• Energy Research

The photoelectrochemical conversion of CO2 into value-added products emerges as an attractive approach to alleviate climate change. One of the main challenges in deploying this technology is, however, the development and optimization of (photo)electrodes and photoelectrolyzers. This review focuses on the fabrication processes, structure, and characterization of (photo)electrodes, covering a wide range of fabrication techniques, from rudimentary to automated fabrication processes. The work also highlights the most relevant features of (photo)electrodes, with special emphasis on how to measure and optimize them. Finally, the review analyses the integration of (photo)electrodes in different photoelectrolyzer architectures, analyzing the most recent research work that comprises photocathode, photoanode, photocathode-photoanode, and tandem photoelectrolyzer configurations to ideally achieve self-sustained CO2 conversion systems. Overall, comprehensive guidelines are provided for future advancements in developing effective devices for CO2 conversion, bridging the gap towards the use of sunlight as the unique energy input and practical applications. The authors fully acknowledge the financial support received from the Spanish State Research Agency (AEI) through the projects PID2020-112845RB-I00, PID2019-104050RA-100, TED2021-129810B-C21, and PLEC2022-009398 (MCIN/AEI/10.13039/501100011033 and Unión Europea Next Generation EU/PRTR). This project has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No 101118265. Jose Antonio Abarca gratefully acknowledges the predoctoral research grant (FPI) PRE2021-097200.

The development of efficient photoanodes that reduce external energy requirements for the electrochemical conversion of CO2 to formate is essential for the future implementation of this technology. In this work, we explore different photoanode structures based on electrodeposited BiVO4 onto transparent FTO substrates to achieve a more efficient PEC reduction of CO2. Among the tested structures, the photoanode incorporating a Bi2O3 underlayer, which enhances the BiVO4-FTO interface by reducing electron-hole recombination, exhibits the best PEC performance. Integrating this photoanode into a CO2 photoelectrolyzer with back visible light illumination achieves an impressive current density of −29 mA cm−2 at constant −1.8 V (vs. Ag/AgCl). Using a Bi/C GDE as the cathode, the system produces up to 56.2 g L−1 of formate with a Faradaic efficiency of 96 %. In terms of energy performance, illuminating the photoanode reduces energy consumption by nearly 40 %, bringing it down to 317 kWh kmol−1, with an energy efficiency of 38 %. The external bias can be further decreased by increasing the irradiation intensity to 2.5 suns using concentrated solar light, resulting in an additional 10 % reduction in energy consumption (290 kWh kmol−1), while maintaining high conversion efficiencies for CO2 to formate (over 95 % Faradaic efficiency). Besides, energy efficiency improves by 12 %, as the cathodic potential is reduced to −1.65 V (vs. Ag/AgCl). These results represent significant progress in reducing the external bias required for CO2 to formate conversion in PEC systems, marking a step toward the industrial application of CO2 conversion technology. The authors gratefully acknowledge Grant TED2021-129810B-C21 and PLEC2022-009398 funded by MICIU/AEI/10.13039/501100011033/ and by the “European Union NextGenerationEU/PRTR”, and Grants PID2022-138491OB-C31, PID2022-138491OB-C32 and PID2022-138491OB-C33 funded by MICIU/AEI/10.13039/501100011033 and by “ERDF/EU”. This project has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No 101118265. Marti Molera acknowledges AGAUR-Generalitat de Catalunya for 2024 FI-1 00421 predoctoral grant. The authors thank Dr. Julià Lopez Vidrier for the access to the UV–vis equipment. Jose Antonio Abarca gratefully acknowledges the predoctoral research grant (FPI) PRE2021-097200. Ivan Merino-Garcia also acknowledges Grant RYC2023-043378-I funded by MICIU/AEI/10.13039/ 501100011033 and by ESF +.

The photoelectrochemical reduction of CO2 to renewable fuels and valuable chemicals using solar energy is a research topic that has attracted great attention recently due to its potential to provide value-added products under the Sun, solving the issues related to global warming at the same time. This review covers the main research efforts made on the photoelectrochemical reduction of CO2. Particularly, the study focuses in the configuration of the applied reactor, which is a topic scarcely explored in the literature. This includes the main materials used as photoelectrodes and their configuration in the photoelectrochemical reactor, which are discussed for technical uses. The review provides an overview of the state-of-the-art processes and aims to help in the development of enhanced photoelectroreactors for an efficient CO2 utilization. The authors acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) under the project CTQ2016-76231-C2-1-R and Ramoń y Cajal programme (RYC-2015-17080).

This study demonstrates the coupling of Ionic liquids (ILs) with a membrane contactor for post-combustion CO2 capture at moderate pressures and temperatures. ILs 1-ethyl-3-methylimidazolium methyl sulfate([emim][MeSO4]), 1-ethyl-3-methylimidazolium dicyanamide([emim][DCA]), 1-ethyl-3-methylimidazolium ethyl sulfate([emim][EtSO4]) and 1-ethyl-3-methylimidazolium acetate ([emim][AC]) were selected due to their high thermal stability, moderate viscosity and surface tension, as well as high CO2 solubility. No wetting conditions were confirmed for the polypropylene membrane by measuring contact angle, liquid entry pressure (LEP) and SEM of fiber surface before and after the operation. ILs were recirculated in the setup until reaching pseudo-steady-state. All four ILs were able to capture a substantial amount of CO2 during the specified operation time. Initially, very high values of CO2 mass transfer flux and experimental overall mass transfer coefficient were obtained which further decreased with operation time and reached a nearly constant value at pseudo-steady-state. Effect of CO2 loading of the ILs and temperature on enhancement factor and first order rate constant were evaluated. The absorption behavior and kinetics were strongly influenced by the CO2 concentration in the ILs, which divides the absorption process in two steps; an initial faster absorption at the gas-liquid interface and later slower absorption in the bulk of the IL. Finally, a pseudo-steady-state modelling approach was implemented and validated.

Active learning, also called "learning by doing" (LbD), has resulted in positive learning outcomes in several higher education degrees. This paper describes an LbD experience within Chemical Engineering education aiming to enhance learning and transferable competencies using a Life Cycle Assessment course as a vehicle. This compulsory course belongs to the European Project Semester (EPS) program taught in the fourth year of the Chemical Engineering Degree at the University of Cantabria. From the beginning, the activity has targeted LCA practice with a strong emphasis on performance and its application as a decision-making tool in real case studies through close collaboration with regional companies. Working in partnership with industrial companies has favoured a win-win-win situation as students could apply knowledge as future LCA specialists. In contrast, companies gained valuable insights to improve their environmental performance, and lecturers enhanced their industrial networks. A public session carried out at the end of the activity created an enriching debate on subjects from a diversity of points of view (e.g., the selection of impact categories, the proposed improvements for environmental impact reduction, etc.). According to the lecturers, the competencies acquired by students through this LbD experience in life cycle assessment have notably evolved, demonstrating not only an enhanced understanding of environmental impacts across a product life cycle but also a significant improvement in critical thinking, team collaboration, and practical problem-solving skills, thereby bridging the gap between theoretical knowledge and its application in real-world scenarios. This is in line with the student´s perception that considered, such as "problem resolution", "capacity for analysing" and synthesis and "capacity for information" management. These are essential not only for future LCA practitioners but for chemical engineers

In this study, a model was built to investigate the role of Cu2O-/ZnO-based gas diffusion electrodes in enhancing the reduction of carbon dioxide into methanol inside an electrochemical cell. The model was simulated using COMSOL Multiphysics software and validated using experimental results. It showed reasonable agreement with an average error of 6%. The model demonstrated the dependence of the methanol production rate and faradic efficiency on process key variables: current density (j = 5-10 mA cm-2), gas flow rate (Qg/A = 10-20 mL min-1 cm-2), electrolyte flow rate, and CO2 gas feed concentration. The results showed a maximum methanol production rate of 50 -mol m-2 s-1 and faradic efficiency of 56% at -1.38 V vs Ag/AgCl. From the economic point of view, it is recommended to use a gas stream of 90% or slightly lower CO2 concentration and an electrolyte flow rate as low as 2 mL min-1 cm-2. The authors would like to convey special thanks to Prof. Mai Kamal El-Din for her willingness to share her knowledge and expertise that are of significant relevance to this work. J.A. gratefully acknowledges the financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) under Ramon y Cajal program (RYC-2015-17080). The authors from ́ the Chemical Engineering Department, Cairo University, gratefully acknowledge the financial support provided by the Science and Technology Development Fund (STDF) of Egypt under project ID #11872. R.M.E.-M. acknowledges the support from the Oil and Green Chemistry research center and the Enhanced Oil Recovery Lab, Suez University, Egypt, and STDF (Science and Technology Development Fund) [Project ID 12395].

AbstractBACKGROUNDThe recycling of CO2by photoelectrochemical reduction has attracted wide interest due to its potential benefits when compared to electro‐ and photo‐catalytic approaches. Among the various available semiconductors, TiO2is the most employed material in photoelectrochemical cells. Besides, Cu is a well‐known electrocatalyst for the production of alcohols from CO2reduction.RESULTSA photoelectrochemical cell consisting of a TiO2‐based membrane electrode assembly (MEA) photoanode and a Cu plate is employed to reduce CO2to methanol and ethanol continuously under UV illumination (100 mW cm−2). A maximum increment of 4.3 mA cm−2in current between the illuminated and dark conditions is achieved at −2 VversusAg/AgCl. The continuous photoelectrochemical reduction process in the filter‐press cell is evaluated in terms of reaction rate (r), as well as Faradaic efficiency (FE) and energy efficiency (EE). At −1.8 VversusAg/AgCl, a maximum reaction rate ofr= 9.5 μmol m−2s−1,FE= 16.2% andEE= 5.2% for methanol andr= 6.8 μmol m−2s−1,FE= 23.2% andEE= 6.8% for ethanol can be achieved.CONCLUSIONSThe potential benefits of the photoanode‐driven system, in terms of yields and efficiencies, are observed when employing a TiO2‐based MEA photoanode and Cu as dark cathode. The results demonstrate first the effect of UV illumination on current density, and then the formation of alcohols from the continuous photoreduction of CO2. Increasing the external applied voltage leads to an enhanced production of methanol, but decreases ethanol formation. The system outperforms previous photoanode‐based systems for CO2‐to‐alcohols reactions. © 2019 Society of Chemical Industry