Building P-Poor Ni2P and P-Rich CoP3 Heterojunction Structure with Cation Vacancy for Enhanced Electrocatalytic Hydrazine and Urea Oxidation

Abstract

Handling hydrazine/urea wastewater through electrochemical oxidation technology (HzOR/UOR) holds significant importance for sewage disposal and nitrogen recycling, as the presence of hydrazine/urea leads to severe environmental issues. On the other hand, hydrazine/urea could potentially serve as a new type of fuel. However, at present, this remains a considerable challenge. The development of a low-cost, highly efficient, and stable electrocatalyst stands as a prerequisite for achieving this goal. In this study, a novel Ni₂P/CoP₃-Zn_vac bimetallic phosphide catalyst is designed and constructed using a hydrothermal-alkali etching-phosphating three-step method. This catalyst integrates P-rich CoP₃, P-poor metallic Ni₂P, and abundant Zn²⁺ cation vacancies into a single structure for HzOR/UOR. Copious P in CoP₃ provides a wealth of negative electrons, which aids in the adsorption of positive reactive intermediates. Meanwhile, P-poor metallic Ni₂P exhibits excellent electrical conductivity, ensuring rapid reaction dynamics. Both physical and electrochemical experiments confirm the successful creation of the Ni₂P/CoP₃-Zn_vac heterojunction, along with the distinctive electron structure of Ni₂P and CoP₃. Electron paramagnetic resonance (EPR) results validate the presence of cation vacancies, which significantly enhance the density of active sites. Consequently, this innovative Ni₂P/CoP₃-Zn_vac heterojunction catalyst displays remarkable electrocatalytic activity, achieving a potential of −47 mV/1.311 V to attain 10 mA·cm⁻² for HzOR and UOR, respectively. The Tafel slopes of 54.3 and 37.24 mV·dec⁻¹ for HzOR and UOR are significantly smaller than those of single-phased Ni₂P and CoP₃, as well as the two-phased phosphide without alkali etching. Building upon the excellent HzOR/UOR performance of the Ni₂P/CoP₃-Zn_vac heterojunction, a two-electrode cell for direct hydrazine fuel cells (DHzFC) and direct urea-hydrogen peroxide fuel cells (DUHPFC) is assembled with a Ni₂P/CoP₃-Zn_vac anode. This configuration demonstrates a maximum power density of 229.01 mW·cm⁻² for DHzFC and 16.22 mW·cm⁻² for DUHPFC. Moreover, both DHzFC and DUHPFC exhibit exceptional stability for up to 24 h. A homemade aqueous Zn-Hz battery, equipped with a Ni₂P/CoP₃-Zn_vac cathode, further demonstrates its practicality for energy conversion. This work underscores a promising avenue for developing cost-effective and highly stable solutions for UOR and HzOR.

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