• International Journal of Technology (IJTech)
  • Vol 12, No 4 (2021)

Chemical Resistance and Catalytic Activity of Copper in the Process of Electrooxidation of Ethanol in Strong Alkaline Media

Chemical Resistance and Catalytic Activity of Copper in the Process of Electrooxidation of Ethanol in Strong Alkaline Media

Title: Chemical Resistance and Catalytic Activity of Copper in the Process of Electrooxidation of Ethanol in Strong Alkaline Media
Klara Tarantseva, Natalia Politaeva, Konstantin Tarantsev, Mikhail Yakhkind, Ajay Kumar Mishra

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Tarantseva, K., Politaeva, N., Tarantsev, K., Yakhkind, M., Mishra, A.K., 2021. Chemical Resistance and Catalytic Activity of Copper in the Process of Electrooxidation of Ethanol in Strong Alkaline Media. International Journal of Technology. Volume 12(4), pp. 676-689

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Klara Tarantseva Department of Biotechnology and Environmental Protection, Penza State Technological University, 1a/11, Baidukova Prospect/Gagarina Street, Penza 440039, Russian Federation
Natalia Politaeva Graduate School of Hydraulic and Power Engineering Construction, Peter the Great St. Petersburg Polytechnic University, 29 Polytechnicheskaya str., St. Petersburg 195251 Russian Federation
Konstantin Tarantsev Department of Machinery Production, Penza State University, 40 Krasnaya str., Penza 440026 Russian Federation
Mikhail Yakhkind Department of Biotechnology and Environmental Protection, Penza State Technological University, 1a/11, Baidukova Prospect/Gagarina Street, Penza 440039, Russian Federation
Ajay Kumar Mishra Academy of Nanotechnology and Waste Water Innovations, University of South Africa, 5th floor Phapha building Florida, Johannesburg 1709, South Africa
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Abstract
Chemical Resistance and Catalytic Activity of Copper in the Process of Electrooxidation of Ethanol in Strong Alkaline Media

The possibility of using copper (Cu) as a catalyst in nonflowing membraneless alkaline fuel cells was investigated. In the present study, the interface between two immiscible liquids served as a virtual membrane. We studied two immiscible liquid systems: Ethanol+K3PO4+H2O (No. 1 electrolyte) and Ethanol+KOH+H2O (No. 2 electrolyte). Cyclic voltammetry, pulse chronoamperometry, and gas-liquid chromatography were used to study the corrosion resistance and catalytic activity of ethanol oxidation on Cu. Analyses of chromatograms obtained after charging and discharging the fuel cells revealed the presence of ethanol oxidation products, mainly in the form of acetaldehyde, which indicates the predominant mechanism of C2 oxidation of ethanol in the studied system. That is, with the release of two and four electrons during the oxidation of alcohol and the formation of acetaldehyde and acetic acid. Anodic currents of Cu dissolution in No. 1 electrolyte were an order of magnitude lower than in the No. 2 electrolyte. The catalytic activity of Cu during the oxidation of ethyl alcohol EtOH in the electrolyte with potassium hydroxide was at least five times higher than in the electrolyte with potassium phosphates, at almost the same pH values. The surface compounds on the Cu electrode in the first electrolyte were composed mainly of monovalent and bivalent Cu compounds, whereas they were composed of trivalent Cu compounds in the second electrolyte. Thus, the possibility of using Cu as a catalyst for the oxidation of EtOH in alkaline nonflowing membraneless fuel cells has been established. To further increase the catalytic activity of Cu and expand its commercial attractiveness as a catalyst, further research is needed to optimize its composition and structure and create bimetallic and multimetallic Cu-based electrodes, including nanostructured ones.

Copper; Ethanol fuel cells; Membraneless

Introduction

        Alternative energy sources, such as fuel cells, have become more attractive in recent years due to their potential in preserving natural fossil fuels and in decentralized energy systems in Industry 4.0 (Berawi et al., 2019; 2020). Due to the growing demand for such energy sources, attention has focused on improving the safety and extending the service life of fuel cells and minimizing pollution from used batteries (Masudin et al., 2019; Kusrini et al., 2020). Ethanol is a safe source of energy in fuel cells that meets all these requirements. However, the ethanol oxidation process is hampered by the need for expensive catalysts based on platinum and palladium, which significantly increases the cost of ethanol fuel cells (Kusrini et al., 2018). Therefore, scientists worldwide continue to search for inexpensive and effective catalysts based on non-noble metals.

The use of copper (Cu) as a catalyst for the electrochemical oxidation of alcohols in alkaline media has attracted considerable attention in recent years due to the development of alcohol fuel cells (Giri and Sarkar, 2016). Copper oxides have attracted particular attention, with a number of studies showing that they can improve catalytic activity and adsorption properties (Gizi?ski et al., 2020; El Attar et al., 2021). Previous studies investigated the catalytic activities of Cu as a base (Wu et al., 2017; Fahim et al., 2018) and as a component in multicomponent catalysts (Freitasa et al., 2014; Almukhlifia and Burns, 2015; Oznuluer et al., 2018). A number of studies showed that metal oxides copper (I) oxide - an inorganic compound (Cu2O) and copper (II) oxide - an inorganic compound (CuO) fixed on the surface of a copper (0) (Cu) electrode (CE) exhibited excellent electrocatalytic activity and stability (Sato et al., 2012; Wan, 2013; Gao et al., 2018; Scherzer, 2019). Li et al. (2014) used CEs with a Cu/Cu2O/CuO layer deposited on the electrode as a sensor for glucose determination. They showed that these CEs had high sensitivity due to a electrocatalytic reaction on the porous surface of crystalline CuO and an improved ability to transfer electrons, which was facilitated by the Schottky transition between Cu and Cu2O. Electrons generated by an electrochemical process can be more efficiently transferred from the oxide to the CE due to the large driving force created by the Schottky barrier at the Cu/Cu2O interface. Thus, Cu2O can serve as a suitable intermediate in a CuO reaction layer.

Prior to cyclic voltammetry, the dependence of the formation of various compounds on the applied potential remained unclear. Cyclic voltammetry gives reproducible results when studying the selectivity of catalysts (Mundinamani and Rabinal, 2014; Aristov and Habekost, 2015; Khalil et al., 2018; Hardi and Rahman, 2020), as the shape and atomic composition of the final films of compounds on metal can be controlled by ramping the potential, reaction duration, and the charge.

Several mechanisms for the oxidation of Cu in alkalis and alcohols have been described (Wan et al., 2013; Liu et al., 2020; Gizi?ski et al., 2020). Paixao et al. (2002) and Bueno and Paixao (2011) studied the oxidation of Cu in 0.1 M KOH. They revealed four peaks in the anodic region associated with the oxidation of Cu. These peaks were due to the formation of Cu2O, CuO, and copper (II) hydroxide is an inorganic crystalline or amorphous substance (Cu(OH)2) layers on the electrode surface. In addition to these, other particles can be deposited on the electrode surface, depending on the potential, pH of the solution, conditions of mass transfer, aging processes, and surface restructuring. X-ray diffraction, together with other methods, revealed three layers on the surface of the electrode after electrolysis: a green layer (Cu2O), a blue layer (CuO), and a black layer (Cu(OH)2 and copper (III) as an intermediate product in the reaction of ethanol electrooxidation (CuOOH-) (Paixao and Beriotti, 2004; Giri and Sarkar, 2016). After electrolysis in 0.1 M KOH, Cu(OH)2 was the dominant component on the electrode surface. Panah et al. (2019) analyzed the formation of various particles of copper oxides and hydroxides in an alkaline medium according to the potential (low or high). They confirmed the formation of Cu2O and CuO at low potential and Cu(OH)2 at high potential. Gizi?ski et al. (2020) showed that nanostructured copper oxides formed via anodizing had a highly-developed surface area and that they exhibited unique adsorption properties to crucial reaction intermediates. Therefore, electrodes with nanostructured copper oxides can be considered as platinum group metals electrodes substituents in fuel cells. El Attar et al. (2021) demonstrated that ethanol molecules are totally oxidized on Cu2O nanodendrites, with the formation of CO2 molecules as a final product (El Attar et al., 2021). Thus, Cu is a very promising catalyst for the oxidation of alcohols. However, there are no reports on the possibility of using Cu as a catalyst in nonflowing membraneless fuel cells. In these fuel cells, the phase boundary between two immiscible liquids serves as the membrane. The main advantage of these fuel cells is the absence of a membrane (as in membrane fuel cells) and laminar fluid flow (as in microfluidic fuel cells), which greatly reduces the cost of the design and simplifies their operation. The absence of a membrane, which cost is up to 30%, reduce the cost of a fuel cell.  The absence of pumps to circulate the fuel an oxidants simplifies the operation and maintenance the fuel cells.

Previously, we proposed a type of membraneless alcohol fuel cell (Tarantseva et al., 2020a; 2020b; 2020c). Our investigation revealed the possibility of separating phases only in a few highly alkaline two-phase ethanol-electrolyte-water systems. The choice of immiscible fluid systems for fuel cells were conducted on the basis of the following requirements: the two phases should not be mixed; both phases shall have electrical conductivity; one phase (anolyte) should contain the maximum amount of alcohol, the other phase (catholyte) should contain the minimum amount of alcohol. In our previous research, we proposed three strongly alkaline two-phase “ethanol–electrolyte–water” systems: based on potassium carbonate (Ethanol+K2CO3+H2O), potassium phosphate (Ethanol+K3PO4+H2O) and potassium hydroxide (Ethanol+KOH+H2O). For these systems, binodal curves were constructed, and the conditions  for the existence of a two-phase system were determined. In the first system, during oxidation of ethanol the deposition of potassium carbonate on the surface of the anode led to blocking of pores and impairment of its operation. Therefore, further studies the process of oxidation of ethanol on copper electrodes were carried out using the other two systems based on potassium phosphate and potassium hydroxide.

There appears to be no published data on the chemical resistance and catalytic activity of Cu in the Ethanol+K3PO4+H2O (No. 1 electrolyte) and Ethanol+KOH+H2O (No. 2 electrolyte) during ethanol oxidation. Some studies have characterized the behavior of Cu during the oxidation of alcohols in buffer solutions of potassium hydroxide (KOH) and sodium hydroxide (NaOH) in membrane fuel cells and microfluidic membraneless fuel cells (Paixao et al., 2002; Abd El Haleem and Abd El Aal, 2006; Wan, 2013; Giri and Sarkar, 2016). To study the possible use of copper in the oxidation of ethyl alcohol in nonflowing membraneless fuel cells, information on the catalytic activity and corrosion resistance of Cu in two proposed systems based on potassium phosphate and potassium hydroxide is required.

In this work, we describe for the first time the chemical resistance and catalytic activity of Cu in ethanol electrooxidation in a new type of nonflowing membraneless fuel cell using Ethanol+K3PO4+H2O (electrolyte No. 1) and Ethanol+KOH+H2O (electrolyte No. 2) systems.

Conclusion

        As shown in the present study, that the catalytic activity of Cu during the oxidation of EtOH in the electrolyte containing potassium hydroxide was at least five times higher than in the electrolyte containing potassium phosphates, at almost the same pH values. Analysis of the chromatograms obtained after charging and discharging the fuel cells revealed the presence of ethanol oxidation products in the form of acetaldehyde, which indicated that the predominant mechanism of oxidation of ethanol in both two-phase systems Ethanol+K3PO4+H2O and Ethanol+KOH+H2O was C2. This indicates that the use of a copper electrode in No.2 electrolyte in membraneless fuel cells is promising. The use of inexpensive copper catalysts instead of platinum catalysts, the absence of a membrane, and an acceptable rate of ethanol oxidation at room temperature allows the proposed fuel cells to be considered for use as a power source for small portable devices or remote sensors.

Acknowledgement

       The study was carried out with the financial support of the Russian Foundation for Basic Research within the framework of scientific project No.19-58-60002 UAR_t “A new type of membraneless fuel cells based on immiscible liquids, intended mainly for renewable fuels".

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