Abstract
Ruthenate perovskites ARuO(3), (A= Ca, Sr or Ba) were prepared by coprecipitation method. The electrocatalytic activity of the prepared perovskites was investigated toward the hydrogen evolution reaction (HER). X-ray diffraction analysis suggested successful incorporation of Ru4+ at the Ca2+, Sr2+ and Ba2+ cations sites confirming the formation of the orthorhombic perovskite phases of CaRuO3 and SrRuO3 and the hexagonal perovskite phase of BaRuO3. The average particle size for the prepared perovskites was 39.0, 42.2 and 101.4 nm for Ca, Sr and BaRuO3, respectively. Scanning electron micrographs showed a well-defined hexagonal crystal structure in case of BaRuO3, while CaRuO3 and SrRuO3 composed of agglomerations of nearly spherical grains. CaRuO3 showed smaller agglomerations and more cavities than SrRuO3. The influence of the type of A-cation on the catalytic activity for HER was studied by Tafel linear polarization. The order of the electrocatalytic activity was BaRuO3>CaRuO3>SrRuO3. This was shown from the values of the exchange current density, -60.7, -13.1 and -884.6 mu A.cm(-2) for Ca, Sr and BaRuO3, respectively. This suggested that the A-site metal ion not only has a strong effect on the stability of the whole crystal configuration but also provides the possibility to improve catalyst performance by synergetic interactions with the B-site metal ion. The activation energy, reaction order and reaction mechanism have been investigated using the electrochemical techniques, Tafel linear polarization and impedance. Again, the calculated values of the activation energy gave similar trend for catalytic activity; 42.6, 86.3 and 17.7 kJ/mol for Ca, Sr and BaRuO(3)respectively. Both Tafel and the impedance data showed that the volmer step is the rate determining step. In addition, the effect of the partial substitution at the A-site in Sr1-xCaxRuO3 was also studied. The catalytic activity of ternary perovskites was about twice higher than that of binary ones. The catalytic activity of ternary oxides increased with increasing the fraction of Ca-doped.