Zhao, Siyu; (2021) Phosphorus modified transition metal electrocatalysts for efficient electrolysis of water. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A variety of approaches to produce hydrogen, such as electrolysis, photo-electrolysis and gasification, have been explored over the past decade. Among these methods, the electrolysis of water, which consists of cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), was intensively studied due to its advantages of easy-to-obtain reactants, stable high-purity outputs, and feasible large-scale production processes. However, it remains a challenge to develop high-performance, stable and cost-effective electrocatalysts for HER and OER. Here, this thesis aims at design and development of transition metal-based nanostructured catalysts for electrolysis of water. Step-by-step research was carried out by firstly investigating efficient electrocatalysts for HER in alkaline media. Then high-efficiency catalysts for HER in neutral media were designed and developed. Finally, electrocatalysts for OER in alkaline media were developed by a facile room-temperature wet chemistry method. The details of these three main PhD projects are as follows: (1) To improve the HER performance of transition metal oxides in alkaline media, Phosphorus modified monoclinic β-CoMoO4 as a low-cost, efficient and stable HER electrocatalyst for the electrolysis of water in alkaline media was fabricated. The optimized catalyst shows a small overpotential of 94 mV to reach a current density of 10 mA cm−2 for the HER with high stability in KOH electrolyte, and an overpotential of 197 mV to reach a current density of 100 mA cm−2.Combined computational and in-situ spectroscopic techniques show that P is present as a surface phosphate ion; that electron holes localise on the surface ions and both (P-O1-) and Co3+-OH- are prospective surface-active sites for HER. (2) To further improve the HER performance in neutral media, which is promising for future study of electrolysis of sea water, N-modified FeP supported on mildly oxidized carbon nanotubes electrocatalysts were fabricated. In general, metal phosphides catalysts are not stable under working condition, which would usually undergo surface reconstruction and weaken the performance. In this work, a facile one-step stabilization of metal phosphides electrocatalysts by doping of N was proposed. The optimized catalyst shows an overpotential of 256 mV at a large current density of 65 mA cm−2, which is even 10 mV lower than that of the commercial 20% Pt/C catalyst. (3) To improve the OER performance in alkaline media, Co4S3 supported on Ni foam catalysts were fabricated by a one-step room-temperature wet chemistry synthesis strategy. Excellent stability was achieved under a constant current density of 100 mA cm-2 for 100 hours and the OER performance of the catalyst was improved after 1400 cycles of linear sweep voltammetry tests in alkaline media. Furthermore, the underpinning mechanism of action was studied by measuring the change of valence states for different elements to elucidate the structural evolution and active species during the electrocatalytic process.

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