The Electrochemical Behavior of Positive Cobalt/Aluminum and Cobalt/Iron Hydroxide Electrodes

Abstract

Doping ~-Co(OH)2 with M 3 § (A13 § Fe 3 § ions by coprecipitation of the nitrate or sulfate salts at pH = 12.5 leads to compounds in which the usual electrochemical inertness of CoOOH is overcome. Products of molar ratio Co:M = 4:1 have the highest charge capacity. They crystallize in the double layer structure of the pyroaurite type, consisting of Co/M hydroxide cations [Co4M(OH)~0]-, M = A1, Fe and anion interlayers [X-, nH20]-X = NO3-, SO42-. The pyroaurite structure favors the formation of various hydrogen bonds. These hydrogen bonds facilitate both the proton availability, which increases as the anion is changed from NO3-to SO42-(as indicated by the increase of capacity and stability of cycles), and also the electrochemical oxidation of cobalt, leading to a maximal oxidation state of + 3.7. The iron compounds are not pure; in addition to the pyroaurite phase, they contain some ~-Co(OH)2, which lowers the capacity.In contrast to the nickel hydroxide electrode (1), there have been very few experimental investigations of the cobalt hydroxide electrode; this is probably a consequence of its irreversibility when acting as a positive electrode. The product formed on charging is CoOOH, which is nearly inert electrochemically and thus practically unable to charge and discharge. Oxidation states of cobalt exceeding +3 have been discussed in earlier thin layer studies of the cobalt hydroxide electrode (2, 3). The discharge of such products appeared possible but further studies were not reported.Partial substitution of Co s § ions by A13 § ions [up to -40 mole percent (m/o) A1] into the layer lattice of ~-Co(OH)2 by coprecipitation of the nitrate salts leads to various products. In one of these cobalt could be electrochemically oxidized in alkaline solution to an oxidation state of +3.7, as we described recently (4). The species with the ratio Co:A1 = 4:1 in Co~l(OH)10NO3 has striking electrochemical properties: of all investigated products it has maximal discharge capacity and can be used as a reversible positive electrode (4). We have continued and extended our studies in this field and prepared, corresponding to the above mentioned products, Co/A1 samples of the ratio Co:A1 = 4:1 from their sulfate salts and Co/Fe species of the ratio 4:1 from their nitrate and sulfate salts. ExperimentalThe compounds were prepared according to former reports (4, 5, 7, 8) by computer-controlled precipitation of cobalt(II)-and aluminum(III)-or iron(III)-salt solutions with potassium hydroxide solutions. Corresponding to former investigations in this field (5), the precipitations were carried out at, or as near as possible to, the isoelectric point of solid (IEPS). Parks (6), on the basis of an empirical equation, calculated the IEPS of Co(OH)2 at pH = 11.2. For Co/Fe precipitates we preferred pH = 12.5; and for Co/A1 precipitates pH = 9.0, bearing in mind the solubility of aluminum hydroxide in more highly concentrated potassium hydroxide solutions. Furthermore, the metal salt solutions had to be acidified to pH=<2, to exclude hydrox...

Fabrication of morphology and crystal structure controlled nanorod and nanosheet cobalt hydroxide based on the difference of oxygen-solubility between water and methanol, and conversion into Co3O4

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Surfactant intercalated α-hydroxides of cobalt and nickel and their delamination-restacking behavior in organic media

“…Divalent hydroxides of the formula M(OH) 2 [M = Ni, Co, Fe] are isostructural with the mineral brucite, Mg(OH) 2 , and consist of hexagonal packings of hydroxyl ions with M 2+ ions occupying alternate rows of octahedral sites [16]. The hydroxides of Co(II) and Ni(II) are extensively studied as they are widely employed as electrode materials in alkaline secondary batteries [17,18]. Studies on the delamination behavior of these solids will form the basis of developing novel composites with enhanced electrochemical properties.…”

Formation of cobalt hydroxide single-crystal platelets from pulsed laser deposited cobalt thin film

“…Cobalt hydroxides prefer to grow into platelets due to their intrinsic lamellar structures, thus organic groups can be intercalated into the interlayer region of cobalt hydroxides to establish the long range magnetic ordering which may form the organic magnetic materials [2]. As an additive, cobalt hydroxides can be used to enhance the electrochemical activity of alkaline secondary batteries [3]. More interestingly, it has been reported that cobalt hydroxide films show electrochromic properties with reversible capability [4], the film also shows catalytic reaction in oxygen production [5].…”