Bonding state synergy of NiF2/Ni2P hybrid with co-existence of covalent and ionic bond as a robust catalyst for energy-relevant water and urea electrooxidation
Electrochemical water and urea oxidation reaction are very significant to solve the growing energy crisis and environmental pollution among the energy-relevant electrochemical reactions. Herein, NiF2/Ni2P hybrid catalyst with the co-existence of covalent and ionic bond is found to be very active for these electrochemical reactions in water electrolysis technique. The bonding state of covalent and ionic bond is verified by the crystal structure and surface chemical state revealed by the spectrum analysis. As a bifunctional catalyst for water and urea electro-oxidation, NiF2/Ni2P hybrid structure demonstrates both higher catalytic activity, kinetics and stability for the catalytic reaction than their counterparts of NiF2 and Ni2P catalyst under the same conditions. Specifically, the overpotential as low as 283 mV can drive the benchmark current density of 10 mA cm-2 for oxygen evolution reaction, significantly lower than the potential required for the NiF2 (393 mV) and Ni2P materials (342 mV); the maximum current density can reach 157.35 mA cm-2 at 1.53V for urea electrooxidation which is much higher than that of NiF2 (23.55 mA cm-2) and Ni2P (102.72 mA cm-2), respectively. The catalytic performance also outperforms the advanced similar catalysts recently reported and the high performance can be attributed to the highly exposed active sites, rough surface area, excellent charge transfer ability, and especially the synergistic effects of the covalent and ionic bond in the catalyst system. With the help of a commercial Pt/C catalyst as cathode, the cell potential for urea assistant water electrolysis can be reduced to 1.5 V to offer nearly 40 mA cm-2 in a two-electrode system (Pt/C||NiF2/Ni2P), about 300 mV less than that for the water electrolyzer in the general alkaline electrolyte. The current work demonstrates the significance of bonding state synergy in the advanced catalyst for water electrolysis technique and sheds some light on the catalysts development in energy chemistry.