Effect of Functional Substrate on the Electrocatalytic Oxygen Evolution Reaction Activity of IrO2 Nanoparticles
Authors: Badam Rajashekar , Asai Yusaku , Gupta Agman , Matsumi Noriyoshi 
Published: July 2021

Water electrolysis (WE) using proton exchange membrane water electrolyser (PEMWE) stands as one of the cleanest methods to produce hydrogen from water which is the greener alternative that can replace fossil fuels. Oxygen evolution reaction (OER) that happens at its anode is both thermodynamically and kinetically demanding. IrO2 is the only known catalyst that can be used in acidic conditions but its high overpotential ~330 mV and cost preclude its future use.
In the past, several attempts have been made to tune the catalytic activity, durability and overpotential by changing the morphology and electronic structure. The high surface area nanostructures could achieve decent activity but the overpotential and durability are still at stake. The high surface area nanostructures could achieve decent activity but the overpotential and durability are still at stake. Norskov1 et al., showed a linear relation between metal oxide d band centre and binding energies to O (intermediates) to the surface (ΔEO) which play a key role in determining the overpotential of the system. Hence, tuning of electronic structure by doping with other metals like Ru, Ni, Co, Pb, Y, Se, Sn, Sr2 etc or with fluorine3 was found to be trending in recent years. But, the leaching of alloying metals (durability) and precise tuning of electronic structure are not achieved. Recently our group has come up with a novel strategy of tuning the electronic structure of IrO2 nanoparticles (nps) by (a) using electrochemically stable carbon and doped carbon substrates4,5 (b) very strong metal substrate interaction and (c) changing the electronic structure of IrO2. By using 10 at% nitrogen doped graphene, overpotential was reduced to 260-270 mV with very high durability. There is always a limit for heteroatom doping using conventional ways. Further very less works on precise tuning of electronic structure and anchoring of IrO2 nanoparticles to substrate gives a scope of research.
In this regard conducting polymers as substrates with coordination sites for anchoring the nanoparticles and tuning the electronic structure was explored in this study. An electropolymerizable monomer bearing α-diimine and thienyle groups was prepared by condensation between acenaphthequinone and 2 equivalents of aminothiophene. Electropolymerization technique was utilized to prepare polythiophene (PTh) on TiO2 nanotube (PTh-TNT) array as substrate. Further, the α-diimine moiety was utilized to prepare polythiophene-Ir metal complex. Ir metal complex centers could act as the nucleation sites for the growth of IrO2 nanoparticles due to which the electronic interaction with IrO2 and the diimine ligand will be still active. This interaction may lead to alter the electronic states of IrO2 like that of Ir in the complex at the same time will anchor the IrO2 nanoparticles strongly that results in high activity and durability during OER. For comparison TNT-PTh-IrO2 material without prior complexation was also prepared. Shift in the electron structure was characterized by X-ray photoelectron spectroscopy. The shift of binding energy (BE) of Ir 4f peak of TNT-PTh-IrO2 (with complex) to a lower BE compared to TNT-PTh-IrO2 (without complex) was a clear indication for increased electron cloud on IrO2. This shift of electron cloud will modulate the bond length of intermediate ions and help in their easy mass transfer leading to higher activity. Electrocatalytic OER activity was studied by linear-sweep voltammetry (LSV) technique in 3 electrode system in 1 M H2SO4. The effect of anchoring through coordination showed twice the activity compared to IrO2 on polythiophene without anchoring (Figure 1). Further a clear reduction in overpotential was also observed. When compared with TNT-IrO2, TNT-PTh-IrO2 with complexation showed at least 10 times higher current density. Further, preliminary chronopotentiometry showed no change in the over potential at 10 mAcm2 for 5 h. Thus, utilization of strong coordination of α-diimine in the polythiophene structure was key in modifying the electronic structure and anchoring resulting very high OER activity in acidic environment.
Acknowledgement: We thank financial support by JSPS, KAKENHI, grant number 19K15674.
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Figure 1