J. Phys. Chem. C 2010, 114, 5049–5059
5049
Anion-Exchange Membrane Fuel Cells: Dual-Site Mechanism of Oxygen Reduction Reaction in Alkaline Media on Cobalt-Polypyrrole Electrocatalysts Tim S. Olson, Svitlana Pylypenko, and Plamen Atanassov* Chemical & Nuclear Engineering Department, Center for Emerging Energy Technologies, UniVersity of New Mexico, Albuquerque, New Mexico 87131
Koichiro Asazawa, Koji Yamada, and Hirohisa Tanaka Daihatsu Motor Co. Ltd., Frontier Technology R&D DiVision Ryuo, Gamo, Shiga 520-2593, Japan ReceiVed: NoVember 5, 2009; ReVised Manuscript ReceiVed: January 20, 2010
The oxygen reduction reaction (ORR) processes in alkaline media that occur on a family of electrocatalyst materials derived from a Co containing precursor and a polypyrrole/C composite material (PPy/C) are investigated here. The effects of Co loading and heat treatment temperature on the CoPPy/C materials are revealed through structural evaluations and electrochemical studies. Principle component analysis (PCA), a mutivariant analysis (MVA) technique, is used to establish structure-to-property correlations for the CoPPy/C materials. In all cases, pyrolysis leads to formation of a composite catalyst material, featuring Co nanoparticles coated with Co oxides and Co2+ species associated with N-C moieties that originate from the polypyrrole structures. Based on these correlations, we are able to propose an ORR mechanism that occurs on this class of non-platinum based fuel cell cathode catalysts. The correlations suggest the presence of a dual site functionality where O2 is initially reduced at a Co2+ containing N-C type site in a 2 e- process to form HO2-, an intermediate reaction product. Intermediate species (HO2-) can react further in the series type ORR mechanism at the decorating CoxOy/Co surface nanoparticle phase. The HO2- species can undergo either further electrochemical reduction to form OH- species or chemical disprotonation to form OH- species and molecular O2. Introduction For the past few decades, the majority of research efforts related to low-temperature fuel cells have focused on proton exchange membrane systems. This trend is largely due to the development of the Nafion membrane by DuPont in 1967. The robust Nafion materials enabled fuel cell systems that would not be plagued by the same technical limitations that have prevented liquid electrolyte based alkaline fuel cells (AFC) from becoming economically viable energy conversion devices. NASA successfully developed and utilized AFCs on the Apollo missions, but they have been deemed not practical for terrestrial applications due to the added complexity of the system and materials design due to the presence of a liquid electrolyte and performance and stability issues that result from contaminates. There still are incentives to develop new and novel materials for AFC systems that would either alleviate or eliminate the technical issues associated with the liquid electrolyte systems. For example, the use of a much more diverse selection of potential fuels becomes thermodynamically favorable in alkaline media. Fuel cell reactions proceed differently in alkaline and acidic environments. The most notable difference in the reaction processes is the consumption and production of water. It is common for anode reaction in fuel cells operating at low pH to consume water, whereas at high pH water becomes a product of the oxidation reaction. Further, the systems will not be limited * To whom correspondence should be addressed. E-mail: plamen@ unm.edu.
to expensive precious metal based electrocatalysts as is required to stand the harsh conditions of proton exchange fuel cell systems. In an attempt to potentially enable liquid electrolyte-free AFCs, a number of groups have began research effort devoted to fabrication and engineering of anion-exchange membranes and ionomer solutions.1-7 A number of critical technical issues still need to be overcome, such as increased stability and conductivity. Anion-exchange membrane fuel cell (AMFC) performance in the absence of a liquid electrolyte has yet to show obtainable current densities that are comparable to stateof-the-art Nafion based fuel cell systems. Although, a hydrazine hydrate/air AMFC operating with dilute KOH added to the anode fuel stream has shown very promising results.8,9 Now, with the new developments of anion-exchange membranes and ionomers, there is also a renewed need for electrocatalyst materials for AFCs. The emerging task is now to develop catalyst materials that are deployable and specifically tailored for AMFCs. This will involve highly collaborative efforts between catalyst and membrane developers. The catalyst, membrane, and ionomer materials must be engineered and designed in a synergistic fashion for each individual AMFC system. Parallel efforts devoted to establishing structure-toproperty relations for the individual materials that could potentially be utilized as a component of an AMFC system must enlist a number of potential catalyst materials based on improved stability in alkaline media. There are several well-known chemistries and classes of nonprecious catalyst materials for the oxygen reduction reaction (ORR) in alkaline media including spinels, metal oxides, and pyrolyzed metal macrocycles
10.1021/jp910572g 2010 American Chemical Society Published on Web 03/03/2010
5050
J. Phys. Chem. C, Vol. 114, No. 11, 2010
(Me-Nx). In general, nonprecious metal catalyst materials are inherently more heterogeneous and less understood as compared to platinum and other precious metal based catalyst materials. Recently, the class of nonprecious metal ORR catalyst that has received the most attention is materials derived from or inspired from pyrolyzed transition metal macrocycles (Me-Nx). Here, we report on ORR mechanistic processes that occur on a family of cobalt-polypyrrole based catalyst materials in alkaline media. We have evaluated non-heat-treated materials as well as materials that have undergone low- and high-temperature pyrolysis treatments in inert atmosphere. The CoPPy/C based catalyst materials evaluated in this study can be more generally classified as Me-N-C type catalyst materials. This class of non-Pt catalyst was initially inspired from biomimic origins. Jasinski was the first to show the potential of utilizing transition metal macrocycles compounds as ORR catalyst precursors.10 Bagotzky et al. demonstrated the advantages of exposing the transition metal macrocycle derived materials to heat treatments (pyrolysis). They showed that after pyrolysis at temperatures from 400 to 800 °C the stability in acidic media increased significantly as well as the catalyst’s ability to heterogeneously decompose peroxide.11 Much of the literature has focused by elucidating the nature of the active sites and the ORR processes that they support. Early in the development of pyrolyzed non-Pt catalysts, three major schools of thought emerged regarding the nature of the active site. These schools were led by van Veen, Yeager, Wiesener, and their respective collaborators. In brief, van Veen and collaborators concluded that Me-N4 centers are the ORR catalyst sites.12-14 Yeager and collaborators proposed that the metallic ion in solution from the dissolved metal oxide phase that forms during pyrolysis adsorbs and coordinates to nitrogen containing surface moiety to from an active ORR site.15,16 While the third school led by Weisener suggests that the metal does not play a role in the ORR and a site composed of C and N serve as the catalyst site.17,18 The exact nature of the active sites is still being debated in recent literature. The authors of this report19,20 and others21,22 have shown evidence of bifunctionality of these materials, where the intermediate reaction product can undergo further reduction or disproponation reactions, while others maintain that the direct 4 e- pathway occurs.23 The variety of conclusions could possibly be a result of the heterogeneous nature of the pyrolyzed materials. Yeager et al. were the first to publish the synthesis and performance of a catalyst material that was derived from separate metal (Co(II) and Fe(II) containing salts) and nitrogen (polyacrylonitrile) containing precursors.24 Since then Dodelet and collaborators have published much of the work in this area.25-33 Here, we evaluate a cobalt-polypyrrole composite catalyst for the ORR in alkaline media. Others have reported similar catalyst synthesis approaches to the one reposted here. For example, it has been shown pyrolyzation of similar carbon supported cobalt-polypyrrole electrocatalyst shifts the overall ORR mechanism toward 4 e- reduction as compared to the unpyrolyzed materials.34 In order to understand which ORR processes are supported on catalyst materials and the resulting implications for use in fuel cell systems, the possible reaction pathways must be known. Equations 1-3 show the series type ORR in alkaline media as well as the corresponding standard reduction potentials. Equation 1 shows that the first reaction process involves the 2 e- reduction of O2 and the consumption of H2O to form the intermediate reaction products, HO2- and OH-. Thereafter, the peroxide radical (HO2-) can either be further reduced in another 2e- to
Olson et al. form OH- (eq 2) or undergo chemical disproponation reaction to from O2 and OH- species (eq 3). The O2 molecule would then be free to be “recycled” in the first ORR step. It is desirable to develop AFC cathode catalysts that can react the HO2- species in either of the given reaction schemes. Complete reaction of the intermediate product would not only result in higher obtainable energy conversion efficiencies but also help mitigate degradation of the electrode and membrane structures from unwanted reactions with HO2- species.
Ο2 + Η2Ο + 2e- f ΗΟ2- + ΟΗ(Εo ) +0.761 V vs RHE pH ) 14) (1) ΗΟ2- + Η2Ο + 2e- f 3ΟΗ(Εo ) +1.693 V vs RHE pH ) 14) (2) 2HO2- f O2 + 2OH-
(3)
In this study we establish structure-to-property correlations for a family of cobalt-polypyrrole based ORR catalyst materials for AMFCs. Specifically, we investigate the effect of Co loading and the pyrolysis temperature used in the catalyst synthesis. In depth X-ray photoelectron spectroscopy (XPS) is performed to identify the surface chemical species present in these materials after certain synthesis conditions. Then structure-to-properties correlations are established through application of principle component analysis (PCA) applied to the data sets consisting of XPS determined composition and rotating ring-disk electrode (RRDE) determined electrochemical performance. Utilizing this analysis approach and physical reasoning, we are able to propose an ORR mechanism that occurs on pyrolyzed CoPPy/C materials. We have used a similar experimental method to infer upon the reaction mechanism that occurs on non-Pt ORR catalyst materials derived from pyrolyzed CoTMPP in acidic media.19 Experimental Section CoPPy/C Catalyst Synthesis. The cobalt-polypyrrole based catalyst materials were synthesized with a commercially available 20 wt % polypyrrole supported on a carbon black (Vulcan XC-72, Cabot Corp.) composite material, PPy/C (Aldrich 577065) and Co(NO3)2 · 6H2O. First, 3 g of the PPy/C material was mixed with 67 mL of water while being heated and stirred for 30 min. Next, 17 mL of water and Co(NO3)2 · 6H2O were added and mixed for an additional 30 min. Different amounts of the Co(NO3)2 · 6H2O precursor were added to obtain CoPPy/C materials with a projected final Co wt % of 1.7, 3.4, 5.1, and 17. Next, 5.23 g of NaBH4 and 0.37 g of NaOH were added to the mixture. The resulting material was washed with hot water and allowed to dry at 90 °C. The dried CoPPy/C material that was collected here will be referred to as the “non-heat-treated” catalyst material. Rotating Ring-Disk Electrode (RRDE) Preparation and Testing. The CoPPy/C catalyst layer was applied to the disk electrode as follows: 10 mg/mL solutions of the CoPPy/C materials were prepared by adding 10 mg of the CoPPy/C material, 0.9 mL of water, and 0.1 mL of isopropyl alcohol. Next 1 mg/mL solutions were prepared by adding 0.1 mL of the concentrated 10 mg/mL solution, 0.75 mL of water, 0.1 mL of IPA, and 0.05 mL of 10× diluted 5 wt % Nafion solution. Then 14 µL of the 1 mg/mL solutions were applied to the disk electrode. This results in a catalyst loading of 55 µg/cm2 and a
Anion-Exchange Membrane Fuel Cells
J. Phys. Chem. C, Vol. 114, No. 11, 2010 5051 TABLE 1: XPS Analysis Showing Elemental Concentration of Non-Heat-Treated and Heat-Treated at 300 °C and 700 °C Co-PPy/C Materials sample Co loading (wt %)
Co
N
C
O
1.7 3.4 5.1
0.9 1.8 2.8
1.6 1.6 1.7
91.1 89.8 87.7
6.4 6.8 7.8
300
1.7 3.4 5.1 17
0.8 1.4 2.2 5.0
1.4 1.5 1.4 1.1
92.0 92.2 90.5 84.6
5.8 4.9 5.9 9.3
700
1.7 3.4 5.1 17
0.4 0.6 2.3 3.2
0.7 1.1 0.5 0.5
93.4 95.8 91.4 89.9
5.5 2.5 5.8 6.4
heat treatment (°C) non-heat-treated
Figure 1. TGA performed on the 5.1 wt % Co-PPy/C sample. Analysis was performed under N2 atmosphere to stimulate the conditions the Co-PPy/C catalyst materials would be subjected to during pyrolysis.
catalyst/Nafion weight ratio of 3:1. When Nafion is used in these quantities, it serves only as a binding element in the catalyst layer. The RRDE experimental setup utilized a Pine Instruments bipotentiostat. A Pine E7 RRDE assembly consisting of a glassy carbon disk and Pt ring working electrodes was utilized. The electrochemical cell consisted of a Hg/HgO reference electrode and a Pt wire counter electrode. All data were collected in 1.0 M KOH electrolyte and at room temperature. The presented data were collected during a 10 mV/s disk potential scan rate and a rotation rate of 1600 rpm. The Pt ring potential was held constant at 0.6 V vs Hg/HgO (1.5 V versus RHE). Results and Discussion To examine the effect of exposing the CoPPy/C materials to heat treatments in inert atmospheres (pyrolysis), thermal gravimetrical analysis (TGA) was used to determine specific temperature treatments of interest. Figure 1 shows the wt % of CoPPy/C with temperature in N2 atmosphere. The curve can be described in three distinct regions of weight loss. The first occurs under 200 °C due to the loss of water. Then between 200 and 300 °C, there is another distinct weight loss. Thereafter, the weight loss becomes more gradual until approximately 550 °C where the third temperature region of weight loss is observed. A “low-temperature” heat treatment of 300 °C and a “hightemperature” heat treatment of 700 °C were selected based on this data. All formulations of the CoPPy/C materials (Co wt % of 1.7, 3.4, 5.1, and 17) were pyrolyzed under N2 gas flow at both temperatures for 4 h. Therefore, a total of 12 different CoPPy/C based materials were synthesized and characterized in this study. XPS Analysis of CoPPy/C Materials. XPS analysis was performed on 11 materials with Co loadings of 1.7, 3.4, and 5.1 wt % before and after heat treatment at 300 or 700 °C. First, we will discuss non-heat-treated materials with varied amounts of cobalt. Based on elemental composition, detected with XPS (Table 1), material with 3.4 wt % has twice and material with 5.1 wt % three times as much cobalt as 1.7 wt % material. This is in agreement with the expected amounts of cobalt following a 1:2:3 ratio. As the amount of cobalt increases, carbon concentration decreases and oxygen concentration increases. The trend for the ratio of oxygen to cobalt is, however, opposite that for oxygen concentration (7.1, 3.9, and 2.7, respectively, for 1.7, 3.4, and 5.1 cobalt wt % materials). This might indicate that some of the introduced cobalt comes associated with
elemental concentration (atom %)
oxygen. All three samples have a similar amount of nitrogen, suggesting that the N content in all samples evaluated here is dictated by the polypyrrole and not the N that is present in the Co precursor (Co(NO3)2). The most interesting trend is the ratio of nitrogen to cobalt. The first sample with 1.7 wt % cobalt has about 1.7 nitrogen atoms per cobalt atom, and it drops down to 0.9 and 0.6 nitrogen atoms per cobalt atom for the other two samples. Again, this suggests that some of the cobalt is not associated with nitrogen and could be bonded to oxygen or carbon instead. The number of peaks and their binding energies used to curvefit high-resolution spectra of Co 2p, N 1s, O 1s, and C 1s are the same for all non-heat-treated materials. Curve-fitting was carried out using individual peaks with 70% Gaussian/30% Lorentzian line shape and width constrained to 1.0 eV for C 1s and N 1s, and 1.3 eV for O 1s and Co 2p components. Only 2p3/2 components of the Co 2p spectra were curve-fitted. Thus, Figure 2a and b demonstrates only the portion of Co 2p spectra that corresponds to the curve-fitted 2p3/2 components. Also, components due to X-ray satellites, detected at binding energies higher than 785 eV, are not included in the quantification. Figures 2a,c and 3a,c show representative deconvoluted highresolution spectra of Co 2p, N 1s, O 1s, and C 1s. Detailed quantification and peak assignments for all non-heat-treated samples can be found in Tables 2-4. Generally speaking, all non-heat-treated catalyst materials consist of (1) graphitic carbon (peak at 284.3 eV), aliphatic carbon (peak at 285.3 eV), and carbon species in which carbon bonded to oxygen and or/ nitrogen (peaks at binding energies above 286 eV); (2) various oxygen species where oxygen is associated mostly with carbon and possibly nitrogen (peaks at 532, 532.9, and 531.1 eV), small amount of cobalt oxide (peaks at 529.9 and 531.1 eV), oxygen in carbonate and nitrate-like species (peaks at 534 and 535 eV), and oxygen from adsorbed water (peaks at 534-537 eV); (3) nitrogen species in the form of pyrrolic nitrogen (peaks at 399.8 and 400.4 eV), pyridinic nitrogen (398.4 eV), nitrogen complexed with cobalt (Co-Nx, peak at 399.2 eV), graphitic/ quaternary nitrogen (peak at 401.2 eV), nitrogen in imine (peak at 397.6 eV), and oxide (peaks at 402-403 eV) forms; and (4) cobalt species in the form of complexes, such as Co-Nx and/ or Co-Ox (peak at 782 eV), cobalt in oxide/hydroxide forms, including CoO, Co(OH)2, Co2O3, and Co3O4 (peaks at 781 and 780 eV), and cobalt bonded to oxygen in a carbonyl-like type of species (peaks at 783-785 eV). Distribution of carbon species is essentially the same in all non-heat-treated materials, as the sources of carbon (carbon
5052
J. Phys. Chem. C, Vol. 114, No. 11, 2010
Olson et al.
Figure 2. High-resolution XPS spectra of Co-PPy/C materials (exemplified by material with 5.1 wt % cobalt loading): (a) Co 2p spectrum of non-heat-treated Co-PPy/C; (b) Co 2p spectrum of Co-PPy/C heat-treated at 700 °C; (c) N 1s spectrum of non-heat-treated Co-PPy/C; (d) N 1s spectrum of Co-PPy/C heat-treated at 700 °C.
Figure 3. High-resolution XPS spectra of Co-PPy/C materials (exemplified by material with 5.1 wt % cobalt loading): (a) O 1s spectrum of non-heat-treated Co-PPy/C; (b) O 1s spectrum of Co-PPy/C heat-treated at 700 °C; (c) C 1s spectrum of non-heat-treated Co-PPy/C; (d) C 1s spectrum of Co-PPy/C heat-treated at 700 °C.
black Vulcan XC72 and polypyrrole) were identical for each catalyst material. In the oxygen spectra, the general trend is that as cobalt loading increases the amount of species at 534, 535, and 532.9 eV also increases. These species can be assigned to nitrates and carbonates and most probably are a mixture of both. The source of nitrates is the cobalt precursor, cobalt nitrate. Carbonate-like species are most probably formed during impregnation with cobalt and are similar to species previously observed during the analysis of materials derived from metal porphyrins.35,36 No major differences or trends were observed in Co 2p and N 1s spectra of non-heat-treated materials, indicating that in terms of nitrogen and cobalt species these materials are very similar. Nitrogen and cobalt spectra confirmed that Co-Nx centers are present in all non-heat-treated materials. The ratio of the peak at 399.2 eV in the N 1s spectrum to the
peak at 782 eV in the Co 2p spectrum allows us to evaluate the number of nitrogen atoms per cobalt atom (Table 5). Interestingly, ratios obtained for N399.2/Co782 are very close to those calculated from elemental concentrations. In case of 1.7 wt % cobalt materials, both the elemental N/Co ratio and N/Co ratio in Co-Nx are close to 2. As the amount of cobalt is increased to 3.4 wt %, only one nitrogen atom is available per cobalt atom. Further, the increase of cobalt to 5.1 wt % results in the amount of cobalt atoms exceeding that of nitrogen atoms. A N399.2/Co782 ratio of 0.6 indicates that part of cobalt complexes, contributing to a binding energy (BE) of 782 eV, are not Co-Nx but rather Co-Ox. Very small changes in the elemental concentrations are observed when materials with various cobalt loadings are pyrolyzed at 300 °C (Table 1). Generally, oxygen, cobalt, and
Anion-Exchange Membrane Fuel Cells
J. Phys. Chem. C, Vol. 114, No. 11, 2010 5053
TABLE 2: XPS Analysis of Co 2p Spectra of Non-Heat-Treated and Heat-Treated at 300 °C and 700 °C Co-PPy/C Materialsa ID: major phase
Co, CoxNy
ID: minor phase position (eV) heat treatment (°C)
Co (wt %)
non-heat-treated
1.7 3.4 5.1
Co3O4, Co
Co2O3
CoO
CoO, Co3O4 780
Co(OH)2, Co2O3 781
Co-complex (Co-Nx, Co-Ox) Co(OH)2 782
Co-nitrate, Co-carbonate
Co-nitrate, Co-carbonate
783
784
Co-nitrate, Co-carbonate
778
779
785
0 0 0
0 0 0
6.5 7.0 5.4
14.4 14.0 11.5
15.0 14.8 13.0
14.2 14.2 12.7
10.9 10.9 11.4
10.8 12.0 13.3
relative amounts as % of Co 2p envelope
300
1.7 3.4 5.1 17
0 0 1.4 1.2
0 2.3 10.5 9.9
6.6 10.5 15.1 14.5
16.0 16.0 16.9 18.2
16.2 14.2 12.6 13.6
12.8 11.6 8.9 9.6
10.2 8.7 6.7 6.6
11.0 10.2 7.1 7.3
700
1.7 3.4 5.1 17
2.7 3.4 2.4 1.2
9.5 12.2 10.2 6.8
12.4 16.2 13.5 10.8
17.1 16.3 10.9 13.6
13.4 11.6 10.0 12.4
9.3 8.4 9.5 10.4
7.2 5.4 8.0 8.1
6.9 6.5 10.5 10.1
a
Quantification is shown as % of Co 2p envelope. Positions and identification of peak components are indicated.
TABLE 3: XPS Analysis of N 1s Spectra of Non-Heat-Treated and Heat-Treated at 300 °C and 700 °C Co-PPy/C Materialsa ID position (eV)
imine, CoxNy 397.6
pyridine 398.4
1.7 3.4 5.1
6.3 6.5 6.8
12.6 12.5 11.0
16.6 17.8 12.8
31.2 32.9 31.0
21.5 21.3 22.9
7.6 6.9 9.6
4.2 2.0 5.9
300
1.7 3.4 5.1 17
11.9 10.7 10.0 9.9
19.5 17.2 17.2 20.2
17.0 15.8 13.8 14.5
19.4 24.4 21.8 17.5
18.3 18.8 21.8 19.2
10.0 8.8 9.1 13.2
3.8 4.3 6.3 5.5
700
1.7 3.4 5.1 17
20.4 16.8 15.5 11.2
22.0 24.0 23.3 27.2
12.6 12.3 15.2 11.1
11.3 11.3 10.7 16.5
16.6 18.9 14.3 20.1
11.4 9.9 14.7 6.3
5.8 6.6 6.2 7.5
heat treatment (°C)
Co (wt %)
non-heat-treated
a
Co-Nx 399.2
pyrrole, C-NdO 399.8 400.4
graphitic, quaternary 401.2
oxide 402-403
relative amounts as % of N 1s envelope
Quantification is shown as % of N 1s envelope. Positions and identification of peak components are indicated.
TABLE 4: XPS Analysis of O 1s Spectra of Non-Heat-Treated and Heat-Treated at 300 °C and 700 °C Co-PPy/C Materialsa ID position (eV)
Co-oxides 529.9
Co-oxides, oxygen bonded to carbon and/or nitrogen 531.1
oxygen bonded to carbon and/ or nitrogen 532
oxygen bonded to carbon and/ or nitrogen 532.9
oxygen in nitrate, carbonate, and adsorbed water 534
oxygen in nitrate, carbonate, and adsorbed water 535
oxygen in nitrate, carbonate, and adsorbed water 536-537
heat treatment (°C)
Co (wt %)
non-heat-treated
1.7 3.4 5.1
6.3 6.5 6.8
12.6 12.5 11.0
16.6 17.8 12.8
31.2 32.9 31.0
21.5 21.3 22.9
7.6 6.9 9.6
4.2 2.0 5.9
300
1.7 3.4 5.1 17
11.9 10.7 10.0 9.9
19.5 17.2 17.2 20.2
17.0 15.8 13.8 14.5
19.4 24.4 21.8 17.5
18.3 18.8 21.8 19.2
10.0 8.8 9.1 13.2
3.8 4.3 6.3 5.5
700
1.7 3.4 5.1 17
20.4 16.8 15.5 11.2
22.0 24.0 23.3 27.2
12.6 12.3 15.2 11.1
11.3 11.3 10.7 16.5
16.6 18.9 14.3 20.1
11.4 9.9 14.7 6.3
5.8 6.6 6.2 7.5
a
relative amounts as % of O 1s envelope
Quantification is shown as % of O 1s envelope. Positions and identification of peak components are indicated.
nitrogen amounts decreased while the amount of carbon increased slightly. Now, the material with 3.4 wt % has 1.8 and the sample with 5.1 wt % has 2.9 times as much cobalt as the 1.7 wt % sample, which is somewhat lower but still close to values observed for the non-heat-treated sample. Changes in the elemental composition resulted from heat treatment at 700 °C are much more drastic than the changes observed for samples treated at 300 °C (Table 1). For example,
after pyrolysis at 700 °C, concentrations of cobalt and nitrogen in 1.7 wt % cobalt catalyst samples had decreased at least by a factor of 2. Co 2p, N 1s, O 1s, and C 1s spectra were acquired and curve-fitted for materials with various cobalt loadings pyrolyzed at 300 and 700 °C. Figures 2b,d and 3b,d demonstrate representative spectra shown for 5.1% cobalt material pyrolyzed at 700 °C. Nitrogen speciation undergoes significant changes even at low-temperature treatment. Pyrrolic nitrogen, the major
5054
J. Phys. Chem. C, Vol. 114, No. 11, 2010
Olson et al.
TABLE 5: XPS Analysis Showing Nitrogen to Cobalt Ratiosa sample
ratio
heat treatment (°C)
Co loading (wt %)
N/Co
N399.2/Co782
non-heat-treated
1.7 3.4 5.1
1.7 0.9 0.6
1.9 1.1 0.6
300
1.7 3.4 5.1 17
1.9 1.1 0.6 0.2
1.9 1.2 0.7 0.2
700
1.7 3.4 5.1 17
1.9 1.9 0.2 0.2
1.8 2.0 0.4 0.1
a Ratios of elemental nitrogen to elemental cobalt are calculated from values indicated in Table 1. Ratios of nitrogen species detected at BE of 399.2 eV to cobalt species detected at BE of 782 eV are based on values in Tables 2 and 3, after accounting for elemental concentrations of cobalt and nitrogen, indicated in Table 1.
form of nitrogen in unpyrolyzed materials, becomes less and less dominant as materials are pyrolyzed at 300 and 700 °C. Pyridinic nitrogen content, on the other hand, is increasing with pyrolysis. An increase in the peak at 397.6 eV, in unpyrolyzed materials assigned to imine nitrogen, is most probably associated with formation of cobalt nitride (CoxNy) species. The nonpyrolyzed materials and materials pyrolyzed at 300 °C have about the same amount of nitrogen in Co-Nx centers, which then somewhat drops when materials are pyrolyzed at 700 °C. The same trend is observed for Co-Nx/Co-Ox centers observed at 782 eV in Co 2p spectra. To draw a certain trend in the changes of graphitic carbon alone was not possible due to the dominance of the signal from the support. Remarkably, the trend in the ratios of elemental N/Co and N/Co present in Co-Nx centers, observed for non-heat-treated materials, is preserved in materials heat-treated at 300 °C and not preserved in materials heat-treated at 700 °C. Moreover, values of N/Co and N399.2/Co782 obtained for materials non-heat-treated and heat-treated at 300 °C are very close. This suggests that Co-Nx centers do not undergo any major changes when materials are pyrolyzed at low
temperatures. When comparing materials with different cobalt loadings pyrolyzed at 700 °C, several observations can be made. First, in 1.7 wt % cobalt material, ratios of N/Co and N399.2/ Co782 do not change at both low and high pyrolysis temperatures. The initial values of about 1, observed for N/Co and N399.2/ Co782 ratios in unpyrolyzed and 300 °C pyrolyzed 3.4 wt % cobalt materials, do change at higher temperature and are close to those observed for 700 °C treated 1.7 wt % cobalt material. Contrary to the case of 3.4 wt % cobalt material, in 5.1 wt % cobalt material pyrolyzed at 700 °C, both ratios, N/Co and N399.2/ Co782, decrease significantly. Significant trends were elucidated while examining the other changes observed in Co 2p spectra for materials pyrolyzed at various temperatures. As a result of pyrolysis at 300 °C, peaks due to nitrates and carbonates located at 783-785 eV decrease and peaks due to CoO and Co2O3 located at 781 and 780 eV increase. In the case of 1.7 and 3.4 wt % materials, pyrolysis at 700 °C leads to further decrease in the peaks at 783-785 eV and further increase in the peaks at 780-781 eV. Materials with higher cobalt loadings, however, after pyrolysis at high temperature, have higher amounts of species at 783-785 eV and lower amounts of species at 780-781 eV than materials with the same cobalt loadings pyrolyzed at 300 °C. This indicates that at high cobalt loadings high pyrolysis temperature results in some new species located at the same BEs as nitrates and carbonates, present in unpyrolyzed materials. The most important change in the Co 2p spectra, however, is the formation of new species at 779 and 778 eV. Species at 779 eV are due to Co3O4 and metallic cobalt and are first observed at 300 °C for all cobalt loadings but 1.7 wt %. Species at 778 eV are due to metallic Co and cobalt nitride (CoxNy) and are first observed at 300 °C for materials with cobalt loadings 5.1 wt % and above. Formation of cobalt in mixed oxide, metallic, and nitride forms and formation of new species at 783-785 eV might be related to the change (decrease) in the ratios of N/Co and N399.2/Co782 that are observed as occurring simultaneously. Analysis of O 1s spectra allows us to conclude that, for all cobalt loadings but 17 wt %, the concentration of cobalt oxides increases as the temperature of pyrolysis increases. Also, peaks due to nitrates, carbonates, and adsorbed water located at binding energies 534-537 eV decrease when materials are pyrolyzed at 300 °C. The concentration of species observed in this binding energy range for the materials pyrolyzed at 700 °C is higher
Figure 4. Dependence of the formation of the decorating CoxOy/Co particle phase as illustrated by TEM micrographs of Co-PPy/C materials on cobalt content and pyrolysis temperature. Six TEM micrographs are arranged to illustrate the observed trend of CoxOy/Co particle formation during Co-PPy/C catalyst synthesis.
Anion-Exchange Membrane Fuel Cells
J. Phys. Chem. C, Vol. 114, No. 11, 2010 5055
Figure 5. High-resolution TEM micrograph of a CoxOy/Co particle phase on the catalyst surface. The metallic Co particle core as well as the CoxOy native surface oxide have been marked.
than that for the materials pyrolyzed at 300 °C, indicating formation of new types of species, possibly also observed in cobalt spectra at 783-785 eV. Transmission Electron Microscopy (TEM) Analysis of CoPPy/C Materials. Figure 4 shows the progression of the appearance of the decorating CoxOy/Co surface particle phase with respect to the Co loading and pyrolysis synthesis temperature. It is seen that no decorating nanoparticle phase is observed on the non-heat-treated materials regardless of the Co loadings, as exemplified by the TEM images for the 3.4 and 17 Co wt % materials. The TEM micrographs in Figure 4 of the heat-treated materials indicate that low- and high-temperature pyrolysis results in the formation of the nanoparticle phase. Further, the nanoparticle density present on the catalyst surface is dependent on the Co loading as well as the pyrolysis temperature. Over the range of Co loadings and pyrolysis temperatures reported here, the nanoparticle phase density increases with both parameters. Figure 5 is a TEM micrograph of a CoxOy/Co particle on the catalyst surface. As marked on the figure, the particle appears to have a metallic Co core and the particle surface is coated with a native CoxOy layer. It is conceivable that a nanoparticle phase consisting of a structure of this nature could play a role in the electrocatalysis processes that occur in an AFC. The metallic Co core would supply the needed electric conductivity for adequate electron transport from the electrode to the electroactive species, and the native oxide surface film would provide the necessary surface conditions for selective electrocatalytic processes. RRDE Data and Analysis of CoPPy/C Catalyst Materials. In order to investigate the ORR processes that occur on CoPPy/C based catalyst materials in alkaline media, RRDE experiments were performed with all the CoPPy/C catalyst formulations described in the Experimental Section. The RRDE data are presented in Figures 6-8 where the ring current data are shown in (a) and the disk currents in (b) for all figures. Here, the effect of Co loading is shown in the individual figures for the nonheat-treated materials (Figure 6), low-temperature pyrolysis (Figure 7), and high-temperature pyrolysis (Figure 8). Comparison of the RRDE presented here indicates that the magnitude of the disk current plateau for non-heat-treated materials is much more sensitive to the Co loading as compared to the materials that were exposed to low and high pyrolysis treatments. In fact, the disk currents obtained in the transport limiting region for the heat-treated materials show almost no dependence on the Co loading. Analysis of the kinetic limiting region for the CoPPy/C materials can be done by inspection of the current-potential
Figure 6. RRDE data for the non-heat-treated Co-PPy/C materials. The ORR disk electrode current (b) and the ring electrode oxidation currents (a) are shown for non-heat-treated Co-PPy/C materials with cobalt contents from 1.7 to 17 wt %.
relationship for the disk electrodes in the potential region of 0.0 to -0.06 V vs Hg/HgO as shown in Figure 9. Figure 9a shows the data obtained for the non-heat-treated materials and (b) and (c) contain the data for the materials exposed to low and high pyrolysis conditions, respectively. There is an observable trend in the non-heat-treated data with respect to the Co loading. The materials containing low Co loadings show more desirable current-potential ORR curves in the kinetic limiting region. This trend can be correlated to the N/Co ratio (Table 5) as determined by XPS analysis. This correlation suggests that the Co-Nx coordination plays a role in the initial ORR processes on this class of materials in alkaline media. This trend is not observed for the heat-treated materials. We can speculate that the catalyst surface after the heat treatments results in the increased complexity and formation of new active sites that support additional ORR processes. If the heat treatment of CoPPy/C materials results in two distinct surface species that promote dual site functionality in an overall series reaction scheme, establishing structure-to-property correlations becomes much more difficult. This advocates the need of structure-toproperty type correlations to be under the aid of PCA (see below). An interesting observation can be made when the nonheat-treated 1.7 wt % Co current-potential curve is compared to the CoPPy/C heat-treated materials as shown in Figure 9b and c. The non-heat-treated material performance is comparable to that of the heat-treated materials. This is in sharp contrast to what is typically reported for materials derived from pyrolyzed
5056
J. Phys. Chem. C, Vol. 114, No. 11, 2010
Olson et al.
Figure 7. RRDE data for the Co-PPy/C materials subjected to lowtemperature pyrolysis (300 °C). The ORR disk electrode current (b) and the ring electrode oxidation currents (a) are shown for the CoPPy/C materials with cobalt contents from 1.7 to 17 wt %.
transition metal macrocycles and other Me-Nx type catalyst systems tested in acidic conditions. Although, one report describing a novel non-Pt cathode catalyst fuel cell system utilized an in situ polymerization technique without any heat treatment showed very promising performance.37 It has been shown that optimization of the heat treatment synthesis step is essential to obtain ORR catalytic activity. This observation supports the idea that the initial steps in ORR processes, such as O2 adsorption and intermediate reaction production formation, are relatively unchanged after the materials undergo heat treatments. Table 6 presents data extracted from the RRDE data shown in Figures 6-8 and further analysis of this data. Included in the table are Id and Ir values taken at the diffusion limiting disk potential of -0.2 V vs Hg/HgO, the corresponding Id/Ir ratio, and %HO2- detected at the ring electrode assuming a collection efficiency of 40% (N ) 0.4) as determined by the manufacturer. The %HO2- generated was calculated using eq 4, where N is the collection efficiency of the ring electrode for the intermediate reaction product, HO2-. Figure 10 is a bar graph showing the trend in the Id/Ir values for CoPPy/C materials. The effect of the heat treatments can be more clearly seen in this representation. Generally, the Id/Ir values increase after the low pyrolysis treatment and then again following the high-temperature pyrolysis
(I /I ) r
-
%HO2 )
d
N
× 100
(4)
Figure 8. RRDE data for the Co-PPy/C materials subjected to hightemperature pyrolysis (700 °C). The ORR disk electrode current (b) and the ring electrode oxidation currents (a) are shown for the CoPPy/C materials with cobalt contents from 1.7 to 17 wt %.
Structure-to-Property Correlations. Individual inspection of the tabulated data, even though it allows us to identify the main trends, does not provide clear correlations for all peaks in the complete XPS data set. Instead, PCA was used first to find correlation and anticorrelation between species identified in materials with various cobalt loadings and heat treatments and second to link them with the electrochemical performance. The first data set used to draw correlations consists of quantified chemical species detected at various binding energies for a number of samples, where the number of samples corresponds to the number of rows and the number of variables (chemical components) corresponds to the number of columns in the matrix. PCA allows for such a data set to be transformed from a large number of variables into a smaller number of variables (principal components). First, it rotates the data to find a new axis (principal component 1 (PC1) axis), that will go through the maximum variation in the data set and thus will explain the most variance in the data set. The following axes (principal component 2 (PC2) and so on) are found on the condition that each next one is completely uncorrelated to the previously found principal components and explains the next most variance remaining in the data. Results of PCA performed on two data sets are shown in Figures 11 and 12 as biplots. Here, PC1 axis is shown on the X-axis, and PC2 axis is shown on the Y-axis. Here, scores (reflecting contributions of samples) and loadings (reflecting contributions of species at various binding energies) are shown for PC1 and PC2. The further away certain scores or loadings are from the axis, the more their contribution to particular component, and the closer they are to the axis the more negligible effect they have to a certain
Anion-Exchange Membrane Fuel Cells
J. Phys. Chem. C, Vol. 114, No. 11, 2010 5057
Figure 10. Id/Ir ratios obtained for all the Co-PPy/C materials at 0.2 V vs Hg/HgO as seen in Figures 6-8.
Figure 9. ORR Tafel region for the non-heat-treated (a), lowtemperature pyrolysis treated (b), and high-temperature pyrolysis treated (c) Co-PPy/C materials. The effect of Co content is shown for each set of materials. The performance of the non-heat-treated Co-PPy/C material with 1.7 wt % Co is included in (b) and (c) as a dashed line for reference.
Figure 11. PCA biplot showing scores (samples, marked as black circles and annotated in black) and loadings (quantified chemical species of cobalt, nitrogen, oxygen, and carbon, determined by XPS, marked as gray triangles and annotated in gray) of PC1 and PC2. This plot demonstrates separation of the samples in groups based on their composition that is effected by the cobalt loading and heat-treatment conditions.
variability, exemplified by the component. Positive and negative values of scores and loadings simply mean that certain samples are found on the extremes of their composition. On the other hand, samples located in close proximity, either on the positive or negative side of the axis, have similar composition. The location of loadings along the same axis tells what exactly in the composition of these samples is similar and what is different. In Figure 11, PC1 separated materials in groups based on the compositional differences consequential of temperature of the
heat treatment. PC2 on the other hand reflects correlations/ anticorrelations that are based mainly on the differences in the cobalt loading. Along the axis for PC1, the first group of samples (group I) is non-heat-treated materials. This group has high contributions of species present in precursor materials (pyrolic nitrogen, nitrates, carbonates) as well as species formed between polypyrrol and metal (Co-Nx centers). The effect of the cobalt loading on the composition of the samples in this group of nonheat-treated materials can be seen from their distribution along
TABLE 6: Electrochemical Performace Parameters Obtained from RRDE Data for All CoPPy/C Materialsa non-heat-treated materials
low temperature pyrolysis: 300 °C
-
high temperature pyrolysis: 700 °C
Co wt % Id (mA) Ir (mA) Id/Ir %HO2 (N ) 0.4) Id (mA) Ir (mA) Id/Ir %HO2 (N ) 0.4) Id (mA) Ir (mA) Id/Ir %HO2- (N ) 0.4) 1.7 3.4 5.1 17 a
0.61 0.49 0.51 0.43
0.1 0.145 0.14 0.17
6.1 3.4 3.6 2.5
-
41 74 69 99
0.55 0.56 0.51 0.55
0.1 0.075 0.095 0.1
5.5 7.5 5.4 5.5
45 33 47 45
0.59 0.54 0.57 0.55
0.09 0.095 0.078 0.081
6.6 5.7 7.3 6.8
38 44 34 37
The disk current (Id), ring current (Ir), disk to ring current ratio (Id/Ir), and peroxide species yield at -0.2 V vs Hg/HgO are given.
5058
J. Phys. Chem. C, Vol. 114, No. 11, 2010
Olson et al.
Figure 13. Model of the active pyrolyzed Co-PPy/C catalyst surface. The active surface species and the ORR mechanistic processes that they support are included.
Figure 12. PCA biplot showing scores and loadings of PC1 and PC2. Scores correspond to samples, marked as black circles and annotated in black. Loadings correspond to elemental nitrogen, cobalt, and oxygen and quantified chemical species of cobalt and nitrogen, determined by XPS, and electrochemical parameters, that include current densities measured on the disk (Id) and ring (Ir) and their ratios Id/Ir, determined from RRDE. Loadings are marked as gray triangles and annotated in gray. This plot demonstrates separation of the samples in groups based on their composition and electrochemical performance that is effected by the cobalt loading and heat-treatment conditions.
the axis of PC2. Higher cobalt loadings result in an increase of various cobalt-oxygen and carbon-oxygen groups, and do not lead to higher amount of Co-Nx centers. Group II contains materials heat-treated at low temperature. At 300 °C, materials are partially decomposed and converted to a pyridinic-type of nitrogen and cobalt oxide and for higher cobalt loadings metallic phases. Group III combines materials pyrolyzed at high temperature. These materials are even richer in pyridinic nitrogen and metal/oxide and nitride cobalt phases than materials pyrolyzed at 300 °C. Interestingly, the separation between samples with different cobalt loadings, as exemplified by PC2 being higher in this group than in groups I and II. It appears that the reason for that is the difference in the concentration of carbon-oxygen containing species. These are most probably formed to a higher extent when materials in unpyrolyzed form have higher amount of various cobalt-oxygen and carbonoxygen containing species as is the case with high cobalt loading materials. As decomposition progresses and cobalt metallic phase is being formed, oxygen content in the cobalt-oxygen-carbon species as well as in cobalt oxides decreases, and the fraction of oxygen groups associated with the carbon support increases. Thus, the biplot in Figure 12 allows identification of groups of samples and follows the composition of the groups of materials as they decompose during pyrolysis. Two decomposition routes, one for materials with low cobalt loadings and one for materials with high cobalt loadings, are identified and highlighted on the biplots along with groups of materials. A biplot demonstrating correlation between materials with various cobalt loadings and heat-treatment conditions, their XPS composition and electrochemical properties exemplified by currents, measured on the disk and ring, and their ratios is shown in Figure 12. All non-heat-treated samples highly correlate with Ir currents. All non-heat-treated materials correlate with pyrrolic nitrogen, nitrogen in Co-Nx centers, and cobalt species that are most probably due to unreacted/undecomposed cobalt precursor. What is interesting is that, out of all these species, the pyrrolic type of nitrogen (nitrogen in the polypyrol that did not react with cobalt) appears to be contributing to high Ir values
the most. Also, as the amount of cobalt in non-heat-treated materials increases, the amount of cobalt coordinated with oxygen rather than nitrogen also increases and so does the amount of peroxide detected on the ring. High currents on the disk are observed for materials that, though lower in the total amounts of cobalt and nitrogen, have higher amounts of Co-Nx centers and pyridinic nitrogen. Materials pyrolyzed at 700 °C have shown highest Id/Ir ratios, and it appears that this behavior is associated with the formation of metallic cobalt and cobalt oxide phases and enrichment with pyridinic and graphitic types of nitrogen. For pyrolyzed materials with low cobalt weight, some decrease in Co-Nx centers seems to be compensated by the formation of the new cobalt phases. For materials with higher cobalt loadings, however, an excess of unreacted cobalt precursor leads to the formation of not only metallic/metal oxide phases but also cobalt that is associated with oxygen and carbon, with the latter contributing to a higher amount of peroxide, lowering the overall Id/Ir. Dual Site Functionality ORR Mechanism. Based on the structure-to-property correlations that have been established here, we can propose an ORR sequence that occurs on pyrolyzed CoPPy/C materials in alkaline media. The structure-to-property correlations strongly suggest that two chemically and structurally different surface species contribute to a series type reaction process. Figure 13 is an illustration of the CoPPy/C catalyst surface and the ORR processes that are supported. The CoPPy/C materials have two distinct classes of active surface species, and both are represented in the figure. The Co-Nx type site (shown as a Co-N4 complex) supports the initial adsorption of the O2 molecule and conversion of O2 to the intermediate reaction product, HO2-, by a 2 e- reduction reaction as shown in eq 1. The HO2- species that are formed in the first reaction step can further react at a decorating CoxOy/Co nanoparticle phase. Based on the RRDE data alone, it is difficult to conclusively distinguish between which series type reaction is supported on the CoxOy/Co nanoparticle particle phase, and Figure 13 shows both possibilities. Nevertheless, the appearance of the decorating CoxOy/Co nanoparticle particle phase on the catalyst surface and the decrease of the flux of HO2- species are strongly linked. This allows us to propose a hypothesis where the Co-oxide surface phase plays a role in HO2- electroreduction and/ or disprotonation catalyst. Similar observations were made previously for the cocatalyst derived from transition metal macrocycles such as porphyrins and phalthocyanides.38,39 Such a mechanism is similar in principle to the one described here by the coauthors in a study involving ORR on pyrolyzed porphyrin catalyst in acidic media.19 Conclusions We have studied the effect of Co loading and heat treatment on CoPPy/C based catalyst materials and the ORR mechanism
Anion-Exchange Membrane Fuel Cells that occurs in alkaline media. We were able to establish structure-to-property correlations based on XPS surface analysis and RRDE performance using a multivariable analysis technique. The structure-to-property correlations suggest the presence of a dual site ORR mechanism. The series type reaction proceeds initially by a 2 e- reduction of O2 on a Co-Nx type site where HO2- is formed. The HO2- species can further react via either electrochemical reduction to form OH- or chemical disportonation to form OH- and O2. It is unclear which of these possible reactions is occurring, but the decorating CoxOy/Co nanoparticle particle phase appears to be the site of HO2destruction. Utilization of the dual site functionality that is present on this class of catalyst materials will be essential to their development and potential use in AMFC systems. Increased utilization of the second reaction process in the overall series type ORR mechanism would potentially result in increased energy conversion efficiency for the fuel cell system as well as a diminished performance degradation effect from unwanted reactions with HO2- species and electrode materials. Acknowledgment. The UNM part of this work was supported by a contract with Daihatsu Motor Co. Ltd. References and Notes (1) Chempath, S.; Einsla, B. R.; Pratt, L. R.; Macomber, C. S.; Boncella, J. M.; Rau, J. A.; Pivovar, B. S. J. Phys. Chem. C 2008, 112, 3179. (2) Macomber, C. S.; Boncella, J. M.; Pivovar, B. S.; Rau, J. A. J. Therm. Anal. Calorim. 2008, 93, 225. (3) Varcoe, J. R.; Slade, R. C. T. Fuel Cells 2005, 5, 187. (4) Varcoe, J. R.; Slade, R. C. T. Electrochem. Commun. 2006, 8, 839. (5) Varcoe, J. R.; Slade, R. C. T.; Yee, E. L. H.; Poynton, S. D.; Driscoll, D. J.; Apperley, D. C. Chem. Mater. 2007, 19, 2686. (6) Danks, T. N.; Slade, R. C. T.; Varcoe, J. R. J. Mater. Chem. 2002, 12, 3371. (7) Danks, T. N.; Slade, R. C. T.; Varcoe, J. R. J. Mater. Chem. 2003, 13, 712. (8) Asazawa, K.; Sakamoto, T.; Yamaguchi, S.; Yamada, K.; Fujikawa, H.; Tanaka, H.; Oguro, K. J. Electrochem. Soc. 2009, 156, B509. (9) Asazawa, K.; Yamada, K.; Tanaka, H.; Oka, A.; Taniguchi, M.; Kobayashi, T. Angew. Chem., Int. Ed. 2007, 46, 8024. (10) Jasinski, R. J. Electrochem. Soc. 1965, 112, 526. (11) Bagotzky, V. S.; Tarasevich, M. R.; Radyushkina, K. A.; Levina, O. A.; Andrusyova, S. I. J. Power Sources 1978, 2, 233.
J. Phys. Chem. C, Vol. 114, No. 11, 2010 5059 (12) Vanveen, J. A. R.; Colijn, H. A.; Vanbaar, J. F. Electrochim. Acta 1988, 33, 801. (13) Vanveen, J. A. R.; Vanbaar, J. F.; Kroese, C. J.; Coolegem, J. G. F.; Dewit, N.; Colijn, H. A. Ber. Bunsen-Ges. 1981, 85, 693. (14) Vanveen, J. A. R.; Vanbaar, J. F.; Kroese, K. J. J. Chem. Soc., Faraday Trans. I 1981, 77, 2827. (15) Scherson, D.; Gupta, S. L.; Yeager, E.; Kordesch, M.; Eldridge, J.; Hoffman, R. W. J. Electrochem. Soc. 1982, 129, C316. (16) Yeager, E.; Scherson, D. J. Electrochem. Soc. 1981, 128, C127. (17) Franke, R.; Ohms, D.; Wiesener, K. J. Electroanal. Chem. 1989, 260, 63. (18) Wiesener, K. Electrochim. Acta 1986, 31, 1073. (19) Olson, T. S.; Pylypenko, S.; Fulgrum, J.; Atanassov, P. J. Electrochem. Soc. 2010, 157, B54. (20) Artyushkova, K.; Pylypenko, S.; Olson, T. S.; Fulghum, J. E.; Atanassov, P. Langmuir 2008, 24, 9082. (21) Maldonado, S.; Stevenson, K. J. J. Phys. Chem. B 2004, 108, 11375. (22) Bonakdarpour, A.; Lefevre, M.; Yang, R. Z.; Jaouen, F.; Dahn, T.; Dodelet, J. P.; Dahn, J. R. Electrochem. Solid-State Lett. 2008, 11, B105. (23) Tarasevich, M. R.; Zhutaeva., K. A. R. G. V. Russ. J. Electrochem. 2004, 40, 1369. (24) Gupta, S.; Tryk, D.; Bae, I.; Aldred, W.; Yeager, E. J. Appl. Electrochem. 1989, 19, 19. (25) Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. P. Science 2009, 324, 71. (26) Jaouen, F.; Dodelet, J. P. J. Phys. Chem. C 2009, 113, 15422. (27) Jaouen, F.; Lefevre, M.; Dodelet, J. P.; Cai, M. J. Phys. Chem. B 2006, 110, 5553. (28) Guillet, N.; Roue, L.; Marcotte, S.; Villers, D.; Dodelet, J. P.; Chhim, N.; Trevin, S. J. Appl. Electrochem. 2006, 36, 863. (29) Lefevre, M.; Dodelet, J. P.; Bertrand, P. J. Phys. Chem. B 2005, 109, 16718. (30) Villers, D.; Jacques-Bedard, X.; Dodelet, J. P. J. Electrochem. Soc. 2004, 151, A1507. (31) Marcotte, S.; Villers, D.; Guillet, N.; Roue, L.; Dodelet, J. P. Electrochim. Acta 2004, 50, 179. (32) Lefevre, M.; Dodelet, J. P. Electrochim. Acta 2003, 48, 2749. (33) Jaouen, F.; Marcotte, S.; Dodelet, J. P.; Lindbergh, G. J. Phys. Chem. B 2003, 107, 1376. (34) Lee, K.; Zhang, L.; Lui, H.; Hui, R.; Shi, Z.; Zhang, J. J. Electrochim. Acta 2009, 54, 4704. (35) Artyushkova, K.; Levendosky, S.; Atanassov, P.; Fulghum, J. Top. Catal. 2007, 46, 263. (36) Pylypenko, S.; Mukherjee, S.; Olson, T. S.; Atanassov, P. Electrochim. Acta 2008, 53, 7875. (37) Bashyam, R.; Zelenay, P. Nature 2006, 443, 63. (38) Bouwkamp-Wijnoltz, A. L.; Visscher, W.; van Veen, J. A. R. Electrochim. Acta 1998, 43, 3141. (39) Wiesener, K.; Ohms, D.; Neumann, V.; Franke, R. Mater. Chem. Phys. 1989, 22, 457.
JP910572G
