Rapid and scalable synthesis of bismuth dendrites on copper mesh as a high-performance cathode for electroreduction of CO₂ to formate

Highlights

• 1 Networked Bi electrode is prepared via a rapid and scalable synthesis method.

• 2 The as-prepared electrode has a large electroactive surface area.

• 3 The as-prepared electrode improves mass transfer of the reactants.

• 4 Excellent performance towards ERCF is obtained.

Abstract

Bi-based electrode is believed to be one of the promising cathodes for the electroreduction of CO₂ to formate. However, it suffers from energy- and/or time-consuming preparation process and mass transfer limitation of the reactants. Here, we fabricated a novel networked Bi-based electrode by formed in situ from Bi dendrites integrated into a Cu mesh substrate via a facile replacement reaction with energy and time savings. The as-prepared electrode was directly used as a cathode for electroreduction of CO₂ to formate without further treatment. It exhibits a formate Faraday efficiency of ≈ 100 %. Additionally, partial current density for formate is as high as 68.51 ± 4.04 mA cm^-2. This performance outperforms other results obtained on Bi-based electrodes—even most gas-diffusion electrodes under similar conditions. The excellent performance seen here is most likely attributed to its large surface area and fast transport of the reactants. This originates from the Bi dendrites catalysts and the unique network structure of the Cu mesh substrate. This work offers a facile route to obtain an efficient and robust electrode for electroreduction of CO₂ to formate, which makes it easy to scale up in industrial applications.

Introduction

The electroreduction of CO₂ in aqueous solutions has received great attention because it can reduce atmospheric CO₂ concentrations [[1], [2], [3]]. It can also produce useful carbon-based fuels under mild conditions (e.g., moderate temperature and atmospheric pressure) [[2], [3], [4]]. Formic acid/formate (depending on pH, “formate” is used hereafter to represent both forms) is a common electroreduction of CO₂ product [3]. Compared to gaseous products such as hydrocarbons and CO, formate can be easily stored and transported. Furthermore, formate is a high-value chemical intermediate and hydrogen storage material [5,6]. The electroreduction of CO₂ to formate (ERCF) is appealing industrially [[7], [8], [9], [10]]. However, there are still some challenges in large-scale ERCF [[11], [12], [13]]. One of the key challenges is the development of high-performance electrode materials [[11], [12], [13], [14], [15]].

Various metal-based electrodes (such as Pd, Pb, Hg, Cd, In, Tl and Sn) have been extensively investigated for ERCF [11,[14], [15], [16], [17], [18], [19], [20], [21]]. Unfortunately, these electrodes usually suffer from one or more of the following problems: toxicity, environmental pollution, poor product selectivity, low catalytic activity, low availability, and high cost [11,22]. More recently, bismuth (Bi)-based electrodes have received great attention for ERCF because Bi is a non-toxic and low-cost material. It also exhibits high selectivity toward ERCF [[23], [24], [25]]. Several Bi-based catalysts such as Bi nanosheets [23], Bi nanospheres [25], and nanowires [25] have been developed for ERCF. In order to fabricate Bi-based electrodes, the as-prepared Bi-based catalysts are usually coated onto a conductive substrate (e.g., carbon paper) with insulating binder (e.g., Nafion) [[23], [24], [25]]. This electrode preparation procedure is complicated. Moreover, the insulating binder will lower the electrical conductivity of the electrode leading to deterioration in electrode performance [[26], [27], [28]]. In addition, the synthesis of Bi-based catalysts is generally highly energy- and time-consuming. It either requires state-of-the-art synthesis methods and/or large amounts of energy (e.g., thermal energy) [25,26]. These problems limit large-scale applications of Bi-based electrodes.

There are several Bi-based electrodes that are fabricated via directly electrodeposition of Bi-based catalysts (e.g., Bi nanoflakes, Bi dendrites) on a flat conductive substrate (e.g., copper foil) [[29], [30], [31], [32], [33]]. This electrode preparation method avoids the employment of a binder and simplify the electrode synthetic procedure. Nevertheless, it also requires electrical energy.

Other than the energy- and/or time-consuming preparation processes, current Bi-based electrodes often suffer from mass transfer limitation of the reactants, especially at regions of high current density [28,34]. This reduces the advantages of Bi-based electrodes. As a result, the enhancement of mass transport appears to be an important factor for improving ERCF performance. Unfortunately, few efforts have focused on this issue [10,[28], [29], [30], [31], [32], [33], [34]]. It has demonstrated that gas-diffusion electrode has the potential to enhance mass transfer [28,35]. However, it requires complex fabricating technique to assemble different porous layers for optimal electrode performance [35]. Consequently, a rapid, scalable and cost-effective synthesis of high-performance Bi-based electrodes with enhanced mass transfer is still an urgent need and important for large-scale production in ERCF.

Herein, we report a novel networked Bi-based electrode (N-Bi) for ERCF. The N-Bi was prepared by in situ synthesis of Bi dendrites on a Cu mesh substrate via a facile replacement reaction. This process requires no additional energy supply. Moreover, it occurs simply by immersing a substrate in the solution that contains the ionic precursors of metal to be deposited [36]. Thus, this fabrication procedure is energy and time savings vs. the traditional preparation method for Bi-based electrode. The electrode structure of this type will facilitate electron transfer because it avoids the use of a Nafion binder [12]. The Cu mesh was selected as the conductive substrate based on the following considerations: 1) it is a highly conductive and low-cost substrate; 2) the unique open area of the Cu mesh benefits the flow of electrolyte and provides an extra lateral surface vs. the traditional flat conductive substrate of Cu foil [37]. The N-Bi was characterized and its performance for ERCF was studied. Moreover, the performance stability of the N-Bi for ERCF was monitored. Bi-based catalysts were loaded on the Cu foil (T-Bi) via the same procedure as the N-Bi. The performance of the T-Bi for ERCF was also studied as a control.