Wenchao Sheng

Efficient water oxidation using nanostructured α-nickel-hydroxide as an electrocatalyst.

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.

Synthesis of monodispere Au@Co3O4 core-shell nanocrystals and their enhanced catalytic activity for oxygen evolution reaction.

DOI: 10.1002/adma.201400336 substrate, which makes the Co 3 O 4 more easily oxidized. [ 6b ] Our electrochemical study shows that our Au@Co 3 O 4 NCs have an OER activity 7 times as high as a mixture of Au and Co 3 O 4 NCs or Co 3 O 4 NCs alone, and 55 times as high as Au NCs, most likely due to a strong synergistic effect between the core and the shell, and this effect does not exist between the physically mixed NCs. Some Au and Co 3 O 4 hybrid NCs have been reported, [ 10 ] however, none of them have well-defi ned core–shell structures and uniform sizes. High-quality Au@Co 3 O 4 core–shell NCs are still desired. In our experiment, a three-step approach ( Figure 1 a) was adopted to synthesize Au@Co 3 O 4 NCs, comprising synthesis of the Au NC, growth of the Co shell, and oxidation of Co to Co 3 O 4 . First, Au NCs were prepared by reducing HAuCl 4 with tertbutylamine borane (TBAB) in the presence of oleylamine (OAm) as the ligand, following the procedure described in a previous study by Peng et al. [ 11 ] Second, Co shells were grown on the Au NC cores to prepare Au@Co NCs by using Co(acac) 2 , where acac is acetylacetonate, as the cobalt source and TBAB as the reducing agent. OAm and oleic acid (OA) were introduced to control the shape and uniformity. Third, the Au@Co NCs were loaded on carbon and then the Co shells were oxidized to Co 3 O 4 by calcination in air. The experimental details are described in the Supporting Information. Figure 1 b shows the transmission electron microscopy (TEM) image of the as-obtained Au NCs. They have a narrow size distribution with a diameter of 3.6 ± 0.5 nm. Five-fold symmetry is found in the high-resolution TEM (HRTEM) image (Figure S1, Supporting Information), which is in agreement with the literature, indicating the multiple-twinned structure of the Au NCs. [ 11 ] Figure 1 c shows a TEM image of Au@Co core–shell NCs that were synthesized by growing Co shells on the pre-synthesized Au cores with 0.5 mmol OA. A dark core corresponding to Au can clearly be seen located at the center of the NC, and a uniform lighter Co shell caps around it. The Au@Co NCs are nearly monodisperse with an overall diameter of 8.1 ± 0.7 nm. The thickness of the Co shell is ca. 2 nm. The Au@Co NCs are highly uniform so that they can assemble into an ordered structure (Figure 1 d). The energy dispersive spectrometry (EDS) spectra (Figure S2a, Supporting Information) show the signals of Au and Co (atomic ratio 1:4), which confi rms the hybrid Au@Co composition. The Co shell seems to be amorphous because no clear lattice fringe can be seen in the HRTEM image (Figure 1 e). This may be due to the lattice mismatch between Au (face-centered cubic, fcc) and Co (hexagonal close packed, hcp). The multiple twinned nature of the Au core may also infl uence the crystallinity of the Co shell. It is noted that Co NCs cannot be synthesized under the same condition The hydrogen economy can provide an effi cient energy system that is free from environmental issues related to the combustion of coal, oil, and natural gas. [ 1 ] However, such a system requires a clean and sustainable source of hydrogen, which can be provided by splitting of water either electrochemically or photoelectrochemically. [ 2 ] One of the key problems in splitting water is the kinetically sluggish anode reaction, i.e., oxygen evolution reaction (OER, 4OH − → 2H 2 O + 4e − + O 2 in base). An overpotential of several hundred millivolts is often required to achieve a current density of 10 A g catalyst −1 . [ 3 ] Recent studies have shown that spinel-type Co 3 O 4 has relatively good OER activities. [ 2e , 3c , 4 ] Hybrid materials have been proposed to further promote the OER activity of Co 3 O 4 , such as doping Co 3 O 4 with other metals to make substituted cobaltites [ 5 ] or growing Co 3 O 4 on a special substrate. [ 6 ]

Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy

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.

Non-precious metal electrocatalysts with high activity for hydrogen oxidation reaction in alkaline electrolytes

A ternary metallic CoNiMo catalyst is electrochemically deposited on a polycrystalline gold (Au) disk electrode using pulse voltammetry, and characterized for hydrogen oxidation reaction (HOR) activity by temperature-controlled rotating disk electrode measurements in 0.1 M potassium hydroxide (KOH). The catalyst exhibits the highest HOR activity among all non-precious metal catalysts (e.g., 20 fold higher than Ni). At a sufficient loading, the CoNiMo catalyst is expected to outperform Pt and thus provides a promising low cost pathway for alkaline or alkaline membrane fuel cells. Density functional theory (DFT) calculations and parallel H2-temperature programmed desorption (TPD) experiments on structurally much simpler model alloy systems show a trend that CoNiMo has a hydrogen binding energy (HBE) similar to Pt and much lower than Ni, suggesting that the formation of multi-metallic bonds modifies the HBE of Ni and is likely a significant contributing factor for the enhanced HOR activity.

Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy

A universal correlation is established between HOR/HER activity and hydrogen binding energy on platinum-group metals. 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.

Hollow Chevrel‐Phase NiMo3S4 for Hydrogen Evolution in Alkaline Electrolytes

Electrochemical water splitting to generate molecular hydrogen requires catalysts that are cheap, active, and stable, particularly for alkaline electrolyzers, where the cathodic hydrogen evolution reaction is slower in base than in acid even on platinum. Herein, we describe the synthesis of new hollow Chevrel-phase NiMo3 S4 and its alkaline hydrogen evolution reaction (HER) performance: onset potential of -59 mV, Tafel slope of 98 mV per decade, and exchange current density of 3.9×10-2  mA cm-2 . This Chevrel-phase chalcogenide also demonstrates outstanding long-term stability under harsh HER cycling conditions. Chevrel-phase nanomaterials show promise as efficient, low-cost catalysts for alkaline electrolyzers.

Electrochemical reduction of CO2 to synthesis gas with controlled CO/H2 ratios

The electrochemical carbon dioxide reduction reaction (CO2RR) to simultaneously produce carbon monoxide (CO) and hydrogen (H2) has been achieved on carbon supported palladium (Pd/C) nanoparticles in an aqueous electrolyte. The synthesis gas product has a CO to H2 ratio between 0.5 and 1, which is in the desirable range for thermochemical synthesis of methanol and Fischer–Tropsch reactions using existing industrial processes. In situ X-ray absorption spectroscopy in both near-edge (XANES) and extended regions (EXAFS) and in situ X-ray diffraction show that Pd has transformed into β-phase palladium hydride (β-PdH) during the CO2RR. Density functional theory (DFT) calculations demonstrate that the binding energies of both adsorbed CO and H are significantly weakened on PdH than on Pd surfaces, and that these energies are potential descriptors to facilitate the search for more efficient electrocatalysts for syngas production through the CO2RR.