Green fabrication of nanostructured Ni(OH)2/Ni/Carbon felt electrodes with water-containing deep eutectic solvent for enhanced water electrolysis performance

Abstract

To produce green hydrogen, the development of high-performance, environmentally friendly, and cost-effective electrocatalysts for efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is imperative. This study explores the potential application of water-containing deep eutectic solvents (DESs) in the synthesis of nanostructured electrocatalysts for water electrolysis. Ni(OH)2/Ni/carbon felt (CF) composite electrodes are developed via one-step electrodeposition in a water-containing choline chloride/urea DES as a green reaction medium. Heterostructured Ni(OH)2/Ni nanosheets are formed and uniformly deposited on the CF substrate with the addition of 10 wt% water to DES (CU10W), which increases active sites for HER and OER. The initial confinement of water molecules by the DES is found to be crucial in forming the hierarchical structure of the electrocatalysts. Notably, the CU10W electrode exhibits excellent bifunctional activity toward HER and OER, with reduced overpotentials of 192 and 564 mV, respectively, at 100 mA cm−2. This renders the CU10W electrode-based electrolysis system stable with a reduced cell voltage of 1.69 V and a high Faradaic efficiency of over 99% for the HER. The augmented electrochemical active surface area and reduced charge transfer resistance of the CU10W electrode account for the improved HER/OER performance associated with the special hierarchical Ni(OH)2/Ni nanostructure.