New Avenues to Transition Metal-Based Water Splitting Electrocatalysts

Abstract

Solar energy is by far the most abundant renewable resource available to mankind. However, it is diffused and intermittent, and often geographically separated from that of the production results in underwhelming utilization of this resource. Inspired by photosynthesis, various efforts were made to store solar energy in form of chemical bonds than can be used when the sun is not shining. A promising approach is to produce hydrogen, a carbon-neutral energy carrier is via water splitting which requires electrocatalysts to accelerate the two half-cell reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The state-of-art catalysts used for HER is Pt and for OER is IrO2/RuO2 that are prohibitively expensive. We have developed new synthesis methodologies for various earth-abundant electrocatalysts supported heteroatom-doped carbon nanostructures and exploited for water splitting. An in-situ solid state route was developed to integrate ruthenium nanoparticles with N-doped graphene sheets which exhibited an HER activity rivalling state-of-art Pt/C over a wide pH range. In order to find further cost-effective materials, we sought inspiration from NiFe-hydrogenase (the most efficient catalyst for HER) to develop a general solid state method for bimetallic MFe@ N-doped carbon core-shell nanostructures (M = Ni, Cu, Co, Zn, Mn) as efficient total water splitting catalyst. Thereafter, a new, phosphine-free, solid state method to hybridize Co2P with N, P co-doped CNTs was developed which could also be extended to synthesize Fe2P, Ni2P and Cu3P. Moreover, glucose oxidation was attempted as a possible replacement for the kinetically sluggish OER half-cell reaction, wherein Co2P/N, P-CNTs were demonstrated to be an efficient non-enzymatic glucose sensor for the first time. Thereafter, Co-imidazolate frameworks (ZIF-67) were transformed into hierarchal Co-N-Se nanosheets via a simple selenization method. Investigations were carried out to establish a structure-property correlation between the nanostructures evolved over various interval of time along with their OER activity. Finally, an in-situ strategy was developed to hybridize N-doped graphitic carbon seets with Ni and MoxC (Mo2C and MoC) nanoparticles which exhibited resilient HER activity besides effectively accelerating OER, thereby resulting in overall water splitting that can be attributed to favorable electronic modulation between various strongly coupled components.