Electrochemical Behavior of Cobalt Hydroxide Used as Additive in the Nickel Hydroxide Electrode

Abstract

As an attempt to understand better how cobalt hydroxide additives improve the nickel electrode performance, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML1" overflow="scroll"> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mo>OH</mml:mo> <mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">CoOOH</mml:mi> </mml:math> redox system has been investigated through electrochemical cycling starting from a commercial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML2" overflow="scroll"> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mo>OH</mml:mo> <mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> sample. A study of the influence of texture and morphology as well as cycling parameters was performed. For charge rates greater than C/5, relative to the amount of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML3" overflow="scroll"> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mo>OH</mml:mo> <mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> , the electrochemical oxidation was found to be a solid‐state process. This process led to a nonstoichiometric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML4" overflow="scroll"> <mml:msubsup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> <mml:msubsup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> <mml:msub> <mml:mo>OOH</mml:mo> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msub> </mml:math> phase having a mosaic texture with enhanced electronic conductivity due to the presence of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML5" overflow="scroll"> <mml:msup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:math> ions. For lower charge rates (C/100), the reaction rate is slower, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML6" overflow="scroll"> <mml:msup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:math> can dissolve in the electrolyte, leading to a less conductive phase having a stoichiometric composition (CoOOH) and a monolithic texture. When present, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML7" overflow="scroll"> <mml:msup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:math> ions are reduced to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML8" overflow="scroll"> <mml:msup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:math> , at 1.05 V while other reductions <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML9" overflow="scroll"> <mml:msup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML10" overflow="scroll"> <mml:msup> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo>→</mml:mo> <mml:mi mathvariant="normal">Co</mml:mi> <mml:mo>°</mml:mo> </mml:math> take place at a lower potential, 0.67 and 0.0 V, respectively. These two reactions are both associated with a dissolution of Co(II) species, followed by a migration of cobalt toward the current collector, with the overall result being an electrode degradation. © 2000 The Electrochemical Society. All rights reserved.