Hydrogen evolution reaction on Ni–P alloys: The internal stress and the activities of electrodes

Abstract

The hydrogen evolution reaction (HER) was studied on Ni–P electrodes prepared by electrodeposition at temperatures varying from 23 °C to 65 °C. The activities for the HER of the electrodes first decreased slowly with the increasing temperature of the Ni–P preparation. A sudden decrease in the HER activity occurred on the electrode prepared at 65 °C. Similar dependence was found for the variation of the amounts of absorbed hydrogen with the electrodeposition temperature of the Ni–P electrodes. The behavior of Ni–P electrodes prepared from Ni–P powders was quite different. (Ni–P powder was prepared by peeling off the Ni–P layer and by milling the leaves of the Ni–P alloy in a vibrating ball mill.) The Ni–P powder electrodes displayed little activity independently of the temperature of the Ni–P powder preparation. It was followed from the results that the high activity for the HER of the layer Ni–P electrodes prepared at T ≤ 53 °C was caused by the internal stress in the layer. The stress originated during the electrodeposition of the Ni–P layer by co-deposited and absorbed hydrogen.

Introduction

The hydrogen evolution reaction (HER) is technologically important for such processes as water electrolysis and a chlorine production. At the same time the HER is a relatively simple electrochemical reaction. For these reasons the HER ranks among the most extensively studied electrode reactions. In the past several years, there has been a great effort to develop new materials that have good electrocatalytic properties for the HER [1], [2], [3], [4], [5], [6], [7], [8], [9]. Particular attention has been paid to nickel and nickel-based alloys [10], [11], [12], [13], [14], [15], [16], [17], [18], namely those involving a non-metallic element—phosphorous, sulfur or boron [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. A great interest has been devoted to the study of Ni–P electrodes. Conflicting reports were published on the activity of Ni–P (or Co–P) electrodes for the HER. It followed from the published results that some Ni–P electrodes were considerably active [25], [26], [27], [28], [29], [30], [37], [38], [39], while other electrodes, even with the comparable P content, were significantly less active [31], [32], [33], [41].

Considerable level of uncertainty exists regarding the role of the phosphorous content and whether Ni–P deposits are crystalline or amorphous. Similar uncertainty exists regarding the influence of the crystal size on the activity of the HER. Burchardt et al. [38] assigned the good electrocatalytic performance of the Ni–P electrodes to their amorphous phase and showed that the amorphous phase conversion into crystalline caused a drop in the Ni–P activity. On the other hand, some authors [30], [33] attributed higher activity of Ni–P electrodes only to their large surface areas.

A great uncertainty also exists regarding the role of heat treatment on Ni–P alloys. The activity of considerably active Ni–P layers prepared by the electrodeposition significantly decreased after heating at T > 150 °C [26], [38]. On the other hand, some authors [32], [41] observed a slight increase in the electroactivity for the HER after heating the electroless prepared Ni–P electrodes.

Some authors [21], [24], [25], [26], [38] found out that Ni–P and Ni–S alloys, which have high activity for the HER, were able to absorb a large amount of absorbed hydrogen. They supposed that absorbed hydrogen could change the electronic structure of the alloy and by this way might cause the high activity of these alloys. It was shown in the paper [29] that the high activity of the HER was not caused by the electronic interaction between absorbed hydrogen and the electrode material, but by an internal stress in the Ni–P deposits. The stress has been originated during the electrodeposition of the Ni–P (or Ni–S) alloy because, simultaneously with the alloy deposition, the hydrogen evolution also occurred and hydrogen was dissolved to a great extent in the amorphous alloy.

Ni–P layers can be prepared either by the electrodeposition or the electroless deposition methods from solutions containing the metal salt and a phosphorous compound—mostly NaH₂PO₂. Both methods lead to phosphorous and nickel co-deposition and this process is also accompanied by hydrogen evolution. Comparing the activities for the HER of electrodes prepared by the processes mentioned above, it has been found that electrodes prepared by the electrodeposition have usually been considerably more active (at an approximately equal P content) than those prepared by electroless deposition.

The present paper was aimed at explaining, why Ni–P layers prepared by the electrodeposition and by the electroless deposition methods have such different electrocatalytic properties for the HER, and aimed at showing the great influence of the internal stress on the electrocatalytic properties of the Ni–P electrodes.