Unveiling the role of surface P–O group in P-doped Co₃O₄ for electrocatalytic oxygen evolution by On-chip micro-device

Highlights

• The electronic/electrochemical signals of P-Co₃O₄ thin-film are collected in time using the OER microdevice.

• The electronic coupling effect between the P–O groups and Co₃O₄ species is clarified at nanoscale.

• In situ I-V measurement and theoretical simulation reveal the promoting effect of P–O groups on reaction kinetics.

Abstract

Transition metal phosphides or partially phosphatized oxides usually suffer from surface reconstruction during oxygen evolution reaction (OER), but still possess enhanced catalytic activity than directly synthesized oxides, which has aroused great interest in exploring the causes of such high catalytic activity. To monitor electronic property of catalyst during the OER can provide crucial insights into catalytic ability. Here we design a planar electrochemical microdevice based on individual thin-film catalyst, and for the first time explore the continuous electric conductance evolution of lattice P-doped oxides during the electrochemical activation process. Moreover, combining on-chip electrochemical impedance spectra measurements, in situ I-V measurements, and theoretical simulations of reconstructed lattice P-doped oxides, the effect of P–O groups on new-formed oxides is clarified. The induced electronic coupling between new-formed oxides and P-O groups has been studied and demonstrated. The coupled P–O groups effectively promote the metal–oxygen covalency of new-formed oxides, which accelerates electron transfer between active metallic center and oxygen adsorbates, thus leading to the enhanced electrocatalytic activity. Our study highlights the role of surface P–O groups in Co₃O₄ during the OER, and such unique on-chip electrochemical microdevice platform can also be applied in other related fields to understand the dynamic behavior of energy materials at nanoscale.

Introduction

Nowadays, water splitting is recognized as an environmentally friendly route for hydrogen production [1]. Anodic oxygen evolution reaction (OER) in this process, is a crucial half-reaction but severely limits the efficiency of hydrogen evolution for its sluggish four-electron transfer kinetics [2], [3], [4]. Transition metal phosphides (TMPs) and phosphorus doped transition metal oxides, are generally recognized as efficient catalysts for the OER in recent years and hold promise to replace the noble metal-based oxides like IrO₂ and RuO₂ [5], [6], [7], [8], [9]. However, different from most stable oxides or (oxy)hydroxides, TMPs or P-doped oxides often suffer from severe surface reconstruction after the OER while show enhanced catalytic activity than the pristine synthesized TM oxides or (oxy)hydroxides [9], [10]. It has been widely reported there are three components in OER-reconstructed TMPs or P-doped oxides: internal unreacted structure, surface new-formed TM-O species and residual P–O groups [7]. The internal phosphatized structure and surface TM-O species serve as conductive support and active species, respectively [11]. While the P–O groups are generally considered as unavoidable impurities and rarely involved in the analysis of oxygen evolution mechanism of reconstructed TMPs or P-doped oxides. Interestingly, through rigorous computational simulations, Fabris et al. predicted that the presence of surface P–O groups near the active Co sites may affect the OER reaction mechanism of the cobalt-phosphate [12]. Inspired by such work, it is necessary to unveil the intrinsic role of P-O groups in surface TM-O species towards OER, which would affect our understanding of oxygen evolution mechanism.

Generally, the OER activity of catalysts is closely related to their electronic properties. Thus, monitoring the electronic property evolution during the OER is of great importance to predict the catalytic activity. Previously, most of fundamental research used first-principles computations (such as density of states) and advanced spectroscopy technologies (such as X-ray photoelectron spectroscopy) to understand the electronic structure of catalysts [13], [14], [15], [16], [17]. Unfortunately, due to the presence of binders, conductive carbon and conductive substrate in traditional OER testing system, it is impossible to accurately measure the actual electric conductance of catalyst during the OER, which hinders the exploration on catalytic mechanism of TMPs or P-doped oxides. In view of this, a planar electrochemical thin-film microdevice is designed to probe the electronic and electrochemical signals of the catalyst. In the device, the catalyst is contacted with two metal microelectrodes in the form of individual thin-film. By this means, the electronic signal from thin-film will be collected in time, which is helpful to understand the catalytic promotion effect of P-O groups in reconstructed phosphorus-containing catalyst [18], [19], [20], [21]. In addition, no binders and conductive carbon additives are used in individual thin-film catalyst, avoiding the interference of irrelevant factors. Finally, we aim to establish the correlation between structure, electrical properties and electrochemical performance of the catalysts.