Electrochemical Cl₂ generation has attracted lots of interest due to the tremendous applications of Cl₂. Basically, the electrochemical water splitting requires two half-cell reactions that include O₂ or Cl₂ evolution as an anodic reaction and H₂ or ORR as a cathodic reaction depending upon the electrolyte and pH conditions. The water splitting to generate O₂ and H₂ are promising strategies for storing electric energy as fuel cells and rechargeable metal-air batteries [1]. Cl₂ production has technological application at the industrial level [2]. Kinetically Cl₂ evolution is more convenient and favors at pH ˂ 4 by adjusting the chloride ion concentration [3]. Thermodynamically, the O₂ evolution is favorable as a parasitic side reaction in neutral and higher pH electrolyte [4]. To address this issue, the electrocatalyst plays a pivotal role to reduce the excess energy requirement and efficient electrochemical water treatment.Herein, an attempt has been made to synthesize the anisotropically aligned particles using cobalt oxalate as a precursor to form hcp-Co rods and investigated their electrochemical properties for O₂ and Cl₂ evolution. The synthesis of cobalt oxalate precursor was performed by using the hydrothermal method at 120 °C. By controlling the thermodynamic and kinetic parameters during thermal decomposition of cobalt oxalate rods in air leads to the change in morphology of cobalt-based nanostructures. Further, Co₃O₄ was reduced in H₂ that leads to the formation of hcp-Co. The slow heating rate favors anisotropic particle growth for Co₃O₄ and hcp-Co which assembled to form rods. However, the fast-heating rate leads to the formation of agglomerated nanoparticles. The kinetically controlled synthesis considerably affecting the morphology of the cobalt-based nanostructures. Anisotropically aligned Co₃O₄ and hcp-Co rods show overpotential corresponds to 358 mV, 240 mV for O₂ evolution in basic media and 120 mV, 26 mV for Cl₂ evolution in acidic media. In comparison to agglomerated Co₃O₄ and hcp-Co nanoparticles show overpotential 394 mV, 352 mV for O₂ and 260 mV, 110 mV for Cl₂ evolution. These findings elaborated that anisotropic particle growth to form rods act as a promising electrocatalyst with better efficiency towards O₂ and Cl₂ generation than agglomerated nanoparticles. Further, wastewater treatment was done by using in situ generated chlorine and the methyl orange was degraded with 99 % degradation efficiency.

References:

[1] X. Lu, C. Zhao, Highly efficient and robust oxygen evolution catalysts achieved by anchoring nanocrystalline cobalt oxides onto mildly oxidized multiwalled carbon nanotubes, J. Mater. Chem. A. 1 (2013) 12053–12059.

[2] R.K.B. Karlsson, A. Cornell, Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes, Chem. Rev. 116 (2016) 2982–3028.

[3] J.G. Vos, T.A. Wezendonk, A.W. Jeremiasse, M.T.M. Koper, MnOx/IrOx as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution, J. Am. Chem. Soc. 140 (2018) 10270–10281.

[4] M. Jiang, H. Wang, Y. Li, H. Zhang, G. Zhang, Z. Lu, X. Sun, L. Jiang, Superaerophobic RuO2-Based Nanostructured Electrode for High-Performance Chlorine Evolution Reaction, Small. 13 (2017) 1–8.