Temperature and Ramp Rate-Controlled Phase Engineering of Copper Oxides in Non-Crystalline Carbon Tubes for Enhanced Alkaline Water Splitting

Trapping active metal oxides within noncrystalline carbon tubes (nCTs) offers unique advantages for electrocatalysis, including abundant dangling bonds, a high surface-to-volume ratio, excellent conductivity, and enhanced long-term durability. However, precise control of the oxide phase during heat treatment remains a key challenge. Herein, we systematically investigate the phase composition of copper oxides (Cu_<i>x</i>O, <i>x</i> = 0,1,2) confined within nCTs, depending on temperature and ramp rate, and explore their application in electrochemical alkaline water splitting. Comprehensive spectroscopic and microscopic analyses reveal that both parameters critically influence the structural and compositional integrity of the composites. Among the synthesized materials, the optimized electrode, nCT-Cu₂O/CuO, delivers superior electrocatalytic performance, achieving a current density of 10 mA/cm² at relatively low overpotentials of 168 mV for the hydrogen evolution reaction (HER) and 284 mV for the oxygen evolution reaction (OER). Favorable Tafel slopes of 73 for HER and 62 mV/dec for OER further underscore its excellent kinetics. The electrolyzer based on the as-synthesized electrodes delivers a current density of 10 mA/cm² at a cell potential as low as 1.57 V. Experimental observations, supported by density functional theory calculations for both HER and OER, attribute these outstanding properties to the combined effects of the well-dispersed CCO nanoparticles within the nCT matrix, which enhance active site density, promote charge transfer, and introduce confinement effects. This work presents a cost-effective and scalable strategy for fabricating efficient bifunctional electrodes, offering insights for advancing sustainable hydrogen production technologies.