Tailoring crystal planes in Co3O4 for enhanced chlorine evolution reaction electrocatalysis
As a crucial chemical,chlorine(Cl2)boasts an annual output exceeding 30 million tons in China,finding extensive application in the production of plastic monomers such as polyvinyl chloride as well as in water treatment,bleaching,and fine chemical synthesis.The primary method for producing chlorine involves the electrolysis of saturated brine using ion exchange membrane electrolyzers within the chlor-alkali industry,where electrochemical chlorine evolution(CER)occurs at the anode.The dimensionally stable anode(DSA),composed of noble metal oxides,including Ru and Ir,has been the predominant commercial CER electrode since its development in the mid-1960s owing to its outstanding CER activity/selectivity and high stability in acidic electrolytes.Nevertheless,the scarcity and high cost of precious metals such as Ru and Ir necessitate the development of high-performance,non-precious metal-based CER electrocatalysts.Despite this,research on non-noble metal-based CER catalysts remains limited,primarily because balancing the high activity of CER with the stability required for the high oxidation potential of acidic electrolytes proves challenging.Recent advancements have shown that cobalt-based oxides exhibit exceptional acidic oxygen evolution reaction(OER)activity at the anode during water electrolysis in proton exchange membranes.Given that chlorine and oxygen evolution reactions partially share an active site—specifically,the metal-oxygen bond serves as an adsorption intermediate for both reactions—it is theorized that catalysts effective for oxygen evolution may also facilitate chlorine evolution.Consequently,we postulate that the Co-based material system can serve as an effective CER catalyst.In this study,we use a strategy to enhance the CER activity and stability of spinel-structured Co3O4 by manipulating its crystal planes.In particular,exposure of the{112} crystal planes is enhanced through low-temperature induction of α-Co(OH)2 monocrystalline nanosheets.Based on XPS and other test results,the {112} crystal plane exposes a higher density of octahedral sites(Cooct)compared to the more stable {111} plane.The charge from the oxygen anion is more readily transferred to the metal cation at the octahedral position,leading to a strong orbital overlap interaction between the oxygen anion and the octahedral cation.This results in a robust charge transfer capability at the Cooct site,thereby enhancing catalytic activity.When tested with 4 mol L-1NaCl(pH 2)as the electrolyte,the CER overpotential of Co3O4 exposed to the {112} crystal plane at a current density of 100 mA cm-2is only 170 mV,considerably lower than the 330 mV of Co3O4 on the {111} plane and better than the 320 mV of commercial RuO2.In addition,it features the lowest Tafel slope and a Faraday efficiency approaching 100％.Following a constant current test at 10 mA cm-2 for 17 h,the cobalt dissolution was minimal at only 8 ppb(1 ppb=1 μg L-1),and there was no significant voltage increase during an 8-h galvanostatic test at a current density of 100 mA cm-2,indicating excellent stability of the catalyst.Density functional theory(DFT)calculations further demonstrated that the {112} surface possessed superior adsorption energy for Cl-.This study provides a new approach for the rational design of efficient Co-based non-precious-metal CER catalysts by leveraging specific crystal planes.
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