The kinetics of the hydrogen oxidation reaction ͑HOR͒ and hydrogen evolution reaction ͑HER͒ on polycrystalline platinum ͓Pt͑pc͔͒ and high surface area carbon-supported platinum nanoparticles ͑Pt/C͒ were studied in 0.1 M KOH using rotating disk electrode ͑RDE͒ measurements. After corrections of noncompensated solution resistance from ac impedance spectroscopy and of hydrogen mass transport in the HOR branch, the kinetic current densities were fitted to the Butler-Volmer equation using a transfer coefficient of ␣ = 0.5, from which HOR/HER exchange current densities on Pt͑pc͒ and Pt/C were obtained, and the HOR/HER mechanisms in alkaline solution were discussed. Unlike the HOR/HER rates on Pt electrodes in alkaline solution, the HOR/HER rates on a Pt electrode in 0.1 M HClO 4 were limited entirely by hydrogen diffusion, which renders the quantification of the HOR/HER kinetics impossible by conventional RDE measurements. The simulation of the hydrogen anode performance based on the specific exchange current densities of the HOR/HER at 80°C illustrates that in addition to the oxygen reduction reaction cell voltage loss on the cathode, the slow HOR kinetics are projected to cause significant anode potential losses in alkaline fuel cells for low platinum loadings ͑Ͼ130 mV at 0.05 mg Pt /cm anode 2 and 1.5 A/cm anode 2 ͒, contrary to what is reported for proton exchange membrane fuel cells.

Electrochemical water splitting is a clean technology that can store the intermittent renewable wind and solar energy in H2 fuels. However, large-scale H2 production is greatly hindered by the sluggish oxygen evolution reaction (OER) kinetics at the anode of a water electrolyzer. Although many OER electrocatalysts have been developed to negotiate this difficult reaction, substantial progresses in the design of cheap, robust, and efficient catalysts are still required and have been considered a huge challenge. Herein, we report the simple synthesis and use of α-Ni(OH)2 nanocrystals as a remarkably active and stable OER catalyst in alkaline media. We found the highly nanostructured α-Ni(OH)2 catalyst afforded a current density of 10 mA cm(-2) at a small overpotential of a mere 0.331 V and a small Tafel slope of ~42 mV/decade, comparing favorably with the state-of-the-art RuO2 catalyst. This α-Ni(OH)2 catalyst also presents outstanding durability under harsh OER cycling conditions, and its stability is much better than that of RuO2. Additionally, by comparing the performance of α-Ni(OH)2 with two kinds of β-Ni(OH)2, all synthesized in the same system, we experimentally demonstrate that α-Ni(OH)2 effects more efficient OER catalysis. These results suggest the possibility for the development of effective and robust OER electrocatalysts by using cheap and easily prepared α-Ni(OH)2 to replace the expensive commercial catalysts such as RuO2 or IrO2.

This paper discusses the mechanisms of surface area loss of supported platinum (Pt) electrocatalysts in lowtemperature fuel cells. It is argued that submonolayer dissolution of Pt nanoparticles governs the surface area loss at high voltages by increasing the loss of Pt from carbon and coarsening of Pt nanoparticles on carbon.

The hydrogen oxidation/evolution reactions are two of the most fundamental reactions in distributed renewable electrochemical energy conversion and storage systems. The identification of the reaction descriptor is therefore of critical importance for the rational catalyst design and development. Here we report the correlation between hydrogen oxidation/evolution activity and experimentally measured hydrogen binding energy for polycrystalline platinum examined in several buffer solutions in a wide range of electrolyte pH from 0 to 13. The hydrogen oxidation/evolution activity obtained using the rotating disk electrode method is found to decrease with the pH, while the hydrogen binding energy, obtained from cyclic voltammograms, linearly increases with the pH. Correlating the hydrogen oxidation/evolution activity to the hydrogen binding energy renders a monotonic decreasing hydrogen oxidation/evolution activity with the hydrogen binding energy, strongly supporting the hypothesis that hydrogen binding energy is the sole reaction descriptor for the hydrogen oxidation/evolution activity on monometallic platinum.

The slow reaction kinetics of the hydrogen evolution and oxidation reactions (HER/HOR) on platinum in alkaline electrolytes hinders the development of alkaline electrolysers, solar hydrogen cells and alkaline fuel cells. A fundamental understanding of the exchange current density of the HER/HOR in alkaline media is critical for the search and design of highly active electrocatalysts. By studying the HER on a series of monometallic surfaces, we demonstrate that the HER exchange current density in alkaline solutions can be correlated with the calculated hydrogen binding energy (HBE) on the metal surfaces via a volcano type of relationship. The HER activity varies by several orders of magnitude from Pt at the peak of the plot to W and Au located on the bottom of each side of the plot, similar to the observation in acids. Such a correlation suggests that the HBE can be used as a descriptor for identifying electrocatalysts for HER/HOR in alkaline media, and that the HER exchange current density can be tuned by modifying the surface chemical properties.

Understanding how pH affects the activity of hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is observed for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13), suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange current density and HBE is also observed on the four metals, indicating that they may share the same elementary steps and rate-determining steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence against the promoting effect of adsorbed OH on HOR/HER.

In situtransformation of Pd into β-PdH is the origin of the high selectivity for CO in the electrochemical CO2reduction reaction using Pd as the electrocatalyst.

Rotating disk electrode measurements of acid-treated "Pt 3 Co" nanoparticles showed specific oxygen reduction reaction (ORR) activity (∼0.7 mA/cm Pt 2 at 0.9 V vs RHE in 0.1 M HClO 4 at room temperature), twice that of Pt nanoparticles. Upon annealing at 1000 K in vacuum, the ORR activity at 0.9 V was increased to ∼1.4 mA/cm Pt 2 (four times that of Pt nanoparticles). High-resolution transmission electron microscopy and aberrationcorrected high-angle annular dark-field in the scanning transmission electron microscope was used to reveal surface atomic structure and chemical composition variations of "Pt 3 Co" nanoparticles on the atomic scale. Such information was then correlated to averaged Pt-Pt distance obtained from synchrotron X-ray powder diffraction data, surface coverage of oxygenated species from cyclic voltammograms, and synchrotron X-ray absorption spectroscopy. It is proposed that ORR activity enhancement of acid-leached "Pt 3 Co" relative to Pt nanoparticles is attributed to the formation of a percolated structure with Pt-rich and Pt-poor regions within individual particles, while the increase in the specific ORR activity of annealed "Pt 3 Co" nanoparticles relative to Pt can be attributed to the presence of surface Pt segregation.