Investigation of the catalytic performance of tin nanowires produced by aluminum anodic oxide template method for electrochemical CO2 reduction

Abstract

Climate change causes various dramatic situations, including the melting of the ice mass, flooding, and drought, and it affects even the daily life of human beings. The increased amount of anthropogenic CO2 gas in the atmosphere is one of the driving forces for global warming and climate change. Therefore, mitigation of CO2 gas has become mandatory to eliminate the harmful effects of global warming. There are mainly two mitigation methods: carbon capture-storage and carbon capture-utilization. By the utilization of carbon dioxide, not only the amount of CO2 is reduced, but also it is used as an input for value-added chemical production. Among the utilization methods, electrochemical CO2 reduction over heterogeneous metallic electrocatalyst steps forward thanks to its several advantages, such as high number of available catalysts, its ability to work at ambient conditions, scalability, and the utilization of renewable energy sources. Several products, including organic acids, hydrocarbons, and fuels, can be obtained via electrochemical CO2 reduction reactions. The techno-economic analysis performed for the feasible CO2 reduction systems have shown that formic acid is the most practical chemical to be produced via electrochemical CO2 reduction. Formic acid is used as input in various industrial areas such as agriculture, food, textile, and pharmaceutical. However, recent studies have indicated that formic acid is also a strong candidate for modern engineering applications. For instance, formic acid has an extraordinary capacity to carry H2 gas (590 l per l). This storage performance of formic acid enables the advance in the H2 driven cars. Moreover, formic acid can be used as feed for formic acid fuel cells to generate electricity. The conventional production methods of formic acid require high energy consumption and include highly complex process steps. Hence, the production of formic acid via electrochemical CO2 reduction utilizing renewable energy sources such as wind, solar, etc., is in high demand. The metallic electrocatalysts that produce formic acid or formate (depending on the pH) are Sn, Cd, Tl, In, Pb, Bi, and Hg. Among these metals, tin is one of the most studied electrocatalysts since it has low toxicity, is abundant in nature, and is cost-friendly. Tin is known to produce only formic acid as liquid product under the electrochemical CO2 reduction in liquid environment at ambient conditions. Although high Faradaic efficiency has been achieved for formic acid, there are challenging problems in scaling up the CO2 reduction system. These problems can be listed as low current density, low stability, high overpotentials, and by-product formation. Specifically, low current density due to the low solubility of CO2 in the aqueous solutions and low catalytic activity is a crucial disadvantage. To overcome the low current density, flow-cell systems, gas diffusion electrodes, and gas-phase reduction structures have been proposed since the low solubility problem of CO2 can be outframed. To increase the catalytic activity of the catalysts, meso- and nano-structured electrodes have been proposed rather than using bulk electrodes. Especially nanostructured electrodes, such as nano-rods, wires, tubes, sheets, etc., have shown promising results in terms of efficiency and activity thanks to their unique structure. In the literature, there are several studies on the production of tin-based nanostructured electrodes, tin nanowires have yet to be investigated extensively compared to the other structures. Nanowires can be a great candidate for electrocatalytic CO2 reduction since they offer high surface to volume ratio and enhanced charge and mass transfer. In recent years, a few studies on tin nanowire electrocatalysts have been published, yet the suggested production methods in these works are highly complex. To commercialize the electrochemical CO2 reduction system, the production technique is also crucial, besides the performance of the electrocatalyst. The ideal production method should be simple, low-cost, and scalable. Also, significant structural changes should be obtained by easily altering the working parameters such as potential and duration. In this study, the catalytic performance of tin nanowire electrodes produced via the AAO template method toward formate production under electrochemical CO2 reduction is investigated. By choosing AAO template method, most of the desired properties of the catalyst production technique are provided. A self-standing, interconnected, and branched-like 3D nanowire network was achieved via simple AAO template technique. Moreover, anodic oxidation in alkaline solution was applied to the as-produced tin nanowires to increase the active surface area and improve catalytic performance by increasing the oxide-related content on the electrode surface. The first of the tin nanowire production was the formation of AAO template. To obtain AAO pattern, the aluminum substrate was anodically oxidized in 0.3 M oxalic acid solution at 70 V for 30 min. Then, the zincating process was applied to make the AAO template electrodepositable. The main drawback of the AAO template method is the formation of a non-conductive barrier layer at the bottom of the pores, which prevents the direct usage of the template for electrodeposition. By fast and easy zincating step, AAO templates can be prepared for the electrolytic solutions without any significant change and/or loss of structure. Tin was electrodeposited into the AAO template in tin sulphate-containing bath under constant potential. Then, AAO template and the remnant aluminum were dissolved in 3 M NaOH solution at 60°C until the vigorous H2 gas evolution stops. The obtained tin nanowire electrodes were characterized by SEM and Raman analysis. The catalytic performance tests were conducted in a custom-made H-cell filled with 0.1 M KHCO3 at potentiostatic mode for 1 h. The catalytic behavior was expressed as Faradaic efficiency toward formate. To compare the performance, high purity tin foil was subjected to similar reduction experiments. The SEM analysis showed that tin nanowire electrodes possessed highly ordered and interconnected branched-like structure, including nanowires having 7 µm length and 150 nm diameter. Moreover, it was found via Raman analysis that the surface contained poor crystalline and non-stoichiometric oxide structures. The reduction experiments indicated that tin nanowire electrodes achieved 10 times higher current density values than tin foil at every reduction potential. However, the increase in formate production was not as high as current density values, and only 6-fold higher production amount was observed. Moreover, Faradaic efficiency values reached only 30% on the tin nanowire electrodes, while 60% efficiency was obtained over tin foil. This result indicated that a significant portion of the total charge is consumed by side reactions, i.e., CO and H2 formation rather than formate. The low efficiency values obtained over tin nanowire electrodes were attributed to the destruction of the oxide content on the nanowires during the AAO removal in the concentrated NaOH solution. Since the oxide layer has been proved as the key catalytic part for the formate production over tin-based catalysts by in-situ and in-operando studies, the removal of the oxide content results in the decay of the catalytic performance toward formate. In concentrated alkaline solutions, tin oxides can dissolve, and the electrode ends up containing mostly metallic tin. To further prove this phenomenon, tin foil was immersed in 3 M NaOH solution at 60°C for 2 hours, and then CO2 reduction experiment was run over this electrode. The result showed that the efficiency reduced from 60 to 15% after etching in alkaline solution. To reform the destructed oxide layer, anodic oxidation in 1 M NaOH at room temperature was performed. The applied potential was set to 4 V to obtain a crack-free porous oxide structure over tin nanowires. For comparison, tin foil was also anodically oxidized. The SEM analysis showed that anodically oxidized tin nanowires enlarged approximately 50 nm, and pores formation on the wires with 30-40 nm diameter was observed. Raman analysis gave peaks of the crystalline SnO2 structure along with the SnO and Sn3O4. The catalytic performance was enhanced significantly, and 87% Faradaic efficiency with ca. 15 mA.cm-2 current density at -1.6 V vs. Ag/AgCl was achieved over anodically oxidized tin nanowire electrodes. Moreover, 9- and 12-times higher formate production was achieved over anodically oxidized tin nanowire electrode compared to anodically oxidized tin foil and untreated tin foil, respectively. However, current density values for both tin nanowire and tin foil decreased after anodic oxidation due to the semi-conductive character of the tin oxide structures. The improvement in the catalytic behavior after anodic oxidation was attributed to the increased surface area and oxide layer formation on the electrode surface; the latter was more dominant. The ECSA calculations showed that anodically oxidized tin nanowire electrode possessed only 4-fold higher active area than anodically oxidized tin foil. The formate production amount was much higher than the surface area increase, therefore the improved catalytic activity was not only related to the active surface area. Furthermore, the kinetics of the electrochemical CO2 reduction reactions was faster over anodically oxidized tin nanowire electrodes since lower polarization resistance value was obtained via EIS analysis. As an important indicator for the catalytic performance, stability was tested over both anodically oxidized tin nanowire and tin foil electrodes for 12 h at -1.5 V vs. Ag/AgCl. Within the first 30 min, the oxide layer over the tin foil flaked off from the surface; therefore, the experiment was not continued. The anodically oxidized tin nanowire electrode preserved its structural integrity throughout the experiment and gave ca. 64% Faradaic efficiency. However, after 5 hours, a slight decline was observed in the current density values. The SEM images revealed the enlargement of the nanowires and fillings between the nanowires. The current density loss was related to the decline in the surface area. The Raman analysis indicated the absence of the crystalline SnO2 related peaks but the intensified of the already existing peaks for SnOx structures and new peak formation for SnO structure. These results concluded that the survival of the metastable oxide structures over tin nanowires is crucial for preserving the efficiency value toward formate even if the current density is lost.