Publications

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

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 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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