Potassium hydroxide (KOH), potassium persulfate (K2S2O8), nickel sulfate hexahydrate (NiSO4·6H2O), copper nitrate gerhardite (Cu(NO3)2·3H2O), urea (CO(NH2)2), and ammonium hydroxide (NH3·H2O) were purchased from Chongqing Chuandong Co., Ltd. (Chongqing, China). The carbon cloth (type: HCP331N) was obtained from Shanghai Hesen Co., Ltd. (Shanghai, China). All the chemicals were used without further purification.
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