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

Developing highly efficient catalysts for the oxygen reduction reaction (ORR) is key to the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Herein, we review the most recent advances in the development of Pt-based and Pt-free materials in the field of fuel cell ORR catalysis. This review covers catalyst material selection, design, synthesis, and characterization, as well as the theoretical understanding of the catalysis process and mechanisms. The integration of these catalysts into fuel cell operations and the resulting performance/durability are also discussed. Finally, we provide insights into the remaining challenges and directions for future perspectives and research.

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Increasing energy demands and environment awareness have promoted extensive research on the development of alternative energy conversion and storage technologies with high efficiency and environmental friendliness. Among them, water splitting is very appealing, and is receiving more and more attention. The critical challenge of this renewable-energy technology is to expedite the oxygen evolution reaction (OER) because of its slow kinetics and large overpotential. Therefore, developing efficient electrocatalysts with high catalytic activities is of great importance for high-performance water splitting. In the past few years, much effort has been devoted to the development of alternative OER electrocatalysts based on transition-metal elements that are low-cost, highly efficient, and have excellent stability. Here, recent progress on the design, synthesis, and application of OER electrocatalysts based on transition-metal elements, including Co, Ni, and Fe, is summarized, and some invigorating perspectives on the future developments are provided.

Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction

Ionic liquids (ILs) are liquids consisting entirely of ions and can be further defined as molten salts having melting points lower than 100 °C. One of the most important research areas for IL utilization is undoubtedly their energy application, especially for energy storage and conversion materials and devices, because there is a continuously increasing demand for clean and sustainable energy. In this article, various application of ILs are reviewed by focusing on their use as electrolyte materials for Li/Na ion batteries, Li-sulfur batteries, Li-oxygen batteries, and nonhumidified fuel cells and as carbon precursors for electrode catalysts of fuel cells and electrode materials for batteries and supercapacitors. Due to their characteristic properties such as nonvolatility, high thermal stability, and high ionic conductivity, ILs appear to meet the rigorous demands/criteria of these various applications. However, for further development, specific applications for which these characteristic properties becom...

Application of Ionic Liquids to Energy Storage and Conversion Materials and Devices

The catalytic activity of Pt catalysts towards the oxygen reduction reaction (ORR) was investigated on a catalyst system developed by thermally induced chemical deposition of Pt on carbon. The use of this deposition method made it possible to prepare a practical catalyst system with various Pt loadings on the support. Increasing the Pt loading caused a change in the Pt surface morphology which was confirmed by transmission electron microscopy (TEM) and CO stripping voltammetry measurements. The occurrence of a low and high-potential CO oxidation peak suggested the presence of Pt agglomerates and Pt nanoparticles, respectively. An increase in Pt loading lead to a subsequent decrease in the electrochemical surface area (ECSA, m2
 Pt/gPt) as the platinum surface transitioned from isolated platinum nanoparticles to platinum agglomerates. The specific activity was found to increase with increasing Pt loadings, while the mass activity decreased with loading. The mass and specific activity data from this study was found to follow a ‘master curve’ obtained by the comparison of normalised activities from various different studies in the literature. Pt selectivity was also affected by Pt loading and hence Pt surface morphology. At low Pt loadings, i.e. large interparticle distances, the amount of H2O2 produced was significantly higher than for high Pt loadings. This confirms the presence of a ‘series reaction pathway’ and highlights the importance of the H2O2 desorption-readsorption mechanism on Pt nanoparticles and the ultimate role of Pt interparticle distance on the ORR mechanism.

/pdf/the-effect-of-platinum-loading-and-surface-morphology-on-2tlexjhjur.pdf

The Effect of Platinum Loading and Surface Morphology on Oxygen Reduction Activity

Herein we report the electrochemical activity and selectivity toward the oxygen reduction reaction (ORR) for composite electrodes made of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskite oxide and Acetylene black (AB) carbon in alkaline media. The onset potential and the selectivity, i.e., hydroperoxide formation, toward the ORR exhibit a volcano type behavior as a function of the electrode composition. The HO2– formation measured by rotating ring disk electrode technique decreases from about 60% and 70% for the BSCF and the AB, respectively, to 28% for the electrode having a BSCF/AB weight ratio of 1.25 (values taken at 0.4 V vs RHE). Accordingly, the value for the overall transferred electrons is significantly larger (around 3.5) for the composite electrode compared to individual materials. Therefore, the overall results point toward a beneficial interaction between BSCF and AB. Different scenarios are considered; first an improved electrical connection of BSCF particle agglomerates upon carbon addition. Howe...

Composite Electrode Boosts the Activity of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite and Carbon toward Oxygen Reduction in Alkaline Media

Electrocatalysis of Perovskites: The Influence of Carbon on the Oxygen Evolution Activity

Trioctylphosphonium triflate, trioctylammonium triflate, triphenylphosphonium triflate and triphenylammonium triflate were synthesized and characterized. It was found that phosphonium-based protic ionic liquids (PILs) exhibit higher thermal stability and ionic conductivity than the corresponding ammonium-based PILs, no matter whether the P and N center atoms are bonded to the electron-donating octyl groups or the electron-withdrawing phenyl groups. The ion conduction behavior of the PILs can be adequately described by the Vogel–Fulcher–Tamman (VFT) equation. The higher ionic conductivity of phosphonium-based PILs may be attributed to their weaker hydrogen bond and Coulombic interactions as well as higher carrier ion concentrations, indicated by infrared analysis, lattice potential energy estimation and VFT fitting results. Interestingly, the stronger hydrogen bonds inside trioctylammonium triflate may lead to a much decreased melting point. Furthermore, compared with electron-withdrawing phenyl, electron-donating octyl enhanced the thermal stability of the PILs.

/pdf/physicochemical-properties-of-phosphonium-based-and-ammonium-4a38af4kg8.pdf

Physicochemical properties of phosphonium-based and ammonium-based protic ionic liquids†

Imidazolium methanesulfonate (1) has been studied as a model proton conductor for high temperature polymer electrolyte membrane fuel cells (PEMFCs). It is found that 1 undergoes transformation from crystalline to plastic crystalline and then molten states successively from ambient temperature to 200 °C. The solid–solid phase transition of 1 at 174 °C has been preliminarily verified by differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). At the melting point of 188 °C, 1 displays a low entropy of fusion of around 24 J mol−1 K−1. In particular, a high ionic conductivity of 1.0 × 10−2 S cm−1 is reached at 185 °C in the plastic phase. The activation energy for ionic conduction decreases as 1 is heated from the crystal phase to the melt phase. In the molten state, the contribution of protons to the ionic conductivity of 1 was corroborated electrochemically. In addition, 1 is electrochemically active for H2 oxidation and O2 reduction at a Pt electrode while it shows a high electrochemical window of 2.0 V. Furthermore, a Nafion® membrane has been successfully doped with 1, as identified by infrared spectroscopy, powder XRD, grazing incidence XRD and thermogravimetric analysis. To the best of our knowledge, this may be the first report on a protic organic ionic plastic crystal (OIPC) consisting of protonated imidazole (C3H5N2+) and an organic anion. The good thermal stability, high ionic conductivity, wide electrochemical window, favorable plastic crystal behavior and simple synthesis make 1 a highly interesting model proton conductor for high temperature PEMFCs.

/pdf/imidazolium-methanesulfonate-as-a-high-temperature-proton-h1ufy77qkn.pdf

Olaf Conrad

Papers

The Effect of Platinum Loading and Surface Morphology on Oxygen Reduction Activity

Composite Electrode Boosts the Activity of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite and Carbon toward Oxygen Reduction in Alkaline Media

Electrocatalysis of Perovskites: The Influence of Carbon on the Oxygen Evolution Activity

Physicochemical properties of phosphonium-based and ammonium-based protic ionic liquids†

Imidazolium methanesulfonate as a high temperature proton conductor