# Nanosheets of CuCo\({}_{2}\)O\({}_{4}\) As a High-Performance Electrocatalyst in Urea Oxidation
## 1 Introduction  
In recent years, the utilization of urea as an alternative fuel in fuel cells has grown significantly, since it is highly available, stable, and nonflammable [1]. Fuel cells are devices that convert chemical energy into electrical energy, playing a vital role in overcoming the world energy crisis [2; 3]. Urea is an abundant waste generated in agricultural land, and it is present in industrial and human wastewater as well. Urea is produced from natural gas or can be synthesized from ammonia. When in contact with atmosphere and groundwater, urea breaks down into ammonia and nitrate, causing health and environmental problems [4; 5; 6]. A solution to eliminate these problems caused by urea, and toalso generate clean energy, is the urea oxidation reaction (UOR) [7]. This reaction is described as follows [4; 5]:  
\[\mathrm{CO(NH_{2})_{2}+H_{2}O\to N_{2}+3H_{2}+CO_{2}} \tag{1}\]  
Thermodynamically, water splitting occurs at 1.23 V to generate hydrogen, while urea oxidation could provide hydrogen at a lower potential of 0.37 V [4; 8]. This means that urea oxidation provides cheaper hydrogen production than water electrolysis. In previous reports, it is common to see the intense utilization of nickel-based catalysts to oxidize urea efficiently at low overpotential [3; 9; 10]. [PERSON] et al. reported a study on the use of LaNiO\({}_{3}\) as a catalyst for the electrooxidation of urea in alkaline conditions [4]. The nonstoichiometric spinel structure Ni\({}_{1.5}\)Mn\({}_{1.5}\)O\({}_{4}\) synthesized by [PERSON] et al., using a hydrothermal synthesis process, showed excellent electrochemical properties with the highest activity toward electro-oxidation of urea in alkaline solutions [1]. The development of porous Ni\({}_{3}\)N nanosheet arrays on carbon cloth studied by [PERSON] et al. also showed high performance and was a durable electrocatalyst for urea oxidation [11]. A possibility to use the same principles, but with a different component, led us to develop and produce copper-based catalysts that can oxidize urea efficiently to improve the properties of fuel cells. Electrode materials made of transition metal oxides are often used as electrocatalysts because of their high electrochemical performance, natural abundance, and low cost. In addition, copper and cobalt salts are considered a promising electrode material for energy generation and storage devices due to its nontoxicity and high electrical conductivity. [PERSON] et al. have shown that cobalt-based compounds are excellent for hydrogen adsorption [12]. Mesostructured CuCo\({}_{2}\)S\({}_{4}\)/CuCo\({}_{2}\)O\({}_{4}\) nanoflowers were synthesized by [PERSON] et al. and showed an excellent electrochemical performance for energy applications [13]. An asymmetric supercapacitor based on CuCo\({}_{2}\)S\({}_{4}\)/CuCo\({}_{2}\)O\({}_{4}\) as the cathode and graphene aerogel as the anode delivered a high energy density of 33.2 Wh/kg and power density of 13.3 kW/kg. [PERSON] et al. used NiMoO\({}_{4}\) nanosheets to study the performance in the urea oxidation reaction. They found a larger number of exposed active sites, faster electron transport, and lower adsorption energy of urea molecules. Comparing the cyclic voltammetry (CV) curve recorded in KOH solution, the anodic current density of NiMoO\({}_{4}\) increased largely after adding urea, demonstrating its high catalytic response activity for electro-oxidation of urea [5]. As reported by [PERSON] et al., a nickel hydroxide electrode with nanocup architecture offered better electrocatalytic performance during electrolysis of urea [14]. Nickel oxide\(-\)nickel hybrid nanoarrays on nickel foam (NiO\(-\)Ni/NF) was studied by [PERSON] et al. for water splitting and urea oxidation [9]. The electrode showed excellent activity for urea oxidation with the need of a low potential in 1.0 M KOH with 0.33 M urea. [PERSON] et al.