We present a method for calculating the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations. We used that method in combination with detailed density functional calculations to develop a detailed description of the free-energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias. This allowed us to identify the origin of the overpotential found for this reaction. Adsorbed oxygen and hydroxyl are found to be very stable intermediates at potentials close to equilibrium, and the calculated rate constant for the activated proton/electron transfer to adsorbed oxygen or hydroxyl can account quantitatively for the observed kinetics. On the basis of a database of calculated oxygen and hydroxyl adsorption energies, the trends in the oxygen reduction rate for a large number of different transition and noble metals can be accounted for. Alternative reaction mechanisms involving proton/electron transfer to ...

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Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode

The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.

Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs

A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

Rod Borup is a Team Leader in the fuel cell program at Los Alamos National Lab in Los Alamos, New Mexico. He received his B.S.E. in Chemical Engineering from the University of Iowa in 1988 and his Ph.D. from the University of Washington in 1993. He has worked on fuel cell technology since 1994, working in the areas of hydrogen production and PEM fuel cell stack components. He has been awarded 12 U.S. patents, authored over 40 papers related to fuel cell technology, and presented over 50 oral papers at national meetings. His current main research area is related to water transport in PEM fuel cells and PEM fuel cell durability. Recently, he was awarded the 2005 DOE Hydrogen Program R&D Award for the most significant R&D contribution of the year for his team's work in fuel cell durability and was the Principal Investigator for the 2004 Fuel Cell Seminar (San Antonio, TX, USA) Best Poster Award. Jeremy Meyers is an Assistant Professor of materials science and engineering and mechanical engineering at the University of Texas at Austin, where his research focuses on the development of electrochemical energy systems and materials. Prior to joining the faculty at Texas, Jeremy workedmore » as manager of the advanced transportation technology group at UTC Power, where he was responsible for developing new system designs and components for automotive PEM fuel cell power plants. While at UTC Power, Jeremy led several customer development projects and a DOE-sponsored investigation into novel catalysts and membranes for PEM fuel cells. Jeremy has coauthored several papers on key mechanisms of fuel cell degradation and is a co-inventor of several patents. In 2006, Jeremy and several colleagues received the George Mead Medal, UTC's highest award for engineering achievement, and he served as the co-chair of the Gordon Research Conference on fuel cells. Jeremy received his Ph.D. in Chemical Engineering from the University of California at Berkeley and holds a Bachelor's Degree in Chemical Engineering from Stanford University. Bryan Pivovar received his B.S. in Chemical Engineering from the University of Wisconsin in 1994. He completed his Ph.D. in Chemical Engineering at the University of Minnesota in 2000 under the direction of Profs. Ed Cussler and Bill Smyrl, studying transport properties in fuel cell electrolytes. He continued working in the area of polymer electrolyte fuel cells at Los Alamos National Laboratory as a post-doc (2000-2001), as a technical staff member (2001-2005), and in his current position as a team leader (2005-present). In this time, Bryan's research has expanded to include further aspects of fuel cell operation, including electrodes, subfreezing effects, alternative polymers, hydroxide conductors, fuel cell interfaces, impurities, water transport, and high-temperature membranes. Bryan has served at various levels in national and international conferences and workshops, including organizing a DOE sponsored workshop on freezing effects in fuel cells and an ARO sponsored workshop on alkaline membrane fuel cells, and he was co-chair of the 2007 Gordon Research Conference on Fuel Cells. Minoru Inaba is a Professor at the Department of Molecular Science and Technology, Faculty of Engineering, Doshisha University, Japan. He received his B.Sc. from the Faculty of Engineering, Kyoto University, in 1984 and his M.Sc. in 1986 and his Dr. Eng. in 1995 from the Graduate School of Engineering, Kyoto University. He has worked on electrochemical energy conversion systems including fuel cells and lithium-ion batteries at Kyoto University (1992-2002) and at Doshisha University (2002-present). His primary research interest is the durability of polymer electrolyte fuel cells (PEFCs), in particular, membrane degradation, and he has been involved in NEDO R&D research projects on PEFC durability since 2001. He has authored over 140 technical papers and 30 review articles. Kenichiro Ota is a Professor of the Chemical Energy Laboratory at the Graduate School of Engineering, Yokohama National University, Japan. He received his B.S.E. in Applied Chemistry from the University of Tokyo in 1968 and his Ph.D. from the University of Tokyo in 1973. He has worked on hydrogen energy and fuel cells since 1974, working on materials science for fuel cells and water electrolysis. He has published more than 150 original papers, 70 review papers, and 50 scientific books. He is now the president of the Hydrogen Energy Systems Society of Japan, the chairman of the Fuel Cell Research Group of the Electrochemical Society of Japan, and the chairman of the National Committee for the Standardization of the Stationary Fuel Cells. ABSTRACT TRUNCATED« less

Scientific aspects of polymer electrolyte fuel cell durability and degradation.

Platinum-based supported nanocatalysts for oxidation of methanol and ethanol

The borohydride oxidation reaction (BOR) was studied on Vulcan XC72-supported Pt active layers by cyclic voltammetry in 0.1 M NaOH / 10−3 mol L-1 NaBH4. The BOR onset potential and the number of electrons (ne-) exchanged per BH4- anion (faradaic efficiency) were investigated for active layers of different thickness and/or compositions, to determine whether the residence time of reaction intermediates does influence the reaction. The results indicate that the ne- value exchanged in the BOR strongly depends on the thickness of the active layer. While thin (ca. 0.5 µm-thick) Pt/C active layers yield ne- < 4, thick layers (approximately 3 µm) yield ne- ≈ 8, which can be ascribed to the sufficient residence time of the molecules formed within the active layer (H2 by heterogeneous hydrolysis or BOR intermediates). This puts into evidence that not only the nature of the electrocatalyst is important to reach high BOR efficiency, but also the structure/thickness of the active layer.

Borohydride Oxidation on Pt-Based Electrodes: Evidence of Residence Time Effect on the Reaction Onset and Faradaic Efficiency

PEM fuel cells fed by hydrogen from ethanol dehydrogenation reaction: Unveiling the poisoning mechanisms of the by-products

. The kinetics of the oxygen reduction reaction (ORR) was studied in KOH electrolyte on ultra thin layer electrodes formed by Pt-V, Pt-Cr and Pt-Co electrocatalysts dispersed on a carbon powder (Pt-M/C). The electronic and structural features of the materials were studied by in situ XANES (Xray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure). Cyclic voltammograms and steady state polarization curves for the ORR were obtained using the rotating ring/disk electrode technique. The XANES results for the composites in the oxidized state have shown an increase of the occupancy of the Pt 5d band while the EXAFS analyses showed some reduction of the Pt-Pt interatomic distance in the different metal alloys compared to Pt/C. The electrochemical measurements indicated that the 4-electrons mechanism is mainly followed for the ORR in these platinum alloys. The highest electrocatalytic activity was shown by the Pt-V/C alloy, after treated at 850 oC under hydrogen atmospher.

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Electrochemical Performance of Dispersed Pt-M (M =V, Cr and Co) Nanoparticles for the Oxygen Reduction Electrocatalysis

The electrochemical carbon dioxide reduction reaction (CO2RR) process can allow the production of chemicals under ambient conditions on nanostructured copper materials. However, the reaction selectivity is still a main drawback due to strong competition reactions at close electrode potentials. Herein, we introduced a novel three-dimensional electrode composed of mesoporous carbon-embedded copper nanoparticles that are capable of selectively producing formic acid and short-chain hydrocarbons at low overpotentials. The mesoporous electrocatalyst was synthesized from a facile and one-pot green chemistry process using bio-sourced Tannin mimosa (polymeric flavonoids) acting as both reducing agent and carbon precursor. The role of different electrode potentials on the product selectivity (methane and ethylene) was probed by on-line differential electrochemical mass spectrometry (DEMS). During the CO2 electrolysis, chronoamperometry experiments allowed the evaluation of the electrocatalytic performance towards CO2RR with a distinguishable production of formic acid and hydrocarbons. It should be noted that methane was first detected at a potential of −0.52 V, while ethylene showed up at −0.72 V vs. RHE. Moreover, the thin-layer Cu/C ex-tannin porous electrode surface exhibited current densities ranging from 5.9 to 42.1 mA cm−2, which are higher than those previously reported on copper-based electrodes. This suggests that the Cu/C ex-tannin electrocatalyst surface facilitates charge and mass transfers towards accessible active sites through mesostructured carbon paths, boosting the performance for CO2RR. The Cu/C ex-tannin electrode described here may provide a promising lead for the development of effective electrocatalyst structures for scalable electrochemical CO2 reduction electrolyzers.

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Edson A. Ticianelli

Papers

Platinum-based supported nanocatalysts for oxidation of methanol and ethanol

Borohydride Oxidation on Pt-Based Electrodes: Evidence of Residence Time Effect on the Reaction Onset and Faradaic Efficiency

PEM fuel cells fed by hydrogen from ethanol dehydrogenation reaction: Unveiling the poisoning mechanisms of the by-products

Electrochemical Performance of Dispersed Pt-M (M =V, Cr and Co) Nanoparticles for the Oxygen Reduction Electrocatalysis

CO2 electroreduction to fuels on mesoporous carbon-embedded copper nanoparticles