We study the effect of H⁺ and OH^– diffusion on the hydrogen evolution reaction in unbuffered aqueous electrolyte solutions of mildly acidic pH values. We demonstrate that the cathodic polarization curves measured on a Ni rotating disk electrode in these solutions can be modeled by assuming two irreversible reactions, the reduction of H⁺ and that of water molecules, both following Erdey-Grúz–Volmer–Butler kinetics. The reduction of H⁺ yields a transport-limited and thus, rotation rate-dependent current at not very negative potentials. At more cathodic potentials the polarization curves are dominated by the reduction of water and no mass transfer limitation seems to apply for this reaction. Although prima facie the two processes may seem to proceed independently, by the means of finite-element digital simulations we show that a strong coupling (due to the recombination of H⁺ and OH^– to water molecules) exists between them. We also develop an analytical model that can well describe polarization curves at various values of pH and rotation rates. The key indication of both models is that hydroxide ions can have an infinite diffusion rate in the proximity of the electrode surface, a feature that can be explained by assuming a directed Grotthuss-like shuttling mechanism of transport.

中文翻译：

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电化学放氢：H +或H2O还原？转盘电极研究

我们研究H的效果⁺和OH ^-在温和的酸性pH值的缓冲电解质水溶液的析氢反应扩散。我们表明，在这些解决方案的镍旋转圆盘电极测量的阴极极化曲线可以通过假设2个不可逆反应，H的降低被建模⁺和水分子，无论是以下Erdey-格鲁兹-沃尔默-巴特勒动力学。H ⁺的减少在不是非常负的电势下产生了一个受运输限制的电流，因此产生了与转速有关的电流。在更多的阴极电势下，极化曲线以水的还原为主，并且似乎没有传质限制适用于该反应。虽然表面上这两个过程可能似乎独立地进行，通过有限元模拟数字的手段，我们表明，强耦合（由于H的重组⁺和OH ^-水分子）它们之间存在。我们还开发了一种分析模型，可以很好地描述各种pH值和旋转速率下的极化曲线。两种模型的关键指标是氢氧根离子在电极表面附近可以具有无限的扩散速率，这一特征可以通过假设有类似格罗特斯式的直接穿梭机制来解释。