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Electrochemical reduction of CO 2 using cuprous oxide particles supported on carbon paper substrate

Received : 27 July 2019 Received in revised: 23 August 2019 Accepted : 30 August 2019 Available online : 27 November 2019 DOI: 10.26655/AJNANOMAT.2020.2.1 Electrochemical reduction of CO2 is so important in mitigating the greenhouse related environmental concerns. Recently, oxidized forms of metals instead of pure metals have gained a great deal of attention due to the difference in product selection between the two classes of electrode materials. Since copper has been widely used in producing carbon-intensive products, various studies have been dedicated to evaluate its oxidized form. In this research study, we focused on using cuprous oxide particles supported on hydrophobic carbon paper substrate. The structure of the carbon paper provides unique reaction sites while the micron-sized particles can help to provide new insight about using smaller surface area to volume ratio as compared to previous reports on oxidized copper nanoparticles. Formic acid, ethylene, and CO were produced as a result of our experiments which show improved product selection compared with the pure copper nanoparticles. The potential and time dependence of these products are presented in this study along with a discussion on the origin of CO2 reduction.


Introduction
Renewable energy sources such as solar and wind have started to play an increasing role in the energy landscape of our planet. However, a major drawback of these sources has been the intermittent supply and lack of energy stability. Research about the scenarios for sustainable energy suggest that the electrochemical reduction of CO2 (CO2RR) can help to address this issue by utilizing the excess energy from renewable energy sources by converting CO2 into some chemical compounds such as methane, methanol or carbon monoxide (for Syngas), which can serve as energy vectors and can be used to compensate the energy fluctuations from the renewable energy sources [1].
To date, most of the studies on CO2 reduction have focused on metal electrodes in which copper [2,3] has been the most popular due to its propensity for producing higher order carbon products. Recently, metal-oxide catalysts have begun to gain attention as well due to their decent faradaic efficiency and selectivity for electrochemical reduction of CO2. Metal oxides have also shown difference in product distribution compared to pure metals [4].
Among metal oxides, cuprous oxide (Cu2O) has proven to be the most active one for generating multi-carbon products [5][6][7]. The first such study on Cu2O was reported by Frese et al. in 1991 [8] who had compared results between air-oxidized and anodized copper and had observed methanol as the major product. Le et al. [9] in 2011 had also studied oxidized copper (Cu2O) and had reported production of methanol along with traces of CO while the following year Li et al. [10] used thicker layers of anodized Cu and decreased the onset potentials for CO formation.
More lately, efforts are being made towards improving the selectivity of multi-carbon products by changing the morphology and sizes of Cu2O particles. Wang et al. [11] in 2019 was able to improve the efficiency towards ethylene (C2H4) by using Cu2O catalysts grown on graphite sheets functionalized with ionic liquid. CH4, C2H6, and other products were also reported in the study. Similarly in 2019, Tan et al. [12] developed a novel metal organic framework in Cu2O-based electrodes and reported improved faradaic efficiency towards CH4 while also producing other higher order carbon products.
The role between oxide layer and metal phase and their contribution towards product selectivity in CO2RR has also been an issue of debate [13,14]. It was demonstrated by Kim et al. [14] that electrodeposited Cu2O can aid in the selective formation of C2H4 over CH4 and had concluded that both the oxide component and metal affect the selective electroreduction of CO2 toward multi-carbon products. Previous works have shown that the product selectivity of Cu2O is dependent upon the morphology and size of the oxide particles while the formation of Cu 0 with O vacancies within these Cu2O particles during CO2RR is also considered as an important factor in the efficacy of Cu2O as catalysts [15,16]. Wang et al. [11] had suggested that the oxide matrix plays a key role in maintaining high performance of oxidederived Cu 0 and that the grain boundaries impact the product selectivity during CO2RR.
In this study, we report the use of micronsized cuprous oxide (Cu2O) particles to analyze the impact of using smaller area to volume ratio on product selectivity. Carbon fiber paper (CP) was chosen as the substrate material due to its hydrophobic composite of carbon and carbon fibers which apart from being conductive also allow higher porosity and cheaper price compared with that of the glassy carbon substrates while offering enhanced structural rigidity than carbon cloth. The surface porosity in electrode structures has been reported to change both reagent and product transport pathways [17] and these changes in transport phenomena have shown to increase current density and change product selectivity [18,19]. Since most studies have used smaller sizes of Cu2O particles in combination with either copper metal [20][21][22][23] or glassy carbon [12,24], it was our hypothesis that the use of larger size of Cu2O particles supported on a structurally porous CP substrate would offer new product distributions.

Materials and Methods
The Cu2O powder was commercially procured from Sigma Aldrich vendor. The method of preparation for this powder was based on chemical reduction approach which typically involves the reduction of metal salts in various solvents and agents, which allows micron-size particles to form [25][26][27][28][29]. Cu2O particles were deposited on a commerciallyprocured fuel cell carbon paper (TPI-120T). A solution was prepared with ethanol as the solvent and nafion as the adhesive material along with the Cu2O particles in the ratio 30:5:1, respectively. The mixed solution was treated ultrasonically for 30 min to form a homogeneous ink. A 50 μL solution was then drop-casted onto 1 cm 2 of carbon paper (CP), which acted as a substrate. The control electrodes in the experiments were CP without the Cu2O particles.
Home-made H-type leak-tight electrochemical cells were used for CO2 reduction, with 100 cm 3 volume and 70 mL of electrolyte. The end-products were collected for further analysis. The working and reference electrodes were put in the same compartment and separated from the counter electrode by a nafion film (DuPont: N117). Ag/AgCl reference electrode was employed while a platinum electrode was used as the counter electrode. A working device for the reduction of CO2 requires a source of protons and electrons and so platinum was used for the oxygen evolution reaction (OER), which served as the other halfreaction for this purpose. Water was oxidized as: 2 H2O → 4 H + + 4 e − + O2. 0.5 M KHCO3 with a pH of 8.59 was used in the cathode and anode electrolyte. The pH changed to 7 after CO2 purging for 25 min prior to the experiment while the pH changes during the experiment were not observed. Since CO2 reduction is a redox reaction, evolution of O2 takes place at the counter electrode and so the separation between the working and counter electrodes helps to ensure that the recombination of the products produced on these two electrodes can be effectively prohibited. The gas product was then sampled manually through syringe using HP 6890 Gas Chromatographer (GC). The liquid phase products were determined by Ion Chromatography (IC) and collected with a Thermo Scientific Dionex ICS-5000 system. The Panalytical X'Pert X-ray diffraction machine was used to study the different peaks originating from the substrate and Cu2O. Hitachi SU8020 high resolution scanning electron microscope with EDX analysis was used for characterization while Hitachi Miniscope TM-1000 was used for images of CP substrate. Electrochemical experiments were performed using Solartron 1280C potentiostat while a stir bar was used within the electrochemical cell. Electrodes were prepared by affixing a copper wire on one side of the CP substrate with kapton tape, which was then covered with aryldyte to ensure that there was no interaction between the electrolyte and the copper wire. The electrolyte was purged with CO2 or Ar for 25 min prior to experimental or control trials, respectively. All experiments were conducted in ambient conditions.

Results and Discussion
A loading amount of 1 and 2 mg/cm 2 was tested for CO2 electrochemical reduction. SEM/EDX analysis determined the particle size distribution of Cu2O to be between 2 μm and 5.5 μm. The relatively large size of Cu2O grains in our study allowed for a different surface area and orientation from the previously reported studies. The SEM image of CP in Figure 1 and Cu2O on CP ( Figure 2) distinguishes between the structure of the substrate and electrode with catalyst loading of 2 mg/cm 2 .   The relative IV response in CO2 saturated and inert environment ( Figure 3) revealed a nominal difference in current densities, reflecting that hydrogen evolution reaction (HER) takes place in the inert environment. In the CO2-saturated environment, on the other hand, HER competes with CO2RR. A stirrer at 350 rpm had been employed to ensure that the bubble formation did not impact the current densities. It is pertinent to mention that even if all other conditions are constant, the absence of stirrer in an EC setup can drastically change the IV response. As an example, for loading amount of 1 mg/cm 2 , it was reported by Chang et al. [30] that the current density for Cu2O on carbon cloth with no stirring, was 6 mA/cm 2 at -1.7 V vs SCE. However, we observed current densities of almost 11 mA/cm 2 with a stirrer at around the same potential and loading. Experiments conducted without a stirrer at exactly the same conditions gave us a current density of about 7 mA/cm 2 .
The electrochemical reduction of CO2 using Cu2O particles with loading amount of 2 mg/cm 2 produced formic acid as the major product. Ethylene and carbon monoxide were produced as the secondary products. The faradaic efficiency of these products varied with the cathodic potential and are shown in Figure 4a. A base-10 log scale is used for the y-axis to better represent the trend between FE and cathodic potential. The experiments were conducted for 35 min. The ethylene presence was detected from the onset of CO2 reduction at -1300 mV vs SHE. The FE for ethylene was 2.8% at the onset potential, which decreased to almost 0.7% at -1600 mV vs SHE, and then went up again to 5.9% at a potential of 1800 mV vs SHE. The faradaic efficiency for CO followed a similar trend to C2H4, whereby its FE decreased at potentials of -1500 mV and -1600 mV but increased to almost 5.1% at -1800 mV. The formic acid however showed an opposite trend with a FE of 13.3% at -1300 mV, which reached upto 31% at -1600 mV and then decreased to about 8.8% at -1800 mV.
This inverse trend between faradaic efficiency of formic acid and C2H4, CO can possibly reflect different origins of CO2RR. Control electrodes (carbon paper) generated formic acid during CO2RR (Figure 4b) but the maximum faradaic efficiency was only 7% at -1300 mV vs SHE. However, the faradaic efficiency of the formic acid was significantly increased when cuprous oxide particles were employed. It can be observed that at potentials of -1500 and -1600 mV vs SHE, the Cu2O catalysts aid in enhancing the faradaic yields of the primary product (formic acid) being produced on the CP substrate during CO2RR. For potentials outside this range, Cu-based catalyst played a more active role in formation of the CO and C2H4. It has been reported that CO formation on Cu2O aids in suppressing HER [31] and increases conversion of hydrocarbon products since CO acts an as intermediate specie [32] for higher order carbon products. This is the reason why CO and C2H4 followed a similar trend in which the increase of CO lead to an increase of C2H4 while a decrease in CO led to a decrease in C2H4. Interestingly, methane and methanol were not detected.
For the experiments conducted at potentials more negative than -1800 mV vs SHE, the current density increased to more than 20 mA/cm 2 , causing slight color degradation of the electrodes, which may be a reflection of Cu2O erosion. Increased nafion binder ratio during the electrode preparation can possibly help to maintain electrode stability [33]. Electrodes with Cu2O loading of 2 mg/cm 2 were experimented for multiple time duration of 35 min, 60 min, 90 min and 120 min at -1300 mV vs SHE and -1600 mV vs SHE and this time dependence of the products is shown in Figure  5. A subtle trend could be observed for both potentials: a gradual decrease of FE for formic acid was observed while C2H4 especially at -1300mV vs SHE slightly increased with time. This can originate from the thermodynamic reducibility of the Cu2O phase under CO2 reduction conditions, which has been a major concern for attempts to use the Cu oxidation state to control selectivity for CO2 reaction products [22]. The purboix diagram for Cu in aqueous solutions [34] indicates that Cu 0 is more stable at negative potentials vs SHE at neutral pH. From the results presented here, it can be speculated that at least a partial reduction of the Cu2O component has been taking place with time which impacts the product selectivity during CO2RR. Since energy level calculations have shown that it is the reduction of the bulk phase which is more likely than surface reduction of Cu2O grains [35,36], it can be speculated that the surface layer may exist as a hybrid structure made up of oxidized Cu at the interface with H2O molecules, along with a neighboring Cu 0 component beneath. To analyze the impact of loading amount of catalyst on the trend originating from the Cu2O/CP, 1 mg/cm 2 of Cu2O was tested for CO2RR and the faradaic results are shown in Figure 6. Formic acid continued to remain the dominant product from CO2RR and the maximum faradaic yield of 13% was seen at -1500 mV. In comparison with the control electrode (carbon paper), the FE towards formic acid was still almost twice greater. Lower loading of catalysts show that product selectivity is affected by the competing H2 reaction on the carbon paper and the sizable diminishing of Cu2O active sites is suggested to decrease the overall activity for CO2RR. C2H4 faradaic efficiency remained around 1% for the entire potential range. Our experiments with 0.5 mg/cm 2 loading did not provide consistent reproducability while increasing the loading amount to 4 mg/cm 2 also did not significantly impact the product distribution and so their results are not discussed here. Energy dispersive X-ray (EDX) spectroscopy was used to analyze the elemental composition of cuprous oxide electrode for loading amount of 2 mg/cm 2 and is shown in Table 1. The Cu/O ratio was approximately 1.5 due to the higher presence of Cu in the Cu2O structure. The possibility of substrate oxidization and margin of error in EDX measurement may be the reason for the deviation from the expected Cu/O ratio of 2. Carbon was detected from the substrate while potassium presence was a result of the reduction of K+ ions [11] present in the electrolyte. Fluorine was detected due to the hydrophobic polymers on CP that assist in suppressing the CO2RR activity on CP.
The XRD data of our electrode in Figure 7 shows only Cu2O and carbon peaks before and after CO2RR. The peak positions are at a good agreement with those for Cu2O powder obtained from the International Center of Diffraction Data card (JCPDS file no. 05-0667), confirming the formation of a single cubic phase Cu2O with a cuprite structure [37], where the oxygen atoms form a body centred cubic lattice while copper atoms form a face-centred cubic lattice such that every copper atom is halfway between two oxygen atoms. The peaks with 2θ values of 29.6, 36.5, 42.4, 61.5, 73.6 and 77.6 correspond to the crystal planes of 110, 111, 200, 220, 311 and 222 of crystalline Cu2O, respectively. Peaks originating from the substrate CP were observed at 2θ value of 26.5 and 54.6 [38]. No characteristic peaks of Cu metal or CuO were observed in the XRD patterns for samples measured after CO2RR. An in-situ raman spectroscopy study by Mandal et. al. [39] for the electrochemical reduction of CO2 on Cu2O, had reported that the reduced Cu phase in Cu2O can easily and instantaneously undergo re-oxidation after CO2RR experiments which may explain the absence of any Cu phase in our XRD measurements.  The production of formic acid during the CO2RR experiments is suggested to originate from the difference in binding energy for the intermediate species. CO production occurs primarily through a key carbon-bound intermediate, *COOH, while formic production proceeds through a key oxygen-bound intermediate, *OCHO [40]. Since formic acid was also observed from experiments involving control electrodes (CP), it reflects that the surface of carbon paper has a propensity towards *OCHO and the presence of Cu-based catalysts assist in enhancing the faradaic yields of formic acid. The modification of reaction energy for formic acid generation due to Cu has been previously reported in DFT studies of Peterson [41] and Shin [42].
The selectivity towards C2H4 instead of CH4 is also an important observation. Hori et al. [43] had previously compared a series of different orientations of single crystal Cu and had shown that C2H4 formation was strongest on Cu (711) surfaces while Baturina et al. [44] and Riske et al. [45] had also suggested that C2 products are produced during CO2RR on low-coordination sites and low-index facets (edges, defects, and corners) due to their similarity to Cu (311), Cu (511), and Cu (711). We believe that the partial reduction of Cu2O particles and the oxygen vacancies has an impact on the grain boundaries [14][15][16] and allow formation of low coordination surface sites which cause preferential production of C2H4. A factor in the performance of Cu2O during CO2 electrolysis is believed to originate from the hydroxide ions (OH -) that are generated during CO2RR at the electrode surface. The rise of local pH close to the surface layer has been reported to limit the reduction of Cu2O to metallic Cu at the electrolyte interface, which assists in the continuous generation of C2H4 [46].
In summary, this study demonstrates selectivity towards C2 ethylene with no CH4 presence. It is our contention that the use of particles with smaller area to volume ratio was able to allow a Cu2O structure, wherein the reduced Cu 0 with oxygen vacant sites had an impact on the grain boundaries and allowed selective production of C2H4 from among the possible multi-carbon products. CO, which is an intermediate specie for C2H4, was also observed. Formic acid production is believed to have originated from the CP substrate, while the presence of Cu-based catalysts caused a potential dependent enhancement of its faradaic efficiency. The production of formic acid as liquid product is also more important due to its higher relative profitability index; a term defined by the ratio of economic value to the required amount of electrical energy per mole of products [47].

Conclusions
The electrochemical reduction of CO2 was attempted on the Cu2O particles with loading amounts of 1 mg/cm 2 and 2 mg/cm 2 . Electrochemical reduction of CO2 showed that formic acid was the major product along with C2H4 and CO while hydrogen gas was the parasitic product from the experiments. The use of these catalysts with relatively bigger size on a hydrophobic carbon paper substrate helped to narrow the selectivity of the products in comparison with the copper metal, which generally produced more number of products during CO2RR. We contend that this work can inspire to further study micron-sized cuprous oxide catalysts for achieving similar selectivity with higher faradaic efficiency of C2 products from CO2RR.

Disclosure Statement
No potential conflict of interest was reported by the authors.