Temperature Effects On Hydrogen Temperature Effects On Hydrogen Evolution And Oxygen Reduction Reaction At Pt (111) Electrode

The hydrogen evolution reaction and the redox reaction are important electrode reactions in clean energy techniques such as electrolyzing water to produce hydrogen and fuel cells. For their technical significance and that they are rather simple reactions compared to more complex ones, the hydrogen evolution reaction and the redox reaction are always used as important model reaction system in the field of electrocatalysis to help understand how catalysis and electricity cooperate to control reaction rate. The impact of catalyst on the activity of electrode reaction has been widely studied, and the results have shown that the reaction rate of hydrogen evolution or redox reaction is relevant to the adsorption energy of H or O atom on catalysts as the Volcano Relationship. The Pt catalyst has always been under great concern as it is the best catalyst for the activities of hydrogen evolution and redox reaction. As for the control of reaction rate with electricity, the discussions in this reference are usually based on the commonly accepted phenomenological theory of Butler-Volmer equation. This theory predicts only the elementary reduction reaction which only correlates with electron transfer, and the the activation energy decreases with the potential shifts to be more negative. However, for more complicated reactions which involve multi-electron transfer, this article hasn’t shed much light on a series of fundimental scientific questions like how the activation energy changes with potential, whether the activation energy can be regarded as an important index for discussing reaction activity of catalysts, or what the relationship between theoretically calculated energy barrier and the reaction activation energy measured experimentally is like.In order to reveal the essence of how the change in electrode potential controls reaction rate, and to solve the questions above, this article studies the thermal effect on the hydrogen evolution redox reaction on Pt(111) electrode,and analyzes parameters like Tafel slope、activation energy and pre-exponential factor, as well as the relationship between temperature and electrode potential, using B-V equation and Arrhenius equation, and discussed their reaction mechanisms.Furthermore,the kinetics for O2and OHad adsorption/desorption and ORR at Pt(111) electrode in0.1M HClO4 have been investigated with the aim to find the key factors which limit ORR kinetics at Pt.The conclusions of this dissertation is summarised as follows:1. Temperature effect of hydrogen evolution reaction atPt(111) electrode:under temperature range of278-298K, the hydrogen evolution reaction in0.1M HClO4at Pt(111) electrode was studied using RDE(rotating disk electrode). We discovered that:In potential range of-0.12-0V, with a demarcation point of0.06V, the Tafel slope of low and high overpotential areas were-60mV/dec and-120mV/dec, respectively. In the low overpotential area, the Tafel slope decreased as the temperature increased; while the high overpotential area met an opposite trend. The exchange-current density was about several mA/cm2, and increased when the temperature rose. With the increase of overpotential, the reactional apparent activation energy added up from55mV/dec to62mV/dec at first, but then decreased to54mV/dec. The logarithm of the pre-exponential factor went on a trend of first increasing and then decreasing, just the same with the activation energy trend.2. Temperature effect of redox reaction on Pt(111) electrode:under temperature range of278-313K, the temperature effect of the redox reaction in0.1M HClO4and H2SO4of0.05M on Pt(111) electrode was studied using RDE. The virtual dynamic current was obtained taking out the effect of mass transfer of oxygen in the potential area which dynamics and diffusion control. Studying the relationship between the dynamic current, temperature and electrode potential, we discovered that:In0.1M HClO4, in the potential range of0.8-0.95V, with a demarcation of0.88V, the Tafel slope of low and high overpotential areas were64mV/dec and78mV/dec, respectively. The reactional apparent activation energy was about48.4±0.6kJ/mol. The logarithm of the pre-exponential factor decreased as the potential increased.For H2SO4of0.05M, merely one Tafel slope of approximately120mV/dec was discovered in the range of0.6-0.9V The reactional apparent activation energy was about48.4±3.4kJ/mol, and the logarithm of the pre-exponential factor decreased as the potential increased.3. O2adsorption and OHaddesorption at Pt(111) andits implication to oxygen reduction reaction kinetics:Kinetics of dissociative O2adsorption, OHad desorption and oxygen reduction reaction (ORR) at Pt(111) electrode in0.1M HClO4has been investigated. We found that i) reversible OHad adsorption/desorption occur fast at potentials from0.6to1.0V (vs RHE) with exchange current density of ca.50mA cm-2at0.8V; ii) at10V/s ORR current in the positive-going potential scan is smaller than that in negative-going scan due to that some OHad formed from dissociative adsorption of O2at lower potentials can only be oxidized at higher potentials under such fast potential scan; and iii) the open circuit potential of Pt/0.1M HClO4interface increases promptly from0.9V to1.0V after switch from O2free-to O2-saturated solution; Our results indicate that the dissociative adsorption of O2is not the rate determining step for ORR, the slow kinetics for ORR is determined by the thermo-equilibrium of OHad+H++e←�'H2O.