Ultra-Small Iridium Oxide Nanoparticles Decorated Carbon Nanostructures; Highly Efficient Electrocatalysts for the Oxygen Evolution Reaction
Authors: Badam Rajashekar , Hara Masanori , Yoshimura Masamichi , Matsumi Noriyoshi 
Published: February 2020

The research in the field of energy is more demanding than ever before. There is no doubt that the future of energy lies in the renewable energy sources. The storage of intermittent energy produced from renewable energy sources has become a major bottleneck. In this regard, storage of hydrogen produced from water electrolysis has surfaced as a plausible cleanest solution. The anodic reaction in water electrolysis i.e., oxygen evolution reaction (OER) with low reversibility, high over-potential and slow kinetics forced researchers to look for a better catalyst beyond platinum. Among many metal and metal oxide catalysts, iridium oxide was found to be the only catalyst that is active in acidic medium for its use on polymer electrolyte membrane water electrolysers (PEMWE). The two important problems for IrO2 is stability of nano-particles (nps) and to reduce the over-potential for OER (~330 mV). Till date there are very few works on nanostructured IrO2 prepared using a scalable procedure1. Moreover, the electronic structure of the IrO2 has been modulated by using it with other metals or metal oxides like RuO2, Ni or Pt2. The addition of other transition metals would either leach out or increase the cost furthermore. Hence, there arises a need for simple yet workable strategy.

In the recent past we have developed several carbons based OER catalysts loaded with ultra-small IrO2 nps (~1.5-1.7 nm). Improving the active surface area by reducing the particle size and increasing the metal substrate interaction (SMSI) resulted very high activity and durability in the case of CNT-IrO2 based catalysts3. Due to aforementioned properties CNT-IrO2 showed 10 times higher mass activity with four times lower Ir loading compared to several recent reports1,2. Along with above properties, improving the electrode electrolyte interface of functionalized acetylene black (FAB-IrO2) catalyst showed very high activity along with durability (~150 times higher activity compared to commercial IrO2 powder).

In the present work, along with the morphological and interfacial tuning, electronic structure of IrO2 was tuned to improve the mass transfer during the catalysis with the use of nitrogen doped graphene (N-graphene) as a substrate for IrO2 nps. N-graphene not only provided nucleation sites to produce ultra-small nps of ~1.5 nm (Fig 1). It was also found from XPS that N-graphene increased the electron cloud on IrO2 nps with which an electronic interaction could be established between nps and substrate that modifies the electronic structure of catalyst to reduce the overpotential. Ultra-small nps provide a large active surface area to provide very high activity towards OER. The increased electron cloud on the IrO2 nps will enable easier mass transfer to reduce the corrosion of catalyst for long durability. On the other hand, this will establish a strong electronic interaction between nps and substrate which limits the diffusion of nps during the long-term use. Electrochemical OER studies revealed that the prepared catalysts showed very promising results. The overpotential was found to be reduced by ~70 mV compared to bare IrO2. The activity or the current density was found to be 5 to 10 times higher at 1.48 V vs RHE compared to bare nanoporous1 IrO2 and Pt-IrO2
2 respectively. The modulated electronic structure and the strong electronic interaction between nps and substrate resulted in very high durability between 1.20-1.65 V vs RHE for 1000 potential cycles.

The above results show that carbon based IrO2 electrocatalysts not only claim very high activity but also are highly stable compared to commercial IrO2 catalysts. These materials can be a promising candidate with low cost and high activity for their use in PEMWE.

(1) Li, S. Li, M. Xiao, J. Ge, C. Liu and W. Xing. Nanoscale, 9, 9291 (2017).

(2) C. da Silva, M.R. Fernandes, and E. A. Ticianelli. ACS Catalysis, 8, 2081 (2018).

(3) Badam, M. Hara, H.H.Huang, M. Yoshimura.
Int

. J. Hydrog. Energy, 43, 18095 (2018)

Figure 1