In situ electrochemical methods for production and porosity characterization of composite Ni-P+TiO₂ catalyst coatings

Bożena Łosiewicz

Quarterly No. 1, 2010 pages 52-58

DOI:

keywords: composite coatings, impedance, electrodeposition, Ni-P+TiO2, porosity

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abstract The present work deals with the development of new electrochemical ways to improve the electrode material activity towards hydrogen evolution reaction (HER). An in situ composite electrodeposition technique has been proposed to prepare the porous Ni-P+TiO2 coatings. Production of this catalyst material was carried out by simultaneous codeposition of amorphous nickel with titanium dioxide (anatase) powder on a copper substrate from the bath in which TiO2 particles were held in suspension by magnetic stirring. The electrodeposition of the Ni-P+TiO2 coatings was carried out under galvanostatic conditions at room temperature from the plating bath containing nickel sulfate, ammonium chloride, sodium hypophosphite, sodium acetate, boric acid, and an addition of TiO2 powder. The chemical and physical characteristics of the obtained deposits have been discussed. Surface morphology was carried out using a scanning electron microscopy (SEM). Structural investigations were conducted by X-ray diffraction method (XRD). The chemical composition of the coatings was determined by means of atomic absorption spectroscopy (AAS). The in situ method of electrochemical impedance spectroscopy (EIS) was applied to the porosity studies of the coatings. For comparison, the Ni-P and Ni coatings were also obtained and investigated in the same manner. The SEM observations of the surface morphology revealed that the composite Ni-P+TiO2 coatings exhibit the presence of TiO2 particles (diameter of particle below 40 um) uniformly embedded into the amorphous nickel matrix. The structural investigations of the deposits exhibit the composite structure with an amorphous nickel matrix into which the solid crystalline TiO2 particles are embedded. The optimum production conditions of the porous coatings containing 30.3 wt.% of TiO2 micro-particles, were proposed. Such electrochemical codeposition method may be a good alternative in the field of composite large-surface electrodes for HER. The pore geometry was determined by impedance modeling and to explain the impedance behavior of the electrode materials, three electrical equivalent circuits containing: (i) the constant-phase element (CPE), (ii) two-CPE elements and (iii) the finite length porous model, were compared and verified. The electrochemical impedance behavior of the composite Ni-P+TiO2 electrode may be well described using the two-CPE model related to the presence of pear-like shape pores with the diameter of 0.37  cm2 on the surface, and a simple CPE model for the Ni-P and Ni electrodes containing the flat pores. It has been found that in situ electrochemical methods can be successfully applied for both production and characterization of the large-surface composite coatings with a high performance towards HER.