CO2 conversion to synthetic fuels using non-CRM catalysts

EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 1/9 A0711 CO2 conversion to synthetic fuels using non-CRM catalysts M. Fazio*, S. C. Zignani, M. Pascale, V. Chiodo, S. Maisano, N. Mondello, A. Carbone, A. Aricò Institute of Advanced Energy Technologies (ITAE) of the Italian National Research Council (CNR); *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract To date CO2 increasing emissions into the atmosphere represent a significant environmental threat that needs to be halted. Electrochemical reduction of CO2 (CO2RR) is deemed to be one of the most promising techniques to convert CO2 and water into green fuels, thus reducing CO2 emissions and storing renewable energy. Past studies have shown that the range of products that can be obtained from CO2RR are highly dependent on the electrocatalyst employed. Among the most commonly used electrocatalysts for electrochemical CO2 reduction in alkaline conditions there is copper, which electrochemically converts CO2 into more than 30 products, including hydrocarbons and alcohols [1], however, Cu-based electrodes present poor selectivity towards the formation of specific products. Nevertheless, a higher efficiency of the catalyst toward CO2RR can be obtained by proper engineering the catalytic surface in order to have a sufficient number of active sites. Experimental data demonstrated that a mixture of Cu-based structures with oxidizing states going from 0 to 2 can be obtained for the cathode through the oxalated method whilst a NiFeOx based catalyst was prepared for the anode according to the coprecipitation procedure. A membrane electrode assembly (MEA) was developed by cold pressing anode, cathode and a commercial anion exchange membrane as a polymeric solid electrolyte. Electrodes were prepared by spray coating deposition of a catalytic inks, prepared by sonicating a certain amount of the synthetized powder in ethanol, on a suitable support respectively a Sigracet GDL for the cathode and Bekaert Ni felt for the anode. Electrochemical experiments were carried out with a complete zero-gap cell operating under alkaline conditions at a 300 mA cm2 current density. Data from gas-chromatographic (GC) analyses of liquid and gaseous effluents from the cathode outlet stream were found to be in line with results featured in the literature regarding the promotion of intermediates that may lead to secondary reactions with production of gases such as H2, CO, C2H4 alongside to carbonaceous fuels like Et-OH and Pr-OH. EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 2/9 Introduction One of the most pressing challenges of the modern time is the transition to a sustainable and environmentally-friendly economy. In the transition period towards renewable energies, decarbonization by CO2 recycling through carbon neutral processes can be considered a significant path to pursue [2]. As the increase of the carbon dioxide (CO2) levels is considered to be one of the main contributors to global climate change, scientific and industrial research has been focusing on the development of new sustainable technologies that can efficiently reduce CO2 emissions by converting them into valuable products. Electrochemical reduction of CO2 (CO2RR) could represent a potential solution for the environmental problem by using renewable electricity to convert the excess of CO2 into efuels and valuable chemicals: by employing electrical energy, in fact, it is possible to convert CO2 into smaller compounds including carbon monoxide (CO), formic acid (HCOOH), methane (CH4), ethylene (C2H4), and alcohols. Efficient catalyst design represents a fundamental step to achieve significant results with this technology: properly structured catalysts, based on CRM-free materials, can, as a matter of fact, promote the desired reactions with high selectivity, stability, and cost-effectiveness as their configuration is directly related to their electrochemical properties [3-5]. By employing a suitable catalyst, it is possible to reduce the activation energy of these reactions, thus conveying chemical transformations toward target products as well as increasing the overall efficiency of the process. Among the most commonly used electrocatalysts for CO2RR in alkaline conditions, copper-based electrocatalysts represent the most promising material for CO2RR as copper can electrochemically converts CO2 into more than 30 products, including hydrocarbons and alcohols [1], however, at the same time, they show poor selectivity towards the formation of specific products. This may represent a major drawback for the development of advanced and selective materials; however, it proves that a higher efficiency of the catalyst toward CO2RR could be obtained by proper engineering the catalytic surface in order to have a sufficient number of active sites [6]. This study focuses on the advanced characterization of various catalysts designed to optimize CO2 conversion via electrolytic processes. Multiple experimental techniques were employed to assess the physical and chemical properties of the developed electrocatalysts in order to determine the major factors that influence their performance. Moreover, the electrochemical performance of the developed catalysts in CO2 reduction was thoroughly investigated by the means of polarization curves, impedance spectroscopy, and chronoamperometry experiments with the purpose of evaluating their reactivity and electrical stability. Results from electrochemical studies are utilized to understand the charge transfer efficiency, as well as the dynamics of CO2 molecule adsorption and desorption at the catalyst’s active sites. 1. Scientific Approach 1. Electrocatalyst development Non-CRM electrocatalysts for both cathode and anode evolution reactions have been developed and assessed in order to be employed in CO2RR. In particular, the investigated catalysts are, respectively, the NiFe-Layered Double Hydroxide (LDH) for the anode and a Cu-based catalyst that has been designed for the cathode. The developed materials are properly engineered to be operated at high current densities with low overpotentials, thus displaying a high active surface area and minimal degradation rates. A cost-effective and efficient co-precipitation method is employed as the main synthetic route in order to obtain catalyst powders with the desired characteristics [7]. Both ex-situ and in-situ EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 3/9 characterizations of the developed materials are carried out in order to evaluate the catalyst properties and include the determination of crystallite size and active phase dispersion and the catalysts’ efficiency and stability under working conditions. The in-situ performance assessments are conducted in the presence of an AEM (Anion Exchange Membrane) ionomer and electrolyte in a zero-gap cell electrolyser. 1.1 Oxygen evolution electrocatalyst NiFe oxide-hydroxide (LDH) based anode catalyst was synthesized with the purpose of improving the catalytic activity and stability in alkaline environment for oxygen evolution reaction. The bimetallic catalyst effectively contributes to stabilize reaction intermediates and improve the overall reaction kinetics. The unique layered structure of LDHs supplies a large surface area and abundant active sites, which are essential for OER, and promote the insertion and mobilization of hydroxide ions, assisting the electrocatalytic process. 1.2 CO2 evolution electrocatalyst Copper-based non-noble metal oxides are considered promising electrocatalysts for the electrochemical reduction of CO2, a greenhouse gas, for their capability of converting it into more than 30 products that include valuable hydrocarbons and alcohols, thereby contributing to carbon recycling and the development of sustainable energy sources. Copper (Cu) compounds are especially effective for the CO2 reduction process for their ability to stabilize reaction intermediates. Moreover, the formation of several copper oxidation states (Cu0, Cu1+, Cu2+) permits different reaction routes with the development of various products of interest such as methane (CH4), ethylene (C2H4), and ethanol (C2H5OH) [8]. As a results, different synthesis pathways to design copper-based oxides with different morphologies and compositions (e.g., copper oxide (CuO) and cuprous oxide (Cu2O)) have been under investigation in order to obtain advanced electrocatalyst powders for CO2RR with suitable catalytic properties, such as surface area, porosity, and ion conduction and enhance the catalyst’s CO2 reduction efficiency. Copper-based oxide catalysts’ performances can be influenced by numerous factors such as electrolyte composition, reaction conditions (e.g., pH, temperature) along with their physical properties, therefore, in order to achieve high activity and selectivity in CO2 conversion, an optimization of these elements is necessary. 2. Experiments 2.1. Synthesis of OER catalyst Nickel Nitrate Hexahydrate (Ni(NO3)2∙6H2O, Sigma Aldrich) and Iron Nitrate Nonahydrate (Fe(NO3)3∙9H2O, Sigma Aldrich) were dissolved in ultrapure distilled water, then the solution was placed in a heated water bath, where it was stirred at 60 °C. When the solution reaches the desired temperature, a 2 M solution of sodium hydroxide (NaOH) solution is added dropwise until pH 9 is achieved. The suspension is then maintained at pH 9 and 60 °C and then stirred for 3 hours. Afterwards, the formed precipitate is filtered and washed with hot ultrapure distilled water (approx. 40 °C) and, lastly, dried at 80 °C in an oven for 24 hours. 2.1.1 Physicalchemical characterizations The physicochemical properties of the synthesized electrocatalyst were thoroughly investigated by several characterization techniques including XRD and XRF. For the X-ray EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 4/9 diffraction (XRD) investigation was used a D8 Advance diffractometer (Bruker AXS, Germany), operating with a Ni b-filtered Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range 5– 100° at 40 kV and 20 mA and a scan step of 0.03° s−1. XRD technique was used to analyze the crystalline phase of the NiFe anode catalyst developed. Fig. 1 shows the registered XRD patterns in which three different structures related, respectively, to Ni(OH)2∙0.75H2O , Ni(OH)2 and FeO(OH). Both broad and thinner peaks are observed in the patterns and are related to a mixture of fine and larger crystallite sizes of the particles. Through the Scherrer formula, it was possible to calculate an average crystallite size of 45.7 Å (4.57 nm). Fig.1 XRD patterns of the NiFe (LDH) anodic catalyst. X-ray fluorescence analysis (XRF) was carried out using an S8 TIGER spectrometer (Bruker AXS, Germany) to evaluate the catalyst elemental composition. The instrument, equipped with a rhodium anode tube (power 4 kW and 75 µm Be window and LiF 220 crystal analyze), allowed to obtain semi-quantitative results of the atomic ratio of Ni:Fe oxide-hydroxide. The composition of the synthesized catalyst was shown in Table 1. Table 1. Atomic composition of anodic NiFe (LDH) electrocatalyst as assessed by XRF analyses. 2.2. Synthesis of HER catalyst Copper-based electrocatalysts that are usually employed on the cathode side for the CO2 reduction are synthetized by co-precipitation method. Precursors of the materials are dissolved in distilled H2O, then the solution is placed into a Teflon beaker installed in a bainmarie at 60 °C. A 1 M solution of sodium hydroxide (NaOH) is added dropwise to achieve pH 9, afterwards the dispersion is stirred for 4 hours in order to facilitate the precipitation of the compound that is, then filtered and washed with hot water. Finally, the catalyst is dried in an oven at 80 °C, treated in a ball milling system at 160 rpm for 17 hours and subsequently sieved. % at Ni Fe NiFe 85.4 14.6 EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 5/9 2.2.1 Physicalchemical characterizations XRD analysis reported in Fig. 2 shows the diffraction peaks of the synthesised Cu-based catalyst. A particle size of 8 nm was registered. Fig. 2 XRD patterns of the CuO-based cathodic unsupported catalyst. 2.3 Membrane electrode assembly (MEA) development and electrochemical study The catalytic inks were prepared by dissolving the catalyst powder (67 wt.%) in ethanol and mixing a Fumatech ionomer (33 wt.%). The dispersion is, then, sonicated for 30 min. A slurry of the anode ink was deposited using a spray coating technique, respectively, on a Nickel Felt backing layer (Bekaert) with a total metal loading of 2.5 mg cm-2 for the anode. Cold-assembly procedure was adopted to prepare the membrane-electrode assemblies (MEAs) with 5 cm2 active area for catalyst screening and durability studies. Before the assembling, it is necessary to exchange both electrodes and membrane for 24 hours with hydroxide ions containing solution in order to activate all the components. The MEAs were assembled in a single-cell housings made of nickel plates characterized by a serpentine flow field channel. Teflon® gaskets were used to seal the cell and avoid any leakage of the electrolyte solution. Cell compression was 2.5 N m per each tie rod. During all experiments, the anode side was fed with 1 M KOH solution recirculated at a flow rate of 1 ml min-1 cm-2 using a peristaltic pump. The MEAs were electrochemically characterized to determine the performance of the developed electrocatalysts. The electrochemical investigations, concerning galvanostatic polarization curves (cell potential as a function of current density) and galvanostatic durability tests (cell voltage versus time), were carried out with a Keithley power supply system (Tektronic). Electrochemical impedance spectroscopy (EIS) analysis was carried out with a PGSTAT Autolab 302 Potentiostat/Galvanostat equipped with a current booster (Metrohm) and a Frequency Response Analyzer (FRA). The impedance measurements were performed at three different cell voltages (1.5 V, 1.8 V, 2 V), in a frequency range between 100 MHz - 10 mHz [7]. 2.3.1 Electrochemical characterization of the anode catalyst A slurry of the anode catalysts dispersed in ethanol was deposited on nickel felts supplied by Bekaert with a total oxide loading of 2.5 mg cm-2. The Pt/C cathode has been prepared with a total metal loading of 1 mg cm-2. A Fumatech FAA3-50 anion exchange membrane in the OHform was used as separator between anode and cathode compartments. Membrane-electrode assemblies (MEAs), with 5 cm2 geometrical area, were prepared by a EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 6/9 cold-assembly procedure. KOH 1M solution was supplied by a peristaltic pump to the anode compartment, at a flow rate of 4 mL min. The overpotential of the Ni-Fe oxide anode with respect to the thermoneutral potential for oxygen evolution reaction (1.48 V) was thus determined from single cell experiments using a benchmark Pt/C cathode according to the formula: ηNiFe− anode = ECell − ηPt− cathode − IR − Eth where ηNiFe-anode is the anodic overpotential versus the thermoneutral potential for the NiFe oxide anode, Ecell is the overall cell potential, ηPt-cathode is the overpotential of the Pt/C benchmark cathode, IR is the ohmic drop with R determined from the series cell resistance and Eth is the thermoneutral potential. The overpotentials of the cathodic catalysts were determined by employing the same Ni-Fe oxide as before through the formula: ηcathode =ECell− IR− Eth − ηNiFe− anode in which ηcathode is the overpotential of the CRM free cathode catalyst. 3. Results 3.1 Electrochemical characterization of the anode catalyst NiFeOx anode electrocatalyst was prepared at CNR-ITAE and tested in single cell, using Pt/C as cathode and Fumatech as electrolyte. This cell was fed with 1 M KOH to the anode side. Fig. 3 compares the polarisation curve and the IR-free curve of the NiFe as anodebased cell investigated at 50 °C. At 1.7 V vs. RHE (IR-free), the mass activity and the current density are, respectively, 140 A/g and 350 mA/cm2. Fig. 3 Polarisation curves of MEAs based on Pt/C as cathode, a Fumatech membrane and NiFeOx as anode for anode material characterization. Fig. 4 displays EIS analyses carried out at different potentials (1.5V – 1.8V – 2 V). The MEA that mounted NiFe LDH at the anode and Pt/C at the cathode showed low series and polarization resistances (Rs-intercept at high frequency with the x-axis), a phenomena that is more evident in the EIS analysis recorded at 1.8V and 2V. EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 7/9 Fig. 4 EIS of the MEA based on Pt/C as cathode -Fumatech membrane and NiFeOx as anode for anode material characterization. 3.2 Electrochemical characterization of the cathode catalyst Electrochemical characterization of the cathode material was also carried out through polarization curves and impedance spectroscopy analysis by developing a MEA (membrane-electrode assembly) that mounted NiFe at the anode side on a Ni felt porous transport layer (Bekaert) and CuxO at the cathode side coated on Sigracet gas diffusion layer. The total catalyst loading was 2.5 mg cm-2 for the anode and 1 mg cm-2 for the cathode. Humidified CO2 was fed to the cathode compartment at 65 °C, with a flow rate of 40 mL per minute. In terms of electrochemical performance, the co-electrolysis cell achieves 3 V/cell at 0.25 A cm-2 and 2.6 V/cell at 1 A cm-2 in the polarization curve carried out at the end of the test (Fig. 5a). A galvanostatic test was carried out by applying a 300 mA cm-2 current density for 2 hours to evaluate cell performance (Fig. 5b) and the mean cell voltage in durability test was just below 2.5 V/[email protected] A cm-2. Figs 5c-d show the EIS analysis carried out at the beginning and at the end of the stability test. The series resistances (Rs) and polarization resistances (Rp) at the end of the durability test results lower compared to Rs and Rp in EIS BoT (Fig. 5c). In terms of Faradaic efficiency, the co-electrolysis cell achieves about 15% of gaseous carbonaceous products and 0.45% of liquid carbonaceous products. The sum of carbonaceous products is less than 20% and the total carbonaceous products and hydrogen is 100%. EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 8/9 Fig. 5 a) polarization curves (BoT) (EoT), b) durability test 300 mA cm-2 at 50 °C for 2 hours, c) EIS investigations at 1.5V, 1.8V, 2V BoT; d) EIS investigations at 1.5V, 1.8V, 2V EoT; e) and f) Faradaic efficiency for gaseous carbonaceous, and liquid carbonaceous products. 3.3 Conclusion In anion exchange membrane (AEM) electrolysis technologies, the employment of NiFe as an anode catalyst presents several advantages in terms of efficiency and long-term stability. The high electrocatalytic activity of NiFe in addition to an optimal utilization of active sites derived by the synergism between nickel and iron facilitates the oxygen evolution reaction (OER) by improving ion transport and minimizing energy losses, thus, enhancing the overall electrolysis process. Furthermore, as a consequence of the material’s mechanical strength and resistance to corrosion, NiFe can be employed for extended operation in AEM electrolysis system. Cathodic CuO-based electrocatalysts showed significant potential for the electrochemical reduction of CO2, as they can selectively convert CO2 on the catalyst’s surface, into suitable e-fuels and chemicals. Selectivity towards target hydrocarbons and compounds can be further improved by proper engineering the material’s structure, composition, and morphology, therefore, enhancing the overall conversion efficiency. By combining NiFe for OER and CuO for CO2RR, it is possible to improve the energy efficiency and sustainability of AEM-based electrochemical systems, increase the performances with respect to reaction selectivity and system stability and offer a practical route for transiting to a low-carbon economy as it directly addresses carbon utilization and mitigation. Future research efforts should focus on further optimizing catalyst design, exploring real-world operating conditions, and scaling up these technologies. Developing these improvements will be crucial in enabling commercial deployment and maximizing the environmental benefits of these innovative electrochemical processes. EFCF 2025: Fuel Cells, Electrolysers & H2 Processing 1 - 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244115 A0711 Page 9/9 Acknowledgements The authors acknowledge the financial support provided by EU H2020 GREEN DEAL “ECO2FUEL” project “Large-scale low-temperature electrochemical CO2 conversion to sustainable liquid fuels” Grant Agreement number: 101037389. References [1] S.C. Zignani, M. Lo Faro, A. Carbone, A. Palella, L. Spadaro, A.S. Aricò, Alkaline electrolysis using CuOx cathode for the conversion of carbon dioxide into liquid fuels, Materials for Renewable and Sustainable Energy 12 (2023) 141–146. [2] J.H. Wesseling, S. Lechtenböhmer, M. 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