Electrochemical CO2 reduction to CO on dendritic Ag–Cu electrocatalysts prepared by electrodeposition
Introduction
Carbon dioxide (CO2) gas from industries and vehicles, is a major contributor to the greenhouse effect responsible for bringing about serious global warming [1]. Among various methods to convert CO2 into useful chemicals [2], [3], [4], [5], electrochemical CO2 reduction on electrocatalysts facilitates a clean process when combined with external electricity from renewable sources (e.g. sunlight, wind and etc.) [6], [7]. There are several reusable forms of carbon produced by the electrochemical CO2 reduction in aqueous electrolytes such as formic acid/formate, methanol, methane and carbon monoxide [6]. Among these, CO is a component of syn-gas which is a useful feedstock for the Fischer–Tropsch process to generate hydrocarbon fuels [8]. Furthermore, unlike other liquid products, the gaseous CO has an advantage: it does not need to be extracted from the electrolyte.
The main challenges in the electrochemical reduction of CO2 are the low energy efficiency and poor selectivity of the process [9]. The electrochemical reduction of CO2 to CO is relatively simpler compared to other products primarily because it is a two-electron reduction process: CO2 + 2H+ + 2e− → CO + H2O, E0 = −0.53 V [9]. Still, a large overpotential is required for this process because CO2 is thermodynamically and chemically stable. Therefore, the hydrogen evolution reaction (HER) competes with the electrochemical CO2 reduction in CO2-saturated aqueous electrolytes. Thus, there is an urgent requirement for the development of electrocatalysts, both with robust activity and high selectivity for CO production.
Many studies have been conducted on various metal electrodes for electrochemical CO2 reduction [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Among them, the noble metal electrocatalysts, such as gold (Au) [13], [14], [15], [16] and silver (Ag) [17], [18], [19], [20] exhibit high CO selectivity at moderate overpotentials. The catalytic performance of these catalysts is significantly related to their nanoparticle sizes [13], [14], [17], morphologies [15], [18], [19], crystal orientations [20] and oxidation states [16]. In recent reports, Zhu et al. have demonstrated the variation in activities of size-controlled monodisperse Au nanoparticles for electrochemical CO2 reduction in an aqueous 0.5 M KHCO3 electrolyte [14]. They have reported that Au nanoparticles with a size of 8 nm showed the highest CO faradaic efficiency (FE) of ∼90% at −0.67 VRHE. A computational study tries to explain this catalytic performance of Au nanoparticles based on the presence of an edge site, which promotes the adsorption and stabilization of a key intermediate COOH∗, which exists on 8 nm Au nanoparticles at an optimum level [14]. For Ag electrocatalysts, Lu and his colleagues have recently reported the morphological effect of Ag on electrochemical CO production [18]. At −0.50 VRHE, the CO FE values with various morphologies of Ag catalysts have been recorded as ∼90% for nanoporous Ag, ∼40% for an Ag nanoparticle, negligible for polycrystalline Ag and negligible for an Ag nanowire. In addition, the nanoporous Ag showed good stability maintaining its CO FE for 8 h. However, these noble metal electrocatalysts are expensive, which is one of the main hurdles for their commercialization.
Cu is one of the most cost-effective electrocatalysts for electrochemical CO2 reduction [21], [22], [23], [24], [25], [26], [27], and the activity and selectivity of Cu electrocatalysts are known to be related to the size of the nanoparticles [21], morphologies [22], [23], crystal orientations [24], [25], [26] and oxidation states [27]. When Cu is used as an alloy or an overlayer with Au and Ag, its catalytic performance for CO production is enhanced and modified at lower costs [28], [29], [30], [31], [32]. Kim et al. prepared Au–Cu bimetallic nanoparticles by co-reducing the gold and copper precursors using a one-pot synthesis method [28]. With the Au–Cu bimetallic nanoparticles as catalysts, various products were obtained in the electrochemical reduction of CO2 and their CO FE was dependent on the Au/Cu ratio. The Au3Cu catalyst exhibited a better turnover rate (TOR) than pure Au, which can be explained by the fact that Au–Cu bimetallic nanoparticles have a Cu atom near the Au–C primary bond, which can bind the O atom of COOH, so that the stabilization of COOH is easier. Additionally, Au–Cu nanoparticles also showed better mass activity.
In the present study, our strategy to enhance CO production with an emphasis on cost-effectiveness, focuses on the preparation of Ag–Cu dendritic structures using electrodeposition. Electrodeposition has some advantages over the conventional catalyst synthesis methods; it is a very simple and cheap process enabling easy control of composition and shape of the catalysts. Under controlled deposition potentials and electrolyte composition, our simple electrodeposition method facilitates morphological and compositional control of Ag–Cu dendrites providing a large surface area for the electrochemical reduction of CO2 to CO. A careful examination of the material properties of Ag–Cu dendrites revealed that a synergetic effect between Ag and Cu exists in these catalysts, which may play a role in cost-effectiveness for practical applications.
Section snippets
Preparation of Ag–Cu catalysts
The catalysts were prepared on a Cu foil substrate using an electrodeposition method. KAg(CN)2 and CuCN were used as the Ag and Cu precursors, respectively, for electrodeposition, with KCN playing the role of a complexing agent. The concentrations of KCN and CuCN were fixed at 0.4 M and 0.2 M, respectively, while various concentrations of KAg(CN)2 from 10 mM to 30 mM were used in order to vary the composition ratios of Ag and Cu. Pure Ag and pure Cu electrodes were also prepared as controls. 0.5 M
Results and discussion
Fig. 1 demonstrates the LSV analysis of Ag and Cu electrolytes on the Cu foil substrate. In order to fabricate the Ag–Cu deposits with various compositions, the concentration of KAg(CN)2 in the electrolytes was varied from 10 to 30 mM at a fixed CuCN concentration (0.2 M). For both the LSV analysis and electrodeposition, an Ag/AgCl (sat. KCl) electrode was used as the reference electrode. Three clear reduction regions were observed in the LSV plots for all the electrolytes as marked in Fig. 1
Conclusions
In the present research, we fabricated Ag–Cu catalysts by an electrodeposition method and analyzed their activities towards electrochemical CO2 reduction. The surface structure and the composition of Ag–Cu catalysts were controlled by changing the concentration of precursors in the electrolyte. For Ag–Cu catalysts, it was found that they have hierarchical dendritic structures, while pure Ag has a multi-pod structure and pure Cu has a film-like structure with a prominences-terminated surface.
Acknowledgements
This work was supported by a grant from the Korea CCS R&D Center (KCRC) funded by the Korea government (NRF-2014M1A8A1049349).
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