Surface modified tatanium anodes for electroplating of manganese dioxide

English: The valve metal characteristic of Ti provides, together with a number of other physical properties, the appropriate anode material for the production of battery grade electrolytic MnO2 (EMD). However, electro-oxidation of Mn(II) to Mn(IV) in an acidic medium creates conditions for anodic oxidation of the surface of a Ti metal electrode. The ohmic potential drop in an interelectrode gap, ∆VΩ, is inter alia a function of the extent of anodic oxidation on an electrode’s surface. The magnitude of electrical currents in large-scale electrolysis cells compels attention to ∆VΩ since it equates to power consumption that has large economic incentives. Electrocatalytic transition metal oxide electrodes, termed dimensionally stable anodes (DSA’s), not only provide high electrical conductivity, but it also has the added advantage of lowering electrode polarization via catalytic considerations. DSA’s have numerous other applications, but it is last mentioned properties that draw attention to these electrodes as substrates for MnO2 electroformation. The basic construction of a DSA is a mixture of a transition metal oxide (more than often a Pt group metal) and a valve metal oxide that adheres to a valve metal substrate. The primary objective of preparing and evaluating DSA’s using different precursor solutions in a thermal decomposition technique is combined with an electrosynthesis method to grow RuOx.nH2O films. Interpretation of X-ray diffraction (XRD) measurements demonstrated the ability of a higher calcination temperature to form larger quantities of RuO2 on Ti. It was also conclusive that an increase in the Ru metal concentration in a precursor solution results in a measurable increase in intensity of the (1 1 0) reflection from RuO2. Electrolysis experiments in conjunction with Auger electron spectroscopy (AES) depth profiling and AES surface spectra vividly illustrated the dominance of the surface concentration of Ru over the total depth distribution of Ru in lowering electrode polarization in the current density range between 155A.m-2 and 170A.m-2. The parameter referred to as polarization slope were derived from electrolysis measurements and a value of 1.63 ± 0.03mV/A.m-2 places the DSA prepared with a 0.8wt/wt% Ru precursor solution at the top of the rankings in terms of preparation cost and electrocatalytic properties. Repetitive potential cycling between –200mV and 1000mV vs. Ag/AgCl in 5mM RuCl3.3H2O was used to electroform RuOx.nH2O films. Cyclic voltammetry yielded results that indicate that the growth of RuOx.nH2O is inextricably associated with a complex redox process, while it was furthermore observed that the peak current density of the cyclic voltammogram increases with cycle number. This suggests another complicating factor that most likely results from the electrocatalytic property of the growing oxide film. XRD conclusively showed that subsequent annealing results in a phase transformation of the hydrous Ru oxide that affects its behaviour when used as a DSA for MnO2 electroplating. AES depth profiling was used to arrive at the conclusion that an unannealed RuOx.nH2O film is most probably porous, but electrochemistry showed that it lacks stability under anodic load. With a polarization slope of 1.590 ± 0.006mV/A.m-2, an annealed Ti/RuOx.nH2O electrode (30min at 600°C) is the most advantageous in terms of MnO2 electroplating of all RuOx.nH2O films studied. Electrode polarization measurements on Ti/TiC electrodes as well as commercial Ti-Mn and Ti-Pb electrodes showed promising results, but none of these materials are in contention when compared with said DSA’s. The study was complemented with an investigation into the effect of an acidic MnSO4 solution on Ti metal. The reactivity of Ti towards atmospheric O2 gives stability to the metal in the form of a superficial oxide film. Dissolution of this passive film can occur under appropriate conditions. Single electrode potential measurements were employed to observe metal activation that is accompanied by H2 evolution at temperatures above 52°C. An activation potential of –0.67 ± 0.01V vs. Ag/AgCl has proven to remain constant at room temperature after activation was induced via temperature perturbation to 86 ± 2°C. A hypothesis is presented that describes spontaneous H2 evolution as a supporting reduction half reaction to the reduction of TiO2. Electroplating of MnO2 onto Ti substrates that were subjected to spontaneous H2 evolution shows a linear increase in electrode polarization (at 100.5A.m-2) as a function of exposure time with a slope of 0.19 ± 0.02V.hr-1 vs. Ag/AgCl. This attests existing theories that the change in free energy for the formation of TiO2 from TiH2 is more negative than for oxidation from Ti metal or that unrecombined hydrogen is present on the Ti surface at the onset of electrolysis.