(220) facet exposed layered Co3O4: A bifunctional catalyst for electrochemical urea oxidation, and rapid 4-nitrophenol reduction to 4-aminophenol

Releasing urea-enriched wastewater from urea industries and 4-nitrophenol (4-NP) threatens human health and aquatic environments. The electrochemical urea oxidation reaction (UOR) offers a dual benefit: reducing wastewater pollution and producing green hydrogen via wastewater splitting. Meanwhile, 4-NP reduction to 4-aminophenol (4-AP) provides an added advantage, as 4-AP is widely used in pharmaceutical and food industries. In this study, layered Co₃O₄ nanosheets with exposed (220) facets were synthesized via a one-pot hydrothermal post-air-annealing method using cobalt acetate and urea without additives. The air-annealing temperature was critical, with the sample annealed at 500 °C (H₁₂₀-500) exhibiting optimized nanosheets with exposed (220) facets, enhancing diffusion and electron transfer during UOR. H₁₂₀-500 demonstrated superior performance, achieving a high turnover frequency (TOF) of 0.1493 s⁻¹, a current density of 250 mA cm⁻², stability over 72 h, a Tafel slope of 22.4 mV·dec⁻¹, and requiring only 1.37 V vs RHE to reach 10 mA cm⁻², surpassing state-of-the-art cobalt-based UOR electrocatalysts. Additionally, H₁₂₀-500 achieved 99.64 % efficiency in reducing 4-NP to 4-AP within 2 min, with a pseudo-first-order rate constant (k) of 2.63 × 10⁻² s⁻¹ and durability over seven cycles. This approach offers a sustainable pathway for environmental remediation and green hydrogen production.