An Oxygen-insensitive Hydrogen Evolution Catalyst Coated by a Molybdenum-based Layer for Overall Water Splitting

Monday, July 31, 2017

Harvesting solar energy in chemical bonds will ultimately disrupt dependency on fossil fuels and, consequently, susceptibility to climate change. In this regard, H2 is considered one of the most promising energy carriers. If H2 is generated from H2O using renewable energy (for example, photocatalytic overall water splitting), it is possible to obtain a sustainable fuel without any carbon footprint. One of the significant issues associated with water splitting is the facile reverse reaction; that is, the catalyst for the hydrogen evolution reaction (HER) is also active in undesired water-formation reactions, such as the oxygen reduction reaction (ORR), or the thermal reaction of H2 and O2. Therefore, inhibition of these back reactions is of paramount importance for photocatalytic water splitting.

An international research team, including scientists from the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and SSRL, has developed a new hydrogen evolution catalyst with a molybdenum coating that prevents this problematic back reaction and works well in realistic operating conditions.

In a new study, featured in Angewandte Chemie, the research team reports the structural and electrochemical properties of a Mo-coated Pt HER catalyst that is highly active, oxygen-insensitive, and stable in acidic media for overall water splitting. By using SSRL Beam Lines 6-2 and 4-1, operando high-resolution x-ray absorption spectroscopy (BL6-2) and EXAFS (BL4-3) studies characterized the working electrode in action. The results show that adequate modification of a Pt electrode with MoIV polyanionic species render the electrode insensitive towards ORR and HOR while preserving high HER performance. The inhibition of water-forming reactions and the superior selectivity towards H2 generation in the presence of O2 seems to originate because the Mo coating likely acts as a membrane, confining the availability of O2 and H2 near Pt.

Indeed, this electrocatalytic concept was reflected in photocatalysis experiments, as overall water splitting proceeded without the back reaction for MoOx/Pt/SrTiO3. The Mo-based modifier developed and characterized in this study is the only acid-tolerant material to date that can selectively prevent the water-forming back reaction.

Figure
In-operando Mo-K edge XAS measurements and ab initio simulations. (a) Mo K-edge XANES spectra of the MoOx/P electrode taken at various potentials (0.1 M KClO4, pH 1.8, 298 K) along with MoO2 and MoO3 references. (b, c) In-operando EXAFS measurements. (b) EXAFS k2∙χ(k) functions at Mo K-edge and (c) corresponding Fourier transforms (without phase correction) for the MoOx/P electrode taken at various potentials (0.1 M KClO4, pH 1.8, 298 K) along with the MoO2 reference. The open symbols represent experimental data, and the full lines indicate spherical wave theory. (d) The theoretical XANES spectra of the dimeric and trimeric structural motifs that best represents the experimental Mo-K edge XANES spectra under H2 production conditions. The normalized (with respect to the experimental spectrum) Mo K-edge XANES spectra of the [Mo2O10]12− and [Mo3O16]16− isopolymolybdates was obtained via ab initio calculations with the FDMNES code. The computations are shown together with the experimental spectrum measured with an applied potential of −0.15 V vs. RHE (0.1 M KClO4, pH 1.8, 298 K).
Primary Citation: 

A. T. Garcia-Esparza, T. Shinagawa, S. Ould-Chikh, M. Qureshi, X. Peng, N. Wei, D. H. Anjum, A. Clo, T.-C. Weng, D. Nordlund, D. Sokaras, J. Kubota, K. Domen and K. Takanabe, "An Oxygen-insensitive Hydrogen Evolution Catalyst Coated by a Molybdenum-based Layer for Overall Water Splitting", Angew. Chem. 56, 5780 (2017) doi: 10.1002/ange.201701861

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