从银电极上氢析出的温度效应得到的反应动力学启示

利用循环伏安法研究了多晶银电极在0.1 mol/L HClO4溶液中氢析出反应的温度效应. 发现当从析氢起始电位负向扫描至零电荷电位（-0.4 V）时，氢析出反应的表观活化能（Ea,app）和指前因子（A）均随着电势的负移而增大（对应的Ea,app从24 kJ/mol增大至32 kJ/mol）.继续负向扫描至零电荷电位以后，Ea,app随电势的负移而减小但A不随电势变化. 推测Ea,app和A在零电荷电位前的反常变化来自于反应物和过渡态之间内能和熵值变化．在零电荷电位以正电位，电极附近的水分子以氧端吸附在电极表面上，且水分子之间有氢键作用。电势变化产生的能量主要用于使水分子脱附并使氢键网络改变．在E〉PZC的电位区，电位负移导致的正的熵效应（指前因子增加）补偿了活化能的升高导致的负效应，从而显示反应的净电流随电位负移而增加．结果表明溶剂的动力学和相关的熵因子项可能会对析氢这类涉及质子和电子转移的电极反应的动力学产生很大的影响．

Kinetic Implication from Temperature Effect on Hydrogen Evolution Reaction at Ag Electrode

Hydrogen evolution reaction （HER） at polycrystalline silver electrode in 0.1 mol/L HClO4 solution is investigated by cyclic voltammetry in the temperature range of 278-333 K. We found that at electrode potential φa,app decreases with φ, while pre-exponential factor A remains nearly unchanged,which conforms well the prediction from Butler-Volmer equation. In contrast, with φ nega-tive shifts from the onset potential for HER to the potential of zero charge （PZC≈-0.4 V）, both Ea,app and A for HER increase （e.g., Ea,app increases from 24 kJ/mol to 32 kJ/mol）. The increase in Ea,app and A with negative shift in φ from -0.25 V to PZC is explained by the increases of both internal energy change and entropy change from reactants to the transition states, which is correlated with the change in the hydrogen bond network during HER. The positive entropy effects overcompensate the adverse effect from the increase in the activation energy, which leads to a net increase in HER current with the activation energy negative shift from the onset potential of HER to PZC. It is pointed out that entropy change may contribute greatly to the kinetics for electrode reaction which involves the transfer of electron and proton, such as HER.