The Synergistic Influences of OH[sup −] Concentration and Electrolyte Conductivity on the Redox Behavior of Ni(OH)[sub 2]/NiOOH

Abstract

The synergistic influences of the OH − concentration and electrolyte conductivity on the redox behavior of NiOOH/Ni͑OH͒ 2 for nickel oxide-coated graphite electrodes are clearly demonstrated by voltammetric and impedance analyses. The increase in the OH − concentration and electrolyte conductivity effectively promote the utilization of active nickel species and the electrochemical reversibility of NiOOH/Ni͑OH͒ 2 , indicating the simultaneous involvement of OH − and cations in the redox transition. The upper limit for utilizing Ni oxyhydroxides is mainly determined by the OH − concentration, which is facilely reached by increasing the electrolyte conductivity ͑adding Na 2 SO 4 ͒. The synergistic phenomena could be very important in the applications of Ni oxidebased batteries, supercapacitors, sensors, electrochromic devices, and organic synthesis.Nickel-based oxyhydroxides in various structures show distinctive electrochemical properties, which are widely studied for several applications, e.g., nickel-based batteries, 1,2 supercapacitors, 3 electrocatalysis in organic synthesis, 4 sensors, 4-6 and electrochromic devices. 7 To achieve a higher utilization of electroactive species, Ni-based oxyhydroxides were widely investigated in concentrated alkaline media ͑generally Ն1 M MOH, M: Li, Na, K͒. 8,9 A model, the so-called Bode diagram, 10 was proposed to clarify the redox transitions among ␣-Ni͑OH͒ 2 , ␤-Ni͑OH͒ 2 , ␤-NiOOH, and ␥-NiOOH. The exact oxidation states of these well-defined oxyhydroxides are unclear, although their structures seem to be clarified. Actually, certain nonstoichiometric intermediates between these well-defined structures were proposed, 4,11,12 indicating the complicated structures of various nickel oxyhydroxides.It is well known that an increase in the OH − concentration can effectively promote the utilization of electrochemically active nickel species ͑estimated from voltammetric charges͒ and electrochemical reversibility of NiOOH/Ni͑OH͒ 2 ͑from the peak potential difference͒, 8,9 suggesting that this complicated redox reaction involves the exchange of OH − . This action, however, results in an increase in the electrolyte conductivity, probably favoring the redox transition due to an increase in the cation concentration. 13,14 Recently, the redox transitions among various oxyhydroxides on Ni͑OH͒ 2 ultrathin films were systematically investigated by means of the electrochemical quartz microbalance ͑EQCM͒. 13-18 These studies proposed several redox mechanisms involving the exchange of cations, differing in whether H + or OH − is transferred. 18 However, contrary results were usually found when the mass of nickel oxyhydroxides was varied, 14,16,17 probably due to the influences of electronic conductivity of oxyhydroxides and/or the diffusion issues of ionic species involved in the redox transitions. Due to the much higher loading of Ni͑OH͒ 2 for sensors, batteries, supercapacitors, and electrochromic devices than that studied in EQCM, the above issues have to be carefully considered in these app...

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