Rejuvenating the Geometric Electrocatalytic OER Performance of Crystalline Co₃ O₄ by Microstructure Engineering with...

Despite significant research on its electrocatalytic OER activity, the geometric performance of Co₃ O₄ has remained unsatisfactory compared to relatively amorphous Co-based materials. In particular, the activity of Co₃ O₄ prepared through annealing always gets inferior compared to its amorphous precursor. This demands the development of synthetic techniques to prepare Co₃ O₄ with superior activity as the unpredictable crystal structure of the amorphous materials makes it difficult to understand their structure-activity relationships despite higher geometric activity. In this article, we have shown that incorporation of sulfate in pre-annealed materials plays a pivotal role in boosting the OER activity of annealed Co₃ O₄ irrespective of the pre-annealed phase. In contrast to commonly used nitrate or carbonate that leaves the structure upon annealing and renders the resulting Co₃ O₄ with poor activity, sulfate remains in the annealed structure due to its thermal stability and causes a dramatic enhancement in the geometric electrocatalytic OER activity of resulting Co₃ O₄ compared to the pre-annealed phase. This was due to the "pore-alteration ability" and "crystallization hindrance effect" of sulfate ions that significantly alter the microstructure of the resulting Co₃ O₄ during annealing process by dramatically improving the surface area, pore size, and pore volume. Moreover, sulfate incorporation provided structures with considerably higher mesoporosity that is known to be conducive for reactant and product diffusion within the network. The improved textural properties led to better exposure of the catalytic centres to the electrolyte leading to higher geometric OER activity despite identical intrinsic activity of both sulfate free and incorporated Co₃ O₄ as confirmed from their specific activities. Further, the Co₃ O₄ synthesized by annealing sulfate incorporated precursor was found to be rich with oxygen defects that are known to increase the potency of a material towards electrocatalytic OER. The sulfate ions also etched out in the electrolyte during electrocatalysis leading to complete unblocking of the pores thereby helping in sustaining the high geometric OER activity. To our knowledge, this is the first report where the geometric electrocatalytic OER activity of an annealed Co₃ O₄ is significantly better compared to its pre-annealed phase and is in fact comparable to the activity of amorphous Co-hydroxide based compounds.