Figure 4. a) In-situ Raman spectrum for Ni-CoP and CoP collected under multi-potential steps. b) Formation free energy of vacancy P in alkaline environment. Gibbs free energy along the reaction steps for: c) OER and d) HER.
3. Conclusion
To summarize, CoP-based catalyst with quasi-covalent bonding mode is prepared through a hydrothermal-phosphating strategy, which is used as a catalytic model. Compared with noble metal catalysts, it demonstrated excellent water electrolysis efficiency (overpotential 1.59 V) and stability in a two-electrode system (current density 100 mA cm-2 for 100 h). Theoretical calculations and in situ spectroscopy characterizations indicate that CoP-based catalysts undergo a deeper configuration evolution process in OER compared to HER: Based on the lower *OH adsorption/proton desorption energy barrier provided by DFT calculations and AIMD diagram, P vacancies and transition state Co-OX caused by POX coordination and broken Co-P bond are identified as an important initial reconstruction process. At higher anode potentials, the above configurations follow the reconstruction path guided by the lattice oxygen mechanism of traditional transition metal oxides, which is mainly shown as cobalt-based oxyhydroxides and superoxides. The influence of heteroatom Ni on reconstruction behavior and water splitting performance of the original CoP configuration is analyzed through DFT calculations, which exhibit promoted configuration evolution and reaction kinetics by providing lower P vacancy formation energy and key intermediate adsorption energy barrier. This work provide an intuitive strategy to observe complex structural evolution process, and offer an novel understanding toward the correlation among structure-composition-activity.
4. Experimental Method
4.1. Reagents and chemicals
Potassium hydroxide (KOH) was purchased from Shanghai Sinopharm Chemical Reagent Co. Carbon cloth (CC), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), Iron nitrate nonahydrate (Fe(NO3)3·9H2O), Chromium nitrate nonahydrate (Cr(NO3)3·9H2O), ammonium fluoride (NH4F), urea (CO(NH2)2), Sodium hypophosphite (NaH2PO2), Ruthenium oxide (RuO2) and Platinum-carbon (Pt/C) were obtained from Sigma Aldrich. The ultrapure deionized (DI) water with (18.25 MΩ cm−1) was purified through a Millipore system. In note, the purity of the above reagents is analytical grade, which does not require further purification.