Self-supported electrocatalysts of tantalum-based nitrides developed through precursor derived route for hydrogen evolution
RAVI K. 1, MUKKAVILLI R. 1, THIYAGARAJAN G. 1, B R A. 1, GRAF D. 2, FISCHER T. 2, WILHELM M. 2, CHRISTIANSEN S. 3, MATHUR s. 2
1 Indian Institute of Technology Madras, Chennai, India; 2 University of Cologne, Cologne, Germany; 3 Fraunhofer IKTS, Dresden, Germany
Nano structuring Self-Supported Electrocatalysts (SSE’s) as an efficient alternative to the conventional drop casted electrodes which suffer from poor stability & adhesion along with expensive cost due to the binders involved is the way forward to realise lab-to-fab transfer of earth abundant electrocatalysts. In this regard, Tantalum nitride by virtue of its near zero hydrogen Gibbs adsorption free energy was chosen to develop SSE’s. The corresponding nitrides were developed using Chemical Vapor deposition and Vacuum annealing on Nickel foam and Tantalum foil respectively. The overall objective was to eliminate two major detrimental factors that can result in poor hydrogen kinetics: oxygen and high temperature processing. In the first part, vacuum annealing of Ta and Nitrogen sources (melamine / thiourea) was carried out at (1000 °C). which resulted in mixed phases of TaCN/TaN/Ta foil. At a fixed over potential of 350 mV vs RHE, Ta / thiourea delivered 8 mA/cm2, whereas only 0.8 mA/cm2 was obtained with Ta/melamine. Due to the presence of Ta-S bonds on the surface as exemplified by the XRD and EDS studies, catalyst using thiourea as the nitrogen source exhibited better electrocatalytic performance compared to the catalyst obtained using melamine. However, Lack of composition and thickness control necessitated development of single source precursors which can be later deposited on highly porous and conducting nickel foam. To achieve this, Chemical vapor deposition (CVD) of tantalum precursor, tris(diethylamino)(ethylimino)tantalum(V), ([Ta(NEt)(NEt2)3]) with preformed Ta–N bonds was grown on nickel foam. The morphological analysis revealed the formation of thin film of core–shell structured TaNx(Oy) coating (ca. 236 nm) on NF. In 0.5 M H2SO4, TaNx(Oy)/NF exhibited enhanced HER activity with a low onset potential as compared to the bare NF (−50 mV vs. −166 mV). The TaNx(Oy)/NF samples also displayed higher current density (−11.08 mA cm−2vs. −3.36 mA cm−2 at 400 mV), lower Tafel slope (151 mV dec−1vs. 179 mV dec−1) and lower charge transfer resistance exemplifying the advantage of TaNx(Oy) coating towards enhanced HER performance. The enhanced HER catalytic activity was attributed to the synergistic effect between the amorphous TaNx(Oy) film and the nickel foam.