Three-dimensional crystalline-Ni5P4@amorphous-NiOx core-shell nanosheets as bifunctional electrode for urea electro-oxidation and hydrogen evolution

The electrochemical urea oxidation (UOR) involves the generation of gases bubbles, which often cause the blockage of the electrode surface leading to decline activity. Herein, we report the growth of Ni5P4 nanosheets on nickel foam as bifunctional electrode (CA-Ni5P4@NiOx/NF) for urea electrolysis. The results indicated that a thin layer of amorphous NiOx formed on the surface of Ni5P4 nanosheets to yield a unique crystalline@amorphous core-shell structure, which can be maintained during long time urea electrolysis. The outstanding UOR performance of CA-Ni5P4@NiOx/NF was attributed to the unique structure, which not only combine the good conductivity and abundant active sites, but also rendered the CA-Ni5P4@NiOx/NF superhydrophilic and aerophobic that significantly facilities the release of gases bubbles formed during UOR to avoid the blockage of the active sites. As a result, the CA-Ni5P4@NiOx/NF electrode exhibited excellent performance for UOR, which only needs 1.45 V (vs. RHE) to deliver a current of 100 mA/cm2. When used as anode for direct urea fuel cell (DUFC), it could reach a maximal power density as high as 3.4 mW/cm2 with an OCV of 0.76 V. The CA-Ni5P4@NiOx/NF also exhibited excellent activity for hydrogen evolution reaction (HER), with -0.089 V (vs. RHE) to deliver a current of 10 mA/cm2. The electrolyzer constructed with CA-Ni5P4@NiOx as both anode and cathode could run for more than 10 h at a high current load about 100 mA/cm2 without appreciable potential change, indicating the superb stability of the CA-Ni5P4@NiOx electrode. Our work provides new insights for understanding as well as the development of advanced electrode for DUFC and urea electrolysis.

Automated summary

In this study, we report the growth of Ni5P4 nanosheets on nickel foam as a bifunctional electrode for urea electrolysis. The electrode exhibits a unique crystalline@amorphous core-shell structure, which leads to outstanding urea oxidation performance. The electrode is superhydrophilic and aerophobic, facilitating the release of gas bubbles and preventing blockage of active sites. The electrode also shows excellent stability in both urea electrolysis and hydrogen evolution reactions.