Development of non-noble catalysts for hydrogen and oxygen evolution in alkaline polymer electrolyte membrane electrolysis

Abstract

Hydrogen is seen as the ‘energy carrier of the future’ due to the element’s relative abundance, the formation of water as opposed to the green house gases when utilised as a fuel in fuel cells, and the ability to be produced by electrolysers powered from renewable energy sources such as wind, water and sunlight. The development of hydrogen production through electrolysis is hindered by the high costs associated with the technology, specifically the ion exchange membranes and electrocatalysts that are employed in the membrane electrode assemblies used in polymer electrolyte electrolysers. This research focused on the development of non-noble catalysts suitable for hydrogen and oxygen evolution in alkaline electrolysis. Synthesis of NiO was achieved through thermal decomposition, chemical bath deposition and solution growth techniques. A mixed metal oxide, NiCo2O4, was synthesized through thermal decomposition of metal nitrate salts. Cyclic voltammetry and steady state electrochemical experiments on the electrodes were conducted in an electrochemical half cell electrolyser. A thin film of pure NiO was formed onto a titanium substrate through chemical bath deposition followed by thermal decomposition. The performance of the electrode at 1.73 V, relative to the mass of the catalyst loading, produced 0.25 A cm-2 mg-1 in 1 M NaOH at 25°C (IrO2 produced 0.44 A cm-2 mg-1 in the same electrolyser). The electrode’s performance is attributed to the nanoporous structure of the catalyst film (20 – 200 nm pore diameters), which was formed from the chemical bath deposition method used to prepare the catalyst films. Unfortunately this procedure has a limited film thicknesses so higher loadings could not be achieved. Higher loadings of other non-noble electrocatalysts were made possible with addition of a PVDF binder to the catalyst film. Physical analysis through XRD was performed on the most promising catalysts for the oxygen evolution reaction to confirm their composition. A blend of α-Ni(OH)2 and 4Ni(OH)2•NiOOH•xH2O formed through the chemical bath deposition technique produced higher current densities (104 mA cm-2 at 0.8 V vs. Hg/HgO) than another non-noble metal catalyst, NiCo2O4 (97 mA cm-2) II in 1 M NaOH at 25°C. An alkaline polymer electrolyser free from noble metals was developed with a membrane electrode assembly that utilised a partially fluorinated membrane, a PVBC/PVC ionomer in the catalyst layers, 1.0 mg NiMoO4 cm-2 in the cathode and 0.7 mg NiCo2O4 cm-2 in the anode. It produced 0.4 A cm-2 in 1 M KOH at 25° at a potential of 1.9 V.