 900 �C, at 10 mV/s and 1000 rpm in RT 0.5 M H2SO4; (b) Mass-transfer correctedKoutecky–Levich plots of the data in Fig. 3a.

0.00 0.01 0.02 0.03 0.04 0.050

200

400

600

800

1000 pH= 0.53 pH=1.5 pH=2.6 pH=4.14 pH=5

Theoretical,

ω-1/2 (rpm-1/2)

i l-1(A

/cm

2 )-1

Fig. 4. Typical 1/i vs. x�1/2 Levich plots (10 mV/s) for Co-pda/VC on GC rde (HT at900 �C, two Nafion overlayers) all in O2-saturated and RT solutions of varying pH.

106 A. Mani, V.I. Birss / Journal of Electroanalytical Chemistry 687 (2012) 102–110

ber of electrons involved in the ORR as a function of pH. This, inturn, gives a good measure of the amount of H2O2 generated inthe reaction and could help to understand the limiting current datain Fig. 3 The fact that straight lines are obtained in Fig. 4 confirmsthat the number of electrons involved in the ORR (n) is indepen-dent of rotation rate. Values of n were then calculated from theslope of the Levich plots (at 0.05 V) for all catalysts using Eq. (2)[17]:

il ¼ 0:62 nFAD23o x1

2 e�16 Co ð2Þ

where il is the limiting diffusion current density, F the Faraday con-stant, Do the oxygen diffusion coefficient (1.77 � 10�5 cm2/s) [28],x is the rotation rate, in rpm, t is the kinematic viscosity(0.01 cm2/s), and Co is the oxygen solubility in the electrolyte(1.26 mM in O2-saturated H2SO4) [3a].

This analysis showed that, for the Co-pda catalyst in the rangeof pH from 0.53 to 2.6, the amount of H2O2 generated increasesfrom ca. 35% (in agreement with the RRDE data in Table 1 at lowpH) to 48%, while 44% H2O2 generation is seen at pH 5. A very sim-ilar outcome was seen at the Co-en/VC materials, although the nvalues are lower and the H2O2 yields higher than observed forthe Co-pda catalyst. It is possible that these data reflect an increas-ing H+ gradient in the catalyst layer down to pH 4, and then watermay be the reactant at pH’s above this. As there is no shortage ofwater present, the gradient disappears and the limiting currentsreturn back to normal.

5. Influence of solution temperature on ORR activity

State-of-the-art PEMFCs operate at temperatures of 70–80 �Cand thus it is critical to determine the effect of solution tempera-ture on the ORR (activity, stability) at the sol–gel derived non-no-ble metal ORR catalyst materials under development here. Thepresent work therefore focuses on the ORR at the Co (pda)- andCo (en)-based catalysts, optimized in terms of their heat treatmenttemperatures (900 �C and 700 �C, respectively) and examined atvarious temperatures only in 0.5 M H2SO4, the solution most sim-ilar to what is encountered in PEM fuel cells. In this part of thework, a rotating disk electrode was used and thus the H2O2 yieldwas again determined from the n value obtained from the limitingcurrent densities and subsequent Levich analyses.

The CV response for the Co-pda/VC catalyst in N2-saturated0.5 M H2SO4 was examined to obtain a sense of catalyst stabilityat higher solution temperatures. In Fig. 5a, the CVs are seen to in-crease in size with increasing solution temperature, all at a scanrate of 10 mV s�1. In fact, a roughly linear increase in the capacitivecurrent is seen with increasing solution temperature (Fig. 5b), sug-gesting that higher solution temperatures cause the carbon pow-der and/or the catalytic material to become more porous, crack,or break apart, thus exposing more internal surface area. In N2-sat-urated H2SO4, Fig. 5a shows that the CVs give capacitive featuresthat are very similar to those seen at carbon powder alone, beingindependent of electrode rotation rate and giving an approxi-mately linear relationship between the CV currents and sweeprate. The peaks centered at ca. 0.6 V in Fig. 5a are typical of thepseudo-capacitive processes that occur at carbon surfaces and noadditional redox peaks, which could be ascribed to the presenceof Fe or N at the catalyst surface, are seen. A capacitive charge of�24 mC is obtained from the CVs (at 22 �C) and as the BET surfacearea of this catalyst is 230 m2/g, and the catalyst loading is1.4 � 10�4 g, this gives a charge density of 0.75 C m�2, which isquite typical for carbon materials.

Fig. 6 shows the mass-transfer corrected Koutecky–Levich plotsderived from the CVs of the Co-en/Vulcan carbon and Co-pda/Vul-can carbon catalysts in O2-saturated 0.5 M H2SO4 at different tem-peratures, all at 1000 rpm and 10 mV/s. Both the Co-pda and Co-encatalyst activity improves with increasing solution temperature(up to 42 �C), as expected. Above this, stability issues were a prob-lem. To overcome this, the catalyst layers were coated with a stan-dard aliquot of 1% Nafion solution to enhance their stability. TheORR response of the catalyst with and without the overlying Nafionlayer was found to be identical in the activation controlled region,yielding the same activity, Tafel slope (50–70 mV for the Co-pdacatalyst), and a values. This shows that the 1% Nafion overlayerdoes not negatively affect the performance of the catalyst.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-4.0x10-4

-2.0x10-4

0.0

2.0x10-4

4.0x10-4

(a)(b)

32 oC42 oC52 oC

62 oC72 oC

22 oC

Cur

rent

(A)

E vs. RHE

Fig. 5. (a) CV response of GC/Co-pda/C, heat-treated at 900 �C and coated with an aliquot of 14 lL of 1% Nafion solution, in N2-saturated, 0.5 M, RT, H2SO4 at 10 mV/s and1000 rpm. (b) Current at 0.6 V from CVs in (a) vs. solution temperature.

0.70 0.72 0.74 0.76 0.78

-5.0

-4.5

-4.042 oC

60 oC

52 oC

68 oC

K-Le

vich

Log

[(I lxI

)/(I l-I)

]

E vs. RHE (V)

22 oC

(a)

0.76 0.78 0.80 0.82

-4.5

-4.0

-3.5

62oC

K-Le

vich

Log

[(I lx

I)/(I l-I

)]

E vs. RHE (V)

52oC

32oC

42oC

22oC

(b)

Fig. 6. Mass-transport corrected Koutecky–Levich plots derived from the CVs (not shown) of: (a) Co-en/VC and (b) Co-pda/VC catalysts as a function of temperature of 0.5 MO2-saturated H2SO4, all at 1000 rpm and 10 mV/s. Catalysts were coated with 1% Nafion solution.

-5

-4

-3

Eact= 34 (kJ/mol)

i (m

A/cm

2 )

A. Mani, V.I. Birss / Journal of Electroanalytical Chemistry 687 (2012) 102–110 107

However, at temperatures above �45 �C, the catalyst activitywas seen to decrease due to degradation and/or physical loss of ac-tive sites and/or catalyst material, even with the Nafion coating inplace, although the catalysts do not lose any further activity afterseveral full potential cycles. The problem of the lack of stabilityof non-noble metal ORR catalysts is well known [29]. One explana-tion is that Co dissolves in the acidic media, consistent with its wellknown poor resistance to corrosion [30], thus decreasing the num-ber of metal-containing (active) sites. Indeed, in Dodelet’s earlywork [31], the presence of Fe and Co in their metallic states wasconfirmed in these types of catalysts. Other explanations are re-lated to N site protonation (followed by anion binding), thus block-ing of the active ORR site [10d,e,32]. Despite the loss of ORRactivity at high solution temperatures, the Tafel slopes (and relateda values) could still be tracked (Table 2), however, as Tafel slopesare related to reaction mechanism and not to the number of activesites available. In fact, the a values remain close to 1 (Table 2) forthe Co-pda catalysts up to a 0.5 M H2SO4 solution temperature of

Table 2Tafel slopes and alpha values for ORR at Co-pda/VC and Co-en/VC.a

T (�C) 12 22 32 42 52 62 72

Tafel slope (Co-pda) 50 58 56 58 66 69 74a (Co-pda) 1.0 1.0 1.0 1.0 0.95 0.95 1.0Tafel slope (Co-en) – 81 – 96 95 112 156a (Co-en) – 0.7 – 0.65 0.5 0.6 0.5

a Co-pda and Co-en catalysts (heat-treated at 900 �C and 700 �C, respectively),examined in 0.5 M H2SO4 at 10 mV/s and 1000 rpm at various temperatures.

72 �C. For Co-en, the a value becomes ever closer to 0.5 as the solu-tion temperature increases, showing again that these two catalystshave a different distribution of active sites.

The activation energy (Eact) of the ORR was obtained (Fig. 7)both for the currents in the diffusion controlled region (e.g., at0.05 V vs. RHE) and in the kinetically controlled region (at 0.78 Vvs. RHE), using the temperature data in Fig. 6 (Co-pda/VC, HT at900 �C) and Arrhenius analysis [33]. Here, R and T have their usualmeanings.

3.0x10-3 3.2x10-3 3.4x10-3 3.6x10-3

-8

-7

-6

Eact= 8.8 (kJ/mol)

Ln

1/T (K-1)

Ln i (Diffusion Controlled Region) Ln i (Kinetically Controlled Region)

Fig. 7. Arrhenius plots of diffusion controlled and (b) kinetically controlled currentsobtained for ORR at Co-pda/VC catalyst, heat treated at 900 �C, on GC at 10 mV/sand 1000 rpm in O2-saturated 0.5 M H2SO4 at various temperatures.

108 A. Mani, V.I. Birss / Journal of Electroanalytical Chemistry 687 (2012) 102–110

Eact ¼ �2:303Rd logi

d 1T

" #ð3Þ

Eact for the diffusion-controlled transport of O2 to the Co-pda/VCcoated GC electrode is found to be 8.8 in kJ/mol (Fig. 7, curve a),which is typical for the mass transport limited region in acidicsolutions, even at Pt/C electrodes [34].

In the kinetically controlled region, Eact is 34 kJ mol�1 for Co-pda/VC (Fig. 7 at 0.78 V vs. RHE) (Note that the Eact value couldnot be determined for the Co-en catalyst at this potential due toits comparatively low activity). This value compares favourablywith previously measured Eact values of 38.5 kJ mol�1 (0.24 eV) at0.93 V (kinetically controlled region) for the ORR at unsupportedpolycrystalline Pt and its alloys [35]. It has been noted that theactivation energies are higher in the low current density region.Beattie et al. reported activation energies of 23.8 and 39.1 kJ mol�1

for the ORR at Pt/BAM (Ballard’s Advanced Material) interfaces atlow and high current densities, respectively [36]. As expected,the activation energy decreases with increasing overpotential.

In order to determine the number of electrons involved in theORR as a function of solution temperature (i.e., to determine the%H2O2 produced), Levich plots were constructed (Fig. 8) from thediffusion controlled currents at 0.1 V vs. RHE, seen at the Co-en/VC and GC/Co-pda/VC catalysts, both nominally 22% Co content,in O2-saturated 0.5 M H2SO4. Linear trends have been fittedthrough the data, verifying that the ORR is proportional to the sup-ply of O2 from the electrolyte to the RDE, as predicted by the Levichequation [37].

The number of electrons involved in the ORR was calculated[3a,28] from the Levich slopes, giving a value of close to 30%H2O2 generation in the case of Co-pda/VC at 22 �C, and droppingto ca. 4% at 62 �C. In contrast, for Co-en/VC, the% H2O2 is ca. 33%at 22 �C, increasing to 95% at 68 �C. This reveals a distinct differ-ence in reaction mechanism for the ORR at these two catalysts,consistent with their very different Tafel slope values.

6. Mechanism of ORR at Co–N–C-based ORR catalysts

The observed a value of 1, obtained for the best-performing Co-pda catalysts at all solution pHs and at temperatures from 12 to72 �C (Fig. 3 and Table 2), can be interpreted classically as indica-tive of a slow chemical step following a rapid first electron transferreaction [17], assuming that b = 0.5, as is normally the case. At Ptcatalysts, a Tafel slope of 60 mV (a of 1) for the ORR is also seen,but it is typically attributed [19,20] to a pathway in which the firstelectron transfer step is slow, with the mechanism involving the

0.00 0.01 0.02 0.03 0.04 0.050.0

2.0x102

4.0x102

6.0x102

8.0x102

1.0x103

oCoC

oCoC

i l-1(A

/cm

2 )-1

ω-1/2(rpm)-1/2

22 52 60 68

(a)

Fig. 8. Typical Levich plots showing the variation of the limiting current with x1/2 in 0.5solution on catalyst surface). (a) Co-en/VC and (b) Co-pda/VC.

adsorption of reaction intermediates or solution ions [19], havinga surface coverage that depends on the potential. However, thereis no evidence in the CVs (e.g., Fig. 5a) of the Co-pda catalyst foradsorption processes of this kind. Further, the Tafel plots do not ex-hibit a slope of ca. 120 mV at higher overpotentials, as would beexpected if this mechanism did apply. For these reasons, here weuse the classical interpretation of Tafel slopes as reflecting differentslow steps in the ORR pathway.

Commencing with the more active Co-pda catalyst, the Tafelslope and reaction rates were seen here to be independent of thesolution pH at pH < 2.5, demonstrating that no protons are in-volved in any of the reactions preceding or including the slow stepof the ORR under these conditions. This is a new and importantclue to the ORR reaction mechanism at this class of NPM catalysts.While the Co-pda catalysts do produce some H2O2, water is the pri-mary product. Therefore, a H2O2-free mechanism is suggested firstfor the Co-pda catalyst, as shown in reactions (4–9). This mecha-nism is consistent with both the observed a value of 1 and thepH independence of the reaction rate.

Importantly, as there is no direct evidence from the CV data inN2-saturated H2SO4 (Fig. 5a) for any redox activity of the Co sites,we do not include redox mediation of the ORR by oxidized Co sitesin the mechanism below, similar to what was concluded by othersfor similar M–N–C ORR catalysts [6e,38]. This is contrary to whatwas indicated in a recent study of the ORR at pyrolyzed Fe-polyan-iline catalysts [23], where redox mediation through the surfacebound Fe2+/3+ redox couple was proposed. In this previous work[23], a correlation was seen between the magnitude of the surfaceredox peaks (e.g., Fig. 5a) and the amount of O2 that was adsorbed,while in our work, the surface redox peaks were always very small,even when very good ORR activities were obtained. While it hasalso been argued [39] that the metal sites, in an oxidized form,are needed to break the strong O2 bond, there are other studies[40] that argue that metal-free N–C catalysts give very good ORRperformances [41]. Therefore, the following proposed mechanismdoes not include the mediation by Co2+/3+ surface sites.

O2 $ O2;ðadsÞ ð4Þ

e� þ O2;ðadsÞ $ O�2ðadsÞ ð5Þ

O�2ðadsÞ $ O�ðadsÞ þ OðadsÞ ðrdsÞ ð6Þ

O�ðadsÞ þHþ $ HOðadsÞ ð7Þ

OðadsÞ þHþ þ e� $ HOðadsÞ ð8Þ

0.00 0.01 0.02 0.03 0.04 0.050

1x102

2x102

3x102

4x102

5x102

oCoC

oCoC

(b)oCoC

ω-1/2 (rpm-1/2)

i l-1(A

/cm

2 )-1

22 32 42 52 62 72

M H2SO4 solutions of varying temperature (with the addition of 14 lL of 1% Nafion

A. Mani, V.I. Birss / Journal of Electroanalytical Chemistry 687 (2012) 102–110 109

2� ½HOðadsÞ þHþ þ e� $ H2O� ð9Þ

In this mechanism, O2 is adsorbed at the catalytic site and isthen reduced in reaction (5), forming O�2 ðadsÞ, which then splitsinto two surface species in reaction (6), suggested to be the rate-determining step (rds). Following reaction (6), protons are in-volved, leading to the formation of only water at this particularreactive site. While this proposed mechanism is not common, ithas been observed that the rds for the ORR at Co–Pd3 alloys is achemical step following a fast first electron transfer step, with nopH dependence observed [25b], similar to what is reported herefor the Co-pda catalyst.

As seen in our work, some H2O2 is produced at the Co-pda cata-lyst, even though the reaction rate and Tafel slope (a = 1) both con-tinue to be pH independent. It has been argued earlier that there aretwo parallel ORR pathways for these catalysts, based partly on thestrong dependence of H2O2 production and ORR activity onthe heat-treatment temperature. [6d,10a,10d,11a,11d,13b] Thus, itis proposed that, at the H2O2 generated sites, the ORR mechanism,with reaction (6) still rate determining, could include step [7a],which would follow reaction (7), forming HO�2 , instead of reactions(8) and (9):

HOðadsÞ þ O�ðadsÞ $ HO�2 ð7aÞ

Reaction (7a) would then be followed by a protonation step,generating the observed H2O2.

The less active Co-en derived materials always gave lower a val-ues, ranging from 0.5 (first electron transfer step is slow) to a max-imum of 0.75 (Tables 1 and 2), with the ORR rate and Tafel slopealso pH independent at pH < 2.5. An a value of 0.5 is typically asso-ciated with the first electron step as rate determining [17], whilevalues between 0.5 and 1 may indicate the presence of severalreaction pathways (with different a values) or the onset of the per-turbation of the Tafel slope by distributed potentials within a por-ous layer [27a]. However, as both the Co-pda and Co-en catalystlayers were formed identically, they should have a very similarporosity, and therefore explaining the lower Tafel slopes seen forCo-en in this way does not seem reasonable. Therefore, atpH < 2.5, the treatment below for the Co-en catalyst is based onan a value of 0.5, a pH independent reaction rate and Tafel slope,and the production of significant amounts of H2O2, especially athigher temperatures (Tables 1 and 2). Reaction (5) is now therds, instead of reaction (6). This is followed by a different seriesof reaction steps (10,10a), giving a possible reaction scheme gener-ating H2O2.

O2 $ O2;ðadsÞ ð4Þ

O2;ðadsÞ þ e� ! O�2ðadsÞ ðrdsÞ ð5Þ

O�2ðadsÞ þHþ þ e� $ HO�2ðadsÞ ð10Þ

HO�2ðadsÞ þHþ $ H2O2 ð10aÞ

Invoking these same reactions, a water-forming pathway couldalso exist, as shown below:

HO�2ðadsÞ $ HO�ðadsÞ þ OðadsÞ ð10bÞ

HO�ðadsÞ þHþ $ H2O ð11Þ

OðadsÞ þ 2Hþ þ 2e� $ H2O ðmulti -stepÞ ð12Þ

The treatments above (reactions (4)–(9), (7a), (4), (5), (10),(10a), (10b), (11), (12)) are all related to a solution pH < 2.5,conditions under which Fig. 3 shows that the ORR is completelyindependent of pH. At pH > 2.5, for both the Co-pda and

Co-en-derived catalysts, the rate of the ORR is now pH dependent,the Tafel slope is around 120 mV (a = 0.5), and the H2O2 yield hasincreased (n has decreased). The ORR mechanism shown below forthese less acidic conditions involves both a hydrogen peroxideroute reaction (13a) and a water route (13b):

O2 () O2;ðadsÞ ð4Þ

O2;ðadsÞ þ e� þHþ ) HO2ðadsÞ ðrdsÞ ð13Þ

HO2ðadsÞ þHþ þ e� () H2O2 ð13aÞ

HO2ðadsÞ þ e� þHþ () H2Oþ OðadsÞ ð13bÞ

OðadsÞ þ 2Hþ þ 2e� () H2O ðmulti -stepÞ ð14Þ

As is always the case, the reactions that occur after the pro-posed rds in any of the sequences described above are not knownwith certainty, as information only up to and including the rds hasbeen obtained in the present work.

Overall, the tracking of the pH and temperature dependence ofthe ORR at non-noble metal catalysts has not been studied in depthpreviously, and thus new insights into reaction mechanism at thisimportant class of cathode materials have been obtained in thepresent work. In terms of the ORR activity of our Co-based NPMcatalyst, this is similar to what has been reported by others forNPM catalysts formed using other methods [7b,9b,c,42], includingthe CoTCPP + CoTMPyP/C (1wt% Co/C) catalysts produced by Okadaet al. [42]. At the same time, our Co-pda catalyst, formed from thesimple precursors used in the present work, is more active thanmany other reported catalysts, e.g., those formed from Co tetra-phenylporphyrins (CoTPP) [31,43].

Acknowledgements

The authors would like to thank Dr. A.H. Sirk of the University ofVictoria (Victoria, BC) for useful discussions, as well as the NaturalSciences and Engineering Research Council of Canada (NSERC) forthe overall financial support of this research and for scholarshipsupport of AM.

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