Behavior of Porous Titanium and Catalytically Active Electrodes Based on It in Acidic Chloride Solutions

The behavior of porous titanium and electrodes based on it, which are activated with Pt, Au, RuO₂, Co₃O₄, and MnO₂, in 20-% LiCl solution (pH –0.4 to –0.5) is studied. On porous titanium in the potential ranges 0.1 < E< 0.5 and 0.5 < E< 1.1 V (NHE), the formation of titanium hydrides and passive oxide layers, respectively, is observed; the processes decay with time. In the ranges E< 0.1 and E> 1.1 V, the dissolved oxygen reduction and chlorine evolution, respectively, are observed on porous titanium at high overpotentials. On porous titanium activated with thin-layer Pt, Au, and RuO₂coatings, the functional Evs. pH dependence, which is typical for these electrocatalysts, breaks down due to the conjugate reactions of titanium oxidation. On porous titanium activated with Co₃O₄and MnO₂, at pH below unity, chlorine evolution is observed; its rate is limited by the chlorine mass transfer into the bulk solution. Under a gas-diffusion control, the chlorine evolution rate is determined by the diffusion of absorbed hydrogen chloride. The conditions of application of porous titanium as the support for catalytically active electrodes of electrochemical sensors in acidic chloride solutions are considered.     

Behavior of manganese dioxide electrodes in a system containing a solid protonic electrolyte

The behavior of manganese dioxide electrodes containing powders of graphite and titanium as electroconducting additives is studied in a system with a solid protonic hydrated electrolyte (SPHE). The MnO2 reduction is accompanied by ion exchange between formed Mn²⁺ ions and structural protons of SPHE. The open-circuit potentials drift in the anodic direction and stabilize when the MnO² composition at a boundary with SPHE is close to stoichiometric. The drift is due to conjugated reactions of the MnO₂ reduction and the graphite oxidation process that decays with time. With increasing cathodic current density, the solid-phase reduction of MnO₂ via an electronic-protonic mechanism becomes predominant. At low polarizing-current densities, the open-circuit and steady-state potentials remain passably stable until the exchange capacity of SPHE by protons is exhausted or MnO₂ near the interface with SPHE is spent. The feasibility of using MnO₂-SPHE-based amperometric sensors for assaying components whose oxidation in a gas medium yields protons is considered     

Mass transfer in amperometric gas sensors

The mass transfer in amperometric gas sensors intended for atmosphere monitoring is studied theoretically and experimentally. External and internal constituents of the diffusion resistance (DR) of the sensors are determined. The external constituent is defined by conditions of convective diffusion of air under analysis relative to the sensor. The internal constituents are defined by parameters of construction of sensor elements, and electrolyte film, as well as the structure of the indicator electrode, and solubility of the gas under analysis in the electrolyte. Testing a chlorine sensor shows that the DR of the internal constituents is independent of the conditions of convective diffusion of the environment under analysis near the sensor. The similarity criterion of sensors of different types is shown to be the relative share of DRs of individual constituents in the overall DR of the sensor. The results obtained can be used for the development and design of sensors with required range and resolution.