Rejuvenating the geometric electrocatalytic OER performance of crystalline Co3O4 by microstructure engineering with sulphate.

Despite significant research on its electrocatalytic OER activity, the geometric performance of Co3O4 has remained unsatisfactory compared to relatively amorphous Co-based materials. In particular, the activity of Co3O4 prepared through annealing always gets inferior compared to its amorphous precursor. In this article, we have shown that incorporation of sulphate in pre-annealed materials plays a pivotal role in boosting the OER activity of annealed Co3O4 irrespective of the pre-annealed phase. In contrast to commonly used nitrate or carbonate that leaves the structure upon annealing and renders the resulting Co3O4 with poor activity, sulphate remains in the annealed structure due to its thermal stability and causes a dramatic enhancement in the geometric electrocatalytic OER activity of resulting Co3O4 compared to the pre-annealed phase. This was due to the "pore-alteration ability" and "crystallization hindrance effect" of sulphate ions that significantly alter the microstructure of the resulting Co3O4 during annealing process by dramatically improving the surface area, pore size, and pore volume. Moreover, sulphate incorporation provided structures with considerably higher mesoporosity that led to better exposure of the catalytic centres to the electrolyte leading to higher geometric OER activity despite identical intrinsic activity of both sulphate free and incorporated Co3O4 as confirmed from their specific activities. Further, the Co3O4 synthesized by annealing sulphate incorporated precursor was found to be rich with oxygen defects that are known to increase the potency of a material towards electrocatalytic OER. To our knowledge, this is the first report where the geometric electrocatalytic OER activity of an annealed Co3O4 is significantly better compared to its pre-annealed phase and is in fact comparable to the activity of amorphous Co-hydroxide based compounds.