Publications

The electrochemical reduction of carbon dioxide (CO₂) to value-added materials has received considerable attention. Both bulk transition-metal catalysts and molecular catalysts affixed to conductive noncatalytic solid supports represent a promising approach toward the electroreduction of CO₂. Here, we report a combined silver (Ag) and pyridine catalyst through a one-pot and irreversible electrografting process, which demonstrates the enhanced CO₂ conversion versus individual counterparts. We find that by tailoring the pyridine carbon chain length, a 200 mV shift in the onset potential is obtainable compared to the bare silver electrode. A 10-fold activity enhancement at −0.7 V vs reversible hydrogen electrode (RHE) is then observed with demonstratable higher partial current densities for CO, indicating that a cocatalytic effect is attainable through the integration of the two different catalytic structures. We extended the performance to a flow cell operating at 150 mA/cm², demonstrating the approach’s potential for substantial adaptation with various transition metals as supports and electrografted molecular cocatalysts.

Maryam Abdinejad, Erdem Irtem, Amirhossein Farzi, Mark Sassenburg, Siddhartha Subramanian, Hugo-Pieter Iglesias van Montfort, Davide Ripepi, Mengran Li, Joost Middelkoop, Ali Seifitokaldani and Thomas Burdyny

Continued advancements in the electrochemical reduction of CO₂ (CO₂RR) have emphasized that reactivity, selectivity, and stability are not explicit material properties but combined effects of the catalyst, double-layer, reaction environment, and system configuration. These realizations have steadily built upon the foundational work performed for a broad array of transition metals performed at 5 mA cm^–2, which historically guided the research field. To encompass the changing advancements and mindset within the research field, an updated baseline at elevated current densities could then be of value. Here we seek to re-characterize the activity, selectivity, and stability of the five most utilized transition metal catalysts for CO₂RR (Ag, Au, Pd, Sn, and Cu) at elevated reaction rates through electrochemical operation, physical characterization, and varied operating parameters to provide a renewed resource and point of comparison. As a basis, we have employed a common cell architecture, highly controlled catalyst layer morphologies and thicknesses, and fixed current densities. Through a dataset of 88 separate experiments, we provide comparisons between CO-producing catalysts (Ag, Au, and Pd), highlighting CO-limiting current densities on Au and Pd at 72 and 50 mA cm^–2, respectively. We further show the instability of Sn in highly alkaline environments, and the convergence of product selectivity at elevated current densities for a Cu catalyst in neutral and alkaline media. Lastly, we reflect upon the use and limits of reaction rates as a baseline metric by comparing catalytic selectivity at 10 versus 200 mA cm^–2. We hope the collective work provides a resource for researchers setting up CO₂RR experiments for the first time.

Mark Sassenburg, Reinier de Rooij, Nathan T. Nesbitt, Recep Kas, Sanjana Chandrashekar, Nienke J. Firet, Kailun Yang, Kai Liu, Marijn A. Blommaert, Martin Kolen, Davide Ripepi, Wilson A. Smith and Thomas Burdyny

Incorporating (operational) flexibility into process design has been a key approach to cope with uncertainties. The increasing penetration of renewables and the need for developing new low-carbon technologies will increase the demand for flexibility in chemical processes. This paper presents a state-of-the-art review focusing on the origin, definition, and elements of flexibility in the chemical engineering context. The article points out a significant overlap in terminology and concepts, making it difficult to understand and compare flexibility potential and constraints among studies. Further, the paper identifies a lack of available metrics for assessing specific types of flexibility and the need for developing indicators for exploring the potential flexibility of novel chemical processes. The paper proposes a classification of flexibility types and provides an overview of design strategies that have been adopted so far to enable different types of flexibility. Finally, it offers a conceptual framework that can support designers to evaluate specific types of flexibility in early-stage assessments of novel chemical processes.

Martin Kolen, Davide Ripepi, Wilson A. Smith, Thomas Burdyny and Fokko M. Mulder

The ever-growing level of carbon dioxide (CO₂) in our atmosphere, is at once a threat and an opportunity. The development of sustainable and cost-effective pathways to convert CO₂ to value-added chemicals is central to reducing its atmospheric presence. Electrochemical CO₂ reduction reactions (CO₂RRs) driven by renewable electricity are among the most promising techniques to utilize this abundant resource; however, in order to reach a system viable for industrial implementation, continued improvements to the design of electrocatalysts is essential to improve the economic prospects of the technology. This review summarizes recent developments in heterogeneous porphyrin-based electrocatalysts for CO₂ capture and conversion. We specifically discuss the various chemical modifications necessary for different immobilization strategies, and how these choices influence catalytic properties. Although a variety of molecular catalysts have been proposed for CO₂RRs, the stability and tunability of porphyrin-based catalysts make their use particularly promising in this field. We discuss the current challenges facing CO₂RRs using these catalysts and our own solutions that have been pursued to address these hurdles.

Maryam Abdinejad, Keith Tang, Caitlin Dao, Saeed Saedy and Tom Burdyny

Carbon-based products are crucial to our society, but their production from fossil-based carbon is unsustainable. Production pathways based on the reuse of CO2 will achieve ultimate sustainability. Furthermore, the costs of renewable electricity production are decreasing at such a high rate, that electricity is expected to be the main energy carrier from 2040 onward. Electricity-driven novel processes that convert CO2 into chemicals need to be further developed. Microbial electrosynthesis is a biocathode-driven process in which electroactive microorganisms derive electrons from solid-state electrodes to catalyze the reduction of CO2 or organics and generate valuable extracellular multicarbon reduced products. Microorganisms can be tuned to high-rate and selective product formation. Optimization and upscaling of microbial electrosynthesis to practical, real life applications is dependent upon performance improvement while maintaining low cost. Extensive biofilm development, enhanced electron transfer rate from solid-state electrodes to microorganisms and increased chemical production rate require optimized microbial consortia, efficient reactor designs, and improved cathode materials.

Victoria Flexer and Ludovic Jourdin

Up to now, computational modeling of microbial electrosynthesis (MES) has been underexplored, but is necessary to achieve breakthrough understanding of the process-limiting steps. Here, a general framework for modeling microbial kinetics in a MES reactor is presented. A thermodynamic approach is used to link microbial metabolism to the electrochemical reduction of an intracellular mediator, allowing to predict cellular growth and current consumption. The model accounts for CO2 reduction to acetate, and further elongation to n-butyrate and n-caproate. Simulation results were compared with experimental data obtained from different sources and proved the model is able to successfully describe microbial kinetics (growth, chain elongation, and product inhibition) and reactor performance (current density, organics titer). The capacity of the model to simulate different system configurations is also shown. Model results suggest CO2 dissolved concentration might be limiting existing MES systems, and highlight the importance of the delivery method utilized to supply it. Simulation results also indicate that for biofilm-driven reactors, continuous mode significantly enhances microbial growth and might allow denser biofilms to be formed and higher current densities to be achieved.

Oriol Cabau-Peinado, Adrie J. J. Straathof and Ludovic Jourdin

In the past decade, research in the field of microbial electrosynthesis (MES) has been driven forward by the development of cathode materials, electroactive bacteria or microbiome enrichment, and productivity improvements.

As the close of three complete funding cycles for the field is reached, recent reviews have sought to refocus emphasis to the eventual application of MES; a means of measurably reducing CO2 waste via the formation of valuable products.

Using present knowledge of bioelectrochemistry, and by learning lessons from adjacent fields, it becomes apparent that the simplest gains in performance are likely to come from advancements in the reactor rather than the biocatalysts. Varying the reactor and operating conditions of the system, however, require adapting these biocatalysts.

Ludovic Jourdin and Thomas Burdyny

The effect of different alkali metal cations on the oxygen evolution activity and battery capacity of nickel electrodes has recently been studied in low concentration alkali hydroxide electrolytes. As high concentration hydroxide electrolytes are favored in applications due to their high conductivity, we investigate if the cation effects observed in low concentration electrolytes translate to more industrially relevant conditions for both alkaline water electrolysis and nickel iron batteries. We investigate the alkali metal cation effect on the electrochemical properties of nickel electrodes in highly concentrated hydroxide electrolytes by adding Li+, Na+, Cs+ and Rb+ cations to a 6.5 M KOH electrolyte, while keeping the hydroxide concentration constant. For OER we find a trend in activity similar to that at low concentrations Rb+>Cs+>K+>Na+>Li+, where especially larger additions of Rb+ and Cs+ (1 M or 0.5 M) cause a significant decrease in OER potential. Smaller cations interact with the layered hydroxide structures in NiOOH to stabilize the α/γ phases and increase the potential for OER. The intercalation of small cations also causes an increase in battery electrode capacity because of the higher average valence of the Ni(OH)2/NiOOH α/γ pair. Small concentrations of Li+ added to a concentrated KOH electrolyte can therefore be beneficial for the nickel electrode battery functionality and for an integrated battery and electrolyser system, where it increases the battery capacity without a significant increase in OER onset potential.

A.Mangel Raventos, R.Kortlever

Rapid advances in electrocatalytic ammonia synthesis are impeded by laborious detection methods commonly used in the field and by constant risk of external contaminations, which generates misleading false positives. We developed a facile real-time GC-MS method for sensitive isotope NH₃ quantification, requiring no external sample manipulations. This method ensures high detection reliability paramount to accelerate (electro-)catalyst screening.

Davide Ripepi, Riccardo Zaffaroni, Martin Kolen, Joost Middelkoop and Fokko M. Mulder

Finding alternative ways to tailor the electronic properties of a catalyst to actively and selectively drive reactions of interest has been a growing research topic in the field of electrochemistry. In this Letter, we investigate the tuning of the surface electronic properties of electrocatalysts via polymer modification. We show that when a nickel oxide water oxidation catalyst is coated with polytetrafluoroethylene, stable Ni–CFx bonds are introduced at the nickel oxide/polymer interface, resulting in shifting of the reaction selectivity away from the oxygen evolution reaction and toward hydrogen peroxide formation. It is shown that the electron-withdrawing character of the surface fluorocarbon molecule leaves a slight positive charge on the water oxidation intermediates at the adjacent active nickel sites, making their bonds weaker. The concept of modifying the surface electronic properties of an electrocatalyst via stable polymer modification offers an additional route to tune multipathway reactions in polymer/electrocatalyst environments, like with ionomer-modified catalysts or with membrane electrode assemblies.

Anirudh Venugopal, Laurentius H. T. Egberts, Jittima Meeprasert, Evgeny A. Pidko, Bernard Dam, Thomas Burdyny, Vivek Sinha, Wilson A. Smith

The specific identity of electrolyte cations has many implications in various electrochemical reactions. However, the exact mechanism by which cations affect electrochemical reactions is not agreed upon in the literature. In this report, we investigate the role of cations during the electrochemical reduction of CO₂ by chelating the cations with cryptands, to change the interaction of the cations with the components of the electric double layer. As previously reported we do see the apparent suppression of CO₂ reduction in the absence of cations. However, using in situ-SEIRAS we see that CO₂ reduction does indeed take place albeit at very reduced scales. We also observe that cations play a role in tuning the absorption strengths of not only CO₂ as has been speculated, but also that of reaction products such as CO.

S Chandrashekar, HP van Montfort, D Bohra, G Filonenko, H Geerlings, T Burdyny, WA Smith

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO₂ supply has been motivated as a means of integrating conversion with upstream CO₂ capture. The way that CO₂ is formed and transported during CO₂-mediated bicarbonate reduction in flow cells is profoundly different from conventional CO₂ saturated and gas-fed systems and a thorough understanding of the process would allow further advancements. Here, we report a comprehensive two-phase mass transport model to estimate the local concentration of species in the porous electrode resultant from homogeneous and electrochemical reactions of (bi)carbonate and CO₂. The model indicates that significant CO₂ is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO₂. However, the in situ formation of CO₂ and subsequent reduction to CO exhibits a plateau at high potentials due to neutralization of the protons by the alkaline reaction products, acting as the limiting step toward higher CO current densities. Nevertheless, the pH in the catalyst layer exhibits a relatively smaller rise, compared to conventional electrochemical CO₂ reduction cells, because of the reaction between protons and CO₃^2– and OH^– that is confined to a relatively small volume. A large fraction of the CL exhibits a mildly alkaline environment at high current densities, while an appreciable amount of carbonic acid (0.1–1 mM) and a lower pH exist adjacent to the membrane, which locally favor hydrogen evolution, especially at low electrolyte concentrations. The results presented here provide insights into local cathodic conditions for both bicarbonate cells and direct-CO₂ reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode.

Kas R, Yang K, Yewale G, Crow A, Burdyny T, Smith WA

CO₂ electrochemical reduction reaction (CO₂RR) enables conversion of greenhouse gas CO₂ into value-added products, which simultaneously reduces carbon emissions and reduces the usage of fossil fuels as feed materials to produce fuel/chemical products. It also provides the potential to integrate electrocatalytic processes with electricity from renewable sources for storing renewable energy. Membrane-electrode assemblies (MEAs) can be an efficient solution to address the key issues in aqueous electrolyzer design and enable the industrial scale-up. In this paper, we reviewed recent advances in the experimental design and simulation of MEAs. The discussion of existing challenges and future research priorities for guiding MEA development and understanding reaction mechanism is also provided.

Ge L, Rabiee H, Li M, Subramanian S, Zheng Y, Lee JH, Burdyny T, Wang H.

Surface conductivity in the electrical double layer (EDL) is known to be affected by proton hopping and diffusion at solid-liquid interfaces. Yet, the role of surface protolysis and its kinetics on the thermodynamic and transport properties of the EDL are usually ignored as physical models consider static surfaces. Here, using a novel molecular dynamics method mimicking surface protolysis, we unveil the impact of such chemical events on the system’s response. Protolysis is found to strongly affect the EDL and electrokinetic aspects with major changes in ζ potential and electro-osmotic flow.

Max F. Döpke, Fenna Westerbaan van der Meij, Benoit Coasne, and Remco Hartkamp

Incorporating (operational) flexibility into process design has been a key approach to cope with uncertainties. The increasing penetration of renewables and the need for developing new low-carbon technologies will increase the demand for flexibility in chemical processes. This paper presents a state-of-the-art review focusing on the origin, definition, and elements of flexibility in the chemical engineering context. The article points out a significant overlap in terminology and concepts, making it difficult to understand and compare flexibility potential and constraints among studies. Further, the paper identifies a lack of available metrics for assessing specific types of flexibility and the need for developing indicators for exploring the potential flexibility of novel chemical processes. The paper proposes a classification of flexibility types and provides an overview of design strategies that have been adopted so far to enable different types of flexibility. Finally, it offers a conceptual framework that can support designers to evaluate specific types of flexibility in early-stage assessments of novel chemical processes.

Jisiwei Luo, Jonathan Moncada and Andrea Ramirez

The application of a polymer electrolyte membrane (PEM) electrolytic cell for continuous conversion of nitrate, one of the contaminants in water, to ammonia at the cathode was explored in the present work. Among carbon-supported metal (Cu, Ru, Rh and Pd) electrocatalysts, the Ru-based catalyst showed the best performance. By suppressing the competing hydrogen evolution reaction at the cathode, it was possible to reach 94 % faradaic efficiency for nitrate reduction towards ammonium. It was important to match the rate of the anodic reaction with the cathodic reaction to achieve high faradaic efficiency. By recirculating the effluent stream, 93 % nitrate conversion was achieved in 8 h of constant current electrolysis at 10 mA cm⁻² current density. The presented approach offers a promising path towards precious NH₃ production from nitrate-containing water that needs purification or can be obtained after capture of gaseous NO_x pollutants into water, leading to waste-to-value conversion.

Sorin Bunea, Kevin Clemens, Prof.dr. Atsushi Urakawa

Low temperature electrolysis brings the possibility of achieving the production of fuels and chemical feedstocks without any carbon footprint. Power electronic converters are vital components of future electrolyser systems in terms of overall cost and efficiency. This paper presents the current state of art in power electronics for low temperature electrolysis and all the major steps of designing an electrolysis system are discussed from the modeling of electrolysers to the system architecture. The most promising routes are pointed out and backed up with results from both experimental and theoretical studies found in the literature.

Cristian Pascalau, Thiago B. Soeiro, Nils H. van der Blij, Pavol Bauer

Electrochemical CO₂ conversion into fuels or chemicals and CO₂ capture from point or dilute sources are two important processes to address the gigaton challenges in reducing greenhouse gas emissions. Both CO₂ capture and electrochemical CO₂ conversion are energy intensive, and synergistic coupling between the two processes can improve the energy efficiency of the system and reduce the cost of the reduced products, via eliminating the CO₂ transport and storage or eliminating the capture media regeneration and molecular CO₂ release. We consider three different levels to couple electrochemical CO₂ reduction with CO₂ capture: independent (Type-I), subsequent (Type-II) and fully integrated (Type-III) capture and conversion processes. We focus on Type-II and Type-III configurations and illustrate potential coupling routes of different capture media, which include amine-based solutions and direct carbamate reduction, redox active carriers, aqueous carbonate and bicarbonate solutions, ionic liquids CO₂ capture and conversion mediated by covalent organic frameworks.

Ian Sullivan, Andrey Goryachev, Ibadillah A. Digdaya, Xueqian Li, Harry A. Atwater, David A. Vermaas & Chengxiang Xiang

Direct electrochemical reduction of CO₂ to C₂ products such as ethylene is more efficient in alkaline media, but it suffers from parasitic loss of reactants due to (bi)carbonate formation. A two-step process where the CO₂ is first electrochemically reduced to CO and subsequently converted to desired C₂ products has the potential to overcome the limitations posed by direct CO₂ electroreduction. In this study, we investigated the technical and economic feasibility of the direct and indirect CO₂ conversion routes to C₂ products. For the indirect route, CO₂ to CO conversion in a high temperature solid oxide electrolysis cell (SOEC) or a low temperature electrolyzer has been considered. The product distribution, conversion, selectivities, current densities, and cell potentials are different for both CO₂ conversion routes, which affects the downstream processing and the economics. A detailed process design and techno-economic analysis of both CO₂ conversion pathways are presented, which includes CO₂ capture, CO₂ (and CO) conversion, CO₂ (and CO) recycling, and product separation. Our economic analysis shows that both conversion routes are not profitable under the base case scenario, but the economics can be improved significantly by reducing the cell voltage, the capital cost of the electrolyzers, and the electricity price. For both routes, a cell voltage of 2.5 V, a capital cost of $10,000/m², and an electricity price of <$20/MWh will yield a positive net present value and payback times of less than 15 years. Overall, the high temperature (SOEC-based) two-step conversion process has a greater potential for scale-up than the direct electrochemical conversion route. Strategies for integrating the electrochemical CO₂/CO conversion process into the existing gas and oil infrastructure are outlined. Current barriers for industrialization of CO₂ electrolyzers and possible solutions are discussed as well.

Mahinder Ramdin, Bert De Mot, Andrew R. T. Morrison, Tom Breugelmans, Leo J. P. van den Broeke, J. P. Martin Trusler, Ruud Kortlever, Wiebren de Jong, Othonas A. Moultos, Penny Xiao, Paul A. Webley and Thijs J. H. Vlugt

Methanation is a potential large-scale option for CO₂ utilization, and it is one of the solutions for decreasing carbon emission and production of synthetic green fuels. However, the CO₂ conversion is limited by thermodynamics in conventional reaction conditions. However, around 100 % conversion can be obtained using sorption enhanced CO₂ methanation according to Le Chatelier’s principle, where water is removed during the reaction using zeolite as a sorbent. In this work 5%Ni5A, 5%Ni13X, 5%NiL and 5%Ni2.5%Ce13X bifunctional materials with both catalytic and water adsorption properties were tested in a fixed bed reactor. The overall performance of the bifunctional materials decreased on going from 5%Ni2.5%Ce13X, 5%Ni13X, 5%Ni5A, to 5%NiL. The CO₂ conversion and CH₄ selectivity were approaching 100 % during prolonged stability testing in a 100 reactive adsorption – desorption cycles test for 5%Ni2.5%Ce13X, and only a slight decrease of the water uptake capacity was observed.

Liangyuan Wei, Hamza Azad, Wim Haije, Henrik Grenman and Wiebren de Jong

Nowadays, Sn-based electrocatalysts for the electrochemical CO₂ reduction reaction (eCO₂RR) toward formic acid have been reported to reach industrially relevant current densities and Faradaic efficiencies approaching 100%. However, electrocatalyst stability remains inadequate and appears to be a crucial piece to the puzzle, as lifetimes in the range of several thousands of hours should be reached for practical application and economic viability. Here, we provide insights into stability issues related to Sn-based electrocatalysts and electrolyzers for formic acid production. By determining the chemical and physical phenomena that occur during the electrochemical reduction reaction on the surface and bulk of Sn-based catalysts, we intend to elucidate the most common degradation mechanisms that impair long-term electrocatalytic activity of these catalysts. Moreover, highlighting the importance of correctly selected process conditions and an optimized reactor design allows us to unveil all necessary aspects for a stable Sn-based eCO₂RR toward formic acid.

Kevin Van Daele, Bert De Mot, Marilia Pupo, Nick Daems, Deepak Pant, Ruud Kortlever and Tom Breugelmans

Electrochemical CO2 capture technologies are gaining attention due to their flexibility, their ability to address decentralized emissions (e.g., ocean and atmosphere) and their fit in an electrified industry. In the present work, recent progress made in electrochemical CO2 capture is reviewed. The majority of these methods rely on the concept of “pH-swing” and the effect it has on the CO2 hydration/dehydration equilibrium. Through a pH-swing, CO2 can be captured and recovered by shifting the pH of a working fluid between acidic and basic pH. Such swing can be applied electrochemically through electrolysis, bipolar membrane electrodialysis, reversible redox reactions and capacitive deionization. In this review, we summarize main parameters governing these electrochemical pH-swing processes and put the concept in the framework of available worldwide capture technologies. We analyse the energy efficiency and consumption of such systems, and provide recommendations for further improvements. Although electrochemical CO2 capture technologies are rather costly compared to the amine based capture, they can be particularly interesting if more affordable renewable electricity and materials (e.g., electrode and membranes) become widely available. Furthermore, electrochemical methods have the ability to (directly) convert the captured CO2 to value added chemicals and fuels, and hence prepare for a fully electrified circular carbon economy.

R. Sharifian,   R. M. Wagterveld,   I. A. Digdaya,   C. Xiang  and  D. A. Vermaas

Advancing reaction rates for electrochemical CO₂ reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO₂, which is capped at 50% (CO as targeted product) due to unwanted homogeneous reactions. Using bipolar membranes in an MEA (BPMEA) has the capability of preventing parasitic CO₂ losses, but their promise is dampened by poor CO₂ activity and selectivity. In this work, we enable a 3-fold increase in the CO₂ reduction selectivity of a BPMEA system by promoting alkali cation (K+) concentrations on the catalyst’s surface, achieving a CO Faradaic efficiency of 68%. When compared to an anion exchange membrane, the cation-infused bipolar membrane (BPM) system shows a 5-fold reduction in CO₂ loss at similar current densities, while breaking the 50% CO₂ utilization mark. The work provides a combined cation and BPM strategy for overcoming CO₂ utilization issues in CO₂ electrolyzers.

Kailun Yang, Mengran Li, Siddhartha Subramanian, Marijn A. Blommaert, Wilson A. Smith and Thomas Burdyny

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The production of value added C1 and C2 compounds within CO₂ electrolyzers has reached sufficient catalytic performance that system and process performance – such as CO₂ utilization – have come more into consideration. Efforts to assess the limitations of CO₂ conversion and crossover within electrochemical systems have been performed, providing valuable information to position CO₂ electrolyzers within a larger process. Currently missing, however, is a clear elucidation of the inevitable trade-offs that exist between CO₂ utilization and electrolyzer performance, specifically how the faradaic efficiency of a system varies with CO₂ availability. Such information is needed to properly assess the viability of the technology. In this work, we provide a combined experimental and 3D modelling assessment of the trade-offs between CO₂ utilization and selectivity at 200 mA cm⁻² within a membrane-electrode assembly CO₂ electrolyzer. Using varying inlet flow rates we demonstrate that the variation in spatial concentration of CO₂ leads to spatial variations in faradaic efficiency that cannot be captured using common ‘black box’ measurement procedures. Specifically, losses of faradaic efficiency are observed to occur even at incomplete CO₂ consumption (80%). Modelling of the gas channel and diffusion layers indicated that at least a portion of the H₂ generated is considered as avoidable by proper flow field design and modification. The combined work allows for a spatially resolved interpretation of product selectivity occurring inside the reactor, providing the foundation for design rules in balancing CO₂ utilization and device performance in both lab and scaled applications.

Siddhartha Subramanian, Joost Middelkoop and Thomas Burdyny 

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The electrochemical reduction of CO₂ on planar electrodes is limited by its prohibitively low diffusivity and solubility in water. Gas-diffusion electrodes (GDEs) can be used to reduce these limitations, and facilitate current densities orders of magnitude higher than the limiting current densities of planar electrodes. These improvements are accompanied by increased variation in the local environment within the cathode, with significant effect on Faradaic efficiency. By developing a simple and freely available analytical model of a cathodic catalyst layer configured for the production of CO, we investigate the relationships between electrode reaction kinetics, cell operation conditions, catholyte composition and cell performance. Analytical methods allow us to cover parameter ranges that are intractable for numerical and experimental studies. We validate our findings against experimental and numerical results and provide a derivation and implementation of the analytical model.

J.W.Blake, J.T.Padding and J.W.Haverkort

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Direct electrochemical nitrogen reduction holds the promise of enabling the production of carbon emission-free ammonia, which is an important intermediate in the fertilizer industry and a potential green energy carrier. Here we show a strategy for ambient condition ammonia synthesis using a hydrogen permeable nickel membrane/electrode that spatially separates the electrolyte and hydrogen reduction side from the dinitrogen activation and hydrogenation sites. Gaseous ammonia is produced catalytically in the absence of electrolyte via hydrogenation of adsorbed nitrogen by electrochemically permeating atomic hydrogen from water reduction. Dinitrogen activation at the polycrystalline nickel surface is confirmed with 15N2 isotope labeling experiments, and it is attributed to a Mars–van Krevelen mechanism enabled by the formation of N-vacancies upon hydrogenation of surface nitrides. We further show that gaseous hydrogen does not hydrogenate the adsorbed nitrogen, strengthening the benefit of having an atomic hydrogen permeable electrode. The proposed approach opens new directions toward green ammonia.

Davide Ripepi, Riccardo Zaffaroni, Herman Schreuders, Bart Boshuizen and Fokko M. Mulder

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The electrochemical reduction of carbon dioxide (CO2RR) requires access to ample gaseous CO2 and liquid water to fuel reactions at high current densities for industrial-scale applications. Substantial improvement of the CO2RR rate has largely arisen from positioning the catalyst close to gas–liquid interfaces, such as in gas-diffusion electrodes. These requirements add complexity to an electrode design that no longer consists of only a catalyst but also a microporous and nanoporous network of gas–liquid–solid interfaces of the electrode. In this three-dimensional structure, electrode wettability plays a pivotal role in the CO2RR because the affinity of the electrode surface by water impacts the observed electrode reactivity, product selectivity, and long-term stability. All these performance metrics are critical in an industrial electrochemical process. This review provides an in-depth analysis of electrode wettability's role in achieving an efficient, selective, and stable CO2RR performance. We first discuss the underlying mechanisms of electrode wetting phenomena and the foreseen ideal wetting conditions for the CO2RR. Then we summarize recent advances in improving cathode performance by altering the wettability of the catalyst layer of gas-diffusion electrodes. We conclude the review by discussing the current challenges and opportunities to develop efficient and selective cathodes for CO2RR at industrially relevant rates. The insights generated from this review could also benefit the advancement of other critical electrochemical processes that involve multiple complex flows in porous electrodes, such as electrochemical reduction of carbon monoxide, oxygen, and nitrogen.

Mengran Li, Mohamed Nazmi Idros, Yuming Wu, Thomas Burdyny, Sahil Garg, Xiu Song Zhao, Geoff Wang and Thomas E. Rufford

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Bipolar membranes (BPMs) are gaining interest in energy conversion technologies. These membranes are composed of cation- and anion-exchange layers, with an interfacial layer in between. This gives the freedom to operate in different conditions (pH, concentration, composition) at both sides. Such membranes are used in two operational modes, forward and reverse bias. BPMs have been implemented in various electrochemical applications, like water and CO₂ electrolyzers, fuel cells, and flow batteries, while BPMs are historically designed for acid/base production. Therefore, current commercial BPMs are not optimized, as the conditions change per application. Although the ideal BPM has highly conductive layers, high water dissociation kinetics, long lifetime, and low ion crossover, each application has its own priorities to be competitive in its field. We describe the challenges and requirements for future BPMs, and identify existing developments that can be leveraged to develop BPMs toward the scale of practical applications.

Marijn A. Blommaert, David Aili, Ramato Ashu Tufa, Qingfeng Li, Wilson A. Smith and David A. Vermaas

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Oil palm empty fruit bunch (OPEFB) is an abundant waste that is commonly incinerated, causing environmental pollution. In this study, an alternative waste management approach was investigated to produce value-added syngas from OPEFB using solar steam gasification. The three operating variables were temperature (1100–1300 °C), H₂O/OPEFB molar ratio (1.7–2.9), and OPEFB flowrate (0.8–1.8 g/min). Central composite design (CCD) was conducted to investigate and optimise the effects of these operating variables on H₂/CO molar ratio and solar to fuel energy conversion efficiency (ηsolar to fuel). The findings revealed that all investigated operating variables were significant. Experimentally, the highest H₂/CO molar ratio (1.6) was obtained at 1300 °C, H₂O/OPEFB molar ratio of 2.9, and OPEFB flowrate of 1.8 g/min, with a high carbon conversion reaching 95.1%. Results from CCD analysis showed that a higher H₂/CO molar ratio (above 1.8) could be reached at 1200 °C, H₂O/OPEFB molar ratio of ≥3.0, and OPEFB flowrate of ≥2.0 g/min. The maximum ηsolar to fuel of 19.6% was achieved at 1200 °C, H₂O/OPEFB molar ratio of 1.3, and OPEFB flowrate of 1.3 g/min, whereby a favourable energy upgrade factor (1.2) was achieved. The statistical model showed adequacy to predict H₂/CO molar ratio.

Saqr A.A. Al-Muraisy, Lais Americo Soares, Srirat Chuayboon, Shahrul Bin Ismail , Stéphane Abanades, Jules B. van Lier and Ralph E.F. Lindeboom

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CO₂ electroreduction provides a route to convert waste emissions into chemicals such as ethylene (C₂H₄). However, the direct transformation of CO₂-to-C₂H₄ suffers from CO₂ loss to carbonate, consuming up to 72% of energy input. A cascade approach—coupling a solid-oxide CO₂-to-CO electrochemical cell (SOEC) with a CO-to-C₂H₄ membrane electrode assembly (MEA)—would eliminate CO₂ loss to carbonate. However, this approach requires a CO-to-C₂H₄ MEA with energy efficiency well beyond demonstrations to date. Focusing on the MEA, we find that an N-tolyl substituted tetrahydro-bipyridine film improves the stabilization of key reaction intermediates, while an SSC ionomer enhances CO transport to the Cu surface, enabling a C₂H₄ faradaic efficiency of 65% at 150 mA cm⁻² for 110 h. Demonstrating a cascade SOEC-MEA approach, we achieve CO₂-to-C₂H₄ with a ~48% reduction in energy intensity compared with the direct route. We further reduce the energy intensity by coupling CO electroreduction (CORR) with glucose electrooxidation.

A Ozden, Y Wang, Li F, Luo M, Sisler J, Thevenon A, Rosas-Hernández A, Burdyny T, Lum Y, Yadegari H, Agapie T, Peters JC, Sargent EH, Sinton D.

Sub-nanometer zeolite 13X-supported Ni-ceria catalysts were synthesized for CO₂ methanation. XRD and SEM results show the structure and morphology of 13X zeolite after impregnation and calcination. Ce loading affected the catalysts’ metal dispersion, reducibility, basicity and acidity, and thence their activity and selectivity. STEM-EDX elemental mappings showed that Ce and Ni are predominantly highly dispersed. Ce has a positive effect on the reduction of NiO and leads to a relatively high number of medium basic sites with a low Ce loading. Highly stable 5%Ni2.5%Ce13X had high activity and nearly 100% CH₄ selectivity in CO₂ methanation at 360 °C, which is mainly due to the high dispersion of metals and relatively high amount of medium basic sites. It can be inferred that this catalyst synthesis strategy has great potential for good catalyst dispersion, since metal uptake by the zeolite is selective for the metal citrate complexes in solution.

Liangyuan Wei, Henrik Grénman, Wim Haije, Narendra Kumar, Atte Aho, Kari Eränen, Liangfu Wei, Wiebren de Jong

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Zeolite 13X and 5A supported Ni catalysts were synthesized for CO₂ methanation using the evaporation impregnation method. The influence of using different Ni precursors (nitrate, citrate, and acetate) as well as calcination temperatures on the catalyst properties and performance were investigated. XRD, SEM-EDX, TEM, STEM-EDX, N₂ physisorption, H_2-TPR, TPD-NH₃ and TG/DTA were used for detailed characterization of the catalysts. The parent structure of the zeolites did not change during catalyst synthesis. Using nickel citrate and acetate resulted in smaller NiO particle size compared to nitrate. STEM-EDX results showed that all the Ni-precursor complexes entered more efficiently the 13X zeolite structure, which is mainly due to steric hindrance resulting from the smaller pore size of 5A. Methanation experiments revealed that the 13X catalysts synthesized using nickel citrate (5% Ni) displayed clearly higher activity, compared to the catalysts synthesized using nickel nitrate or nickel acetate. A 79% conversion at 320 °C was obtained with 100% selectivity towards CH₄ and the catalyst showed excellent stability during 200 h testing. Overall, it can be concluded that the Ni precursor significantly influences the physico-chemical characteristics and catalytic properties of Ni 13X and Ni 5A zeolite catalysts in CO₂ methanation: complex size and pore size matter.

Liangyuan Wei, Wim Haije, Narendra Kumar, Janne Peltonen, Markus Peurla, Henrik Grenman, Wiebren de Jong

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Methanation is a potential large-scale option for CO₂ utilization, and it is one of the solutions for decreasing carbon emission and production of synthetic green fuels. However, the CO₂ conversion is limited by thermodynamics in conventional reaction conditions. However, around 100 % conversion can be obtained using sorption enhanced CO₂ methanation according to Le Chatelier’s principle, where water is removed during the reaction using zeolite as a sorbent. In this work 5%Ni5A, 5%Ni13X, 5%NiL and 5%Ni2.5%Ce13X bifunctional materials with both catalytic and water adsorption properties were tested in a fixed bed reactor. The overall performance of the bifunctional materials decreased on going from 5%Ni2.5%Ce13X, 5%Ni13X, 5%Ni5A, to 5%NiL. The CO₂ conversion and CH₄ selectivity were approaching 100 % during prolonged stability testing in a 100 reactive adsorption – desorption cycles test for 5%Ni2.5%Ce13X, and only a slight decrease of the water uptake capacity was observed.

Drs. Liangyuan Wei, Drs. Hamza Azad, Dr. Wim Haije, Prof.dr.ir. Wiebren de Jong

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Due to over- and under-supply, renewable energy sources can have a major impact on the electricity market and cause fluctuating electricity prices. In their laboratory, dr. Audrey Iranzo and prof.dr. Fokko Mulder developed a Ni-Fe layered double hydroxide for use as a positive electrode in the integrated nickel-iron battery and alkaline electrolyser.

Dr. Audrey Iranzo and prof.dr. Fokko Mulder

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The deployment of gas diffusion electrodes (GDEs) for the electrochemical CO₂ reduction reaction (CO₂RR) has enabled current densities an order of magnitude greater than those of aqueous H cells. The gains in production, however, have come with stability challenges due to rapid flooding of GDEs, which frustrate both laboratory experiments and scale-up prospects. Here, we investigate the role of carbon gas diffusion layers (GDLs) in the advent of flooding during CO₂RR, finding that applied potential plays a central role in the observed instabilities. Electrochemical characterization of carbon GDLs with and without catalysts suggests that the high overpotential required during electrochemical CO₂RR initiates hydrogen evolution on the carbon GDL support. These potentials impact the wetting characteristics of the hydrophobic GDL, resulting in flooding that is independent of CO₂RR. Findings from this work can be extended to any electrochemical reduction reaction using carbon-based GDEs (CORR or N₂RR) with cathodic overpotentials of less than −0.65 V versus a reversible hydrogen electrode.

Yang K., Kas R., Smith WA., Burdyny T.

Electrolysis-assisted nitrate (NO₃⁻) reduction is a promising approach for its conversion to harmless N₂ from waste, ground, and drinking water due to the possible process simplicity by in-situ generation of H₂/H/H⁺ by water electrolysis and to the flexibility given by tunable redox potential of electrodes. This work explores the use of a polymer electrolyte membrane (PEM) electrochemical cell for electrolysis-assisted nitrate reduction using SnO₂-supported metals as the active cathode catalysts. Effects of operation modes and catalyst materials on nitrate conversion and product selectivity were studied. The major challenge of product selectivity, namely complete suppression of nitrite (NO₂⁻) and ammonium (NH₄⁺) ion formation, was tackled by combining with simultaneous photocatalytic oxidation to drive the overall reaction towards N₂ formation.

Dr. Jordi Ampurdanés, Sorin Bunea, Prof.dr. Atsushi Urakawa

Electrochemical CO₂ electrolysis to produce hydrocarbon fuels or material feedstocks offers a renewable alternative to fossilized carbon sources. Gas-diffusion electrodes (GDEs), composed of solid electrocatalysts on porous supports positioned near the interface of a conducting electrolyte and CO₂ gas, have been able to demonstrate the substantial current densities needed for future commercialization. These higher reaction rates have often been ascribed to the presence of a three-phase interface, where solid, liquid, and gas provide electrons, water, and CO₂, respectively. Conversely, mechanistic work on electrochemical reactions implicates a fully two-phase reaction interface, where gas molecules reach the electrocatalyst’s surface by dissolution and diffusion through the electrolyte. Because the discrepancy between an atomistic three-phase versus two-phase reaction has substantial implications for the design of catalysts, gas-diffusion layers, and cell architectures, the nuances of nomenclatures and governing phenomena surrounding the three-phase-region require clarification. Here we outline the macro, micro, and atomistic phenomena occurring within a gas-diffusion electrode to provide a focused discussion on the architecture of the often-discussed three-phase region for CO₂ electrolysis. From this information, we comment on the outlook for the broader CO₂ electroreduction GDE cell architecture.

Nesbitt NT., Burdyny T., Simonson H., Salvatore D., Bohra D., Kas R., Smith WA.

X-ray absorption spectroscopy (XAS) offers the unique possibility to study metal electrocatalysts such as silver and copper while they are performing electrochemical carbon dioxide (CO₂) reduction. In this work, we present an approach to perform operando XAS experiments on an electrochemical cell performing CO₂ reduction with a gas diffusion electrode (GDE) as cathode. The experimental set-up, advantages and drawbacks, XAS data analysis, and XAS theory are discussed. Results on copper and silver GDEs obtained through the presented procedures are then presented and discussed. Structural and compositional catalyst data acquired under operando conditions can help further density functional theory calculations, and catalytic, and systems studies on CO₂ reduction. Structural and compositional data including crystallite size were obtained while performing high current density (up to 200 mA cm⁻²) CO₂ reduction. On the silver catalysts at higher than 100 mA cm⁻² applied current density, a Ag–X contribution was found and is ascribed to Ag–O. For both silver and copper, the XAS experiments revealed that the crystallite size of the ex situ samples is smaller than the samples during CO₂ reduction. Furthermore, metal particle size polydispersity was found in the silver catalysts by comparing the obtained coordination numbers with theoretical values. The operando EXAFS data was of such high quality (k: 3–14 Å⁻¹) that four shells could be fitted. The value of combining ex situ material characterisation and electrocatalyst performance data with operando XAS experiments is discussed and found to be of great importance to further CO₂ reduction research.

Firet NJ., Burdyny T., Nesbitt NT., Chandrashekar S., Longo A., Smith WA.

Despite substantial progress in the electrochemical conversion of CO₂ into value-added chemicals, the translation of fundamental studies into commercially relevant conditions requires additional efforts. Here, we study the catalytic properties of tailored Cu nanocatalysts under commercially relevant current densities in a gas-fed flow cell. We demonstrate that their facet-dependent selectivity is retained in this device configuration with the advantage of further suppressing hydrogen production and increasing the faradaic efficiencies toward the CO₂ reduction products compared to a conventional H-cell. The combined catalyst and system effects result in state-of-the art product selectivity at high current densities (in the range 100–300 mA/cm²) and at relatively low applied potential (as low as −0.65 V vs RHE). Cu cubes reach an ethylene selectivity of up to 57% with a corresponding mass activity of 700 mA/mg, and Cu octahedra reach a methane selectivity of up to 51% with a corresponding mass activity of 1.45 A/mg in 1 M KOH.

De Gregorio GL., Burdyny T., Loiudice A., Iyengar P., Smith WA., Buonsanti R.

Electrochemical CO₂ reduction has received an increased amount of interest in the last decade as a promising avenue for storing renewable electricity in chemical bonds. Despite considerable progress on catalyst performance using nanostructured electrodes, the sensitivity of the reaction to process conditions has led to debate on the origin of the activity and high selectivity. Additionally, this raises questions on the transferability of the performance and knowledge to other electrochemical systems. At its core, the discrepancy is primarily a result of the highly porous nature of nanostructured electrodes, which are vulnerable to both mass transport effects and structural changes during the electrolysis. Both effects are not straightforward to identify and difficult to decouple. Despite the susceptibility of nanostructured electrodes to mass transfer limitations, we highlight that nanostructured silver electrodes exhibit considerably higher activity when normalized to the electrochemically active surface in contrast to gold and copper electrodes. Alongside, we provide a discussion on how active surface area and thickness of the catalytic layer itself can influence the onset potential, selectivity, stability, activity and mass transfer inside and outside of the three dimensional catalyst layer. Key parameters and potential solutions are highlighted to decouple mass transfer effects from the measured activity in electrochemical cells utilizing CO₂ saturated aqueous solutions.

Kas K., Yang K., Bohra D., Kortlever R., Burdyny T., Smith WA.

The environment of a CO₂ electroreduction (CO₂ER) catalyst is intimately coupled with the surface reaction energetics and is therefore a critical aspect of the overall system performance. The immediate reaction environment of the electrocatalyst constitutes the electrical double layer (EDL) which extends a few nanometers into the electrolyte and screens the surface charge density. In this study, we resolve the species concentrations and potential profiles in the EDL of a CO₂ER system by self-consistently solving the migration, diffusion and reaction phenomena using the generalized modified Poisson–Nernst–Planck (GMPNP) equations which include the effect of volume exclusion due to the solvated size of solution species. We demonstrate that the concentration of solvated cations builds at the outer Helmholtz plane (OHP) with increasing applied potential until the steric limit is reached. The formation of the EDL is expected to have important consequences for the transport of the CO₂ molecule to the catalyst surface. The electric field in the EDL diminishes the pH in the first 5 nm from the OHP, with an accumulation of protons and a concomitant depletion of hydroxide ions. This is a considerable departure from the results obtained using reaction-diffusion models where migration is ignored. Finally, we use the GMPNP model to compare the nature of the EDL for different alkali metal cations to show the effect of solvated size and polarization of water on the resultant electric field. Our results establish the significance of the EDL and electrostatic forces in defining the local reaction environment of CO₂ electrocatalysts.

Bohra D., Chaudhry JH., Burdyny T., Pidko EA., Smith WA.

A successful transition of our global energy system will require changes to our society and industry. One of these changes is the production of electricity from renewable energy sources, which can be directly used today and in our increasingly electrified future infrastructure. The generated green electricity is also foreseen to be essential in “Power-to-X” technologies, where ambient molecules such as H₂O, CO₂, and N₂ are converted into dense-energy carriers.

Here, we provide an example of the combined technologies needed to make Power-to-X into an industry capable of influencing the global energy system (~TW). Using CO₂ electrolyzers as a central technology in the production of green methanol (MeOH), we discuss the integration of direct CO₂ air capture with CO₂ and H₂O electrolyzers and a traditional MeOH synthesis step. The resulting analysis provides tangible scales for CO₂ electrolyzers at an early research stage and perspectives on a future industry powered by dilute solar energy resources.

Smith WA., Burdyny T., Vermaas D., Geerlings H.

Electrocatalytic CO₂ reduction has the dual-promise of neutralizing carbon emissions in the near future, while providing a long-term pathway to create energy-dense chemicals and fuels from atmospheric CO₂. The field has advanced immensely in recent years, taking significant strides towards commercial realization. Catalyst innovations have played a pivotal role in these advances, with a steady stream of new catalysts providing gains in CO₂ conversion efficiencies and selectivities of both C1 and C2 products. Comparatively few of these catalysts have been tested at commercially-relevant current densities (∼200 mA cm⁻²) due to transport limitations in traditional testing configurations and a research focus on fundamental catalyst kinetics, which are measured at substantially lower current densities. A catalyst's selectivity and activity, however, have been shown to be highly sensitive to the local reaction environment, which changes drastically as a function of reaction rate. As a consequence of this, the surface properties of many CO₂ reduction catalysts risk being optimized for the wrong operating conditions. The goal of this perspective is to communicate the substantial impact of reaction rate on catalytic behaviour and the operation of gas-diffusion layers for the CO₂ reduction reaction. In brief, this work motivates high current density catalyst testing as a necessary step to properly evaluate materials for electrochemical CO₂ reduction, and to accelerate the technology toward its envisioned application of neutralizing CO₂ emissions on a global scale.

Burdyny T., Smith WA.

Electrocatalysis of carbon dioxide can provide a valuable pathway towards the sustainable production of chemicals and fuels from renewable electricity sources. One of the main challenges to enable this technology is to find suitable electrodes that can act as efficient, stable and selective CO₂ reduction catalysts. Modified silver catalysts and in particular, catalysts electrochemically derived from silver-oxides, have shown great promise in this regard. Here, we use operando EXAFS analysis to study the differences in surface composition between a pure silver film and oxide-derived silver catalysts – a nanostructured catalyst with improved CO₂ reduction performance. The EXAFS analysis reveals the presence of trace amounts of oxygen in the oxide-derived silver samples, with the measured oxygen content correlating well with experimental studies showing an increase in CO₂ reduction reactivity towards carbon monoxide. The selectivity towards CO production also partially scales with the increased surface area, showing that the morphology, local composition and electronic structure all play important roles in the improved activity and selectivity of oxide-derived silver electrocatalysts. Earlier studies based on X-ray photoelectron spectroscopy (XPS) were not able to identify this oxygen, most likely because in ultra-high vacuum conditions, silver can self-reduce to Ag⁰, removing existing oxygen species. This operando EXAFS study shows the potential for in situ and operando techniques to probe catalyst surfaces during electrolysis and aid in the overall understanding of electrochemical systems.

Firet NJ., Blommaert MA., Burdyny T., Venugopal A., Bohra D., Longo A., Smith WA.