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.