Co3O4 Nanosheets Deposited By the Sacrificial Anode Method and Their Pseudocapacitive Behavior

Spinel cobalt oxide (Co₃O₄), an attractive material possessing great electrochemical properties, has been prepared by various methods such as solvothermal method, chemical bath deposition, electrodeposition, thermolysis and plasma-assisted deposition. However, these methods require either complicated process, high temperature treatment or external power supply to prepare Co₃O₄. To overcome these problems, the sacrificial anode method, which belongs to an electrochemical deposition, is proposed in this study to prepare cobalt hydroxide thin film on a carbon cloth substrate. The obtained thin film is completely converted to Co₃O₄ by a low-temperature ozone treatment. In this facile, self-powered process, an aluminum foil, serving as the sacrificial anode, was immersed into an electrolyte containing Co ions and the donated electrons would flow along the external circuit to the cathode substrate, i.e., the carbon cloth; the deposition of cobalt hydroxide started spontaneously due to the potential difference (as shown in scheme 1). Firstly, a nanosheets structure was observed on the carbon cloth substrate by scanning electron microscope, and the composition of this film was confirmed to be α-Co(OH)₂ and Co₃O₄ before and after the ozone treatment, respectively. By comparing the standard potentials of the possible reactions as well as recording the pH value of the electrolyte and other experimental observations during the deposition, the reaction on the cathode was confirmed to be: NO₃^- + 7H₂O + 8e⁻ ⇔ NH₄⁺ + 10OH^-. By appling X-ray photoelectron spectroscopy, the atomic ratio of N to Co element in the obtained thin film can be calculated, and the chemical formula of the obtained α-Co(OH)₂ film can be further confirmed to be Co(OH)_1.85(NO₃)_0.15. After confirming the reaction occurred on the cathode and the chemical formula of the obtained thin film, the expected mass loading of the film can be calculated from the accumulated charge consumed during the deposition. Moreover, an accurate mass calibration curve was constructed by using the UV-Vis absorption data to precisely estimate the actual mass loading of the film. Comparing the actual and ideal mass loading, the Faraday efficiency of the deposition was finally calculated to be 94% when the accumulated charge for the deposition is 2.0 C. In this process, the rate of the deposition can be easily controlled by tuning the concentration of the electrolyte and the surface area ratio of the anode to cathode. The effects of these two factors on the rate of the deposition and the capacitive performance of the obtained films were also investigated. Finally, a high specific capacitance of 346 F/g was achieved. The proposed method offers advantageous in terms of large-area deposition and scale-up possibility for obtaining Co₃O₄ thin films.