The rising share of renewable electricity is testament to the increasing importance of solar/wind-electric routes to harvest sun light in form of potential differences and free electrons. While some electricity is used directly or stored capacitively, an increasing portion calls for direct conversion into valuable molecular solar fuels or chemicals. This “dark” e-conversion is made possible by heterogeneous electrocatalysis on the surface of solid electrodes coupled to mass and charge transport processes. Sustainable materials synthesis pathways coupled to novel advanced characterization techniques result in a deeper understanding of the origin of reaction kinetic barriers and the origin of transport limitations. This is critically needed for the design of more efficient, electrochemical materials, interfaces, and electrodes for practical electrolytic devices for the production of e-fuels and e-chemicals.

In this presentation, I will report on recent advances in our design and understanding of electrocatalytic materials, interfaces and electrodes relevant to the electrochemical conversion and valorization of CO2 into value-added molecular compounds. Focus will be placed on CO and ethylene as key target e-compounds in conventional liquid cells and Gas Diffusion Electrodes.

Many industrial chemical processes involve a high-energy demand (often still derived from fossil fuels), Urea is an important chemical for the agricultural industry, which accounts for 70% of the global nitrogen-containing fertilizer usage. Currently, urea is produced by the reaction of liquid NH₃ and CO₂^[1] in a process known as the Haber-Meiser process. Despite its importance, this process suffers from high energy demand and CO₂ emissions, mainly due to the NH₃ synthesis step (Haber- Bosch process), which relies on fossil fuel resources. Making urea with electrochemical methods has recently gained interest from the scientific community as this approach provides a way to reduce the high CO₂ emissions currently associated with urea production. Among the various nitrogen sources being explored for urea electrosynthesis, nitrate (NO₃^-) is an attractive one because of its high solubility in water, low dissociation energy and potential to mitigate NO₃⁻ contaminations in water. Even though the mechanism of urea synthesis via CO₂ and NO₃^- coupling is still controversial and not broadly studied, most reports implicate CO* and NH₂* surface intermediates in urea synthesis.^[2] Cu is an attractive electrocatalyst as it can reduce CO₂ to CO and further products and it is also an active catalyst for the reduction of nitrate to ammonia Previous reports on Cu and Cu-based materials showed that urea can be obtained from NO₂^-.^[3,4]  Early research by Shibata et al. with various catalysts demonstrated that the highest faradaic efficiency towards urea from NO₃^- was achieved with a Zn catalyst.^[5] In this presentation, I will show our latest results on the development of Cu bimetallic catalysts (CuZn and CuRh) for urea electrosysnthesis from CO₂ and NO₃^-.

References:

[1]        P. N. Cheremisinoff, Waste Minimization Cost Reduct. Process Ind. 1995, 222–284.

[2]        X. Liu, Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Nat. Commun. 2022, 13, 1–9.

[3]        S. Liu, S. Yin, Z. Wang, Y. Xu, X. Li, L. Wang, H. Wang, Cell Reports Phys. Sci. 2022, 3, 100869.

[4]        N. Cao, Y. Quan, A. Guan, C. Yang, Y. Ji, L. Zhang, G. Zheng, J. Colloid Interface Sci. 2020, 577, 109–114.

[5]         M. Shibata, K. Yoshida, N. Furuya, J. Electrochem. Soc. 1998, 145, 595–600.

Carbon dioxide (CO₂) electrolysis to produce hydrocarbons and oxygenates using copper (Cu) based catalysts has attracted substantial interest due to the direct production of versatile C₂+ feedstocks . Inside the reactor, however, CO₂ electrolyzers produce C2+ compounds occur via a two-step tandem CO₂ to CO and CO to C₂+ steps. Such knowledge has been utilized in catalyst and cathode-to-anode reactor design, but sparingly in the in-plane design of the system. Here we use the knowledge that CO₂ reduction on copper is primarily a tandem reaction, and through modulation of the reactor flow rate achieve C2+ selectivity 84% at CO₂ utilizations of 31%, exceeding theoretical CO₂ utilization efficiencies of 25% for C₂₊ products. We show that higher utilizations are possible when a subset of the reactor performing only CO reduction, instead of CO₂ reduction, preventing excess CO₂ conversion to carbonates. Through use of varied flow field (serpentine, parallel, interdigitated) and pure CO-fed electrolysis, we link these our results to CO residence time. Notably we find that while ethylene production is constant with flow rate (~40%), oxygenates increase substantially at lower flow rates, reaching 45% at 10 SCCM. Finally, we posit that researchers should switch to combined ethylene + ethanol selectivity as a qualifying metric due to the ease of dehydrating ethanol to form ethylene and a demonstrated inability to fully control ethylene:oxygenate branching pathways. Efforts should then shift to the removal of ethanol from membrane electrode assembly systems and downstream recovery through existing commercial processes.

The synthetic methane from hydrogen derived from renewable energy and captured CO₂, so-called e-methane, is one of the promising ways to decarbonize gaseous fuel necessary for realizing carbon neutrality. The e-methane can utilize existing city gas infrastructure, and is particularly effective for high-temperature heat demand of 200°C or higher, which is difficult to replace with electricity. To meet the above-mentioned requirement, Tokyo Gas has started developing electrochemical CO₂ reduction technology using a polymer electrolyte membrane to synthesize e-methane with Osaka University under the “Green Innovation Fund project” in Japan.

Since the CO₂ electrochemical reduction yields chemical species such as CO, ethylene, formic acid, and hydrogen in addition to methane, one of the most important technical challenges is improving methane selectivity.

To solve this challenge and optimize the cell structure, the copper-based catalysts, known to have a potential for methane synthesis, have been designed and evaluated. The progress of catalyst and cell development for improved methane selectivity will be discussed.

In this talk, I will summarize various results on CO electroreduction to the C₂ species ethylene, ethanol and acetaldehyde on Cu-based catalysts.

I will start presenting a pathway for CO electroreduction to ethylene, ethanol and acetaldehyde [1]. An early intermediate in this pathway can be spectroscopically detected, and DFT helps elucidate its structure [2].

Moreover, the pathway predicts that the late stages of ethanol production correspond to the reduction of acetaldehyde [3], while the late stages of ethylene production coincide with the reduction of ethylene oxide [4]. Because these two reactions have different structural sensitivity, one can elucidate the structure of ethanol-producing sites on the rough surface of oxide-derived Cu [5].

Finally, previous work postulated that a *CHO dimer is part of the CO reduction pathway to ethanol [6]. We put that idea to the test by electroreducing glyoxal and determined that the *CHO dimer is unlikely to be a key intermediate of that pathway [7].

Time permitting, I will show that Ag-Cu tandem catalysts open an alternative, ethanol-selective pathway by virtue of their large surface coverage of *CO [8, 9], the structure sensitivity of which can be capitalized on to design active catalysts [10].

A multi-scale first-principles reaction-transport model is derived for the electrochemical reduction of CO2 to fuels and chemicals on polycrystalline copper electrodes. The model utilizes a continuous stirred-tank reactor (CSTR) approximation that captures the relative timescales for mesoscale stochastic processes at the electrode/electrolyte interface
that determine product selectivity. The model is built starting from a large experimental dataset obtained under a broad range of well-defined transport regimes in a gastight rotating cylinder electrode cell. Product distributions under different conditions of transport, applied potential, bulk electrolyte concentration, temperature and catalyst porosity are rationalized by introducing dimensionless numbers that reduce complexity and capture relative time scales for mesoscopic and microscopic dynamics of electrocatalytic reactions on copper electrodes of any porosity. This work demonstrates that one CO2 reduction mechanism can explain differences in selectivity reported for copper-based electrocatalysts when mass transport, concentration polarization effects, and primary and secondary current distributions are taken into account. The reaction-transport model presented in this talk should enable the rational design of CO2 electrolyzers.

Climate change concerns have spurred a growing interest in developing environmentally friendly technologies for energy generation, and the electrochemical reduction of CO₂ (CO₂RR) into value-added chemicals offers a possibility to store renewable energy into chemical bonds. It is therefore of particular interest to develop efficient, selective and durable electrocatalysts. The latter requires fundamental understanding of their structure and surface composition under reaction conditions.

This talk will illustrate how of a multi-technique in-situ/operando experimental approach is able provide in depth mechanistic insights into CO₂RR. A synergistic combination of LC-TEM, EC-AFM, NAP-XPS, XAS, XRD, GC/MS, and Raman Spectroscopy, coupled with machine learning-based data analysis, has been employed to investigate the evolution of the structure/composition of mono (Cu₂O) and bimetallic (ZnO@Cu₂O) cubic nanoparticles and single atom (M-N-C, with M=Fe, Sn, Cu, Co, Ni, Zn) CO₂RR electrocatalysts under reaction conditions.

A main aspect that I will discuss is the use of periodic potential pulses to alter the product selectivity toward desired high order hydrocarbons and alcohols. By screening a variety of product-steering pulse-length conditions for pre-reduced Cu₂O nanocubes, we identified a critical role of the formation of Cu-O_ad or CuOx/(OH)_y species as well as the importance of having an optimal OH_ad versus CO_ad catalyst surface coverage during CO₂RR. For the ZnO@Cu₂O system, we have further unveiled the role of the dynamic interplay between Zn Oxide, CuZn alloy, and metallic Zn formation, the evolution of Cu crystallites, and the adsorption behavior of CO and OH species. These findings pave the road toward improved catalyst design for non-conventional dynamic CO₂RR reaction conditions.

In addition, metal-nitrogen-doped carbons (M-N-C) will be discussed as emerging cost-effective CO₂RR catalysts, with emphasis in the study of their stability during operation. Here operando XAS data revealed drastic differences in the structural evolution of the different M-N-C materials, including reversibly formation of metallic clusters for some of the M-N-Cs studied that can be used to rationalize their distinct electrocatalytic performance and durability.

Finally, our studies are expected to open up new routes for the reutilization of CO₂ through its direct conversion into industrially valuable chemicals and fuels.

Gas diffusion electrodes (GDEs) improve the performance of CO₂ reduction by enhancing the gaseous species transport. GDEs also increase the accessibility of the catalyst and the reaction surface area. A membrane electrode assembly (MEA) electrolyzer is advantageous due to its ease of assembly and low ohmic loss. However, the impact of operational conditions on cell performance and the local variations of mass and charge transfer in the GDE MEA electrolyzer is not well understood, and studies typically focus on individual components rather the complete cell. This study aims to investigate CO₂ electrolysis in a GDE MEA configuration using a porous Ni-based single-site catalyst through a combined computational and experimental approach.

The approach followed here was: i) we developed a model based on the assembly structure and geometry of the MEA cell to represent the experimental setup; ii) we extracted morphological information of the catalyst layer and the diffusion layer by pore-level investigations utilizing the exact mesostructured obtained from tomography, as well as kinetics parameters extracted by a boundary layer model that was fit to the H-cell experiments with the same catalytic material as the one used in the MEA cell; iii) we fed these parameters into the MEA cell model that accounts for the three relevant phases (gas phase for reactant/product transport, membrane phase for ion transport, and solid phase for electron transport) and predicted the cell performance; and iv) we compared the predicted results with the experimental data obtained with the MEA cell to validate our model.

The computational model of the GDE MEA cell was able to predict the behavior of the experiments with less than 15% error. The computed potential was about 0.2 V to 0.4 V lower than the potential in the experiment at the same current, mainly attributed to contact resistances in the system. The model provides local distributions of gaseous species and current density at the catalyst layer and diffusion layer interface. At 15 sccm flow rate, the total current density decreased along the gas flow direction and reached 100 mA/cm² at 50% downstream of the channel. CO₂ concentration depleted after 50% of the channel length. To operate at more negative cell potentials, it was necessary to increase the flow rate or reduce the gas channel length to avoid the competing hydrogen evolution reaction becoming the dominant reaction. For a cell running at 15 sccm, the conversion was approaching 99% at a cell potential of -2.7 V. This was also near the point (-2.6 V) where maximum Faradaic efficiency was reached. Beyond this potential, the additional charge will not contribute to CO evolution but rather hydrogen evolution, because of the depletion of CO₂.

This work offers a practical methodology to provide first insights into the performance-significant design and operational choices of GDE-MEA CO₂ reduction devices. Differences between the model and the experiment are attributed to: i) Competitive adsorption between reactant CO₂ and H₂O on catalytic sites in the experiment; ii) bypass of flow along the GDE, especially at high flow rate, and iii) contact resistance induced potential drops in the cell in between each component (cathode, membrane, anode, current collector, etc). Future work will account for these factors.

CO2 electroreduction into valuable hydrocarbons with high selectivity is of high market interest yet challenging. Cu-based catalysts have a unique catalytic activity towards hydrocarbon (C2+) products in the CO2 electroreduction reaction and are widely studied to promote the corresponding selectivity and activity. The control of the oxidation state of the catalyst is essential in controlling the desired hydrocarbon production. Herein, we steer CO2 electroreduction towards ethylene production with high selectivity in an alkaline flow cell configuration from a copper phosphate catalyst. An optimum faradaic efficiency of 55% for ethylene production was obtained in a 7 M KOH electrolyte for a total current density of 150 mA.cm-2. A high faradaic efficiency for C2+ products of 60% was also obtained. The investigation of the mechanism shows that Cu nanoparticles are derived during electrolysis which may favour the dimerization of *CO to ethylene formation. This work provides new insights into designing catalysts for high-selectivity hydrocarbon formation.

CO₂ reduction provides a carbon neutral means of converting waste CO₂ and surplus electricity into valuable fuels and chemicals to reach the net zero global goal. Only Cu can yield valuable high energy C₂₊ products, due to its unique binding energy with *CO and *H intermediate [1], but only at the Faradaic Efficiencies of up to 52% for ethanol [2] and 87% for ethylene [3], and under high over-potentials and harsh conditions. Single crystal studies have shown that Cu(100) with surface defects (under-coordinated atoms) favours such C₂₊ products even oxygenated products, because of both the correct binding and low barriers for protonation and C-C coupling. Therefore, we hypothesise that by synthesizing nano-porous Cu with (100) orientation, its catalytic activity will be maximised [4 – 6].

Nano-porous materials have many potential applications as sensors, actuators, catalysis, etc, because of their large surface-to-volume ratio and high surface distortion. These structures can be produced by (electro)chemically dealloying a bimetallic alloy, in a regime where one of the components is much more chemically active than the other. As the active species are removed, the remaining material forms a nano-porous ‘sponge’ that has a bi-continuous structure, of which the amount of under-coordinated atoms is significant [7 – 8]. Inspired by the work from Chattot et al [9], where a linear relationship between the surface distortion of Pt and its oxygen reduction reactivity was found, we hypothesise that the CO₂ reduction reactivity of Cu surface could also be tailored by introducing surface distortion via dealloying.

Herein, we systematically studied the corrosion behaviour of Cu₂₀Zn₈₀ brass, using cryo-atomic probe tomography (cryo-APT) and in situ synchrotron X-ray diffraction (XRD), as shown by Figure 1. We successfully synthesised a series of nano-porous Cu materials with different ligament sizes, ranging from tens of nanometres to micrometres, via chemical dealloying by just changing temperatures. Synchrotron XRD measurement indicates that there is a huge but different micro-strain on these dealloyed nano-porous Cu, which is caused by the under-coordinated atoms. Electrochemical CO₂ and CO reduction measurement was then conducted, showing their significantly different catalytic activities, which correlated well with their micro-strains measured.

Electrochemical CO₂ reduction is a candidate for reduced CO₂ concentration in the atmosphere and changes to usable compounds. Zero-gap reactor for CO₂ reduction is expected to realization due to its low applied voltage and high current density.

The zero-gap reactor for CO₂ reduction requires an anion exchange membrane and an anolyte. These requirements are different from the polymer electrolyte electrochemical cell for water electrolysis. Although the membrane is an anion exchange, cations in anolyte pass through from anode to cathode. The existence of a cation in the cathode is also known to be essential for CO₂ reduction, thus, the cation in the cathode has a possibility to affect the properties of CO₂ reduction. This report discusses the effects of the anolyte of KHCO₃ concentration.

 The KHCO₃ concentration was 0.01, 0.1, and 1.0 mol/L, the catalysts of the cathode for CO₂ reduction and the anode for water oxidation were Cu and IrOx, respectively. The electrode area was 5 cm². Since the catalyst for the cathode was Cu, the products in the cathode were mainly CO, CH₄, and C₂H₄, with the satellite reaction product of H₂ from water reduction.

The applied voltage under a constant current density of 200 mA/cm² was lowest at the KHCO₃ concentration of 1.0 mol/L. The maximum faradaic efficiencies of C₂H₄ around 35% were obtained at 4.2, 3.6, and 3.2 V for 0.01, 0.1, and 1.0 mol/L, respectively. From these results, the higher anolyte concentration shows the lower applied voltage at the same current density and at the highest faradaic efficiency of C₂H₄. The reliability was, however, the best at the concentration of 0.1 mol/L because the CO₂ gas flow was strangled by the precipitation of the salt at 1.0 mol/L and the current density increased at 0.01 mol/L. The anolyte concentration affects the CO2 reduction and the 0.1 mol/L KHCO₃ was the best considering all the results.

Electrochemical CO reduction (ECOR) can act as a potential bridge between CO2-to-CO technologies and renewable production of C2+ chemicals. Along with the development of catalyst materials for selective and efficient CO reduction, it is imperative to optimize electrolysis conditions and cell parameters to efficiently reduce CO at industrially relevant current density and produce pure concentrated product streams. In this work, we study ECOR is three different cell configurations, firstly, microfluidic configuration, secondly, a hybrid configuration with anode zero gap, and lastly, a zero gap configuration. We establish the optimal working conditions for each configuration and compare the fundamental differences in each configuration.
The zero gap configuration is optimal for producing a stable product stream of ethylene, acetate, ethanol and propanol for current densities upto 1 Acm-2 at reasonable cell potentials and can sustain long term C2+ production of >70% for upto 10 hours at a current density of 200 mAcm-2. The catalytic activity in this configuration is insensitive to anolyte pH and the cell can run at relatively lower alkaline concentrations. On the other hand, the product selectivity in hybrid cell configuration changes with catholyte concentrations. The catholyte ensures a high local pH near the cathode and therefore the product selectivity can be altered by changing alkaline concentration. This hybrid configuration works stable with anode catalyst coated membranes for current densities of upto 0.8 Acm-2, however the flowing catholyte makes the long term operation difficult. It is interesting to note the substantial consequence on catalytic activity by altering cell configuration and electrolysis conditions. These results stress on the importance of optimising cell configuration and measurement parameters for attaining optimum working conditions for reaching industrially relevant current densities.

Electrochemical reduction of CO2 presents an attractive way to store renewable energy in chemical bonds in a potentially carbon-neutral way. However, current electrolyzers suffer from intrinsic problems, like flooding and salt accumulation, that must be overcome to industrialize the technology. To resolve flooding and salt precipitation issues, researchers have used ultra-hydrophobic electrodes based on either polytetrafluoroethylene (PTFE) gas-diffusion layers (GDL’s), or carbon-based GDL’s with added PTFE. While the PTFE backbone is highly-resistant to flooding, the non-conductive nature of PTFE means that without additional current collection the catalyst layer itself is responsible for electron-dispersion, which penalizes system efficiency and stability. In this work, we present operando results that illustrate the poor current/potential distribution in thin catalyst layers (~50 nm) deposited onto PTFE GDL’s. We then compare the effects of thicker catalyst layers (~500 nm) and a newly developed non-interfering current collector (NICC). The NICC can maintain even current distribution with 10-fold thinner catalyst layers while improving stability towards ethylene (≥ 30%) by approximately two-fold. 

CO2 electrolysis using silver (Ag) and copper (Cu) based catalysts have been widely studied due to their ability to produce CO and multi-carbon products respectively. In industrially relevant membrane electrode assembly configurations, long term operational stability is hampered due to (bi) carbonate precipitation at the cathode triggered by alkali metal cation crossover from the anode. In this work, we investigate the role of cation crossover for Ag and Cu based catalysts by varying cation concentrations and the cation identity. We find that cation crossover from the anode is essential for CO2 activation and cation identity (K+, Na+) affect CO2RR selectivities significantly as shown in a number of previous studies. Further, we find that cation concentration do not alter product selectivity for a Ag catalyst producing CO, but alter product distribution significantly for a Cu catalyst, showing that C-C coupling rates are significantly affected by local cation concentration at the cathode. In contrast, cation crossover is detrimental for long term operation due to (bi) carbonate precipitation at the electrolyte free cathode, that induces flooding of the gas diffusion electrode over time. These results reveal that a proper management of local cation and water concentrations are essential in order to acheive long term operational stability in zero gap CO2 electrolyzers. 

The membrane-electrode assembly (MEA) approach appears to be the most promising technique to realize the high-rate CO2/CO electrolysis, however there are major challenges related to the crossover of ions and liquid products from cathode to anode via the membrane and the concomitant anodic oxidation reactions (AORs). In this perspective, by combining experimental and theoretical analyses, we elaborate on several impacts of anodic oxidation of liquid products in terms of performance evaluation. Firstly, we analyzed the crossover behavior of several typical liquid products through an anion-exchange membrane. Subsequently, two instructive examples (introducing formate or ethanol oxidation during electrolysis) reveals that the dynamic change of the anolyte (i.e., pH and composition) not only brings a slight shift of anodic potentials (i.e., change of competing reactions), but also affects the chemical stability of the anode catalyst. Anodic oxidation of liquid products can also cause either over- or under-estimation of the faradaic efficiency, leading to an inaccurate assessment of overall performance. To comprehensively understand fundamentals of AORs, we further develop a theoretical guideline with hierarchical indicators to predict and regulate the possible AORs in an electrolyzer. We conclude by giving some suggestions on rigorous performance evaluations for high-rate CO2/CO electrolysis in the MEA-based setup.

This talk will analyze CO₂ electrolysis from a device perspective to provide insight into what are the bottlenecks in the field and how to resolve them.  In particular we will focus on our results using synchrotron based wide angle x-ray scattering to observe CO₂ electrolysis devices in real time while concomitantly measuring anode and cathode product distributions.  During higher current density (>100 mA/cm²) CO₂ electrolysis devices are prone to cathodic ‘flooding’ leading to greatly enhanced hydrogen production.  We show that during this degradatory issue there is chaotic oscillatory fluctuations in potential, and product distribution.¹  By analyzing these fluctuations in the presence of a synchrotron, we can monitor salt deposition and even electrolyte penetration by a shift in the background scattering signal.  From the comprehensive analysis we have developed a hypothesis of why we both see the oscillations as well as flooding.  Interestingly, salt solubility and precipitation has a strong role in flooding issues. 

We then analyzed various salts (Li, Na, K, Cs) of differing solubility while doing CO₂ electrolysis.²  The synchrotron analysis clearly demonstrates the importance of salt solubility on stable operation.  We then switched to CO electrolysis as this allowed for more soluble hydroxides compared to the carbonates used during CO₂ electrolysis.  While this allowed for more stable hydration management, it did corrode the anode, as evidenced by IrO2 appearing on the cathode.  This was resolved by switching to a Ni anode, though long term acetate build-up caused pH issues.  The acetate issues was resolved by simply removing the acetate from the anolyte during testing.   This allowed for multiple tests with over 100 hour stability.³

CO₂ electrolysis to value-added products is a promising technology to close the carbon cycle and sequester anthropogenic CO₂ into chemical feedstocks; increasing the current density for multi-carbon products is one of the requirements for practical implementation. [1] The use of gas diffusion electrodes (GDEs) that allow the CO₂ reduction reactions (CO₂RR) to occur at the solid-catalyst/liquid-electrolyte/gaseous-CO₂ interface, effectively accelerates the CO₂RR by solving the problem of the mass transport limitation due to the inherently low diffusion and solubility of CO₂ in water. However, CO₂RR at the three-phase interface is complex, and guidelines for catalyst design toward its high activity have not been fully established. Since macrostructure significantly influences activity, it is challenging to distinguish between surface reactivity at the microscale (molecular level) and material transport properties at the macroscale (three-phase interface level). This presentation summarises our recent studies on high-rate CO₂RR from the point of view of both novel electrocatalyst designs and appropriate electrode assembly.

We successfully applied various metal-doped covalent triazine frameworks (M-CTFs) as platforms for CO₂RR electrocatalysts on GDEs [2-5]. In addition, we conducted systematic first-principles calculations and found that this reaction selectivity correlates with the adsorption energy of the intermediates. As M-CTFs possess the same framework, it is possible to vary only the metal center (catalytic active center) with the identical macrostructure of the catalyst layer. Thus, as we can directly compare the catalytic activity of the metal centers, M-CTFs serve as a model catalyst for establishing design guidelines for gaseous CO₂ electrocatalysts consisting of single metal centers.

We also achieved the ultra-high-rate CO₂ electrolysis to multicarbon products (C₂₊) by designing the triple phase interface composed of ordinary materials. We successfully increased the partial current density for C₂₊ over cupric oxide (CuO) nanoparticles on gas diffusion electrodes in neutral electrolytes to a record value of 1.7 A/cm² [6,7]. Specifically, we highlighted that the thickness of the catalyst layers is a crucial parameter that impacts the maximum current density for C₂₊. Although the GDE and electrocatalyst used in this case are not unique, the optimized assembly elicits their potential.

Electrochemical CO₂ reduction reaction, is the conversion of CO₂ to products through the application of charge in an electrolysis cell. If CO₂ could be selectively converted to a particular high value product at low overpotential and high current density, CO₂ would be turned into an energy storage molecule and/or a chemical feedstock, driving society closer to closing the carbon loop.

Since the early-mid 90s, research into CO₂ reduction reaction materials has primarily centred around copper^1-3.  Copper has the unique ability to bind to key intermediates *CO and H* moderately⁴, improving the chance of hydrogenation or coupling reaction steps⁵. Incremental improvements in cell design and nano structuring have improved the partial current density towards high value C₂₊ products. Size control⁶, oxide derived copper⁷ and facets⁸ among others effects have been tested, but questions remain whether the intrinsic activity towards the CO₂ reduction reaction on copper has been improved. A possible work around, is to alter the localized concentration of key intermediates. Therefore, multiple groups have paired Cu with a secondary, CO producing active site, to form a tandem mechanism^9-11.

In this talk I will be highlighting the advantages of using atomic layer deposition (ALD) in electrocatalysis and how it can form a method towards altering selectivity for the CO₂ reduction reaction.  I will initially present the requirement for altering copper activity through relating activity towards CO₂ reduction reaction to surface roughness of nanostructured copper and oxide-derived copper. Subsequently, trends in selectivity with different metal oxide nanoclusters atop copper nanoparticles are shown towards the CO₂ reduction reaction, with CO reduction reaction being used as a mechanistic tool. Further characterization techniques both ex and in situ will be presented to shed light on the changes in selectivity including XPS, in-situ contact angle measurements and FTIR amongst others.

The electrochemical conversion of CO2 (CO₂RR) into value-added chemicals using renewable energy sources is seen as one of the most promising approaches to achieve defossilization of the chemical industry.[1] The main targets are the direct production of multicarbon products such as ethylene and ethanol and the generation of syngas (CO+H₂), a feedstock in the Fischer Tropsch process. However, even though the CO₂RR process has been extensively investigated for the last 30 years,[2] its industrial application still faces significant challenges. To meet the industrial requirements, many operational parameters must be optimized besides the catalyst material such as the electrode configuration, the reaction microenvironment, and the electrolyzer design.[3] The state-of-the-art CO2RR setups, which consist of gas diffusion electrodes (GDEs) in flow cells and membrane electrode assembly (MEA) electrolyzers, already achieve relevant current densities (≥ 200 mA/cm2) but further research is needed to enhance the GDEs stability during long-term operation.

In this contribution, we present the synthesis of Ni and Cu-based electrocatalysts with high conversion to syngas and multicarbon products, respectively. These materials are synthesized via a single pyrolysis using a benzoxazine monomer (BO) as a precursor,[4] which leads to more active and stable catalyst materials than those produced with just the metal precursor. Besides the catalyst synthesis, we will discuss how the gas diffusion layer, and the hydrophobic/ion conductivity properties of the catalyst layer, strongly influence the GDEs’ stability and suppress the parasitic hydrogen evolution reaction. We show that, upon optimization, the Ni-BO-based GDE exhibits syngas production at -200 mA/cm² with a minimum stability of 2 h while the Cu-BO-based GDE is able to reach current densities up to -600 mA/cm² with high selectivity for ethanol and ethylene. The present work provides insight into important parameters that must be considered to fabricate stable GDEs for the conversion of CO₂ into valuable products at industrially-relevant current densities.

The Single-site MNC catalysts offer a ground-breaking non-precious-metal platform for CO₂ electrolysis, demonstrating exceptional faradaic efficiency towards CO (> 90% FE_CO) at industrial-relevant current densities (> 250 mA cm^-2) in regular membrane electrode assembly electrolyzer.[1] However, achieving such performance requires a cell potential (E_Cell) above 3 V, resulting in energy efficiency (EE) below 40% (EE = E_CO2-to-CO * FE_CO / E_Cell) [2, 3]. To address this challenge, a crucial technical solution involves further enhancing the catalyst material to minimize catalytic potential loss (for reduced E_Cell).

The metal center highly influences the intrinsic activity-potential relationship of the MNC catalysts. FeNx exhibits superior binding energy to the crucial intermediate, *COOH, for CO formation, compared to other metal centers. However, the FeNC ones suffer poisoning during the reaction, restricting their practical application in large-scale electrolyzers at high current densities.[4] Hence, to address this limitation, we designed a novel experimental approach to deactivate and reactivate the FeNC catalyst under CO2 electrolysis conditions using pulsed potentials. Interestingly, coupled with a millisecond-resolution differential electrochemical mass spectrometer, we defined an uncommon but potential-dependent recoverable CO₂ poisoning phenomenon on the FeNC catalyst. This comprehensive investigation unravels the mechanisms of CO₂ poisoning and recovery of the FeNC catalyst at the molecular level. Also, it offers practical guidance for mitigating the poisoning issue in large-scale applications.

The CO₂ reduction reaction (CO2RR) has shown promise for producing C1-based feedstocks, including formic acid, CO, and syngas, under ambient conditions. A great challenge in this effort is to achieve control over the reaction path in order to define the product selectivity. In this work, we exploit the well-defined atomic structure and tunable coordination environment of Bi-based single atom catalysts (SACs)[1] embedded in carbon-nitrogen frameworks to show that the coordination environment of a single metallic species can be used to tune product selectivity. Bi is well-known to be an efficient CO2RR catalyst to produce formic acid,[2, 3] but has also recently been reported for CO generation.[4] While it has been hypothesised that this differing CO2RR product selectivity may arise from different Bi coordination environments, these different products stem from dissimilar catalytic systems. Here, we aim to test this hypothesis on the same catalyst system to exclude other influences on selectivity.

Using Bi SACs within a carbon-nitrogen framework, we find that the CO2RR selectivity can be tuned towards formic acid or syngas production by choosing tailored annealing treatments. Bi SACs anchored on commercially available carbon black were synthesized via a solution-based chemical method followed by inert atmosphere annealing. The single-atomic nature of Bi is confirmed by both scanning transmission electron microscopy and X-ray absorption spectroscopy. Low-temperature (300 °C) annealing of these samples results in oxygen-coordinated Bi SACs and promotes formic acid generation, while high-temperature annealing (800 °C) favours formation of nitrogen-coordinated Bi SACs and syngas production. Since the versatility of a single CO2RR catalyst system for producing two different major products has rarely been reported, our work opens up a new direction of tuning the CO2RR C1 product selectivity using SACs.

Nanoparticle composites are widely used as catalysts in various chemical transformations. However, large-scale production of these materials is crucial for their practical implementation, and more efficient methods are needed. This presentation highlights recent advancements in the production of metal nanoparticles on carbon nanofibers through microwave heating. To achieve this, metal ions were deposited onto the carbon nanofiber surfaces using ethanol solutions, which were subsequently exposed to hexane, an antisolvent for metal ions. The carbon fibers were then rapidly and locally heated via Joule heating, resulting in the instantaneous formation of metal nanoparticles on the substrate surfaces in just a few seconds of microwave exposure, all in an ambient environment.
Furthermore, if nickel salt was used to produce nickel nanoparticles, carbon nanotubes (CNTs) were also produced on the carbon fibers during the process. This was due to several factors, including the vaporization of solvent residue that created a local environment for the reduction of metal salt, the decomposition of carbonaceous precursors on nanoparticle catalysts, and the fast cooling related to the condensation of hexane. We are exploring using the system to conver carbon dioxide to CNTs. Overall, this technique holds great promise for the scalable production of supported nanoparticle composites with potentially significant applications in the field of catalysis.

The intermittent photovoltaic (PV) power generation requires long term storage to stabilize energy system. Conversion of the excess PV power into valuable chemicals (products of CO₂ reduction, for example) represents an attractive solution for the problem. We address this challenge via ‘artificial leaf’ approach – photovoltaic driven conversion of carbon dioxide (CO₂) and water (H₂O) into valuable chemicals. In our experiment an electrochemical cell (EC), responsible for the CO₂ reduction, was directly connected to the highly efficient silicon heterojunction PV module that generates the sunlight-based electricity.

In this work, we study the performance of the directly coupled PV-driven CO₂ reduction cell with emphasis on the effects of realistic temperatures (25-65°C) and irradiances (0.2 to 1 sun) for the PV module. During the experiment we characterized coupling efficiency in the system and product selectivity of the electrolyzer, catalyst stability and finally solar-to-chemical (STC) efficiency. Coupling efficiency in the directly connected PV-EC system is an important performance parameter and is characterized by the coupling factor C_PV-EC, the ratio between the operating power and the maximum power deliverable by the PV device. The current-voltage parameters of both PV and EC system components were preselected to ensure high power coupling factor under standard test conditions of PV (1 sun irradiance A.M 1,5G and 25°C). A 5-cells silicon heterojunction PV module with maximum power point voltage of 3.2V, current of 383.7 mA, aperture area of 51.7 cm², and power conversion efficiency of 23.82% was connected to a commercially available EC cell with an 8.8 cm² CO-targeted silver catalyst [1].

The directly connected PV-EC device withstands realistic temperature and irradiance variation of the PV module maintaining high coupling factor C_PV-EC for the PV-EC device between 0.89 and 1.00. Furthermore, while temperature minimally affects PV-EC working points (V_WP, I_WP), the variations in irradiance between 0.2 and 1 sun significantly influence the voltage at working point (between 2.55 V and 3.44V respectively) and therefore affect electrolyzer products selectivity. The effect of operating voltage on the electrolyzer product selectivity is studied in a separate experiment with constant voltage steps with 1 hour duration using a potentiostat. As the voltage is increased between 2.6 to 3.4V the product selectivity for CO increases from 80% to 95%. In the operation stability study, we have observed high CO selectivity of approximately 90% over 7 hours of experiment at 3.4V and 330 mA. Performance of the EC cell characterized with the potentiostat suggests that under proper coupling, the PV-EC system can operate with solar to chemical efficiency for CO of approximately 10% over the operating voltage range.

In the ongoing experiments we investigate the CO-selectivity, operating stability and solar to CO efficiency with the directly connected PV-EC device in realistic irradiance and temperature conditions. In addition to that, the catalyst degradation rate on microstructure scale is studied using electron microscopy.

The electrochemical reduction of CO₂ (CO₂R) holds tremendous potential as a pivotal component in future energy systems reliant on renewable fuels. CO₂R proceeds through a complex reaction scheme consisting of multiple proton-electron steps with numerous possible intermediates and products, out of which many have been detected only recently. An additional level of complexity arises from the surface dynamics of the Cu catalysts undergoing structural transformations under potential,  with rapid changes in size, shape, and activity. By combining electrochemical measurements with high-resolution imaging, we identify and track dynamic active sites on the surface of electrodes during electrocatalysis.

In this talk, I will show you examples on the surface dynamics of Cu, Cu oxides and Cu electrodes covered by 2D materials during CO2 electroreduction. I will discuss the use of scanning probe microscopy and solid state spectroscopy to track the surface dynamics of electrocatalysts in-situ, and present a few unconventional strategies to reach significant boosts in the catalytic activity of 2D materials, using spin-selective mechanisms.

In the presentation I will go throught the latest developments that we have been developing in the group on one side on the type of materials and how they reconstruct under reaction conditions and how the relationships with the electrolyte can affect the chemistry taking place. The model systems investigated will allow to understand the particularities of highly reconstructed materials under relevant reaction conditions, this is a very crucial point when trying to go beyond the standard traditional models materials. Additionally the electrolytes can change the complete picture of the reactivity due to the generation of low energy paths, alternative paths that might contribute to the reactivity and more importantly new ways that reaction can be driven in different ways. The relevance of the results indicate that there are ways to allow some of the constraints that are limiting the technical implementation of CO2 reduction technologies in a short time.