No abstract

No abstract

Ni and mixed Ni-Co oxide films were formed at Pt substrates by the sol-gel technique and studied electrochemically in 1 M NaOH solutions. All sol-gel films under study have to be found amorphous. The charge densities of these films are quite high compared to oxides formed by the anodic oxidation of metallic substrates. Mixed Ni-Co oxide films display higher charge densities, broader CV peaks, and negatively shifted equilibrium potentials vs. pure Ni oxide films. At slow sweep rates, all films studied showed kinetically reversible surface electrochemical behavior. At higher sweep rates, the redox reactions became kinetically irreversible, being either surface reactions with no diffusion limitations or controlled by the diffusion rate of species inside the film. The effect of the oxide film formation conditions, i.e., the withdrawal rate of the substrate from the sol solution and the firing temperature, on the electrochemical efficiency of the oxide films was also studied. InfroductionNickel oxide/hydroxide and mixed Ni-Co oxide films have been extensively studied in the past, primarily due to their numerous promising applications. These include their use as battery electrodes,' both in aqueous and nonaqueous media, with much work having been focused on the addition of Co to Ni oxides in order to improve their performance. The electrochromic oxidation/reduction in both cobalt and nickel oxide/hydroxide films, which occurs with the injection and expulsion of mobile ions,2 has facilitated their applications in "smart windows,"3 display panels,4 and rearview mirrors.4 Ni-Co-based alloys also appear to be very promising materials for water electrolysis because of their good electrocatalytic properties and reported stability for the oxygen'' and hydrogen9-" evolution reactions. Cobalt based films are also widely used in recording media.'2The most important electrochemical process for Ni oxide electrodes, which occurs in alkaline solutions, can be described asA similar reaction can be written for cobalt oxide electrodes" and 3Co11 (OH)2 + 2 OH-# Co"111O4 + 4H2O + 2e [21 Co"(OH)2 + OW a Co'1100H + H2O + e Co"OOH + OW # Co"TO, + H2O + e [3] [4]However, it is a well-established fact that these redox reactions are more complex than indicated here''4 and that they involve the intercalation/deintercalation of cations in basic solutions1' in which the oxides are stable, i.e., in simplified form'6Ni" (or Co")oxide + 1.5 OW + 0.5Na s Ni"(or CoIv) oxide (1.5 OH)(0.5 Nat) + e [5] Nickel and cobalt oxide-hydroxide films can be prepared by the anodic oxidation of the metallic substrate," cathodic precipitation,' electron-beam * Electrochemical Society Student Member. * * Electrochemical Society Active Member. electrodeposition,"2 etc. Chemical deposition," electrolytic deposition," spray pyrolysis," etc, are utilized to make mixed nickel-cobalt oxide films, which can also be

There has been much interest in the practical applications of Ir oxide films in supercapacitive, 1-3 electrochromic, 4-6 and energy storage devices, 6 as interneural stimulating electrodes, 7,8 electrocatalysts, [9][10][11][12][13] and pH electrodes. 14-16 In many cases, Ir oxide films have been formed electrochemically, i.e., by cycling or pulsing the potential of an Ir metal electrode between critical limits in various aqueous solutions. [4][5][6][17][18][19][20][21][22][23][24][25][26][27] This can result in the formation of relatively thick films (up to several micrometers in thickness), which are hydrous in nature (coded as activated iridium oxide films (AIROFs) in the prior literature 2,15,28-30 ) and exhibit very rapid oxidation/ reduction kinetics when the films are switched between their conducting Ir(IV) and insulating Ir(III) states. However, this method of film growth is both time and energy intensive, requires a metallic Ir substrate (often not fully reacted), and also usually involves some simultaneous loss, through dissolution, of the Ir metal substrate.Other methods 2,15,28,29 which have been used to form Ir oxide films have involved the thermal oxidation of iridium salts, or the sputtering of Ir onto a conductive substrate in an oxidizing plasma environment. Sputtered iridium oxide films (SIROFs) are known to be less hydrous in nature, have a more featureless cyclic voltammetric (CV) response, and yield lower maximum charge densities than do AIROFs. 2,15,28,29 Ir oxide films have also been formed by induction heating 31 and by electrodeposition at constant anodic potentials from alkaline Ir-containing solutions, 32 although insufficient data are available for the properties of these films to be assessed relative to AIROFs and SIROFs.The present paper is a continuation of our previous work, 33 involving the use of the sol-gel (SG) technique for the preparation of Ir oxide films. The SG method has been employed before to prepare IrO 2 34 and mixed oxides such as IrO 2 -Ta 2 O 5 , 34 IrO2-SnO 2 , 35 and RuO 2 -IrO 2 , 36 in powdered form. IrO 2 coatings were also obtained by painting 37,38 and dip-coating 38 techniques, using substrates such as amorphous silica microbeds 37 and glass, 38 followed by drying at temperatures between 350 and 600ЊC. However, only compositional/structural studies of these materials were performed, 34-38 and no prior electrochemical characterization has been reported.In our previous paper, 33 it was shown that the Ir-containing films formed by the SG method actually consist of Ir metal nanoparticles. These can be transformed easily into the oxide by potential cycling within the limits normally used for the electrochemical growth of IrOx films at bulk Ir electrodes. The resulting films, which were not studied in detail previously by us, 33 were shown to exhibit the general properties of AIROFs. In the present work, SG-formed Ir oxide films are demonstrated to be electrochromic, to yield high charge densities, as well as to exhibit excellent (Ir(III)/(IV)) oxidation/ reducti...

Two different types of Co oxide films, each having a distinct electrochemical signature correlated with the film drying temperature, were formed using the sol-gel (SG) technique. Two different states of gelation of the film precursor were also explored. Cyclic voltammograms, collected in alkaline solutions for the low temperature films, displayed two pairs of peaks corresponding to the Co(ll) to Co(lll) and Co(lll) to Co(IV) transitions, centered at 1.2 and 1.4 V, respectively, while the high temperature films underwent only the Co(lll) to Co(IV) redox process at 1.4 V. The charge densities obtained for the lower temperature films (particle sizes 2-10 nm in diameter) ranged between 40 and 70 mC/cm 2 ; charge densities for the higher temperature films (particle sizes of 5-40 nm), otherwise formed identically, were between 10 and 20 mC/cm 2. The more viscous Co oxide gels led to significantly higher charge densities than less viscous gels, as well as greater film stability during electrochemical cycling. Using a wide range of film characterization techniques, it was shown that Co oxide films formed at >180°C are composed mainly of C03O4 spinel, while films formed at <180°C consist predominantly of CoO.