|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|
Copper; Ethanol fuel cells; Membraneless
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
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.
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.
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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