Coaxial Ni–S@N-Doped Carbon Nanofibers Derived Hierarchical Electrodes for Efficient H₂ Production via Urea Electrolysis

Abstract

Electrochemical water splitting into hydrogen is a promising strategy for hydrogen production powered by solar energy. However, the cell voltage of an electrolyzer is still too high for practical application, which is mainly limited by the sluggish oxygen evolution reaction process. To this end, hybrid water electrolyzers have drawn tremendous attention. Herein, coaxial Ni/Ni 3 S 2 @N-doped nanofibers are directly grown on nickel foam (NF), which is highly active for hydrogen evolution reaction. Meanwhile, the Ni 3 S 2 @N-doped nanofibers on NF prepared in an Ar atmosphere display superior urea oxidation reaction performance to previously reported catalysts. The cell voltage is about 1.50 V in urea electrolysis to deliver a current density of 20 mA cm −2 , lower than that of a traditional water electrolyzer (1.82 V). The current density is around 77% relative to its initial value of 20 mA cm −2 after 20 h, superior to Pt/C|Ir/C-based urea electrolysis (14%). It is found that the synergistic effect between metallic Ni and Ni 3 S 2 , as well as the interfacial effect between metal centers and N-doped carbon, favors the initial dissociation of H 2 O and the adsorption/desorption of H* with thermal neutral Gibbs free energy. Meanwhile, the in-situ generated NiOOH on the outer surface of Ni 3 S 2 possessed lower electrochemical activation energy for urea decomposition. Meanwhile, the abundant oxygen vacancies in electrodes could expose more active sites for the adsorption of intermediates, including H* and OOH*. It is also found that the hierarchical nanostructure of densely packed nanowires provides ideal electronic and ionic transport paths for fast electrocatalytic kinetics. The present work indicated that the modulation of compositions and hierarchical nanostructure is effective to prepare efficient catalysts for H 2 production via urea electrolysis.

An Unveiled Electrocatalysis Essence of NiCo Hydroxides through in Situ Raman Spectroscopy for Urea Oxidation

“…In addition, we found that the electrocatalytic urea oxidation rate gradually increased as the temperature of the electrolyte increased, which was related to the rate constant of the chemical reaction. [ 34 ] By transforming the Arrhenius formula, we can see that the Ni 0.8 Co 0.2 DHs had a lower apparent E a value of 12.45 kJ mol −1 , while the values of Ni 0.67 Co 0.33 DHs, Ni 0.5 Co 0.5 DHs, and Ni 0.2 Co 0.8 DHs were 13.88, 23.91, and 37.50 kJ mol −1 , respectively (Figure S5, Supporting Information). This result indicates that Ni 0.8 Co 0.2 DHs possessed a lower apparent barrier for the CN bond cleavage in urea.…”

“…In addition, we found that the electrocatalytic urea oxidation rate gradually increased as the temperature of the electrolyte increased, which was related to the rate constant of the chemical reaction. [34] By transforming the Arrhenius formula, we can see that the…”

Recent Development of Nickel-Based Electrocatalysts for Urea Electrolysis in Alkaline Solution

“…Here, future perspectives are discussed: Developing highly efficient UOR catalysts, in terms of boosted catalytic current, low overpotential and durable catalytic performance, is highly desirable. The majority of the prepared catalysts are nickel oxides and hydroxides at early stages, while nickel sulfides [ 115 , 116 , 117 , 118 ], selenides [ 62 , 63 ], phosphides [ 119 , 120 ] and nitrides [ 111 ] have shown appreciable UOR performance in recent years. In addition, Ni-based Prussian blue analogues [ 17 ] and perovskites [ 121 ] have also been revealed as promising candidates for the UOR.…”

The synergetic effect of the mixed phase of NiMoO4 with a 1D–2D–3D hierarchical structure for a highly efficient and stable urea oxidation reaction

“…The lower apparent barrier of the C-N bond cleavage in urea for a/b-NiMoO 4 @NF indicates its easy oxidation into CO 2 and N 2 . 48 According to the previous studies, 13,49 the rate-determining step for the UOR is CO 2 desorption; hence, the CO 2 temperature-programmed desorption (CO 2 -TPD) measurements are conducted on both the a/b-NiMoO 4 @NF and a-NiMoO 4 @NF electrodes. It is observed from Fig.…”