Increasing human activities and industrial manufacturing in the past few centuries have intensified the excessive consumption of fossil fuels, leading to an alarming rise in global atmospheric CO2 concentration. Among the different approaches that can be used to mitigate CO2 emissions, the electrochemical CO2 reduction reaction (CO2RR) to value-added chemicals and fuels driven by renewable energy sources arouses great interest. CO2RR is a multi-step process, which undergoes multiple proton-coupled electron transfer processes to produce diverse carbon-based products, including carbon monoxide, methane, formic acid, methanol, ethylene, acetic acid, ethanol, propanol, propylene, and others. Among different CO2RR products, liquid fuels and chemicals are particularly intriguing due to their high energy densities and versatile storage and distribution. Formic acid (or formate) is very attractive because it is used in various industries, including pharmaceuticals and biofuel manufacturing, and it also has the potential to serve as a hydrogen carrier. Furthermore, solar-driven CO2 reduction has received significant interest as a key approach to establishing a circular carbon economy and achieving true carbon neutrality. Two main strategies have emerged: photoelectrochemical (PEC) systems, where semiconductor photoelectrodes absorb sunlight to directly generate charge carriers for fuel production, and photovoltaic-driven electrochemical (PV–EC) systems, where solar cells generate a photocurrent that drives the electrochemical reduction reactions. Both approaches aim to integrate solar energy harvesting and CO2 conversion into efficient and sustainable solar-to-fuel processes. Despite remarkable progress, current CO2RR systems remain largely confined to laboratory-scale studies. Realising their potential contribution to global carbon neutrality requires scaling up from small prototypes to industrially relevant devices. Large-scale electrolysers play a central role in this transition, targeting performance metrics such as current densities exceeding 300 mA cm-2, operational stability beyond 100,000 hours, and product selectivity above 80%. However, moving toward industrial-scale systems introduces additional complexities, including precise temperature control, pressure regulation, uniform CO2 distribution, electrode compression, and electrolyte flow optimisation. Overcoming these challenges is essential to ensure the viability of CO2RR as a scalable and sustainable pathway for carbon utilisation. In this context, this PhD work aimed to design, develop, and validate advanced electrocatalytic and photoelectrocatalytic systems for CO2RR in both conventional aqueous media and unconventional organic environments, combining innovative materials, reactor engineering, and solar-powered operation. The research focused not only on the development of new catalytic materials but also on the optimisation of electrode engineering and the design of efficient electrochemical reactors, to improve overall performance and promote the industrial scalability of the developed systems. Specifically, the research activity carried out at the Home Institution (University of Messina - Laboratory of Catalysis for Sustainable Production and Energy - CASPE/INSTM) focused on the development of innovative copper-based electrocatalytic materials for CO2RR. The overall intent of these materials was to address product selectivity toward the production of high-value liquid products, particularly formic acid, by limiting the hydrogen evolution reaction. More specifically, electrocatalysts based on nitrogen-functionalized and copper-doped carbon nanotubes (CNTs) were initially developed. These materials were developed using an innovative organic synthesis process that has the advantage of using moderate temperatures (60°C). This synthesis allowed the selective insertion of a single type of functional group (-NH2 or – N(CH3)3+) onto the CNTs via covalent bonding. These materials were subsequently doped with copper and then subjected to advanced characterisation (Kaiser Test, FT-IR, TGA, SEM-EDX, and XRD), which allowed the functional groups to be identified and quantified. The subsequent research activity was carried out within the framework of a European Project (SUPERVAL, Grant number: 101115456). The general aim of this project was to develop an integrated system capable of capturing and recovering CO2, transforming it into a high-energy organic molecule (formate) through a photoelectrocatalytic route. At the same time, NOx can also be captured and converted, in combination with N2, into ammonia using the hydrogen generated in the CO2 co-electrolysis processes. The work of the present thesis within this project focused on the development of the cathode for the CO2RR, with the aim of reaching high selectivity to a single carbon product (i.e., formic acid), minimising CO formation, and not using critical raw materials. Specifically, the developed materials consists of copper sulphides (Cu-S) in the crystalline form of covellite, which were modified by introducing different amounts of aluminium (Al) into the structure to evaluate their effect on product selectivity and current density. It is known that electrochemical performance does not depend exclusively on the characteristics of the electrocatalytic material; the reactor configuration also plays an important role, influencing factors such as mass transport, the characteristics of the three-phase boundary, and the internal resistance of the cell. For these reasons, in parallel with material development, research has also focused on the design and construction of electrochemical reactors to enhance the electrocatalytic performance of the developed materials. Specifically, flow-by and zero-gap electrolysers were designed and realised, and validated in CO2RR. Subsequently, during research periods abroad at the Forschungszentrum Jülich (Germany) and at the Motor Oil Hellas refinery (Greece), the PhD research work focused on developing and subsequent scaling-up a photoelectrocatalytic system for CO2RR, which operates under unconventional conditions using organic electrolytes (bioethanol or methanol). This work was carried out within the activities of another European project (DECADE, Grant number: 862030), which focused on a new PEC technology for CO2RR using alcohols and waste CO2 as feeds. The main feature of this system was its design to enhance the anodic reaction, aiming to obtain products of interest to both the cathode and the anode. The reactions envisaged in this system are electro-oxidative esterification on the anode side, where two ethanol molecules are converted into ethyl acetate, and electroreduction of CO2 into formate and acetate on the cathode side, which then react with ethanol to produce ethyl acetate and ethyl formate. Analogous reactions can be obtained by replacing ethanol with methanol. An important part of this work was scaling up the system to reach a TRL (technology readiness level) of 5, thus validating the technology in a relevant environment. The thesis is structured into four chapters, each addressing a different aspect of the overall research. The following is a summary of the single chapters of the thesis. Chapter 1 introduces the environmental and technological context of the CO2 reduction reaction. After describing the reaction mechanisms, performance parameters, and cell configurations for CO2RR, strategies based on photocatalysis, photoelectrocatalysis, and integrated PV-EC systems are analysed, presenting the main challenges related to efficiency, selectivity, and scalability and highlighting the role of these systems in the future of sustainable carbon conversion. Chapter 2 focuses on the synthesis and characterisation of nitrogen-functionalized and copper-doped carbon nanotube (CNT) based catalyst, for use in the CO2RR, capable of operating at relatively high current densities (10 mA cm-2), reducing collateral hydrogen production. A home-made flow electrolyser, continuously fed with CO2, was also developed and optimised to test the electrodes. The results demonstrated that nitrogen functionalization plays a crucial role in modulating selectivity: in particular, functionalization of CNTs with the –NH2 and –N(CH3)3+ functional groups successfully decreased hydrogen selectivity while increasing selectivity toward carbonaceous compounds, such as formic acid (–N(CH3)3+: FEHCOOH = 41.6%, FECO = 30.8 %). and carbon monoxide (–NH2: FEHCOOH = 28.3%, FECO = 36.2 %). Chapter 3 presents the work carried out within the SUPERVAL project, which aimed to design and optimise electrocatalysts based on non-critical raw materials, with the goal of modulating the Faradaic efficiency of formate/hydrogen production in CO2RR, minimising CO formation. To this end, a copper sulphide (Cu-S) material with the rare crystalline form of covellite was developed, which showed high selectivity towards formic acid over carbon monoxide (HCOOH: FE = 78%; CO: FE = 0.5% at 27 mA cm-2). Subsequently, the Cu-S was modified by introducing aluminium (Al) in different amounts using two synthetic strategies. The results obtained show that the modification of Cu-S with the introduction of a moderate amount of Al (10% w/w) improved electronic conductivity, leading to higher current densities (36 mA cm-2), while maintaining the Faradaic efficiencies towards formic acid and CO unchanged compared to the Cu-S sample. Furthermore, another significant result was obtained by introducing a lower amount of Al (0.5% w/w), in which case almost complete selectivity towards the production of a single carbon product (formic acid) was obtained at all investigated potentials. In parallel, the research presented in this chapter also addressed engineering aspects related to the reactor configuration. A zero-gap electrolyser was developed and optimised, and its performance was subsequently compared with that of a flow electrolyser. Despite the engineering challenges encountered in developing this type of electrolyser, the final zero-gap system achieved higher current densities than flow systems, confirming the importance of the reactor configuration. However, it also showed a decline in carbon selectivity, thus highlighting the complexity of developing this type of system and the need for further investigation to improve its performance. Chapter 4 focuses on the work carried out within the European DECADE project, with the aim of designing, manufacturing, validating and scaling up an advanced device for the photoelectrocatalytic conversion of CO2 and waste alcohols (particularly bioethanol) into valuable chemical compounds, such as ethyl acetate (EA) and ethyl formate (EF). A laboratory-scale electrocatalytic (EC) device was initially designed and fabricated to optimise materials, operating conditions, and electrolyte composition, achieving stable operation and the formation of target products such as ethyl acetate. Parallel experiments conducted on methanol at the Forschungszentrum Jülich (Germany) confirmed the formation of methyl formate and formaldehyde, demonstrating the system's versatility in various organic media. The EC reactor was then integrated with photovoltaic modules, enabling autonomous solar-powered operation (PV-EC configuration). In the final phase, a larger-scale prototype (electrode area: 121 cm2) was built in collaboration with industrial partner Hysytech and validated with both simulated and real sunlight, first at the University of Messina and subsequently at the Motor Oil Hellas refinery (Greece). Validation tests confirmed EA formation as the primary target product, achieving Faradaic efficiencies of up to 40% under controlled indoor conditions and approximately 13-15% under real sunlight. Overall, the research demonstrated the feasibility of this PEC technology and successfully advanced its Technology Readiness Level to TRL 5, representing a significant step towards the industrial implementation of integrated PV-EC systems for solar-powered CO2 conversion.
CO2 and water co-electrolysis over Cu-based catalysts using a scalable artificial leaf prototype
COSTANTINO, VERONICA
2025-12-17
Abstract
Increasing human activities and industrial manufacturing in the past few centuries have intensified the excessive consumption of fossil fuels, leading to an alarming rise in global atmospheric CO2 concentration. Among the different approaches that can be used to mitigate CO2 emissions, the electrochemical CO2 reduction reaction (CO2RR) to value-added chemicals and fuels driven by renewable energy sources arouses great interest. CO2RR is a multi-step process, which undergoes multiple proton-coupled electron transfer processes to produce diverse carbon-based products, including carbon monoxide, methane, formic acid, methanol, ethylene, acetic acid, ethanol, propanol, propylene, and others. Among different CO2RR products, liquid fuels and chemicals are particularly intriguing due to their high energy densities and versatile storage and distribution. Formic acid (or formate) is very attractive because it is used in various industries, including pharmaceuticals and biofuel manufacturing, and it also has the potential to serve as a hydrogen carrier. Furthermore, solar-driven CO2 reduction has received significant interest as a key approach to establishing a circular carbon economy and achieving true carbon neutrality. Two main strategies have emerged: photoelectrochemical (PEC) systems, where semiconductor photoelectrodes absorb sunlight to directly generate charge carriers for fuel production, and photovoltaic-driven electrochemical (PV–EC) systems, where solar cells generate a photocurrent that drives the electrochemical reduction reactions. Both approaches aim to integrate solar energy harvesting and CO2 conversion into efficient and sustainable solar-to-fuel processes. Despite remarkable progress, current CO2RR systems remain largely confined to laboratory-scale studies. Realising their potential contribution to global carbon neutrality requires scaling up from small prototypes to industrially relevant devices. Large-scale electrolysers play a central role in this transition, targeting performance metrics such as current densities exceeding 300 mA cm-2, operational stability beyond 100,000 hours, and product selectivity above 80%. However, moving toward industrial-scale systems introduces additional complexities, including precise temperature control, pressure regulation, uniform CO2 distribution, electrode compression, and electrolyte flow optimisation. Overcoming these challenges is essential to ensure the viability of CO2RR as a scalable and sustainable pathway for carbon utilisation. In this context, this PhD work aimed to design, develop, and validate advanced electrocatalytic and photoelectrocatalytic systems for CO2RR in both conventional aqueous media and unconventional organic environments, combining innovative materials, reactor engineering, and solar-powered operation. The research focused not only on the development of new catalytic materials but also on the optimisation of electrode engineering and the design of efficient electrochemical reactors, to improve overall performance and promote the industrial scalability of the developed systems. Specifically, the research activity carried out at the Home Institution (University of Messina - Laboratory of Catalysis for Sustainable Production and Energy - CASPE/INSTM) focused on the development of innovative copper-based electrocatalytic materials for CO2RR. The overall intent of these materials was to address product selectivity toward the production of high-value liquid products, particularly formic acid, by limiting the hydrogen evolution reaction. More specifically, electrocatalysts based on nitrogen-functionalized and copper-doped carbon nanotubes (CNTs) were initially developed. These materials were developed using an innovative organic synthesis process that has the advantage of using moderate temperatures (60°C). This synthesis allowed the selective insertion of a single type of functional group (-NH2 or – N(CH3)3+) onto the CNTs via covalent bonding. These materials were subsequently doped with copper and then subjected to advanced characterisation (Kaiser Test, FT-IR, TGA, SEM-EDX, and XRD), which allowed the functional groups to be identified and quantified. The subsequent research activity was carried out within the framework of a European Project (SUPERVAL, Grant number: 101115456). The general aim of this project was to develop an integrated system capable of capturing and recovering CO2, transforming it into a high-energy organic molecule (formate) through a photoelectrocatalytic route. At the same time, NOx can also be captured and converted, in combination with N2, into ammonia using the hydrogen generated in the CO2 co-electrolysis processes. The work of the present thesis within this project focused on the development of the cathode for the CO2RR, with the aim of reaching high selectivity to a single carbon product (i.e., formic acid), minimising CO formation, and not using critical raw materials. Specifically, the developed materials consists of copper sulphides (Cu-S) in the crystalline form of covellite, which were modified by introducing different amounts of aluminium (Al) into the structure to evaluate their effect on product selectivity and current density. It is known that electrochemical performance does not depend exclusively on the characteristics of the electrocatalytic material; the reactor configuration also plays an important role, influencing factors such as mass transport, the characteristics of the three-phase boundary, and the internal resistance of the cell. For these reasons, in parallel with material development, research has also focused on the design and construction of electrochemical reactors to enhance the electrocatalytic performance of the developed materials. Specifically, flow-by and zero-gap electrolysers were designed and realised, and validated in CO2RR. Subsequently, during research periods abroad at the Forschungszentrum Jülich (Germany) and at the Motor Oil Hellas refinery (Greece), the PhD research work focused on developing and subsequent scaling-up a photoelectrocatalytic system for CO2RR, which operates under unconventional conditions using organic electrolytes (bioethanol or methanol). This work was carried out within the activities of another European project (DECADE, Grant number: 862030), which focused on a new PEC technology for CO2RR using alcohols and waste CO2 as feeds. The main feature of this system was its design to enhance the anodic reaction, aiming to obtain products of interest to both the cathode and the anode. The reactions envisaged in this system are electro-oxidative esterification on the anode side, where two ethanol molecules are converted into ethyl acetate, and electroreduction of CO2 into formate and acetate on the cathode side, which then react with ethanol to produce ethyl acetate and ethyl formate. Analogous reactions can be obtained by replacing ethanol with methanol. An important part of this work was scaling up the system to reach a TRL (technology readiness level) of 5, thus validating the technology in a relevant environment. The thesis is structured into four chapters, each addressing a different aspect of the overall research. The following is a summary of the single chapters of the thesis. Chapter 1 introduces the environmental and technological context of the CO2 reduction reaction. After describing the reaction mechanisms, performance parameters, and cell configurations for CO2RR, strategies based on photocatalysis, photoelectrocatalysis, and integrated PV-EC systems are analysed, presenting the main challenges related to efficiency, selectivity, and scalability and highlighting the role of these systems in the future of sustainable carbon conversion. Chapter 2 focuses on the synthesis and characterisation of nitrogen-functionalized and copper-doped carbon nanotube (CNT) based catalyst, for use in the CO2RR, capable of operating at relatively high current densities (10 mA cm-2), reducing collateral hydrogen production. A home-made flow electrolyser, continuously fed with CO2, was also developed and optimised to test the electrodes. The results demonstrated that nitrogen functionalization plays a crucial role in modulating selectivity: in particular, functionalization of CNTs with the –NH2 and –N(CH3)3+ functional groups successfully decreased hydrogen selectivity while increasing selectivity toward carbonaceous compounds, such as formic acid (–N(CH3)3+: FEHCOOH = 41.6%, FECO = 30.8 %). and carbon monoxide (–NH2: FEHCOOH = 28.3%, FECO = 36.2 %). Chapter 3 presents the work carried out within the SUPERVAL project, which aimed to design and optimise electrocatalysts based on non-critical raw materials, with the goal of modulating the Faradaic efficiency of formate/hydrogen production in CO2RR, minimising CO formation. To this end, a copper sulphide (Cu-S) material with the rare crystalline form of covellite was developed, which showed high selectivity towards formic acid over carbon monoxide (HCOOH: FE = 78%; CO: FE = 0.5% at 27 mA cm-2). Subsequently, the Cu-S was modified by introducing aluminium (Al) in different amounts using two synthetic strategies. The results obtained show that the modification of Cu-S with the introduction of a moderate amount of Al (10% w/w) improved electronic conductivity, leading to higher current densities (36 mA cm-2), while maintaining the Faradaic efficiencies towards formic acid and CO unchanged compared to the Cu-S sample. Furthermore, another significant result was obtained by introducing a lower amount of Al (0.5% w/w), in which case almost complete selectivity towards the production of a single carbon product (formic acid) was obtained at all investigated potentials. In parallel, the research presented in this chapter also addressed engineering aspects related to the reactor configuration. A zero-gap electrolyser was developed and optimised, and its performance was subsequently compared with that of a flow electrolyser. Despite the engineering challenges encountered in developing this type of electrolyser, the final zero-gap system achieved higher current densities than flow systems, confirming the importance of the reactor configuration. However, it also showed a decline in carbon selectivity, thus highlighting the complexity of developing this type of system and the need for further investigation to improve its performance. Chapter 4 focuses on the work carried out within the European DECADE project, with the aim of designing, manufacturing, validating and scaling up an advanced device for the photoelectrocatalytic conversion of CO2 and waste alcohols (particularly bioethanol) into valuable chemical compounds, such as ethyl acetate (EA) and ethyl formate (EF). A laboratory-scale electrocatalytic (EC) device was initially designed and fabricated to optimise materials, operating conditions, and electrolyte composition, achieving stable operation and the formation of target products such as ethyl acetate. Parallel experiments conducted on methanol at the Forschungszentrum Jülich (Germany) confirmed the formation of methyl formate and formaldehyde, demonstrating the system's versatility in various organic media. The EC reactor was then integrated with photovoltaic modules, enabling autonomous solar-powered operation (PV-EC configuration). In the final phase, a larger-scale prototype (electrode area: 121 cm2) was built in collaboration with industrial partner Hysytech and validated with both simulated and real sunlight, first at the University of Messina and subsequently at the Motor Oil Hellas refinery (Greece). Validation tests confirmed EA formation as the primary target product, achieving Faradaic efficiencies of up to 40% under controlled indoor conditions and approximately 13-15% under real sunlight. Overall, the research demonstrated the feasibility of this PEC technology and successfully advanced its Technology Readiness Level to TRL 5, representing a significant step towards the industrial implementation of integrated PV-EC systems for solar-powered CO2 conversion.Pubblicazioni consigliate
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