Investigating reactive sputtered IrxNiyOz electrocatalysts for the oxygen evolution reaction

Abstract

Alkaline water electrolysis (AWE) is a promising, simple and environmentally friendly technology, when coupled with renewable energy sources (e.g. wind and solar), to produce high purity hydrogen gas for clean energy conversion and storage. In spite of the significant advances and progress made in AWE, modern electrolysis cells are performing at somewhat limited operating current densities with high energy consumption. Contributing to this unsatisfactory performance is the slow oxygen evolution reaction (OER) kinetics on the anode which requires the use of efficient electrocatalysts to lower initial electrical energy input. From the numerous papers being published in the field of electrocatalysis for the OER, it is evident that the development of an efficient electrocatalyst to serve as the anode for the OER, is a necessity to realise optimal industrial performance. Among the various electrocatalysts that have been developed and studied (Co, Mn, Fe and Ni, along with their oxides etc.), Ni and Ni-based oxides have received much attention, exhibiting superior performance in terms of activity and stability of the studied non-noble metals for the OER in alkaline conditions. However, improvement in performance is still required to compete with the performance of the best electrocatalysts in acidic media, Ir and IrO2. Electrocatalyst research is multifaceted and depends on many factors contributing to the observed activity. In this study, focus was placed on two main aspects, namely the supporting structure of the electrocatalyst and the elemental composition of the electrocatalyst. The development of support structures for electrocatalysts (i.e. graphite, Vulcan carbon (VC), graphene etc.), has received a great deal of attention over the last decade. Carbon support structures are known to increase the surface area, stability and activity of electrocatalysts in most cases, and can be used to overcome the delamination of thin films from electrode substrates. Two types of electrode substrates were used in this study, which include an Au/SiO2 wafer and glassy carbon (GC) disk inserts. In an attempt to (i) obtain surface structures and areas on Au/SiO2 wafer electrode pads, for combinatorial high-throughput sputtering and screening, that are comparable to GC, (ii) eliminate delamination of the electrocatalyst, and (iii) increase activity and stability, the first part of the study focused on the preparation technique of VC:Nafion support. Four different VC:Nafion inks were prepared and used as carbon support on GC electrode inserts to analyse their effect on the activity of sputtered Ni thin films towards the OER in alkaline media. Scanning Electron Microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were employed for physical characterisation of the VC:Nafion supported GC before and after electrochemical characterisation. Linear sweep voltammetry (LSV) and chronopotentiometry (CP) were employed for electrochemical characterisation to compare the catalytic activity and stability of these sputtered Ni thin films on the various VC supports. Results exhibit improved performance achieved by the sputtered Ni on VC:Nafion (1:0.67) support for the OER in alkaline media (in comparison to unsupported Ni), indicating improved Ni utilisation as well as improved short-term stability of the Ni thin films. These results validate the use of VC:Nafion as support for sputtered electrocatalysts. Various reactive sputtered IrxNiyOz electrocatalyst combinations were subjected to high-throughput electrochemical characterisation, with the aim of identifying an attractive alternative OER electrocatalyst, showing satisfactory activity and stability in alkaline conditions. The concept of investigating a spread of IrxNiyOz electrocatalyst combinations was based on (i) the huge body of literature supporting the activity of Ir/IrO2 in acid media, (ii) the evidence of the activity and stability of Ni and NiO in alkaline media, and (iii) ultimately exploiting the possibility of an optimally modulated mix of these elements resulting from desired synergistic effects. Rotating disk electrode (RDE) techniques, which included LSV and CP were used with the VC:Nafion supported GC as electrode substrate for in-depth electrochemical analysis along with SEM, EDX and X-ray photoelectron spectroscopy (XPS) as physical characterisation techniques of the best IrxNiyOz electrocatalyst combinations. Overpotential, Tafel slopes and exchange current along with results from physical characterisation were employed as key performance indicators. Overall the IrxNiyOz electrocatalyst combinations containing higher amounts of Ir (Ir92Ni8Ox, Ir68Ni32Ox and Ir62Ni38Ox) performed the best of the tested mixed metal electrocatalysts with overpotentials after stability testing of 389, 390 and 530 mV, respectively with Ni performing the best out of all testes electrocatalysts with 278 mV. However, evident from this study was the fact that the combination of Ir with Ni did not result in a mixed metal electrocatalyst that could outperform pure Ni. Nonetheless, it is also clear that a synergy does indeed exist for the IrxNiyOz combination, however, in this study it was not optimal (maybe due to various factors) to satisfy the compromise between electrocatalyst performance and cost.