Search

CoOOH nanolayers were rst prepared by the successive ionic layer deposition (SILD) method using aqueous Co(OAc) 2 and K 2S 2O 8 solutions. The obtained nanolayers were investigated by SEM, EDX, XRD, FTIR spectroscopy and electrochemical techniques. SEM images showed that the layers formed by nanosheets of size approximately 80-100 nm which had a hexagonal crystal structure. Electrochemical study of nickel foam electrodes modi ed by CoOOH layer prepared by 50 SILD cycles demonstrates that speci c capacitance of the lm is 1520 F/g at current density 1 A/g. Repeated cycling for 1000 charge-discharge cycles demonstrates 2% capacitance fade, so such electrodes may be used as pseudocapacitor electrodes.

Cobalt oxyhydroxide thin films were grown by anodic electroprecipitation on a ITO/glass substrate from a cobalt nitrate solution. The electrochromic characteristics of the material, such as electrochromic efficiency, optical contrast, durability, optical memory, and response time are presented in this work. The total optical contrast between fully bleached and fully coloured states was 40%, and the electrochromic efficiency in the visible range was 20 cm 2 °C −1 . The film can withstand more than 10 4 coloration/bleaching cycles, without significant decrease of the optical contrast. Results indicate that this anodic electrochromic material has properties suitable for a smart window or rearview mirror application.

International audience; The stability of the Na0.6CoO2 phase, as intended to be used as a conductive additive at the positive electrode of Ni–MH batteries, was tested in electrochemical cycling conditions, in KOH alkaline electrolyte. Simple aging of Na0.6CoO2 in the electrolyte entails its spontaneous transformation into a mixture of y and B(III) cobalt oxyhydroxides. This reaction, occurring within the solid state, is based on an original mechanism, which involves cationic exchange processes and passes through an intermediate interstratified phase. Electrochemical oxidation of Na0.6CoO2 in the alkaline electrolyte, as well as chemical oxidation, leads to a y cobalt oxyhydroxide, which can be totally reduced to B(III), depending on the reduction conditions. It is important to understand such an evolution mechanism of the Na0.6CoO2 additive because it will have a significant and positive influence on the electrochemical cycling of the positive electrode in specific deep discharge or low potential storage conditions.

Decomposition of ferrate(VI) in the presence of cobalt compounds in different oxidation states is investigated in the article.It is shown that the decomposition rate of the alkaline solution of sodium ferrate increases with the introduction of Со compounds in the system in conjunction with the following number: Co2O3−CoO(OH)−СоSO4·7H2O−Co(OH)2. The greatest effect is destabilizing substances which contain Co(II). It was established experimentally that Co(II) compounds in an alkaline environment are rapidly oxidized by ferrate(VI) anion preferably to cobalt oxyhydroxide СоО(ОН). If FeO42− concentration of is significantly higher than Co(II) concentration, as it happens in the majority of cases, the reaction between them is as follows:3Со(OH)2+FeO42−+Н2О→3СоO(OH)+[Fe(OH)4]−+OH−.Therefore, Co(III) compound, instead of Co(II), may be regarded as catalysts of the ferrate decomposition. It was shown that the crystalline K2FeO4 and BaFeO4, containing impurities of cobalt as Co2O3, are more stable in comparison with solutions with similar concentration of cobalt compounds. The results are interested for the development of effective technologies for producing ferrate(VI) of various metals.

The Na0.60CoO2 phase, obtained by a classical solid-state reaction, is tested as a conductive additive in the nickel oxide electrode. Though the process was not optimised in terms of additive repartition, the experiments show a good efficiency of the Na0.60CoO2 phase even at low cobalt content, compared to usual additives like CoO. Moreover, it increases the stability of the electrode at low potential. The added Na0.60CoO2 phase is shown to transform, during the first cycles, into a γ-type cobalt oxyhydroxide phase that is more stable at low potential than the usual additives.

Water splitting offers the opportunity for storing solar energy and, thus, producing carbon-neutral and renewable solar fuels. The process, known as artificial photosynthesis, is limited by the electrocatalytic conversion of water into molecular oxygen. Electrocatalysts are essential for reducing energy losses and improving the efficiency of the oxygen evolution reaction. This thesis describes the catalytic properties of first-row transition metal oxides for the OER and provides new insights into the mechanism of the reaction. Decades of research have yielded various transition metal oxides as efficient electrocatalysts for the OER, yet their stability is not well established. Here, a combination of techniques was employed to investigate the stability of unary, and mixed metal oxides; electrochemical activity measurements, electrochemical quartz crystal microbalance, inductively coupled plasma optical emission spectrometry and electrochemical impedance spectroscopy. The change of mass, composition, and activity metrics over time resolved the stability profiles of CoOx, CoFeOx, CoFeNiOx, NiOx, and NiFeOx. According to ICP-OES, the composition of all the five catalysts changed over time, while the CoOx and CoFeOx exhibited the most significant mass loss during an initial period of electrolysis. During a dynamic exchange of metal ions between the catalyst and the electrolyte, cobalt ions dissolve into the electrolyte, and iron from the electrolytic solution incorporates into the catalytic film. Once this dynamic exchange reaches equilibrium, the catalysts are stable. The mechanism of the OER mediated by amorphous cobalt oxyhydroxide was investigated by in situ spectroscopic and electrokinetic experiments. In situ surface-enhanced Raman and X-ray absorption spectroscopy supported a Co(IV)-O species at potentials before the onset of OER. The gradual transformation of the catalyst into a CoO2 concurred with the appearance of a superoxidic species. In situ SERS coupled with 18O and H/D isotope experiments provided evidence for the participation of the superoxidic species in the oxygen evolution reaction. The catalyst exhibits a Tafel slope of 60 mV/dec, a first-order dependence of the catalytic activity, and a proton-coupled electron-transfer pre-equilibrium. These findings propose a new OER mechanism, in which the rate-determining step is the release of oxygen from a Co-superoxo intermediate. Iron incorporation into the cobalt lattice enhances the catalytic activity greatly. Even though there are several studies on the OER mechanism in Co-Fe oxides, many vital questions on the role of iron, the true nature of active sites, and the oxidation states of Co and Fe remain. In situ SERS and XAS indicated the presence of Co(IV)-O species and a Fe valency higher than three at potentials prior to the oxygen evolution. An O-O- band, attributed to the superoxidic species, emerged at potentials lower than the onset of OER. In situ SERS coupled with 18O isotope experiments confirmed that both the lattice oxygen and superoxidic species participate in the oxygen evolution. The Tafel slope is about 37 mV/dec indicating that both the pre-equilibrium and rate-determining step involve a single-electron transfer. These findings support that neither the Co(III)/Co(IV) oxidation nor the O-O bond formation are controlling the rate of the reaction.