As Europe races to decarbonize industry, it has been clearly seen that the ability to transform carbon dioxide from a waste emission into valuable chemical feedstock is imperative. In the REUSE project, researchers are advancing electrochemical CO₂ reduction to formic acid using morphology-engineered tin-based catalytic systems, solar-assisted electrodeposition, and next-generation in-situ monitoring.

In this feature, Dr. Stella Balomenou unpacks how catalyst shape, corrosion science, and sustainability metrics converge to define what scalable carbon utilization could look like, and what still stands between laboratory innovation and industrial impact.

1 – The REUSE project explores template-free synthesis of tin-based catalysts with striking morphologies like rods, dendrites, thorns, and flower-like structures. What inspired this morphology-driven strategy, and how do these shapes fundamentally influence CO₂ activation and selectivity toward formic acid (FA)?

Our morphology-driven strategy is based on a fundamental principle: in catalysis, and particularly in electrochemical CO₂ reduction, the catalyst cannot be understood only by its composition. Its microstructural features are equally decisive, as they determine the electrochemically active surface area, the accessibility of active sites, how intermediates are stabilized, how the local pH evolves, and how mass transport develops near the interface. All these factors ultimately define the reaction rate, conversion, and selectivity – that is, the overall electrochemical performance.

Tin-based materials are widely recognized for their potential in selectively converting CO₂ into formate or formic acid. However, their performance is highly dependent on surface structure and behavior under operating conditions, particularly in stabilizing the intermediates that influence selectivity.

This is why, in REUSE, we employ template-free synthesis to generate diverse architectures and morphologies, such as rod-like, dendritic, thorn-like, or flower-like structures. These are not just visually distinctive, they represent different distributions of active sites, edge densities, and transport pathways. A more compact morphology may offer better mechanical stability or conductivity, but fewer highly undercoordinated active sites. In contrast, more branched or hierarchical structures can provide a significantly larger electrochemically accessible area and a broader distribution of reactive environments, enhancing CO₂ activation and the stabilization of -OCHO intermediates associated with the formate pathway.

Yet, greater structural complexity does not always translate to improved performance. Highly branched or sharp features may experience local electric field intensification, making them more susceptible to restructuring or corrosion. Similarly, a catalyst with very high initial electrochemically active surface area may not retain this over time and may be prone to faster degradation.

Therefore, the central scientific question is not identifying which morphology delivers the highest initial activity, but determining which structure optimally balances selectivity toward formate, robustness, and long-term stability under realistic operating conditions. In REUSE, catalyst morphology is a key design variable, bridging synthesis, reaction mechanisms, and durability.

2. How does integrating photovoltaic energy into catalyst synthesis change the way these materials form? Could this approach redefine how we think about scalable, sustainable catalyst manufacturing?

In REUSE, the integration of photovoltaic energy into catalyst synthesis is primarily driven by sustainability and system-level thinking, rather than by a deliberate attempt to alter the fundamental growth mechanisms of the materials.

We use electrodeposition as a key synthesis route because it is scalable, controllable, and directly compatible with electrode fabrication. In fact, within REUSE, we explore both conventional and solar-assisted electrodeposition methods for producing shape-controlled tin-based catalysts. The role of photovoltaic energy is therefore to ensure that, as we move toward larger electrode areas and pilot-scale systems, the energy required for catalyst and electrode fabrication remains aligned with the overall concept of low-carbon CO₂ utilization. In other words, we avoid introducing an additional carbon footprint at the materials production stage.

So, the main contribution is not so much redefining how catalysts form at the microscopic level, but rather how we think about manufacturing them at scale, ensuring that both the process and the materials are consistent with the principles of renewable, sustainable technologies.

3. Most electrochemical CO₂ reduction studies examine catalyst stability in aqueous bicarbonate or hydroxide systems. REUSE goes further by testing catalysts in CO₂-captured phase-changing solvents. What new stability challenges emerge in these environments, and why is understanding corrosion under these conditions critical for real-world deployment?

This is one of the most important aspects of REUSE, because it brings the discussion of catalyst stability much closer to the conditions where industrial implementation will actually become challenging. A large part of the CO₂ reduction literature evaluates catalysts in relatively idealized aqueous electrolytes, where the chemistry is well understood and the environment is easier to control. In REUSE, however, the catalytic interface is exposed to much more complex media, as it is directly coupled with CO₂ capture.

This shift is deliberate. Moving toward the direct use of CO₂-captured, amine-based solvents inside the electrochemical cell has the potential to significantly reduce overall process complexity and energy demand, by avoiding separate CO₂ desorption, purification, and compression steps. In this sense, integration is not only a scientific challenge, but also a key opportunity for improving system-level efficiency.

At the same time, this integration introduces a broader and more complex concept of what is often referred to as corrosion. In this context, we are not dealing with classical corrosion at near-equilibrium conditions, but rather with electrochemically and chemically induced degradation processes under operation. These include catalyst dissolution or instability, morphological restructuring and loss of active surface area, chemical interactions with the solvent or capture species, and also degradation phenomena at the device level.

This is particularly important in REUSE, because we do not operate in a conventional liquid-phase electrochemical cell, but in a zero-gap electrolyser configuration, based on a membrane electrode assembly. In such systems, the catalyst is directly integrated with the membrane and the transport layers, so degradation is not limited to the catalyst surface, but involves the full electrode-membrane interface. This includes changes in interfacial resistance, ion transport limitations, flooding or dry-out effects, and even mechanical degradation such as catalyst layer detachment.

When moving to CO₂-loaded, phase-changing solvents, additional challenges emerge. The solvent can significantly alter speciation, ionic conductivity, wetting behavior, and the local availability of proton donors. Capture-related species may interact with the catalyst surface through adsorption or coordination, while the interface itself becomes highly dynamic as solvent composition evolves during operation. These effects influence mass transport, local concentration gradients, and the thermodynamic driving forces for dissolution or oxide formation.

For tin-based catalysts, this is particularly critical because selectivity toward formate is strongly linked to surface chemistry. Any restructuring, passivation, dissolution, or redeposition of the active phase can directly alter catalytic behavior. As a result, a catalyst that appears highly selective under short-term or idealized conditions may not remain stable in an integrated capture-conversion environment.

Understanding these degradation mechanisms across different scales, from intrinsic material stability to full MEA operation, is therefore essential. It is not a secondary issue, but central to technological credibility. If REUSE aims to bridge CO₂ capture and electrochemical conversion in a realistic process chain, then stability must be assessed under the actual chemical and operational conditions that this integrated system creates.

4. The project integrates in-situ imaging and electrochemical monitoring to observe degradation as it happens. What have these techniques revealed about morphology-dependent corrosion or restructuring? Have any findings challenged conventional assumptions about catalyst stability?

One of the main advantages of combining in-situ imaging with electrochemical monitoring is that it allows us to move beyond the traditional before-and-after view of catalyst stability. In many cases, post-mortem characterization confirms that a catalyst has changed, but it does not reveal when the change started, whether it was gradual or abrupt, or how it relates to electrochemical operation. By observing the system during operation, we can directly connect structural evolution with functional behaviour.

A key insight is that degradation is strongly morphology-dependent, not only in extent but also in mechanism. Different structures do not simply degrade faster or slower, they evolve in fundamentally different ways. Highly branched or sharp architectures tend to undergo localized restructuring at exposed features, while more compact morphologies evolve more uniformly but still experience significant surface chemical changes. This highlights that the relationship between initial morphology and long-term performance is more complex than the conventional assumption that higher roughness or surface area automatically leads to better practical catalysts.

In addition, in-situ observations indicate that morphological evolution can occur even when electrochemical responses appear stable. Surface restructuring, loss of fine features, or material redistribution may take place during operation without immediate drops in current or selectivity. This suggests that the catalyst can remain electrochemically active while its active interface is continuously evolving, challenging the assumption that stable current or faradaic efficiency alone is sufficient evidence of structural robustness. In such cases, performance may appear unchanged, while the nature of the active surface is already evolving.

Electrochemical impedance spectroscopy (EIS) further clarifies how these changes impact performance by distinguishing between ohmic resistance, charge transfer, and mass transport limitations. By tracking these components, we can link structural and chemical shifts to specific electrochemical phenomena, such as increased interfacial resistance or loss of active area.

To deepen this understanding, REUSE is advancing operando electrochemical characterization, including near-ambient pressure XPS, to probe surface chemistry under realistic conditions. This approach provides direct insights into oxidation state changes, surface segregation, and intermediate stability during operation.

Ultimately, these combined techniques provide a comprehensive, multi-scale framework for assessing catalyst performance and degradation, and reveal that stability is not a static property but a dynamic interplay between structure, chemistry, and function. For morphology-driven catalyst design, understanding these real-time transformations is essential for developing reliable design principles and ensuring long-term performance.

5. The project investigates multiple conductive substrates and develops membrane electrode assemblies (MEAs) for operation in zero-gap CO2 electrolysis cells. How do substrate properties, interface design, and charge transfer dynamics influence overall catalytic performance and faradaic efficiency toward formic acid?

In REUSE, substrate properties and interface design are fundamental, because the catalyst does not operate in isolation but as part of a fully integrated membrane electrode assembly in a zero-gap configuration. In this context, catalytic performance and faradaic efficiency toward formic acid are determined by how effectively the catalyst, support, ionomer, and membrane function as a coupled system.

From our work, it is clear that the choice of substrate and microporous layer strongly influences catalyst distribution, and accessibility, and ultimately electrochemical behaviour. For example, carbon nanotube-based microporous layers promote more uniform tin deposition and better-defined structures, which correlate with lower cell voltages and improved activity compared to graphene-based systems. In contrast, less compatible substrates or highly hydrophobic commercial gas diffusion layers tend to result in non-uniform deposition, poor adhesion, or partial detachment of the catalyst layer, which negatively affects both performance and stability.

These observations highlight the importance of the catalyst-support interface. Good interfacial contact and homogeneous ionomer distribution, at an optimal concentration, enable efficient electron and ion transport, reducing interfacial resistance and promoting uniform current distribution. In contrast, discontinuities, weak adhesion, or non-uniform coatings introduce low quality interface, meaning local resistive losses and uneven reaction environments, which can directly impact selectivity, reduce faradaic efficiency toward formic acid, and accelerate degradation.

In this context, charge transfer dynamics are governed not only by the intrinsic catalytic activity, but by how effectively electrons and ions are delivered to the active sites through the electrolyte-substrate-ionomer-catalyst network. Limitations in this coupling, such as poor electrical contact or restricted ion transport, lead to local current heterogeneities, and reduced effective active area.

In a zero-gap MEA, these effects are further amplified. The catalyst layer is directly coupled with the membrane and transport layers, making charge transfer, mass transport, and water management strongly interdependent. Local gradients in reactant availability, ion conduction through the ionomer, and wetting behaviour of the microporous layer can significantly influence reaction pathways and selectivity, while also increasing energy losses at the cell level.

This is why, in REUSE, we focus not only on catalyst development but also on MEA engineering and manufacturing. By controlling parameters such as electrode architecture (e.g., catalyst-coated substrates versus catalyst-coated membranes), ionomer content and distribution, and the hydrophilic-hydrophobic balance of the microporous layer, we can actively tune the interfacial environment and transport properties.

Overall, achieving high faradaic efficiency toward formic acid in REUSE is not only a matter of electrocatalyst activity, but of electrode architecture and MEA engineering. The substrate, microporous layer, ionomer, and catalyst must be carefully co-designed to ensure efficient charge transfer, uniform reaction conditions, and stable operation over time.

6. Life-cycle assessments suggest that electrochemical CO₂-to-FA could reduce emissions by more than eightfold compared to fossil-based routes. In your view, what scientific or engineering milestones must be achieved for REUSE technology to transition from promising research to industrial-scale climate solution?

For this type of technology to become an industrial climate solution, three critical scientific and engineering milestones must converge: performance, durability, and integration.

First, electrochemical performance must meet industrial standards—not just in controlled lab settings, but under realistic conditions. This means sustaining high current densities (~200 mA·cm⁻²), high faradaic efficiency, and competitive energy consumption while ensuring compatibility with practical product recovery and system design.In REUSE, performance is assessed not only at small scales but also on larger electrode footprints, enabling the identification of spatial non-uniformities and transport limitations that become critical in stack-relevant configurations, and ultimately providing confidence in operation under realistic conditions.

Second, long-term durability must be demonstrated under realistic operating conditions. Often, catalysts are optimized in simplified electrolytes, but industrial applications, especially those involving capture-derived solvents, require catalyst stability, electrode integrity, and interfacial reliability in complex, integrated media. REUSE addresses this by focusing on extended operation, allowing degradation to evolve and be properly assessed. This is combined with in-situ/operando diagnostics to correlate performance evolution with material and interface changes. Without this level of understanding, there’s a risk of developing systems that fail under practical conditions.

Third, process-level integration is essential. A climate-relevant solution must be evaluated as a full system, not just an isolated electrochemical cell. REUSE advances this by piloting the coupling of CO₂ capture (via rotating packed bed reactors) with electrochemical conversation, including solvent recirculation. This integrated approach reveals real-world challenges (e.g. process stability, interface compatibility, and operational flexibility), while providing a foundation for scalable optimization.

Ultimately, the path to scale is not about a single breakthrough in catalyst performance. It requires the alignment of catalyst design, durability science, reactor engineering, and system integration into a coherent technological framework. This is precisely the systems-level approach that REUSE aims to establish.

7. Looking ahead, what do you see as the most critical research directions emerging from the REUSE project? For researchers working in electrochemical engineering, materials science, or sustainable catalysis, what is the most important conceptual or methodological takeaway they should carry forward from REUSE’s work?

I see three particularly important research directions emerging from REUSE.

First: the need to design catalysts together with their operating medium, not independently of it. Too often, catalyst development assumes a standard electrolyte and treats the surrounding chemical environment as secondary. REUSE points in the opposite direction: if CO₂ capture and conversion are to be integrated, then the electrolyte or solvent is not a background condition, but a co-determinant of reaction mechanism, stability, and reactor design.

Second: the importance of dynamic characterization. Catalysts for CO₂ reduction are often discussed as if their active structure were fixed, but in reality these materials behave as evolving interfaces under polarization, in complex chemical media, and over time. The combination of electrochemical monitoring with in-situ and operando techniques is therefore not just complementary, it is essential for linking structural evolution to performance and for distinguishing transient behaviour from genuinely stable operation.

Third: the need to embed sustainability thinking upstream in research design. It is no longer sufficient to ask whether a catalyst is active or selective. We must also consider how it is synthesized, how robust it is under realistic conditions, how it integrates into process architectures, and whether its environmental and economic benefits remain valid at scale. This is particularly important for researchers entering the field, because future progress will depend less on isolated record values and more on the coherence between materials, devices, and processes.

To conclude, the most important takeaway from REUSE, in my view, is methodological: electrochemical CO₂ conversion should not be developed as an isolated reaction, but as an integrated system of materials, interfaces, operating media, and process constraints. It is this systems-level perspective that allows promising chemistry to evolve into credible, scalable technology.

This project has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No. 101172954. Views and opinions expressed in this article are however those of the author(s) only and do not necessarily reflect those of the European Union.