Oxygen Reduction Reaction Rate Research Articles

Enhancing surface activity of La0.6Sr0.4CoO3-δ cathode by a simple infiltration process

Nanoscaled La0.6Sr0.4CoO3−δ (LSC) prepared by infiltration has been investigated as an effective catalyst to promote oxygen reduction reaction (ORR) performance for solid-oxide fuel cell. The area specific resistance of original cathode is appreciably reduced by the infiltration of LSC nanoparticles into the porous backbones of LSC cathode. The Arrhenius plots reveal that the reduction is originated from the pre-exponential factor, not the activation energy, suggesting that the rate of ORR has been enhanced by the infiltrated LSC nanoparticles. The infiltrated cell shows a high power density of 800 mW cm−2 at 700 °C, which is more than two times higher than those measured in the same conditions for a similar cell with pristine LSC cathode. Our results demonstrate that simple infiltration process without further heating at high temperature is a potential way to enhance electrochemical performance and to reduce the operating temperature.

The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells.

Substantial progress has been made in reducing proton-exchange membrane fuel cell (PEMFC) cathode platinum loadings from 0.4-0.8 mgPt/cm(2) to about 0.1 mgPt/cm(2). However, at this level of cathode Pt loading, large performance loss is observed at high-current density (>1 A/cm(2)), preventing a reduction in the overall stack cost. This next developmental step is being limited by the presence of a resistance term exhibited at these lower Pt loadings and apparently due to a phenomenon at or near the catalyst surface. This issue can be addressed through the design of catalysts with high and stable Pt dispersion as well as through development and implementation of ionomers designed to interact with Pt in a way that does not constrain oxygen reduction reaction rates. Extrapolating from progress made in past decades, we are optimistic that the concerted efforts of materials and electrode designers can resolve this issue, thus enabling a large step toward fuel cell vehicles that are affordable for the mass market.

In Situ FTIR Spectroscopy: Probing the Electrochemical Interface during the Oxygen Reduction Reaction on a Commercial Platinum High‐Surface‐Area Catalyst

AbstractIn situ observation of anion adsorption on industrial high‐surface‐area catalysts is used for the first time under oxygen reduction reaction (ORR) conditions with a defined mass transport. For this purpose, a specially fabricated electrode is used for which the catalyst layer is spray‐coated on top of a structured Au contact layer and applied to our recently developed in situ attenuated total reflectance FTIR wall‐jet electrode. The designed interface allows us to track anion adsorption and measure the reaction rate simultaneously under mass controlled conditions. The observed absorption bands are caused by anion interaction with the active phase and also the carbon support. If we analyze the absorption band intensity of adsorbed anions as a function of the oxygen reduction reaction rate, the band intensity decreases with the onset of the ORR. This shows that ORR inhibition is a complex interplay between site blocking caused by anion adsorption and oxide formation.

Hybrid electrocatalysts for oxygen reduction reaction: Integrating enzymatic and non-platinum group metal catalysis

For the first time, oxygen reduction reaction has been demonstrated on a system which integrates enzymatic and non-platinum based catalysts simultaneously. This achievement is of a great importance as it offers the possibility of exploring concomitantly two very different types of catalysts, combining the advantages of both in enhancing oxygen reduction reaction rate. The engineered catalytic hybrid material not only possesses lower overpotentials compared to the purely non-PGM catalyst, but also is capable of achieving higher current densities in comparison to purely enzymatic catalyst. The hybrid catalyst undergoes oxygen reduction with the desired 4 electron transfer process, leading to the formation of water as a final product. The achieved current density of 1.2mAcm−2 is believed to be the highest reported for bilirubin oxidase -based gas-diffusion cathode reported so far.

Probing the Buried Metal-Organic Coating Interfacial Reaction Kinetic Mechanisms by a Hydrogen Permeation Based Potentiometric Approach

Corrosion driven delamination of coatings is crucially determined by the rate of the cathodic oxygen reduction reaction at the buried metal-organic coating interface. Quantitative measurement of this rate at such interfaces by conventional techniques is impeded due to the blocking of ion transport by the coating. A new approach where hydrogen permeation is used as a tool to measure the oxygen reduction rate underneath coatings has been recently introduced. This permeation based potentiometry approach measures the rate of the oxygen reduction reaction by correlating the open circuit potential established as a consequence of the dynamic electrochemical equilibrium between hydrogen oxidation and oxygen reduction reactions on the coated exit side with the hydrogen uptake rate on the entry side. In this work, the interfacial reaction kinetics of the oxygen reduction reaction causing delamination of the coating are investigated by this hydrogen permeation based potentiometric approach. Moreover, thus performed prolonged cathodic polarizations of the palladium/coating interface have been found to destroy the interface, just like it is the case in the cathodic delamination process. Initial results obtained on ultra-thin films of iron on palladium are also presented, showing that the technique is applicable also on technically more relevant metal surfaces.

λ-MnO2 Positive Electrode for Fuel Cell/Battery Systems

The possibility of replacing λ-MnO2 with previously used γ-MnO2 in the positive electrodes of fuel cell/battery (FCB) systems was analyzed. Samples were evaluated in three different modes: In fuel cell mode, the oxygen reduction reaction rate was measured at an elevated pressure of 1.0 MPa. In battery mode, the electrodes were electrochemically cycled at different rates to assess their proton diffusion. Finally, in FCB mode, the discharged electrodes were chemically charged with oxygen for one hour to quantify the chemical charge rate. For all modes, X-ray diffraction analysis was conducted to assess the crystal stability of both species. λ-MnO2 was found to exhibit a considerably better catalytic capability for oxygen reduction reaction and could be chemically charged more quickly with oxygen than γ-MnO2. It is proposed that these results were caused by a lower charge transfer resistance in λ-MnO2. However, γ-MnO2 showed better proton diffusivity than λ-MnO2, mainly owing to its higher surface area. In terms of crystal stability, λ-MnO2 was found to be superior to γ-MnO2, thus making it a promising positive electrode material for FCB systems.

Effects of Ethylene Glycol and Caprolactam on the ORR and HOR Performances of Pt/C Catalysts

Ethylene glycol (EG) and caprolactam are two representative contaminant species either present in the fuel cell system or released by fuel cell system materials. The contamination effects of EG and caprolactam on the electro-catalytic performances of a Pt/C catalyst toward the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) were investigated in acid media using the rotating ring/disk electrode (RRDE) technique. The Pt surface coverage increased with EG and caprolactam concentrations. The H2O2 yield also significantly increased because of the effects of the two contaminants. The EG- and caprolactam-derived adsorbates reduce the ORR and HOR rates and modify the rate-determining step (change in Tafel slope).

Three-dimensional X-ray microcomputed tomography of carbonates and biofilm on operated cathode in single chamber microbial fuel cell.

Power output limitation is one of the main concerns that need to be addressed for full-scale applications of the microbial fuel cell technology. Fouling and biofilm growth on the cathode of single chamber microbial fuel cells (SCMFC) affects their performance in long-term operation with wastewater. In this study, the authors report the power output and cathode polarization curves of a membraneless SCMFC, fed with raw primary wastewater and sodium acetate for over 6 months. At the end of the experiment, the whole cathode surface is analyzed through X-ray microcomputed tomography (microCT), scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDX) to characterize the fouling layer and the biofilm. EDX shows the distribution of Ca, Na, K, P, S, and other elements on the two faces of the cathode. Na-carbonates and Ca-carbonates are predominant on the air (outer) side and the water (inner) side, respectively. The three-dimensional reconstruction by X-ray microCT shows biofilm spots unevenly distributed above the Ca-carbonate layer on the inner (water) side of the cathode. These results indicate that carbonates layer, rather than biofilm, might lower the oxygen reduction reaction rate at the cathode during long-term SCMFC operation.

Electrochemical Characterisation of Cobalt-Oxide Catalysts with Different Cobalt Loading for Oxygen Reduction Reaction in Alkaline Media

Alkaline fuel cells are very promising because of their high efficiencies (1). In fuel cells the reduction of oxygen on cathode side is the performance-limiting step (2, 3). Catalysts, e.g. platinum, are used to enhance the oxygen reduction reaction (ORR) rate and also the performance. Due to the high costs of the fuel cell components, especially for the platinum catalyst (4), the fuel cell is not common for various applications up to now. The alkaline media enables the use of non-precious metals, e.g. cobalt or manganese oxides (5-8), to enhance the kinetic on the cathode side. The electroactivities of different non-precious metal catalysts like cobalt oxide are promising to achieve a similarly high activity as platinum. A cobalt acetate precursor is plasma-treated and resulting cobalt oxide catalysts with different cobalt loading were investigated in different alkaline media and at temperatures between 25 and 75 °C using a rotating ring disk electrode (RRDE) analysis. The advantage of plasma treatment is the better distribution of particles, they are less agglomerated compared due to the common heat treatment (9-11). This leads to a higher activity of the catalyst material (9). Different characterisation methods are used to identify the catalyst material like XRD, TEM and SEM. A custom-built Teflon cell is used for the RRDE analysis to avoid glass corrosion in alkaline media. Glass corrosion affects the electrochemical measurements (12), therefore it is necessary to replace glass components for the experiments.

Mitigation of Cathode Catalyst Degradation during Air/Air Startup Cycling Via the Atmospheric Resistive Switching Mechanism of a Hydrogen Anode with a Pt Catalyst Supported on Ta-Doped TiO2

One of the most important issues for polymer electrolyte fuel cells (PEFCs) is the severe degradation of cathode catalysts during startup and shutdown. The degradation is brought about by the reverse current mechanism [1]. In the present study, we propose a new strategy for reducing the reverse current using unique characteristics of an “atmospheric resistive switching mechanism” (ARSM) of the metal oxide semiconductor support, such that the electrical resistivity changes depending on gas atmosphere (Fig. 1) [2]. 10 atom% Ta-doped TiO2 nanoparticles with a fused-aggregate structure were synthesized by a flame oxide-forming method [3]. The powder obtained was heat-treated at 850 °C in a 4% H2 atmosphere for 2 h to form Ti0.9Ta0.1O2–δ having a surface area of 14.7 m2 g−1. Pt nanoparticles were deposited on the Ti0.9Ta0.1O2–δ by the colloidal method. The powder obtained was heat-treated at 900 °C in a 4% H2 atmosphere for 2 h to produce 19.8 wt% Pt on the Ti0.9Ta0.1O2–δ support catalysts (Pt/Ti0.9Ta0.1O2–δ). We fabricated two membrane-electrode assemblies (MEAs), one using the Pt/Ti0.9Ta0.1O2–δ and the other a graphitized carbon black-supported platinum (Pt/GCB) (TEC10EA50E, Tanaka Kikinzoku Kogyo K. K. (TKK)) as the anode catalysts. Both of the cathode catalysts were Pt/GCB. The electrochemically active surface area (ECSA) values of Pt/Ti0.9Ta0.1O2–δ and Pt/GCB were 30.7 m2 gPt −1 and 49.3 m2 gPt −1, respectively. The resistance of the Pt/Ti0.9Ta0.1O2–δ anode cell was showed approximately one order of magnitude greater resistance in air than in hydrogen, which could be explained by the ARSM of Pt/Ti0.9Ta0.1O2–δ. The overpotentials of the hydrogen oxidation reaction (HOR) of both anodes were compared by the H2 pump method [4]. The HOR overpotential for the Pt/Ti0.9Ta0.1O2–δ anode was negligible, at the same level as that for the Pt/GCB anode, up to at least 1.5 A cm−2. The MEAs were subjected to repeated anode gas switching from air to H2, thus simulating air/air (for anode/cathode) startup cycling. Figure 2 shows the H2/air polarization curves measured at 65 °C with 100% relative humidity. Initially, the Pt/Ti0.9Ta0.1O2–δ anode cell showed slightly lower performance than the Pt/GCB anode cell due to the greater loss of oxygen mass transport in the cathode, which was probably caused by more accumulation of generated water than the latter cell, because back-diffusion of the generated water from the cathode toward the anode must be small due to the limited capability for water expulsion via the hydrophilic anode.. On the other hand, the performance of the Pt/Ti0.9Ta0.1O2–δ anode cell after 1000 cycles exceeded that of the Pt/GCB anode cell. The percentages of retention of the ECSA of cathode at 1000 cycles were 64.7% for the Pt/Ti0.9Ta0.1O2–δ anode cell and 42.4% for the Pt/GCB anode cell. The retention percentages of mass activity at 0.9 V were 51.4% for the Pt/Ti0.9Ta0.1O2–δ anode cell and 39.1% for the Pt/GCB anode cell. Therefore, the low performance degradation of the Pt/Ti0.9Ta0.1O2–δ anode cell at low current density could be attributed to the low ECSA degradation. At high current density (> 0.4 A cm−2), the Pt/Ti0.9Ta0.1O2–δ anode cell showed distinctively higher performance than the Pt/GCB anode cell after 1000 cycles. The thickness of the cathode catalyst layer in the tested MEA with Pt/Ti0.9Ta0.1O2–δ anode was nearly the same as that of the pristine MEA. On the other hand, thinning of the cathode catalyst layer of approximately 40% was observed in the tested MEA with the Pt/GCB anode (Fig. 2 insets). Such a difference indicates that the carbon corrosion of the cathode catalyst during air/air startup cycling was significantly suppressed by the use of the Pt/Ti0.9Ta0.1O2–δ anode. The decrease in the degradations is attributed to a reduction of the reverse current due to a low oxygen reduction reaction rate at the Pt/Ti0.9Ta0.1O2–δ anode, which showed a high resistivity in air atmosphere. These results demonstrate the effectiveness of the ARSM in mitigating the catalyst degradation and the oxygen mass-transport overpotential in the cathode during air/air startup cycling [5]. 1. C.A. Reiser, L. Bregoli, T.W. Patterson, J.S. Yi, J.D. Yang, M.L. Perry, T.D. Jarvi, Electrochem. Solid-State Lett. 8 (2005) A273. 2. Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, M. Watanabe, RSC Adv. 4 (2014) 32180. 3. K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, M. Watanabe, Electrochim. Acta 56 (2011) 2881. 4. K.C. Neyerlin, W. Gu, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 154 (2007) B631. 5. H. Shintani, Y. Kojima, K. Kakinuma, M. Watanabe, M. Uchida, submitted in J. Power Sources. Figure 1

Role of oxygen vacancy in dissociation of oxygen molecule on SOFC cathode: Ab initio molecular dynamics simulation

The oxygen reduction reaction (ORR) at the cathode of the solid oxide fuel cell (SOFC) is investigated by the ab initio MD simulation, focusing on the role of the oxygen vacancy on the oxygen dissociation and reduction reaction. The oxygen molecule easily adsorbs on the nickel atom at the NiO surface as the monodentate superoxide ion (O2−(ad)), which turns quickly to the bidentate peroxide ion (O22−(ad)) regardless of the existence of vacancies at surface. The bidentate O22−(ad) in the vicinity of the oxygen vacancy dissociates easily, while that on the surface without vacancy is kept undissociated. Nickel atoms next to the oxygen vacancy have more electrons compared with those in the bulk NiO, and therefore, are able to donate more electrons to the adsorbed oxygen molecules, which results in the dissociation of the oxygen molecule. Detailed Mulliken charge analyses in term of bond overlap population on the dissociation process have revealed that dissociation undergoes with two series steps of O22−(ad) via O− state either in the form of V••O or O−(ad) into O2− state in the form of V×O or O2−(ad). Moreover, the NEB analyses reveal that the energy decreases monotonically along the reaction path of oxygen dissociation on the NiO surface with vacancies, while there exists the activation energy of about 0.6eV in the case without vacancies. Since the dissociation of O22−(ad) takes place quickly upon encounter with oxygen vacancy at the surface, it is expected that the probability of meeting O22− and V••O per time and area determines the reaction rate of ORR.

Novel strategy to mitigate cathode catalyst degradation during air/air startup cycling via the atmospheric resistive switching mechanism of a hydrogen anode with a platinum catalyst supported on tantalum-doped titanium dioxide

We propose a new strategy for alleviating the reverse current phenomenon using a unique “atmospheric resistive switching mechanism” (ARSM) of a metal oxide semiconductor support, such that the electrical resistivity changes depending on the gas atmosphere. The membrane-electrode assembly (MEA) using Ta-doped TiO2-supported platinum (Pt/Ta–TiO2) as the anode catalyst showed approximately one order of magnitude greater resistance in air than in hydrogen. The overpotential of the hydrogen oxidation reaction was negligible up to at least 1.5 A cm−2. The losses of electrochemically active surface area and carbon corrosion of the cathode catalyst during air/air startup cycling were significantly suppressed by the use of the Pt/Ta–TiO2 anode. The decrease in the degradation is attributed to a reduction of the reverse current due to a low oxygen reduction reaction rate at the anode, which showed high resistivity in air. These results demonstrate the effectiveness of the ARSM in mitigating cathode catalyst degradation during air/air startup cycling.

Surface Modification of LaNi0.6Fe0.4O3-δ Film Electrode by Ce0.9Gd0.1O1.95 Porous Layer

1. Introduction Our previous study on the composite electrode between mixed ionic and electronic conductor (MIEC) of LaNi0.6Fe0.4O3-δ (LNF) and ion-conducting oxide of Ce0.9Gd0.1O1.95(GDC) showed high electrochemical performance [1]. The lack of understanding on reaction mechanism makes the enhancement of composite electrode always connected to the enhancement of active region length of the electrode. However, for electrode which has quite high ionic conductivity such as LNF, another factor may enhance as well [2]. The surface oxygen exchange rate may enhance due to the hetero junction effect with GDC. To prove our hypothesis, LNF film electrode was prepared and GDC porous layer added on top of the surface of the film. In this study, the effect of the GDC was analyzed and discussed. 2. Experimental A LNF film electrode was prepared by pulse laser deposition method by Nd-YAG laser on the GDC electrolyte. Porous LNF as a counter electrode was deposited on the opposite side to the LNF film on the GDC electrolyte. The GDC porous layer was deposited into top of the film surface by screen printing method. The diameter of GDC porous layer was smaller than LNF film electrode. The samples were calcined at 1273 K for 6 hours. Electrochemical measurement was taken to compare two samples between LNF film and LNF film-GDC porous layer as a function of Temperature (973 – 1073 K) and oxygen partial pressure (10-3-1 bar). For the impedance set up, the alumina ring which was covered with Pt mesh was used as a current collector. The diameter of the alumina ring was larger compared to the GDC layer that the alumina ring didn’t touch GDC. In the analyses of the impedance responses, tentative equivalent circuits with two or three R-CPE elements (a resistance, R, and a constant phase element, CPE, connected in parallel) were used to identify their physical meanings. Secondary ion mass spectrometry measurement (SIMS) was also performed to determine the surface oxygen exchange kinetics of the samples. The samples with LNF film and LNF film – GDC porous layer were cut into pieces of 3 mm x 3 mm. The samples were pre-annealed at 973 K under 10-1 bar for 3 hours and then quenched to room temperature. The isotope exchange was performed by flowing 16O2 at 973 K under 10-1 bar for 30 minutes before the gas abruptly changed into 18O2for 10 minutes. The quenched samples after the isotope exchange were used for SIMS measurement. For LNF film – GDC porous sample, the GDC porous layer was removed from LNF film prior to SIMS measurement. The isotope depth profile technique was used to determine the surface exchange kinetics for both of the samples. 3. Results and Discussion From electrochemical impedance spectroscopy (EIS) result, the depressed semicircle of impedance spectra was observed for the LNF film and LNF film – GDC porous electrode. The low frequency arc was related to the electrode reaction on both of the samples for which the area specific conductivity was calculated. Fig. 1 shows the comparison of area specific conductivity (ASC) of LNF film and LNF film - GDC porous. The LNF film - GDC porous layer has higher ASC compared to the LNF film electrode. It shows that the activity of oxygen reduction process enhances due to the GDC existence on the top of LNF film surface. The ASC which was estimated from SIMS were close compared to the EIS result as shown in Fig. 1. The area specific conductivity which was estimated from k*by the following equation [3]: σe= (k*.2.n.F 2.Co )/(R.T) Where the F, and Co are faraday number and oxygen concentration in the bulk sample. The n (between 2 to 4) is dependent on the reaction kinetics. Two measurements by EIS and SIMS proved that GDC on the surface of LNF film enhances the surface oxygen kinetics. Within the surface modification by porous GDC, the ASC enhances significantly. It may be due to the GDC on the LNF gives the oxygen vacancy at the interface between GDC and LNF film surface. The oxygen vacancy from GDC increases the rate of oxygen reduction reaction on the electrode surface. At the current result, the solid conclusion on the mechanism remains unclear. Reference [1] R. A. Budiman et al., ECS Trams., 57(2013) 1873. [2] M. Nishi et al., ECS Trans., 45, 171 (2012). [3] T.Kawada et al., Solid State Ionic, 121 (1999) 271. Figure 1

Effect of Acid-Leaching on the Electrochemical Performance of Carbon-Supported Copper Phthalocyanine Tetrasulfonic Acid Tetrasodium Salt (CuTSPc/C) Catalys

Since the strong O=O bond (498kJ mol-1) with very slow oxygen reduction reaction (ORR) rate at the cathode has a much larger overpotential (or polarization) than that of hydrogen oxidation reaction (HOR) at the anode, the cathodic ORR has become the most challenging step in a fuel cell1. Up to now, Pt is still the best catalyst candidate for the ORR due to its high catalytic activity. As yet, there are still two major challenges for the Pt-based catalysts, one is high cost, and the other is insufficient electrochemical durability2-3. Therefore, the major effort in fuel cell research and development have been put on reducing Pt loading by exploring more effective synthesis technologies, and/or replacing Pt metal using other non-precious metals such as Fe, Co and Cu. In this regard, carbon-based transition metal macrocycle complexes catalysts have become one of the most promising candidates to replace Pt/C catalyst, especially these catalysts synthesized after thermal treatment show high catalytic activity3,4. For revealing the mechanism of ORR and improving the ORR activity, one should know the active site in these catalysts. So far, there is a consensus that carbon, nitrogen and transition metals are all considered to be indispensable elements for the ORR activity5. However, for the transition metals, there is few report clearly point out whether it is a part of the active sites in these macrocycle complexes. In this work, with carbon-supported copper phthalocyanine tetrasulfonic acid tetrasodium salt, CuTSPc/C, as a target, the role of transition metal in macrocycle complexe was investigated by acid-leaching procedures.

Electrocatalysis of Fuel Cell Relevant Reactions in Protic Ionic Liquids

Protic ionic liquids (PILs) are a sub-class of ionic liquids that have attracted attention recently as effective proton carriers for medium-temperature fuel cells because PILs have favorable properties such as high proton conductivity, low vapour pressure and excellent chemical and thermal stability.1,2 PIL-based fuel cells (PILFCs) can conduct protons without deliberate humidification, unlike the polymer electrolytes used in conventional proton exchange membrane fuel cells (PEMFCs), meaning that water management in PILFCs is simpler than in PEMFCs. Moreover, the rate of the oxygen reduction reaction (ORR) in PILFCs can exceed that of the rate of ORR in PEMFCs as PILFCs are able to operate above 100 °C. Despite recent research in the development of PIL-based fuel cells, electrocatalysis in this media has not yet been studied in any great detail. In particular, detailed study on the effect of electrode-adsorbate interactions during electrocatalysis of the ORR at Pt in PILs has been lacking. This contribution investigates the role of ion adsorption on the electrocatalysis of ORR, CO oxidation and hydrazine oxidation in a series of ammonium-based PILs. Surface oxides that form during trace water oxidation play a key role during electrocatalysis in PILs. Pt surface oxides hinder the ORR, and this reaction only occurs at potentials where the Pt surface is in its reduced state. By contrast, the rate of hydrazine oxidation in PILs increases as the extent of residual Pt oxide increases. Moreover, CO oxidation coincides with oxidation of the Pt surface in each PIL, suggesting that the reaction proceeds via a Langmuir-Hinshewood mechanism involving adsorbed CO and surface oxides. The “CO displacement” technique was used to determine whether the cations or anions of the PILs were adsorbed on the Pt surface, and this insight was used to explain the trends observed during electrocatalysis of the ORR in PILs. For example, the onset potential of the ORR in diethylmethylammonium trifluoromethanesulfonate ([dema][TfO], see Figure 1A for structure) was 0.7 V vs RHE at 45 °C, and in diethylmethylammonium bis (trifluoromethanesulfonyl)imide ([dema][Tf2N]) was -0.2 V vs RHE at 45 °C. It is important to note that the cations of these PILs are the same, and their physicochemical properties such as viscosity and conductivity are similar. Figure 1 shows the i-t curve when CO was dosed at various potentials into [dema][TfO] and [dema][Tf2N]. In [dema][TfO], cation displacement was observed negative of 0.25 V vs RHE and anion ([TfO]-) displacement was seen positive of 0.28 V vs RHE (Figure 1A). In contrast, anion displacement was observed at all potentials studied in [dema][Tf2N] (Figure 1B), indicating the strong interaction between Pt surfaces and the [Tf2N]- anion. This result indicates that the binding of the [Tf2N]- anions to the Pt surface causes the high overpotential of the ORR in [dema][Tf2N]. Extension of the “surface electrocatalysis” approach described here to well-defined surfaces will reveal whether ion adsorption from PILs exhibits structural sensitivity and will yield new insights into electrocatalysis in PILs. Ultimately, this approach may lead to the rational design of electrocatalysts for use in PIL electrolytes

Insights into the Oxygen Reduction Reaction Activity of Pt/C and PtCu/C Catalysts

A mechanistic electrochemical study of the oxygen reduction reaction (ORR) on a carbon-supported PtCu core-shell catalyst is presented. The catalyst was prepared by galvanic displacement of nanoporous copper with Pt. The electrochemistry of the catalyst was explored at pH 1 and pH 13. Hydrogen peroxide reduction and rotating ring-disk electrode (RRDE) studies showed that the PtCu/C catalyst facilitates a 4e- direct or series reduction to water in alkaline electrolyte. Tafel data suggests that the ORR rate limiting step for PtCu/C does not change when the catalyst is subject to pH extremes. The ORR activity of the PtCu/C catalyst was found to be 2 to 3 times higher than the ORR activity of commercially available Pt/C (Johnson Matthey). Adsorption of OH- was quantified for PtCu/C and Pt/C, and PtCu/C was observed to have a reduced affinity towards OHads in both acid and alkaline electrolyte, which was found to promote the rate of the ORR relative to Pt/C. Based on this study, we propose that: 1) Pt-based catalyst has a unique rate of change of OHads coverage which is correlated to their ORR activity and 2) All Pt-based catalysts have the same rate of change of OHads coverage during irreversible oxide formation. Figure 1

A Comprehensive Model for Carbon Corrosion during Fuel Cell Start-Up

A transient 1+1 D model has been developed for carbon corrosion during fuel cell start-up. The model considers the species transport in the anode channel and catalyst layer, the double-layer and pseudo capacitances, and the multiple species coverage on carbon and Pt which influences reaction kinetics. Model simulations and analysis have been performed focusing on the reaction kinetics including carbon corrosion rate, the contributions from double-layer and pseudo capacitances, and the effects of initial Pt state at anode and cathode, temperature, and anode O2 diffusivity. It has been found that the remaining O2 amount in the anode catalyst layer, rather than the H2 front, is the key factor for cathode carbon corrosion rate. There exists a charge balance in anode oxygen reduction reaction (ORR) which is the driver for cathode carbon corrosion, cathode oxygen evolution reaction (OER), cathode carbon corrosion reaction (COR), and multiple capacitive currents. Among all capacitive currents, cathode pseudo capacitance contributes most to the protons for anode ORR. Enhancing cathode OER is preferred over cathode pseudo capacitance to reduce COR since the latter approach can increase cathode PtO coverage and Pt dissolution. Higher cathode initial bare Pt surface leads to less cathode carbon corrosion, whereas higher anode initial bare Pt surface enhances anode ORR rate as well as cathode carbon corrosion in the beginning but reduces both afterwards due to faster consumption of the remaining O2. Both reduced temperature and anode O2 diffusivity are shown to decrease the cathode carbon corrosion during start-up, the former being highly effective.

Shift of voltage reversal in stacked microbial fuel cells

We proved that sluggish kinetics on the cathode and the imbalance of cathode kinetics cause voltage reversal in a stacked microbial fuel cell (MFC) equipped with a non-Pt cathode. Catholyte aeration to a unit MFC against passive air diffusion to the cathode in the other unit MFC shifted voltage reversal between the two units, due to improved mass transport and O2 concentration effects in the aerated MFC. The shifted voltage reversal returned to an original status when catholyte aeration was stopped. A Pt-coated cathode increased the rate of oxygen reduction reaction (ORR) by a factor of ∼20, as compared to the non-Pt cathode. As a result, the anodic reaction rate that became slower than the rate on the Pt-cathode limited current density to overpotential in the stacked MFC equipped with the Pt-cathode. This work shows that dominant kinetic bottlenecks, which are the primary cause of voltage reversal, can be shifted between individual MFCs of stacked MFCs or electrodes depending on relative kinetics.

The evidence of limitation of oxygen reduction reaction by proton diffusion in low-concentration acid solutions

By voltammetry with polycrystalline platinum rotating disc electrode in either nitrogen purged or oxygen saturated 1×10−3moldm−3 and 2×10−3moldm−3 HClO4 solutions with 0.1moldm−3 KClO4 supporting electrolyte, the transition was observed from the proton diffusion limited to the oxygen diffusion limited rate of oxygen reduction reaction. The experiments confirmed roughly the calculation based on the Levich equation, that the limitation in the diffusion kinetics of oxygen reduction reaction by proton diffusion terminates for acid concentration higher than 1.69×10−3moldm−3.

Carbon black/sulfur-doped graphene composite prepared by pyrolysis of graphene oxide with sodium polysulfide for oxygen reduction reaction

Sulfur doped graphene nanosheets (S-GNs) are synthesized by using graphene oxide (GO) as carbon source and sodium polysulfide as sulfur source, and then, annealing the prepared precursors (S-GO) at different temperatures up to 1000°C. The structure and morphological characteristics of the fabricated composites are investigated by Field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Electrochemical results show that S-GNs has high catalytic activity toward oxygen reduction reaction (ORR) in an alkaline electrolyte in comparison with GNs. To improve the characteristics of S-GNs and prevent agglomeration of the particles, the S-GNs-1000-CB is fabricated by mixing the S-GNs-1000 with the carbon black (CB). The results show that CB particles inserted between S-GNs layers function as spacers and prevent the restacking of S-GNs during drying, and thus, most probably promote the diffusion of O2 molecules through S-GNs and enhance the ORR rate. The S-GNs-1000-CB composites show excellent electrocatalytic activity in alkaline solutions over all the studied (catalyst) samples. Our findings demonstrate that S-GNs-1000-CB is an appropriate catalyst for ORR in alkaline solutions, and could be a good candidate for replacement of the precious Pt based catalysts for this goal.