Ultrathin cobalt phosphate enfolded with biomass-derived multishelled carbon onion as a proficient electrocatalyst for the oxygen evolution reaction and its green sustainability assessments

Abstract

Water electrolysis is a key factor to generate mobile and sustainable energy sources in the form of H₂ fuel. Herein, we provided a green mechanochemical synthetic method to develop an inexpensive and efficient non-noble electrocatalyst consisting of cobalt phosphate incorporated with pyrolyzed carbon onion (CPC) to generate H₂ in 1 M KOH. High degree graphitised carbon and the proton management of the phosphate group increased the OER activity of the CPC. The high graphitic carbon onion and cobalt phosphate (CoP) synergistically improved the facile electron transfer between the carbon and CoP, which generated surface oxygen vacancies, whereby CoP was easily converted to Co²⁺(Co(OH)₂)/Co³⁺(CoOOH). The greenly synthesized amorphous CPC 4-4/GC sample, where GC is graphitic carbon, exhibited a low overpotential (η) of 301 mV in 10 mA cm⁻² (without iR correction) and low Tafel slope of 83 mV dec⁻¹ compared to the benchmark IrO₂ (371 mV/120 mV dec⁻¹). The turnover frequency (TOF) of CPC 4-4 (0.0102 s⁻¹) was 3.2 times higher than that of IrO₂ (0.0048 s⁻¹). The CPC 4-4/NF sample, where NF is nickel foam, exhibited an overpotential of 271 mV (82 mV dec⁻¹), versus 351 mV (108 mV dec⁻¹) for IrO₂/NF. The overpotential of CPC 4-4/NF was found to be 80 mV lower than that of IrO₂. The CPC 4-4/NF displayed good stability for 110 h with a potential loss of only 2.8%. Also, the electrochemical surface area of CPC 4-4 (90.5 m⁻² g⁻¹) was higher than that of IrO₂ (29.2 m⁻² g⁻¹). Further, the alkaline water electrolysis of CPC 4-4 (CPC 4-4/NF//Pt/C/NF) required only 1.57 V for effective water splitting and was stable for over 110 h with a low potential loss of only 3.2%. In solar-to-hydrogen water splitting, the solar cell structure of CPC 4-4 required 1.57 V for the non-stop evolution of H₂ and O₂, showing good potential to facilitate low-cost, large-scale hydrogen generation.