Publications
Size-modified Poisson–Nernst–Planck approach for modeling a local electrode environment in CO2 electrolysis
Electrochemical reduction of CO2 heavily depends on the reaction conditions found near the electrode surface. These local conditions are affected by phenomena such as electric double layer formation and steric effects of the solution species, which in turn impact the passage of CO2 molecules to the catalytic surface. Most models for CO2 reduction ignore these effects, leading to an incomplete understanding of the local electrode environment. In this work, we present a modeling approach consisting of a set of size-modified Poisson–Nernst–Planck equations and the Frumkin interpretation of Tafel kinetics. We introduce a modification to the steric effects inside the transport equations which results in more realistic concentration profiles. We also show how the modification lends the model numerical stability without adopting any separate stabilization technique. The model can replicate experimental current densities and faradaic efficiencies till −1.5 vs. SHE/V of applied electrode potential. We also show the utility of this approach for systems operating at elevated CO2 pressures. Using Frumkin-corrected kinetics gels well with the theoretical understanding of the double layer. Hence, this work provides a sound mechanistic understanding of the CO2 reduction process, from which new insights on key performance controlling parameters can be obtained.
Esaar Naeem Butt, Johan T. Padding and Remco Hartkamp
An analytical multiphase flow model for parallel plate electrolyzers
Membraneless parallel-plate electrolyzers use electrolyte flow to avoid product crossover. Using a mixture model neglecting inertia, and assuming an exponential gas fraction profile, we derive approximate analytical expressions for the velocity profile and pressure drop for thin plumes. We verify these expressions using numerical solutions obtained with COMSOL and validate them using experimental data from the literature. We find that the wall gas fraction increases rapidly at small heights, but becomes fairly constant at larger heights. These expressions serve as a guiding framework for designing a membraneless parallel-plate electrolyzer by quantifying the maximum possible height. We find that buoyancy driven membraneless parallel-plate electrolyzers with a 3 mm gap can be designed with a maximum height of around 7.6 cm at 1000 A/m2 for operation with 98% product purity at atmospheric pressure. For a forced flow at Re=1000, the same electrolyzer can be made around 17.6 cm tall at 1000 A/m2. These limits can be further improved with smaller bubbles or higher pressure.
A. Rajora and J.W. Haverkort
Energy comparison of sequential and integrated CO2 capture and electrochemical conversion
Integrating carbon dioxide (CO2) electrolysis with CO2 capture provides exciting new opportunities for energy reductions by simultaneously removing the energy-demanding regeneration step in CO2 capture and avoiding critical issues faced by CO2 gas-fed electrolysers. However, understanding the potential energy advantages of an integrated process is not straightforward due to the interconnected processes which require knowledge of both capture and electrochemical conversion processes. Here, we identify the upper limits of the integrated process from an energy perspective by comparing the working principles and performance of integrated and sequential approaches. Our high-level energy analyses unveil that an integrated electrolyser must show similar performance to the gas-fed electrolyser to ensure an energy benefit of up to 44% versus the sequential route. However, such energy benefits diminish if future gas-fed electrolysers resolve the CO2 utilisation issue and if an integrated electrolyser shows lower conversion efficiencies than the gas-fed system.
Mengran Li, Erdem Irtem, Hugo-Pieter Iglesias van Montfort, Maryam Abdinejad & Thomas Burdyny
Electrochemical Reduction of CO2 in Tubular Flow Cells under Gas–Liquid Taylor Flow
Electrochemical reduction of CO2 using renewable energy is a promising avenue for sustainable production of bulk chemicals. However, CO2 electrolysis in aqueous systems is severely limited by mass transfer, leading to low reactor performance insufficient for industrial application. This paper shows that structured reactors operated under gas–liquid Taylor flow can overcome these limitations and significantly improve the reactor performance. This is achieved by reducing the boundary layer for mass transfer to the thin liquid film between the CO2 bubbles and the electrode. This work aims to understand the relationship between process conditions, mass transfer, and reactor performance by developing an easy-to-use analytical model. We find that the film thickness and the volume ratio of CO2/electrolyte fed to the reactor significantly affect the current density and the faradaic efficiency. Additionally, we find industrially relevant performance when operating the reactor at an elevated pressure beyond 5 bar. We compare our predictions with numerical simulations based on the unit cell approach, showing good agreement for a large window of operating parameters, illustrating when the easy-to-use predictive expressions for the current density and faradaic efficiency can be applied.
Isabell Bagemihl, Chaitanya Bhatraju, J. Ruud van Ommen and Volkert van Steijn
Earlier Publications
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The selective separation of ions is a major technological challenge having far-ranging impacts from product separation in electrochemical production of base chemicals from CO2 to water purification. In recent years, ion-selective electrochemical systems leveraging redox-materials emerged as an attractive platform based on their reversibility and remarkable ion selectivity. In the present study, we present an ultrasound-intensified fabrication process for polyvinyl ferrocene (PVF)–functionalized electrodes in a carbon nanotube (CNT) matrix for selective electro-adsorption of formate ions. To this end, a response surface methodology involving the Box–Behnken design with three effective independent variables, namely, PVF to CNT ratio, sonication duration, and ultrasonic amplitude was applied to reach the maximum formate adsorption efficiency. The fabricated electrodes were characterized using cyclic voltammetry.
Sevgi Polat, Ruud Kortlever, Huseyin Burak Eral
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The electrochemical CO2 reduction reaction (CO2RR) is important for a sustainable future. Key insights into the reaction pathways have been obtained by density functional theory (DFT) analysis, but so far, DFT has been unable to give an overall understanding of selectivity trends without important caveats. We show that an unconsidered parameter in DFT models of electrocatalysts─the surface coverage of reacting species─is crucial for understanding the CO2RR selectivities for different surfaces. Surface coverage is a parameter that must be assumed in most DFT studies of CO2RR electrocatalysts, but so far, only the coverage of nonreacting adsorbates has been treated. Explicitly treating the surface coverage of reacting adsorbates allows for an investigation that can more closely mimic operating conditions. Furthermore, and of more immediate importance, the use of surface coverage-dependent adsorption energies allows for the extraction of ratios of adsorption energies of CO2RR intermediates (COOHads and HCOOads) that are shown to be predictive of selectivity and are not susceptible to systematic errors. This approach allows for categorization of the selectivity of several monometallic catalysts (Pt, Pd, Au, Ag, Zn, Cu, Rh, W, Pb, Sn, In, Cd, and Tl), even problematic ones such as Ag or Zn, and does so by only considering the adsorption energies of known intermediates. The selectivity of the further reduction of COOHads can now be explained by a preference for Tafel or Heyrovsky reactions, recontextualizing the nature of selectivity of some catalysts. In summary, this work resolves differences between DFT and experimental studies of the CO2RR and underlines the importance of surface coverage.
Andrew R. T. Morrison, Mahinder Ramdin, Leo J. P. van der Broeke, Wiebren de Jong, Thijs J. H. Vlugt, and Ruud Kortlever
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Copper is one of the most promising catalysts for the CO2 reduction reaction (CO2RR) due to its unique capability of producing multicarbon products in appreciable quantities. Most of the CO2RR research efforts have been directed towards the development of new electrocatalysts to either increase product selectivities or decrease overpotentials. In contrast, only a few studies have systematically tested or benchmarked CO2RR performances of electrocatalysts. In this paper, for the first time, the performances of five different polycrystalline copper foils purchased from different suppliers are benchmarked for their CO2RR performance. Their differences are characterized in terms of microstructural features and the effect that these microstructural properties have on the electrocatalytic behavior during potentiostatic CO2RR experiments are evaluated. It is shown that the potential applied is the dominant factor controlling CO2RR selectivities, leading to the conclusion that microstructural properties of polycrystalline copper electrodes have a negligible effect on the outcome of CO2RR experiments.
Simone Asperti, Dr. Ruud Hendrikx, Dr. Yaiza Gonzalez-Garcia, Dr. Ruud Kortlever
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Electrolysis of water, CO2, and nitrogen-based compounds presents the opportunity of generating fossil-free fuels and feedstocks at an industrial scale. Such devices are complex in operation, and their performance metrics are usually reported as electrode-averaged quantities. In this work, we report the usage of infrared thermography to map the electrochemical activity of a gas-diffusion electrode performing water and CO2 reduction. By associating the heat map to a characteristic catalytic activity, the presented system can capture electrochemical and physical phenomena as they occur in electrolyzers for large-scale energy applications. We demonstrate applications for catalyst screening, catalyst-degradation measurements, and spatial activity mapping for water and CO2 electrolysis at current densities up to 0.2 A cm–2. At these current densities we report catalyst temperature increases (>10 K for 0.2 A cm–2) not apparent otherwise. Furthermore, substantial localized current density fluctuations are present. These observations challenge assumed local conditions, providing new fundamental and applied perspectives.
Hugo-Pieter Iglesias van Montfort and Thomas Burdyny
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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 NOx pollutants into water, leading to waste-to-value conversion.
Sorin Bunea, Kevin Clemens, Prof.dr. Atsushi Urakawa
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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
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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
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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
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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
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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.
Liangyuan Wei, Hamza Azad, Wim Haije, Henrik Grenman and Wiebren de Jong
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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
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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
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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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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, H2-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