Cs⁺ and Rb⁺ have previously been shown to enhance C₂₊ product selectivity during CO₂ electrolysis on Cu catalysts. In a membrane electrode assembly (MEA) electrolyzer, these cations migrate to the cathode surface from the anolyte through an anion exchange membrane (AEM). The long-term durability of the electrolyzer depends significantly on the type of AEM used, as cation transport indirectly influences durability through its role in water transport. Various membrane properties, such as the nature of cationic head groups, ion exchange capacity, and water uptake, play critical roles in determining the diffusion and migration of cations across AEMs. Understanding these transport mechanisms is essential for designing membranes that prevent flooding of the gas diffusion electrode (GDE) while enabling optimal cation transport. In this work, we investigate the transport mechanisms of two specific cations Cs⁺ and Rb⁺, across five different ion exchange membranes. We use in-situ wide-angle X-ray scattering (WAXS) measurements to study the dynamic changes in the catalysts, flow field, membranes, and water transport during electrolysis. The transport of cations is understood from X-ray fluorescence (XRF) spectroscopic measurements performed simultaneously. Overall, we map the key components of the electrolyzer during electrolysis using high-energy X-rays including the electrodes, membrane, water, and cations, and how they change as a function of time. Our findings reveal significant changes when different membranes are used, as well as a strong dependence between the Donnan potential and cation transport. A comparative analysis indicates that structural and compositional differences among membranes influence Donnan potential, preferential cation transport, and solvation shell dynamics, thereby impacting the overall electrochemical durability. The insights from this study are expected to guide the design of ion-selective and durable AEMs for advanced electrolysis systems.

Electrochemical CO2 reduction is one of the potential negative emission technologies for mitigating climate change effects.[1] By synchrotron wide-angle X-ray scattering (WAXS) it is possible to follow transient changes in overall crystallinity and specific crystal phases of the catalyst phase, as well as to monitor background intensity attributed to the electrolyte.[2] By employing a specialized electrolyzer cell, gas diffusion electrodes on top of a microfluidic channel can be investigated along their z axis, delivering a bigger picture of processes in a 2D or even 3D scale down to μm resolution.[3] Precision and interpretation of such measurements, however, depend strongly on the algorithm used to process the vast amount of diffraction patterns. In this contribution, the challenges and possibilities are discussed, including baseline removal and peak picking for complex mixtures in electrolyzer operation. Different methods of baseline removal deliver varying results, indicating the need to optimize data processing strategies. By application of the right analysis method, different Bi-based catalyst materials or perspirating species present at operating conditions.

A decarbonized society is essential to maintain and further improve the standard of living for humankind. While the residential sector and short-distance transport have available solutions for electrification, several processes in the chemical industry (such as plastic manufacturing and base chemicals) still require carbon-based fuels as a resource [1].

One of the carbon-neutral technologies to produce base chemicals is electrochemical carbon dioxide reduction (CO₂R), powered by renewable electricity. CO₂R targets multiple base chemicals ranging from C₁- products like carbon monoxide (CO) to C₂+-products like ethylene (C₂H₄) [2].

CO₂R is already established at a lab scale. Implementation of gas-fed membrane electrode assembly (MEA) ‘zero-gap’ cells, as the state-of-the-art reactor design, demonstrates promising potential for a scale-up onto the commercial level [3,4]. However, the potential upscale MEA CO₂R electrolyzers are inevitably exposed to elevated temperatures, as a substantial part of the input energy dissipates as heat during the CO₂R process. Despite the higher temperatures once scaled up, limited research is available on the phenomena and bottlenecks present in CO₂R at industrial-relevant temperatures exposing a major knowledge gap in the field [5, 6].

This study presents the effects of industrial-relevant temperatures in MEA-based CO₂ electrolysis cells. In the first step, heat balance investigations are shown, proving that industry-size electrolyzers will require heat management and cooling. This acts as a basis for the main motivation why higher than ambient temperatures inevitably need to be investigated. We briefly demonstrate that a corridor of 40°C to 70°C is the most likely operatable range for low-temperature CO₂ electrolysis if industry-scale is achieved.

To investigate higher-than-ambient temperatures the usually present experimental setup has to be extended to provide temperature control for lab-scale MEA cells, the input gas, and the liquid electrolyte input. Thus, we will briefly introduce our experimental setup using a heating oven for the MEA cell and external heating of the anolyte reservoir. Additionally, the setup utilizes several devices to enhance the control over the humidity of the incoming gas-feed CO₂ to increase the control over the water balance of the system. We explicitly share our experimental setup to lower the entree bar to motivate other groups to expand their ambient temperature testing to industry-relevant temperatures.

Extensive flooding and salt formation are two of the main bottlenecks in the current state of the art for CO₂ electrolysis often occurring at the same time or briefly after each other [7-9]. To investigate the effect of higher temperatures on flooding and salt formation experiments in an AEM-based MEA cell, using Ag as the cathode-side catalyst partnered with an oxygen-evolving IrO₂ catalyst at the anode, are executed in a current density range of 100 mA/cm² to 400 mA/cm² in a temperature corridor from 25°C to 70°C degrees using KOH and KHCO₃ as anolytes. Our data shows that elevated temperature benefits the CO₂R runtime in MEA cells by diminishing the issue of salt formation, enabling operation at current densities of >200 mA/cm² even under high electrolyte concentration at temperatures of 50 °C and above. We link this to better solubility at higher temperatures combined with a smaller cation crossover rate. However, flooding remains an issue, especially at current densities higher than 300 mA/cm². Nevertheless,  higher temperatures also mitigate cell flooding time. The exact mechanisms transporting water from the anode to the cathode are currently debated. We suggest osmotic water drag as the main transport mechanism for water transport to the cathode side, backed up by our experimental temperature data with additional control experiments for the osmotic-pressure-relevant variables like anolyte concentration and cation type.

Multi-physical transport processes on multiple scales occur in electrochemical devices and components for CO₂ electroreduction. These complex coupled transport processes determine the local environment in the catalyst layer and subsequently also the reaction rates at the catalytic sites. I will discuss how the coupling between atomistic- and molecular-scale models, microkinetic models, and continuum-models can help in improving the understanding of the elecotrchemical CO₂ reduction. I will focus on silver as catalysts and discuss how (i) the presence of reaction products can affect the performance, (ii) how the electrolyte nature can affect the selectivity and activity [1], (iii) how pulsed operation can help to improve performance, and (iv) how the mesostructure of the electrode can affect the local conditions [2,3]. All these modeling efforts will demonstrate how multi-scale and multi-physics models can be used to guide the design, operation and material choice for electrochemical CO₂ reduction. In fact, such models are essential for the understanding of electrochemical CO₂ reduction since experimental investigations cannot provide the detail and locally resolved information required.

The re-utilization of CO2 via its electrocatalytic reduction (CO2RR) into value-added chemicals and fuels is a promising avenue to minimize the impact of existing technologies on the climate change. This requires the development of low cost, efficient, selective and durable electrocatalysts. However, their rational design requires in depth understanding of the modifications that structurally and chemically well-defined pre-catalysts undergo during operation, especially when the reaction conditions themselves change dynamically.

Here, the transformations that Metal-N-C catalysts (M=Cu, Ni, Co, Fe, Sn, Zn) experience during static and pulsed CO2RR will be unveiled. This will be achieved by a synergistic combination of operando quick X-ray absorption spectroscopy (XAS), high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) and X-ray emission spectroscopy (XES), coupled with unsupervised and supervised machine learning methodologies and density functional theory.

In particular, I will illustrate the astonishing behavior displayed by Cu-N-C catalysts during CO2RR, featuring reversible transformations from single atom sites towards small clusters and Cu nanoparticles. The switchable nature of these species, that can be achieved by applying different potential pulses, holds the key for the on-demand control of the distribution of the CO2RR products and thus, a wide-spread adoption of this process. Moreover, I will elucidate the nature of the ligands formed under CO2RR at singly dispersed Ni sites or Co sites in Ni- or Co-N-C catalysts, which are currently drawing great attention for their excellent CO yields.

Overall, my lecture will feature the importance of operando characterization of electrocatalysts in order to elucidate structure/composition-reactivity correlations during CO2RR.

The current state-of-the-art copper electrode lifetime falls several orders of magnitude short from the 10.000 hours industrially relevant timespan.[1] Most copper catalyst cannot maintain their activity towards ethylene and other multicarbon products for more than 100 hours. Contributors to limited catalyst lifetime are copper restructuring[2] , salt formation[3] , flooding[4]  and impurity deposition[5]. The restructuring of copper remains one of the least understood mechanisms. Scale up of CO₂ electrolysers therefore requires a deeper understanding of the chemical processes leading up to copper instability as well as solutions on both an atomistic and system design level.
Several fundamental works have demonstrated that anodic/cathodic dissolution and redeposition of copper species is one of the main components of the restructuring process.[6] The degree of dissolution and redeposition of the positively charged copper-carbonyl complex was found to proportionate to the applied current density.[7] Based on these results, one would expect that low current CO₂ electrolysis within H-cells results in very high electrode stability. Yet, it is the high current CO₂ electrolysis in gas diffustion electrode (GDE) based cell configurations that demonstrate the most impressive numbers of operational hours.

In this work, restructuring on a poorly in-plane conducting PTFE based copper electrode is captured through ex situ SEM analysis. It is shown that, over time, copper migrates from the center of the electrode to the perimeter closest to the negatively charged current collector. Subsequently, a nonporous structure erupts, favouring the hydrogen evolution reaction (HER) over CO₂ reduction reactions (CO2RR) after 45 minutes to 1 hour of operation at 200 mA cm^-2. We then present confinement procedures that are commonly applied within (GDE) based systems to minimize the dissolution of copper species such as metaloxide nanoparticle coatings and ionomers. In addition, the redeposition location is brought closer to the dissolution location by distributing the potential more uniformly across the GDE. Using one or a combination of the outlined strategies allowed the catalyst lifetime to be lengthened to several hours.

Copper is unique among metals for its ability to reduce CO₂ to high-value products. Despite its often-considered noble nature, copper readily degrades during electrochemical CO₂ reduction (ECR). Through extensive experimental results, theoretical modeling, and literature data, the mechanisms behind copper degradation have been elucidated.¹ A high-surface-area Cu catalyst exhibited significant changes in morphology and product selectivity over several hours under constant cathodic potentials relevant to ECR (-0.8 to -1.1 V vs. reversible hydrogen electrode). The formation of copper complexes with CO₂ reduction intermediates was identified as the main driving force behind this instability.

Our findings additionally suggest that these dissolved Cu species preferentially redeposit on sites with lower intermediate coverage, such as adsorbed CO (*CO). A dynamic equilibrium between dissolution and selective redeposition of these copper complexes drives morphological restructuring, leading to catalyst deactivation. This results in a shift in selectivity away from ECR towards hydrogen production during prolonged operation. The interconnected changes in nanoparticle size, crystallographic facet orientation, *CO coverage, and the CObridge vs. COatop ratio were proposed as the key factors contributing to catalyst deactivation.

To confirm the universality of this effect across copper-based catalysts, experiments on electrodeposited copper nanoparticles and copper foil were conducted, yielding similar trends. Understanding these processes is essential for developing strategies to mitigate instability and improve catalyst stability, addressing one of the critical barriers to the industrialization of ECR.

Metal- and nitrogen-doped carbons (M-N-Cs) represent a promising class of low cost electrocatalysts derived from nature-abundant elements for various electrochemical processes including CO₂RR [1,2]. Traditionally, M-N-Cs containing active metals (M = Fe, Co, Ni) are synthesized by direct pyrolysis of inorganic and organic precursors, a process that often results in the undesired formation of inorganic side phases through carbothermal reduction, impeding the effective integration of active metals like Fe, Co and Ni. Furthermore, comparing the intrinsic activities of different M-N-Cs can be complicated due to variations in catalyst morphology and active site concentration that arise during the pyrolysis.

To address these challenges, we developed an active-site imprinting strategy in which active metals are introduced post-pyrolysis via ion-exchange [3-5]. In this work, we employed the Mg imprinting strategy to produce Co-N-Cs and Ni-N-Cs with comparable morphology and metal dopant concentration. Our approach allows for a more direct comparison of the intrinsic activities that arise from the metal dopant. The Ni-N-Cs derived this way are consistently higher in activity and selectivity than the corresponding Co-N-Cs, exhibiting a CO Faraday efficiency of up to 95% at potentials between -0.5 to -0.8 V_RHE. The Ni-N-C catalyst maintains high stability at -0.65 V_RHE, with 92.5% retention of current density and 97.6% retention of CO selectivity after 100 hours of continuous operation.

While Ni-N-Cs are known to be relatively good CO₂RR catalysts, the origin of its activity remains rather elusive. Contrary to the widely accepted MN₄ active sites in M-N-Cs, NiN₄ sites are theoretically inert to both CO₂RR and HER due to the instability of the *COOH and *H intermediates respectively, often leading to the proposal of alternative but unproven NiN_x sites with various coordination structures to justify their performance. In this study, however, the absence of inorganic side phases allowed us to characterize the tetrapyrrolic NiN₄ coordination structure via Extended X-ray Absorption Fine Structure (EXAFS), suggesting that NiN₄ sites should in fact be CO­₂RR-active. Therefore, we performed mechanistic studies on the tetrapyrrolic NiN₄ sites using density functional theory (DFT), drawing inspirations from recent studies which elucidated the critical role of cations in enhancing CO₂RR activity on noble metal catalysts like Cu, Ag, and Au [6-7]. The presentation will further illustrate the influence of cations on the activity and selectivity of M-N-Cs towards CO₂RR, as well as well as the advantage of the pyrrolic nitrogen coordination in promoting the adsorption of these cations. 

Overall, this work not only highlights the potential of Mg-imprinted strategies in developing high-performance, morphological comparable electrocatalysts for sustainable energy applications and intrinsic activity studies, but also provides mechanistic insights on the involvement of cations in the valorization of the seemingly inert NiN₄ site.

The green transition requires discovery and development of new catalyst materials for sustainable production of chemicals and fuels. However, it is difficult to predict a material, which might have a high catalytic activity for a given reaction, therefore the development of catalysts up until now has been driven mainly by trial and error. It would increase the pace of development, if we could predict a range of promising materials or if we at least could understand the limitations of catalysis. In this context high entropy alloys offer a chemical space of possible materials where the composition can be smoothly varied and where the properties also might vary in a seamless manner. This is good news for catalysis as such a smooth space is easier to explore to determine the interesting regions in composition space. Furthermore, the highly heterogeneous nature of a high entropy alloy surface reveals fundamental effects which are important for chemistry on surfaces in general, but are overlooked in the classic mean field view on catalysis.

Finding affordable, durable, and efficient catalysts is crucial for advancing green hydrogen production and carbon dioxide conversion, both vital for mitigating climate change. Currently, the discovery of new catalysts is hindered by the gap between computational predictions and experimental outcomes. Traditional catalyst discovery is a slow, trial-and-error process relying on decades of expertise. Analyzing a single material takes time, so different parts of the material design space are often studied separately by various research groups. This results in reproducibility challenges, making it difficult to build on previous findings and draw comprehensive conclusions. Progress requires large, diverse experimental datasets that are reproducible and tested under industrially relevant conditions to bridge the gap in discovery. In my talk, I will highlight the development of the Open Catalyst Experiments OCx24 dataset, featuring more than 500 samples synthesized and characterized by XRF and XRD and tested on gas diffusion electrodes in zero-gap electrolysis for CO2 reduction and hydrogen evolution. To find correlations with experimental outcomes and to perform computational screens, we calculated DFT-verified adsorption energies for six adsorbates on approximately 20,000 inorganic materials, requiring 685 million AI-accelerated relaxations. 

To alleviate CO₂ emissions and their impact on climate change, converting carbon dioxide into valuable products such as multi-carbon organic chemicals is of great importance. CO₂ can be converted via different pathways such as electrochemical, photo-electrochemical and biological etc. Each approach offers distinct merits but also certain challenges in terms of process efficiency, product selectivity and implementation at scale etc. Therefore, developing coupled CO₂ conversion systems, for instance bioelectrochemical reactors, can potentially address some of those challenges.^[1] In this work, the focus is on developing cost-efficient, biocompatible, and high activity porous M-N-C catalysts with M = Ni and Co that are atomically dispersed as NiN₄ and CoN₄ active sites in porous carbon matrix. Ni- and Co-N-Cs are prepared by active-site imprinting approach using Mg as an imprinter.^[2][3] Pyrolysis of Mg-N-C is carried out in a salt-melt at high temperatures (≥ 800 ^oC) and followed by an exchange with Ni or Co at low temperatures. N₂-sorption of the materials reveal a micro-mesoporous structure with high surface areas (> 1000 m² g^-1) and a mass-transport enabling pore system. Extended X-ray absorption fine structure (EXAFS) reveal the existence of atomically dispersed single atom active sites with defined active site structure. A variety of Ni-N-Cs and Co-N-Cs were tested for CO₂R activity in a rotating disc electrode (RDE) setup, showing high activity and selectivity towards CO₂R versus the competing HER. Subsequently, these catalysts were implemented in a home-made bio-electrocatalytical system (BES) consisting of a bioreactor coupled to a CO₂ electrolysis cell. Here, CO₂ is first electrochemically converted to CO in the electrolysis cell which is then directly fed to bacteria (Clostridium ragsdalei) in bioreactor who further metabolize it to valuable carbon compounds such as acetate. In the BES, partial pressures of CO reached a maximum of 5.7 mbar and that of hydrogen was 2.7 mbar after 30 h. A specific exponential bacterial growth rate of 0.16 h^-1 was observed with acetate formation rate of 1.8 mg L^-1 h^-1 and an acetate concentration of 0.103 g L^-1 corresponding to acetate formation rate of 0.73 mmol d^-1. As will be discussed in greater details in this talk, we have successfully demonstrated the validity of a coupled bio-electrocatalytical system concept operating with Co- and Ni-N-C catalysts for CO₂ conversion.

The electrochemical conversion of CO₂ has in recent years emerged as a promising technology towards allowing the conversion of CO₂ emissions to valuable chemicals and synthons. In recent years, molecular electrocatalysts have risen to the foreground as a tunable, more mass active and selective alternative to transition metal electrocatalysts. Despite the impressive results reached in recent years for a multitude of systems, transferring molecular systems to the large-scale is not yet a smooth endeavor.
Herein, we present our recent efforts towards a holistic transfer of molecular based assemblies, based on the Ag-BIAN (BIAN: N,N′-bis(arylimino)acenaphthene) structure in industrially relevant CO₂ electrolyzers operating at elevated current densities ≥ 300 mA cm^-2 at 60°C. Notably, we show how the performance and long-term stability of gas diffusion electrodes coated with Ag-BIANs is not only affected by the nature of the ligand system but also the operation conditions. Together with the performance of ex-situ and operando characterization, we show how ink compositions can play a crucial role in unlocking the activity and stability of previously though inactive complexes under electrolytic conditions at 600 mA cm^-2.

Furthermore, the currently running and future scale-up projects of Fraunhofer UMSICHT will be presented opening the discussion on the possible up-scaling of molecular systems.

The electrochemical reduction of CO₂ offers a promising route for converting renewable energy into carbon-based fuels through CO₂ hydrogenation, creating a net-zero carbon emissions energy cycle. Formate is one of the important value-added CO₂ reduction products. It is highly demanded in textile and food preserving industries. So, we targeted formate as selective product in CO₂ reduction in aqueous electrolyte.[1] Among the catalysts explored, Bismuth (Bi) stands out for being earth-abundant, environmentally friendly, cost-effective, and highly stable, with excellent selectivity for formate production. We developed a controlled synthesis process to produce uniform height 2D Bi flakes with (012) facets, using the chemical reduction of a sacrificial 2D BiOCl template under ambient conditions.[2] These Bi flakes demonstrate an impressive formate partial current density of 18.7 mA/cm² at -1.14 V_RHE during chronoamperometry, along with a peak formate Faradaic Efficiency (FE) of around 90% at -0.84 V_RHE. However, due to weak solubility of CO₂ in water, at higher potential, CO₂ saturation at the electrode surface drops which cause severe formate selectivity drop, giving rise to competitive Hydrogen Evolution Reaction (HER). To address this, we strategically modified the 2D Bi flakes with an ultrathin coating of Polyaniline (PANI), a redox-active conducting polymer. The basic amine groups in the polymer chain of PANI attract weakly acidic CO₂ molecules to the electrode surface. We found, PANI is not at all active for CO2RR but when it is coated on 2D Bi flakes, it significantly improved performance, increasing the formate partial current density to 35.5 mA/cm² at -1.14 V_RHE. Moreover, this modification also reduced the formate selectivity drop at higher potentials in comparison to bare 2D bismuth flakes, demonstrating the synergy between PANI and 2D Bi flakes in enhancing CO₂ reduction. We also have postulated a different mechanistic path that shows the role of PANI which is working behind this enhanced activity.

It is widely recognized that CO₂ is one of the driving forces behind the climate change. However, CO₂ is also a valuable resource for the formation of industrially relevant chemicals, and thus carbon capture and utilization, especially its electrical conversion, is becoming increasingly important. A major challenge for electrochemical fixation of CO₂ is the low energy efficiency of electrocatalysts due to high overpotentials. A cost-effective and sustainable material in the field of CCU is nitrogen-rich polymers such as polyethyleneimine (PEI) [1] or polyimidazolium (PI). These polymers are particularly well suited for CO₂ adsorption at low partial pressures, which would allow for the direct utilization of industrial flue gas. However, they lack the necessary electrical conductivity. Therefore, the design of hybrid materials combining porous carbons with high electrical conductivity and the CO₂-absorbing polymer is key. In this work, these nitrogen-rich polymers were incorporated into carbon substrates via a sonication-assisted impregnation process to improve transport properties and conductivity, both of which are important for subsequent electrochemical CO₂ conversion.

The pristine mesoporous carbon substrate was synthesized via zinc oxide-templated carbonization of sucrose at 950 °C and exhibits a specific BET surface area of 1750 m²/g, a high pore volume of 3 cm³/g as well as a hierarchical pore system [2]. These polymer-carbon composites have been characterized by physisorption measurements (N₂/CO₂), thermal response measurements (Infrasorp) and DRIFTS. All these measurements indicate a strong interaction of these polymer-carbon composites with CO₂ even at low pressures. By using a highly porous carbon support matrix, the initial CO₂ uptake is increased at least 10-fold compared to the bulk polymer. In addition, the irreversibility of CO₂ adsorption is proposed to follow a chemisorption mechanism and thus activate the CO₂ molecule.

First electrochemical measurements were performed on a rotating ring-disk electrode (RRDE). The results exemplify how the material can be used as an electrocatalyst for the reduction of CO₂. These hybrids were found to exhibit significant selectivity between the hydrogen evolution reaction (HER) and the CO₂ reduction reaction (CO2RR), as no hydrogen was detected in CO₂-saturated KHCO₃ solutions. It is anticipated that, due to their enhanced affinity for CO₂, a selective conversion process can be achieved without the need for metallic catalyst centers. Current work aims at transitioning from these fundamental RRDE investigations to more application-oriented measurements in a GDE/zero gap setup [3]. The goal is to enhance the faradaic efficiency further towards CO formation under flue gas conditions by using diluted CO₂ as a feed gas.

The rising levels of CO₂ in the atmosphere require the development of novel approaches to carbon management. Moreover, the installed capacity of photovoltaic (PV) systems is expanding at a considerable rate, resulting in a significant discrepancy between the generation of electricity from PV sources and the actual demand for electricity. The long-term storage of energy in molecules such as fuels or other industrially useful chemicals is of particular significance in counterbalancing the seasonal variations in photovoltaic power generation. The direct-coupled photovoltaic-electrochemical (PV-EC) system addresses these issues by converting excess PV energy into chemicals prior to feeding it to the grid. This enhances the utilization of photovoltaic power and improves grid stability. In this study, we have designed and tested a direct-coupled photovoltaic (PV) electrocatalytic (EC) device for the conversion of carbon dioxide (CO₂) into carbon monoxide (CO) and hydrogen (H₂) (see Figure 1). The device employs emulated PV that reproduces the IV characteristics of real PV modules in the field at the most relevant irradiance and temperature combinations (Figure 1(a)). The characteristic operating points of the PV-EC were obtained using the NREL public database for silicon heterojunction (SHJ) modules installed in specific regions of the USA. The newly developed PV emulator routine enables the precise and accurate reproduction of any IV characteristic of a PV module, at a level comparable to that of a Class A+ solar simulator. In our study, a SHJ module with an emulated area of 44.5cm² drives a flow-type stack EC cell (area 9.5cm²) with a silver/gas diffusion layer (GDL) cathode and an iridium oxide anode (see Figure 1(b)). The operation of the PV-EC system was evaluated under dynamic conditions, represented by three sunny days in a cycling procedure. This procedure involved eleven steps of irradiance-temperature pairs, ranging from 0.2 Sun to 1.1 Sun and 20°C to 54°C, followed by idle 'night' periods. The operating voltages achieved were in the range of 2.4 to 3.2V, while the operating currents were between 70 and 335mA corresponding to current densities of 7.4 to 35.2mA/cm2. The solar-to-chemical efficiency was observed to range between 8.7 and 10.8% at a high degree of coupling (0.85 to 1) in the absence of power electronics. Finally, a consistent and stable dynamic operation towards CO as the primary product with 75% faradaic efficiency and H₂ (25%) as a by-product over one to three-day cycles was demonstrated. This coupling, in conjunction with the high selectivity towards CO, renders the approach an attractive one for the decentralized storage of excess PV energy, offering a material-saving route.
 

(Photo)electrochemical conversion of solar energy represents a sustainable approach for generating chemical fuels, such as hydrogen (H₂) and hydrocarbons. Recent advances have highlighted the potential of PEC systems to drive convert carbon dioxide (CO₂) into various hydrocarbons. Despite these advancements, several critical challenges persist, particularly regarding the selectivity of the systems and the suboptimal efficiency and limited long-term stability of current PEC systems. Overcoming these limitations is crucial for realizing the full potential of PEC technology in renewable fuel production and for its broader adoption within the energy sector.

Here we will share the most recent results in our group on two main aspects regarding (photo)electrochemical CO2 reduction. On the one hand, we will focus on the importance of the microenvironment in determining and tuning selectivity by changing the local environment on metal electrodes or the chemical environment of metal centers otherwise selective for H₂ production. On the other hand, we will show examples of stabilized and stable system for CO₂ (photo)electroreduction.

The electrochemical carbon dioxide (CO₂) conversion to fuels and chemicals, powered by renewable electricity, presents a compelling avenue for reducing CO₂ emissions while facilitating large-scale and long-term renewable energy storage. Over the past decade, there have been promising steps in the production of fuels and chemicals (e.g., methane, ethylene, and ethanol) through electrochemical CO₂ conversion. Specifically, advancements in catalyst and system designs have enabled high selectivity for methane and ethylene (>70%) at high current densities (100 – 1000 mA/cm²).

While high selectivity at high current densities has been achieved, the stability of electrochemical hydrocarbon production remains insufficient for practical applications. Copper (Cu)-based materials are the most efficient catalysts for hydrocarbon production, but they experience morphological, structural, and chemical transformations under CO₂ reduction conditions, leading to changes in product selectivity.

In this study, we introduce the concept of reversible catalysts for CO₂ conversion to methane. This approach leverages the dynamic behavior of copper catalysts during CO₂ reduction. Using this method, we achieved CO₂-to-methane conversion at a current density of 200 mA/cm², with a methane Faradaic efficiency exceeding 50% sustained over 1,000 hours of operation.

Electrolysis technologies such as water splitting, CO2 electroreduction, and other emerging reactions, present sustainable alternatives to power large industries such as transport (fuels, e.g., green hydrogen), manufacturing (chemical feedstock) and agriculture. These reactions rely on breaking molecules and reassembling them into desired products with sufficient activity and selectivity. Their viability relies on achieving sufficient performance in metrics in scalable processes.

Conventionally, this has been pursued by innovation at the catalyst level, for example, designing materials with target physicochemical properties. This is challenging, since catalysts change substantially during reaction, which precludes a predictive catalyst design. On the other hand, while water is arguably the main ingredient in water electrolysis, its role at the catalyst environment and in the electrolyte remains underexplored.

I will first present a new strategy to program catalyst reconstruction. Starting from the same precatalyst, this can either lead to unpredictable composition and structure or enable predictable catalysts with improved reliability. I will then focus on the liquid-side of the reaction. I will show examples on how control over water at catalyst interfaces can enable achieving distinct physicochemical properties leading to improved stability during water electrolysis and activity and selectivity during CO2 electroreduction.

To conclude, I will briefly overview sustainability issues in the scale up and path to market of these technologies.

The electrochemical CO₂ reduction reaction (CO₂RR) has garnered significant attention over the past decade due to its distinct advantages, including operation under ambient conditions, compatibility with renewable electricity, and the ability to produce a diverse array of value-added products. Most CO₂RR research to date has utilized pure CO₂ as the feedstock. However, real-world CO₂ waste streams, such as those in flue gas or biogas, typically contain no more than 40% CO₂. This discrepancy poses challenges for the economic feasibility and sustainability of CO₂RR, as CO₂ purification steps prior to electrolysis—such as CO₂/N₂ separation—can cost $70–100 per ton of CO₂ and significantly increase the carbon footprint [1].

In recent years, integrating CO₂ capture with electroreduction into a single unit has emerged as a promising strategy to address these limitations. Many studies have focused on using CO₂ capture solutions as electrolytes, achieving encouraging results in improving the overall cost-efficiency of CO₂RR. Beyond solution- or electrolyte-based capture methods (e.g., amine solutions and ionic liquids), integrating alternative approaches such as solid adsorption and membrane-based processes into CO₂RR systems offers considerable potential. For instance, combining these methods with gas-diffusion electrode designs could enhance the efficiency and practicality of CO₂ capture-electroreduction systems [2,3].

Herein, we showcase the scenarios to integrate adsorption and membrane separation within gas-diffusion electrodes (GDEs) and present our recent research data of CO₂ conversion by an ionic liquid-mediated CO₂-selective GDE. GDEs have been tested with gaseous feed with CO₂ concentration as low as 15% containing O₂, resembling flue gas, and showed high-rate syngas production with a polymer-ionic liquid selective layer.

Bipolar membranes under reverse-bias (r-BPM) offer a possibility of using PGM-free anodes in CO₂ electrolysers. Under ideal circumstances, the OH- generated from the BPM can fully replenish the OH- consumed by the OER, maintaining a stable anolyte pH over time. However, the OH- regeneration rate, and hence the stability of such systems, is dependent on the Water Dissociation Efficiency (WDE). In non-ideal BPM, the transport of co-ions through the membrane lowers the WDE below 100%. In this case, a pH decrease in the anolyte over time is expected, compromising the long-term stability of PGM-free anodes in CO₂ electrolyser.

In our study we aim to explore the feasibility of replacing PGM anodes under industrially relevant conditions. To simulate the long-term stability of a PGM-free CO₂ electrolyser, we have developed a methodology that combines both an experimental and modelling approach. Using a MEA cell architecture, we have determined the WDE of commercial BPMs under various process conditions, including current density, anolyte concentration, or cation identity. The experimental results have been used as the model input to extrapolate the long-term performance of a r-BPM CO₂ electrolyser. Our results suggest that current commercial BPM WDEs are not high enough to allow for the replacement of PGM anodes in CO₂ electrolysers. In addition, we highlight the importance of assuming realistic industrially relevant anolyte volumes when assessing the stability of PGM-free anodes.