A nanocomposite electrode includes a porous copper substrate, Co3O4 and/or Cu/Co(OH)x nanoparticles electrolytically deposited on the porous copper substrate. Fabricating the nanocomposite electrode includes contacting the porous copper substrate with a solution comprising cobalt, and electrodepositing the cobalt on the porous copper substrate to yield the nanocomposite electrode. Reducing nitrate to ammonia includes contacting the nanocomposite electrode with an aqueous solution comprising nitrate, and electrocatalytically reducing the nitrate to yield ammonia.