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Screening of electrocatalysts for hydrogen evolution reaction using bipolar electrodes fabricated by composition gradient magnetron sputtering

https://doi.org/10.1016/j.jelechem.2019.113562Get rights and content

Abstract

This paper reports the facile fabrication of bipolar electrodes using composition gradient magnetron sputtering and subsequent activity screening of the prepared electrodes for hydrogen evolution reaction using bipolar electrochemistry. The electrocatalysts layers based on Ni–Mo, Au–Mo and Au–Ni alloys were deposited in form of compositional gradients on the bipolar electrodes. Concentration of the elements in the gradients varies from 40 to 60 at. % across the bipolar electrodes depending on the deposition conditions. The results of the screening experiments for the hydrogen evolution activity were also confirmed by conventional voltammetry measurements. We find that optimal compositions of the electrocatalyst for hydrogen evolution reaction are Au2Ni, AuMo2, Ni2Mo and Ni2Mo alloy has the highest catalytic activity. Mechanisms of HER on selected alloy compositions was also studied in more detail using voltammetry experiments.

Introduction

A lot of interesting applications of bipolar electrochemistry (BE) have been presented during the past two decades, such as electrical contacts creating [1], compositional and morphological gradients formation [[2], [3], [4]], Janus objects formation [[5], [6], [7], [8]], movement of micro swimmers [9], screening of electrocatalysts [10] and corrosion processes [11], sensing applications [12] and many others [13,14]. Recently Crooks at el. have demonstrated rapid screening of electrocatalysts for oxygen reduction reaction (ORR) [15,16] and hydrogen evolution reaction (HER) [17] by bipolar electrochemistry approach. In their pioneering work arrays of bipolar electrodes (BPEs) were fabricated by photolithography with plasma etching of FTO glass and subsequent physical vapor deposition of indicator chromium layer on anodic pole of BPE accompanying by lithographic process [[15], [16], [17]]. Catalysts formation on the cathodic pole was performed with the help of a piezo dispensing setup. Despite the success of the bipolar screening approach widespread of this method is limited by a relative complexity of the BPEs fabrication. Later, Kiani at el. have simplified the fabrication process by using BE both for the compositional gradient of a catalyst layer deposition on the conductive surface of the DVD-R reflective layer as well as for the catalyst screening [18]. This fast and simple approach suffers from the main disadvantage – a large variability of the catalyst loading through substrate deposited by BE that has a big impact on screening results. Moreover, the morphology of the catalysts deposited by BE also changes across the substrate. In the present study, we focus on formation of BPEs for HER screening by using only composition gradient magnetron sputtering approach that greatly simplifies the process. Sputter deposition also allows to reach the homogeneity of catalyst morphology across overall gradient catalyst layer.

HER is an important reaction for hydrogen production by electrolysis process. The kinetics of the HER on catalysts surface in acidic aqueous media is well studied and it is believed to proceed in two steps [19]. The first one is ‘‘proton discharge step’’ in which a hydrogen atom bounds to an active catalyst site through an electron transfer to a proton. This step is called the Volmer reaction:H++eHads

The second step can be either the Heyrovsky reaction:Hads+H+e-H2or the Tafel reaction:Hads+HadsH2

The dominant reaction mechanism can be determined from the so-called Tafel slopes, which are calculated from the plot of overpotential vs. log10 (current density). Based on Butler-Volmer equation Tafel slopes for three limiting cases can be derived [20]. If the discharge reaction is slow, the Tafel slope should be 116 mV dec−1 at 25 ​°C. If the discharge reaction is fast and Heyrovsky step (ion ​+ ​atom reaction) is rate determining the Tafel slope should be 38 ​mV dec−1. When the Tafel recombination step is rate determining, the measured slope of 29 mV dec−1 should be observed.

In this paper we report facile fabrication of Ni–Mo, Au–Ni and Au–Mo bimetallic electrodes by composition gradient sputtering followed by bipolar electrochemistry screening. This helped us to find compositions with the best activity for HER. In addition to bipolar screening experiments mechanism of HER on selected alloys compositions was studied in more detail by conventional voltammetry experiments.

Ni–Mo alloy is the most investigated catalyst for HER among chosen systems [17,21,22]. We have selected this system to compare our results with data available in the literature to verify validity of our bipolar screening process. Au–Ni and Mo–Au systems have not so far been systematically evaluated in literature for HER activity. Data in literature indicate that NiAu/Au core/shell nanoparticles can be promising catalyst for HER comparable to Pt [23]. Pure bimetallic Au–Mo systems have not been studied so far although Jaouen et al. reported that Au nanoparticles can enhance HER on Mo2N/C catalyst [24]. Yeo et al. also showed that presence of nitrogen atoms does not improve the catalyst activity while synergy effect of Mo and Au was clearly observed [25].

Section snippets

Bipolar electrodes fabrication

Two step approach was used to produce the BPEs (Fig. 1). Microscope glass slides with dimensions 70 mm × 26 mm were chosen as substrates. At the first step composition gradient of catalyst in form of thin film with length of 70 mm and height ≈7 mm was deposited close to one edge of glass slide by magnetron sputtering (see Fig. 1). Setup for the sputtering is schematically shown in Fig. 2. Au and Ni magnetron sources were placed opposite each other while Mo magnetron was placed as it is shown in

Deposition of gradient catalyst layer

Magnetron sputtering approach provides a simple way of deposition of compositional gradients. As seen from Fig. 4 the composition of the Mo-based catalysts (Au–Mo and Ni–Mo) varies by approximately 40 at. % and the Mo concentration changes linearly across the substrate from one edge to another. Au–Ni films variation of the composition differs from Mo-based alloys and can reach 60 at. % as well as Au content does not follow linear relationship. These peculiarities of gradients depositions are

Conclusions

In summary, it has been shown that bipolar electrodes for electrocatalysts screening purposes by bipolar electrochemistry approach can be easily prepared using magnetron sputtering deposition. Compositional gradients of the electrocatalysts were deposited by using commercial sputtering system without any modifications. Microscopy glass slides were used as the substrates. Optimal compositions for HER of the Au2Ni, AuMo2, Ni2Mo alloy catalysts defined by the screening are in a good agreement with

Acknowledgements

This work has been supported by grant VEGA No. 1/0204/18, the grant of the Slovak Research and Development Agency under the contract No. APVV-17-0059 and by the ERDF EU grant under the contract No. ITMS26220120047.

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