Ethylene production processes using alternative carbon sources like biomass or CO₂ could have great potential for the olefins industry. Comparing the benefits and pitfalls of different process routes is challenging due to the vastly different feedstocks and key conversion technologies involved. Here, we performed an ex-ante techno-economic and environmental assessment to explore potential trade-offs of three low technology readiness level ethylene production processes. The three routes were: 1) biobased syngas fermentation to ethanol followed by ethanol dehydration, 2) direct electrochemical conversion of CO₂, and 3) indirect CO₂ and H₂O electrolysis to form syngas followed by a Fischer-Tropsch step. This study found three main takeaways. Firstly, the biobased route significantly outperforms the direct and indirect routes in terms of techno-economic and carbon footprint performance. Secondly, the electrolyzer unit is the main factor limiting the techno-economic performance of the direct and indirect cases, reemphasizing the need for continued technological advancements and cost reductions by researchers in this domain. Finally, the indirect plant design, incorporating two electrolyzers and a Fischer-Tropsch step, is not techno-economically feasible for ethylene production, underscoring the need for further research on Fischer-Tropsch plant designs to advance the replacement of traditional fossil-based refineries.

Catalyst lifetime is a primary technical bottleneck obstructing Cu-based CO₂ reduction (CO₂R), with restructuring via dissolution-redeposition being a commonly reported reason for selectivity loss. Here we examine how atomistic restructuring manifests at the microlevel of gas diffusion electrode (GDE)-based systems, ultimately compromising long-term CO₂R performance. Using a flow-cell CO₂R electrolyzer configuration and a copper-coated PTFE GDE, we first show how voltage gradients result in directional in-plane copper migration and porosity changes, causing a decrease in CO and ethylene production due to blocked catalyst pores. By the incorporation of different ionomer and inert carbon overlayers onto copper, we then demonstrate how in-plane degradation is mitigated by modulating the local pH and voltage homogeneity of the electrode, extending ethylene lifetimes by 10-fold. Ultimately, through-plane compaction of copper then becomes the limiting degradation pathway. Combined, these results provide rationale for the paradox of why copper degradation in membrane-electrode assemblies illustrates 100-fold greater stabilities than H-cell and flow-cell architecture.

CO2 electrolysis is an emerging technology for the sustainable production of fuels and chemicals. Its transition from laboratory-scale research to real-world application is strongly driven by both regulatory and strategic means, aimed at achieving net-zero greenhouse gas emissions. To meet this goal, accelerated progress in CO2 electrolysis research and technological development is essential to ensure economic viability. This requires clear performance targets, reference materials, and standardized testing protocols that serve as a basis for reliable performance comparison within the CO2 electrolysis community. To address this need, a Round Robin experiment was conducted involving well-established R&D entities in the field of CO2 electrolysis. The objective was to identify and mitigate the main sources of experimental variability, thereby enhancing reproducibility. We found that especially the modes of temperature measurements and cell/anolyte heating alongside pressure fluctuations and overpressures during product analysis are considerable differences among labs, while adjustments to the initial electrochemical protocol helped in minimizing voltage spikes in changing operation. As a result of multiple measurement campaigns and in-depth discussions among participants, a recommendation for a standardized testing protocol and test setup requirements for CO2 electrolyzers are provided.

As molecular catalysts are increasingly employed in heterogenized systems such as CO₂ electroreduction, a need arises for more systematic approaches to characterize their preparation, distribution, and activity. Current means of classifying electroactive versus spectator molecules are insufficient, while improvement of ink formulations, depositions, and distributions relies primarily on indirect links to electrochemical performance. In this study, we expand the common utilization of Cyclic Voltammetry (CV) in homogeneous systems toward heterogenized molecular catalyst architectures. We illustrate how, even with porous catalyst layers containing carbon, ionomer, and molecules, a combined redox wave integration and UV-vis analysis can be used as a tool for designing a reproducible deposition procedure. An in-depth CV analysis is then used to study the effect of catalyst aggregation and quantify the number of electroactive sites on carbon supports. We show that FeTPP (Iron(III)meso-tetraphenylporphyrin chloride) gives a non-linear electroactive response when loading is varied, allowing for the identification of distinct loading regions of insufficient, optimal, and excessive coverage. A FeTPP to Vulcan carbon mass ratio of 0.1 provides the highest number of electroactive species, giving the lowest expected aggregation. Overall, the CV approaches are extendable to any redox-active catalysts, providing a versatile means of characterizing porous heterogeneous molecular catalyst systems.

Electrochemical CO₂ reduction is emerging as a compelling route for renewable energy storage and carbon neutrality. Focus on improving catalyst selectivity and energy efficiency resulted in a surge of catalysis-centered research. The advent of artificial intelligence and high-throughput screening enables parallelized catalyst characterization to accelerate discovery, but their implementation into application-relevant device configurations is challenging. We present a scalable, high-throughput platform based on infrared thermography that preserves realistic electrochemical environments from lab to industrially relevant scales. We demonstrate the spatial and electrochemical homogeneity of a 16-well parallel electrolyzer and validate a combinatorial testing approach using copper-based catalysts with varied loadings and precursor chemistries. The results highlight how activity trends can be rapidly mapped under controlled conditions, while also revealing the limitations of activity-only combinatorial testing, particularly for multiproduct electrochemical applications in complex environments like CO₂ electrolysis on Cu. This platform thus provides an efficient pre-screening tool to accelerate catalyst discovery when analyzed appropriately and paired with follow-up single catalyst testing.

The electrochemical CO₂ reduction reaction (CO₂RR) in a membrane electrode assembly (MEA) efficiently turns CO₂ into a feedstock. However, unfavorable steady-state concentrations of ions in the cathode compartment result in salt formation if unaddressed, which restricts the access of CO₂ and causes cell failure. Here, we systematically show the relationship between salt accumulation and four system parameters including cation species, anolyte concentration, membrane thickness, and operating temperature. To compare each metric, we quantified the cation accumulation rate at predefined operating times. Notably, we show that operating at temperatures above 50 °C with properly humidified CO₂ stream fully avoids salt formation. We further show that combining separate operating conditions is also highly effective, showing operation for >144 h with no measurable salt deposition at 200 mA/cm². Collectively, our work systematically demonstrates that salt formation is a prevalent yet surmountable CO₂RR challenge that can be overcome by elevated cell temperatures or switching to more soluble alkali cations.

Correction to: Nature Reviews Materialshttps://doi.org/10.1038/s41578-025-00815-0, published online 16 June 2025. In the version of the article initially published, in Fig. 4b, the labels “Gas-diffusion layer” and “Copper catalyst layer” were switched and have now been amended so that the upper layer is labelled as the “Copper catalyst layer” and the lower layer is the “Gas-diffusion layer”. This correction has been made to the HTML and PDF versions of the article.

In Fig. 4(e) on page 6733 of this article, the legends in the graph for faradaic efficiency of CO and C2+ were misplaced. The original figure should be replaced with an updated one. Note that this correction does not have any impact on the main idea and conclusion of this article. The updated Fig. 4 is as follows. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. (Figure presented).

CO2 electrolyzers show promise as a cleaner alternative to produce value-added chemicals. In the last decade, research has shifted from classifying CO2 reduction activity and selectivity as a catalytic property (zero-dimensional [0D]) to one that includes the complex interactions of gas, liquid, and solid species between the cathode and anode (1D). To scale CO2 electrolyzers, however, 2D and 3D spatial variations in product selectivity, activity, and stability arise due to the design of reactor components, as well as concentration variations of the reactants, intermediates, and products. Conventional “black-box” measurement protocols are then insufficient to characterize CO2 electrolyzers. Here, we discuss the critical multi-dimensional phenomena occurring inside these electrochemical systems, which impact the observed performance. Recent literature is used to demonstrate how a spatial perspective is essential for proper data interpretation, designing effective catalysts, and prolonging CO2 electrolyzer lifetimes. Researchers should then define CO2 electrolysis systems in multiple dimensions (2D and 3D).

Copper and copper-based catalysts can electrochemically convert CO₂ into ethylene and higher alcohols, among other products, at room temperature and pressure. This approach may be suitable for the production of high-value compounds. However, such a promising reaction is heavily burdened by the instability of copper during CO₂ reduction. To date, non-copper catalysts have also failed to supplant the activity and selectivity of copper, leaving CO₂-to-C₂ electrolysis in the balance. In this Perspective, we discuss copper catalyst instability from both the atomistic and the microstructure viewpoint. We motivate that increased fundamental understanding, material design and operational approaches, along with increased reporting of failure mechanisms, will contribute to overcoming the barriers to multi-year operation. Our narrative focuses on the copper catalyst reconstruction occurring during CO₂ reduction as one of the major causes inducing loss of C₂ activity. We conclude with a rational path forward towards longer operations of CO₂-to-C₂ electrolysis.

The electrochemical reduction of carbon dioxide (CO2) presents an opportunity to close the carbon cycle and obtain sustainably sourced carbon compounds. In recent years, copper has received widespread attention as the only catalyst capable of meaningfully producing multi-carbon (C2+) species. Notably carbon monoxide (CO) can also be reduced to C2+ compounds on copper, motivating tandem systems that combine copper and CO-producing species, like silver, to enhance overall C2+ selectivities. In this work, we examine the impact of layered-combinations of bulk Cu and Ag by varying the location and proportion of the CO-producing Ag layer. We report an effective increase in the C2+ oxygenate selectivity from 23 % with a 100 nm Cu to 38 % for a 100 : 15 nm Cu : Ag layer. Notably, however, for all co-catalyst cases there is an overproduction of CO vs Cu alone, even for 5 nm Ag layers. Lastly, due to restructuring and interlayer mobility of the copper layer it is clear that the stability of copper limits the locational advantages of such tandem solutions.

Low-temperature carbon dioxide electrolysis (CO₂E) provides a one-step means of converting CO₂ into carbon-based fuels using electrical inputs at temperatures below 100 °C. Over the past decade, an abundance of work has been carried out at ambient temperature, and high CO₂E rates and product selectivities have been achieved. With scaling of CO₂E technologies underway, greater discourse surrounding heat management and the viable operating temperatures of larger systems is important. In this Perspective we argue that, owing to the energy inefficiency of electrolysers, heat generation in CO₂E stacks will favour operating temperatures of between 40 and 70 °C, far from the ambient temperatures used so far. Such elevated temperatures put further pressure on catalyst and membrane stability and on the stack design. On the other hand, elevated temperatures could alleviate challenges in salt precipitation, water management and high cell voltages, aiding the technology. We reflect on these aspects and discuss the opportunities for waste heat valorization to increase the economic feasibility of the process.

Operando characterization is crucial for understanding the selectivity and stability of the electrochemical CO₂ reduction reaction (eCO₂RR). Existing operando techniques normally use single-compartment cells operating at low currents. However, high current densities on the order of 100 mA cm^-2 are required for practical applications. Under a high current, reaction pathways and electrolyte dynamics can change, and stability issues such as salt precipitation and water crossover become more pronounced. Here, we developed an inline operando NMR method that is compatible with high-current reaction conditions. Demonstrating this on a copper-catalyzed eCO₂RR at 100 mA cm^-2, the operando NMR revealed a fast decrease of Faradaic efficiency for formate and ethanol within half an hour of reaction, accompanied by a pH decrease from 14 to 8 and a continuous accumulation of bicarbonate in the electrolyte. Water crossover was simultaneously observed and quantified via a deuteration technique and became more severe at high currents. This study revealed a highly dynamic electrolyte environment of copper-catalyzed eCO₂RR. Using a gas diffusion flow cell and a benchtop NMR system, this operando approach is accessible by non-NMR experts and readily applicable to a wide range of catalysts, electrolyte compositions, and reactor designs for eCO₂RR.

Electrochemical ammonia (NH₃) synthesis from nitrate (NO₃⁻) offers a promising greener alternative to the fossil-fuel-based Haber-Bosch process to support the increasing demand for nitrogen fertilizers while removing environmental waste. Previous studies have mainly focused on designing catalysts to promote the direct conversion (NO₃⁻ → NH₃) while suppressing the two-step pathway (NO₃⁻ → NO₂⁻ → NH₃). We hypothesize that efficient nitrate reduction is possible on simple catalysts by instead promoting the two-step reaction and using chemical reactor principles in a membrane electrode assembly, despite NO₂⁻ intermediates. Here, we use an unmodified copper catalyst and control reactivity through current density, flow rate, and electrolyte recycling. Balancing the electrolyte flow rate with current density results in ideal residence times for NO₂⁻, allowing for 91% FE_NH3 in a 5 cm² electrolyzer with a NO₃⁻ to NH₃ partial current of 1.8 A. This work shows that traditional engineering principles can substantially boost the NO₃ reduction reaction, even for simple catalysts.

Using copper (Cu) as an electrocatalyst uniquely produces multicarbon products (C₂₊-products) during the CO₂ reduction reaction (CO2RR). However, the CO2RR stability of Cu is presently 3 orders of magnitude shorter than required for commercial operation. One means of substantially increasing Cu catalyst lifetimes is through periodic oxidative processes, such as cathodic-anodic pulsing. Despite 100-fold improvements, these oxidative methods only delay, but do not circumvent, degradation. Here, we provide an interrogation of chemical and electrochemical Cu oxidative processes to identify the mechanistic processes leading to stable CO2RR through electrochemical and in situ Raman spectroscopy measurements. We first examine chemical oxidation using an open-circuit potential (OCP), identifying that copper oxidation is regulated by the transient behavior of the OCP curve and limited by the rate of the oxygen reduction reaction (ORR). Increasing O₂ flux to the cathode subsequently increased ORR rates, both extending lifetimes and reducing “off” times by 3-fold. In a separate approach, the formation of Cu₂O is achieved through electrochemical oxidation. Here, we establish the minimum electrode potentials required for fast Cu oxidation (−0.28 V vs Ag/AgCl, 1 M KHCO₃) by accounting for transient local pH changes and tracking oxidation charge transfer. Lastly, we performed a stability test resulting in a 20-fold increase in stable ethylene production versus the continuous case, finding that spatial copper migration is slowed but not mitigated by oxidative pulsing approaches alone.

Electrochemical CO₂ reduction aims to compete with Power-to-X alternatives but is well behind the scales of water electrolyzers and thermochemical reactors. In a recent issue of Nature Chemical Engineering, Crandall and co-workers demonstrate a 1000 cm² tandem CO₂/CO electrolyzer for acetate production. The work invites discussion on scientific and engineering scale-up challenges.

Molecular catalysts play a significant role in chemical transformations, utilizing changes in redox states to facilitate reactions. To date molecular electrocatalysts have efficiently produced single-carbon products from CO2 but have struggled to achieve a carbon–carbon coupling step. Conversely, copper catalysts can enable carbon–carbon coupling, but lead to broad C2+ product spectra. Here we subvert the traditional redox-mediated reaction mechanisms of organometallic compounds through a heterogeneous nickel-supported iron tetraphenylporphyrin electrocatalyst, facilitating electrochemical carbon–carbon coupling to produce ethanol. This represents a marked behavioural shift compared with carbon-supported metalloporphyrins. Extending the approach to a three-dimensional porous nickel support with adsorbed iron tetraphenylporphyrin, we attain ethanol Faradaic efficiencies of 68% ± 3.2% at −0.3 V versus a reversible hydrogen electrode (pH 7.7) with partial ethanol current densities of −21 mA cm−2. Separately we demonstrate maintained ethanol production over 60 h of operation. Further consideration of the wide parameter space of molecular catalyst and metal electrodes shows promise for additional chemistries and achievable metrics.

Electrochemical CO2 reduction offers a promising method of converting renewable electrical energy into valuable hydrocarbon compounds vital to hard-to-abate sectors. Significant progress has been made on the lab scale, but scale-up demonstrations remain limited. Because of the low energy efficiency of CO2 reduction, we suspect that significant thermal gradients may develop in industrially relevant dimensions. We describe here a model prediction for non-isothermal behavior beyond the typical 1D models to illustrate the severity of heating at larger scales. We develop a 2D model for two membrane electrode assembly (MEA) CO2 electrolyzers; a liquid anolyte fed MEA (exchange MEA) and a fully gas fed configuration (full MEA). Our results indicate that full MEA configurations exhibit very poor electrochemical performance at moderately larger scales due to non-isothermal effects. Heating results in severe membrane dehydration, which induces large Ohmic losses in the membrane, resulting in a sharp decline in the current density along the flow direction. In contrast, the anolyte employed in the exchange MEA configuration is effective in preventing large thermal gradients. Membrane dehydration is not a problem for the exchange MEA configuration, leading to a nearly constant current density over the entire length of the modeled domain, and indicating that exchange MEA configurations are well suited for scale-up. Our results additionally indicate that a balance between faster kinetics, higher ionic conductivity, smaller pH gradients and lower CO2 solubility causes an optimum operating temperature between 60 and 70 °C.

CO2 electrolysis allows the sustainable production of carbon-based fuels and chemicals. However, state-of-the-art CO2 electrolysers employing anion exchange membranes (AEMs) suffer from (bi)carbonate crossover, causing low CO2 utilization and limiting anode choices to those based on precious metals. Here we argue that bipolar membranes (BPMs) could become the primary option for intrinsically stable and efficient CO2 electrolysis without the use of scarce metals. Although both reverse- and forward-bias BPMs can inhibit CO2 crossover, forward-bias BPMs fail to solve the rare-earth metals requirement at the anode. Unfortunately, reverse-bias BPM systems presently exhibit comparatively lower Faradaic efficiencies and higher cell voltages than AEM-based systems. We argue that these performance challenges can be overcome by focusing research on optimizing the catalyst, reaction microenvironment and alkali cation availability. Furthermore, BPMs can be improved by using thinner layers and a suitable water dissociation catalyst, thus alleviating core remaining challenges in CO2 electrolysis to bring this technology to the industrial scale.

CO₂ from carbonate-based capture solutions requires a substantial energy input. Replacing this step with (bi)carbonate electrolysis has been commonly proposed as an efficient alternative that coproduces CO/syngas. Here, we assess the feasibility of directly integrating air contactors with (bi)carbonate electrolyzers by leveraging process, multiphysics, microkinetic, and technoeconomic models. We show that the copresence of CO₃^2- with HCO₃^- in the contactor effluent greatly diminishes the electrolyzer performance and eventually results in a reduced CO₂ capture fraction to ≤1%. Additionally, we estimate suitable effluents for (bi)carbonate electrolysis to require 5-14 times larger contactors than conventionally needed contactors, leading to unfavorable process economics. Notably, we show that the regeneration of the capture solvent inside (bi)carbonate electrolyzers is insufficient for CO₂ recapture. Thus, we suggest process modifications that would allow this route to be operationally feasible. Overall, this work sheds light on the practical operation of integrated direct air capture with (bi)carbonate electrolysis.