Abundant oxygen vacancies assisting NaPO3 supported by porous carbon derived from cotton pulp black liquor as a transition metal-free electrocatalyst for efficiently enhancing the urea oxidation reaction

Abstract

To improve green hydrogen production and to significantly treat urea-rich wastewater, exploring transition-metal-free electrocatalysts with high performance for the urea oxidation reaction (UOR) is significant but challenging. Herein, the acid precipitation method is used to extract biomass from cotton pulp black liquor (CPBL) using different H₃PO₄ treatments to regulate the pH values (pH = 1, 7 and 11). The as-obtained biomass containing sodium salts is employed to prepare the electrocatalysts after high-temperature treatment, and they are named PCMA-x (x: pH = 1, 7 and 11). The physical characterizations confirm that the electrocatalysts are composed of porous carbon and NaPO₃, and typically, PCMA-7 has the highest oxygen vacancies. Experimental results confirm that PCMA-7 delivers the highest electrocatalytic UOR activity with the lowest overpotential of 132 mV and the lowest Tafel slope of 33.8 mV dec⁻¹. Based on physical characterizations, experimental results, and DFT calculations, it is clearly found that PCMA-7 exhibits a superior catalytic UOR efficiency because the porous structure provides a channel for ion transport/electrolyte transmission, abundant oxygen vacancies strengthen intermediate adsorption/activation, and NaPO₃ accelerates the reaction of key intermediates.