Flow Electrolyzers Research Articles

Rational Modulation of Interface Microenvironment and Design of the Flow Electrolyzer for COx Electroreduction to Alcohol.

The electrocatalytic reduction of COx (including CO2 and CO) into value-added fuels and chemicals, particularly multi-carbon (C2+) alcohols, presents a significant opportunity to close the manmade carbon cycle and support sustainable energy systems. The catalytic performance of electrochemical reduction reactions of CO2 and CO (COxRR)is strongly correlated with the local microenvironments, the flow electrolyzer, and the catalysis approaches with flow electrolyzers, which contribute to the kinetic and thermodynamic landscape of the reaction, ultimately determining the efficiency and selectivity of the COxRR toward desired reduction products. However, controllable microenvironment construction, rationally designed flow electrolyzers, and matchable flow electrolyzers derived catalysis approaches chosen for improving COxRR-to-alcohol performance still face challenges. Building upon the foundation laid by previous research, this review article will provide an in-depth summary of the regulation of the catalytic reaction interface microenvironment, the design of flow electrolyzers, and the development of derived stepwise catalysis approaches with the flow electrolyzers, which provide a comprehensive and strategic approach to enhancing the COxRR process for alcohol production, offering valuable insights and innovative solutions that can significantly impact the field of COxRR conversion to alcohol and contribute to the development of more sustainable chemical production methods.

Determination of local pH in CO2 electroreduction.

The electrocatalytic conversion of CO2 and H2O into fuels and valuable chemicals has gained significant interest as a prospective method for the storage of renewable energy and the utilization of captured CO2. In the process of electroreduction of CO2, pH near the surface of the electrocatalysts plays an important role in the catalytic selectivity and activity. However, to elucidate the local pH effect on the fundamental reaction mechanism and modify the catalytic CO2 reduction performance, the localized pH determination method is highly desirable. In this minireview, we present the recent advances in the strategies of the local pH probe for CO2 electrolysis in both H-type cell reactors and GDE-type flow electrolyzers, followed with a better understanding of the local reaction environment in CO2 reduction. Additionally, pertinent advantages and drawbacks of the different localized pH probe techniques are discussed, and perspectives on future research efforts are also provided in this minireview.

Electrochemically Generated Platform Oxidizers for Pharmaceutical Production

Periodates represent one of the most powerful and versatile oxidizers in organic synthesis. Common ways to generate it electrochemically employ lead dioxide anodes. The disintegration of these electrodes cause contaminations which impede sensitive applications.[1] The purification of periodates makes this reagent usually too costly for pharmaceutical applications. With the use of boron-doped diamond anodes, a direct conversion of the inexpensive sodium and potassium iodide within an eight-electron oxidation process renders periodate. The novel process is scalable, heavy metal-free and can be conducted in batch as well as flow electrolyzers.[2] This approach can also be used for the regeneration of periodates from iodates and other iodine sources (waste streams). The process was implemented in a new synthetic approach to levetiracetam, which demonstrates the technical application and recycling of iodates in API synthesis.[3] This reagent can also be employed to make sulfoximines and degrade lignins to elaborated building blocks. [4, 5] The reactor concepts for the efficient preparation of multi-kg quantities will be presented. [6]

(Keynote) Electrochemical CO2 Reduction on NiNC Single Metal Atom Catalysts Under Alkaline to Acidic pH Conditions

The science and technology of the direct electrochemical CO2 reduction reaction on both model electrodes in liquid-electrolyte H-cells and Gas Diffusion Electrodes (GDEs) in flow electrolyzers offer as many formidable challenges as intriguing opportunities. Controlling the selectivity (faradaic efficiency), while maximizing the energy efficiency by lowering the kinetic overpotentials remains key to turn this complex reaction into a practical process embedded in a process chain. In this talk, I will highlight some of our recent advances in the design and characterization of NiNC single site electrocatalysts for the electrochemical reduction of CO2 to CO and CO2/CO mixed feeds into value-added fuels and chemicals in H-cell and Gas Diffusion Electrode (GDE) single cell electrolyzers. Further focus will be placed on the diagnosis of carbonate transport processes during electrolyzer cell operation using the experimentally accessible carbon crossover coefficient, CCC. Performance and its limitations of cell operation in alkaline and acid conditions will be contrasted and discussed.

Translating Catalyst-Polymer Composites from Liquid to Gas-Fed CO2 Electrolysis: A CoPc-P4VP Case Study.

The electrochemical CO2 reduction reaction (CO2RR) in gas-fed flow electrolyzers using gas diffusion electrodes (GDEs) generates industrially relevant activities and provides a promising approach for carbon recycling. Developing effective catalyst systems on GDEs is critical for achieving high activities. Catalyst-polymer composites (CPCs) formed between immobilized molecular catalysts and coordinating polymers exhibit positive synergies for the enhancement of CO2RR activity. However, previous studies of CPCs have been primarily confined to liquid reaction platforms, and there are few examples of translating CPCs to GDE architectures. This suggests a knowledge gap exists in translating between the two platforms. Herein, we identify and bridge that gap by demonstrating a case study for the (poly-4-vinylpyridine)-encapsulated cobalt phthalocyanine (CoPc-P4VP) CPC. We identify a major knolwedge gap in the overlooked factor of CPC's hydrophobicity, which plays a significant role in gas-fed CO2RR but is often neglected in fundamental studies conducted on the liquid reaction platform. We bridge this gap by correlating catalyst hydrophobicity in liquid CO2RR with activity in gas-fed CO2RR by means of water contact angle measurements. Our case study underscores the importance of incorporating an engineering perspective into CPC studies and the necessity to consider hydrophobicity in CPC design and evaluation. This approach will hopefully accelerate the applied studies of this group of promising catalytic materials in gas-fed CO2 electrolysis.

Progress and Understanding of CO2/CO Electroreduction in Flow Electrolyzers

Progress and Understanding of CO₂/CO Electroreduction in Flow Electrolyzers

Bipolar Hydrogen Production from Biomass-Derived Aldehydes and Water in Flow Electrolyzers

It is critical to fight against global warming and confine it to a manageable level of +1.5°C by curbing CO2 emissions, eventually achieving zero net emissions by 2050. This target will involve significant efforts from all industry sectors and require renewable energy storage and conversion, most of which are heavily rely on clean hydrogen production. Water electrolysis, producing hydrogen and oxygen from water and electricity, is a promising route to the decarbonization of industry and transportation. The main source of energy loss in water electrolysis is the unfavorable thermodynamics (standard potential of 1.23 V vs. NHE) and slow kinetics of oxygen evolution reaction (OER). These challenges motivate us to design efficient paired electrolysis systems to substitute OER. In this work, we demonstrate an efficient bipolar hydrogen production system that combines cathodic hydrogen evolution from water and anodic hydrogen generation from aldehyde oxidation, at a low voltage (~0.1V) with co-generation of high valuable carboxylic acid. We used furfural as the model aldehyde, a biomass-derived compound that is produced on a large scale, currently on a scale of 0.43 million tons/year. Unlike conventional combined systems for hydrogen production that require high voltage (> 1.0 V) and are limited by mass transport (<100 mA cm−2) of reactants, this study doubled the hydrogen production rate, obtained industrial-level current density (e.g. 390 mA cm−2, 0.6V), and achieved economic viability ($2.51/kg of H2). These achievements benefit from a thermodynamically favorable hydrogen production pathway from aldehyde, facile kinetics on porous Cu-based catalysts, efficient mass transport in flow cells, and inexpensive size-exclusive separators. The successful implementation of such an energy-efficient process will impact future low-carbon hydrogen production and distributed chemical manufacturing.

Rigorous Evaluation of Liquid Products in High-Rate CO2/CO Electrolysis

In this work, the water crossover in CO2 electrolysis was systematically analyzed with hydrated ions transferring through ion-selective membranes as charge carriers in three-compartment devices. We demonstrate that the water crossover leads to a variation in the electrolyte volume, which plays an important role in the evaluation of the catalytic selectivity of liquid products. Without considering the water crossover, the catalytic selectivity of liquid products could be overestimated with the use of anion exchange membranes and underestimated with cation exchange membranes, especially when ions with large hydration numbers transport through membranes for long-term electrolysis. In addition, a protocol for reliably quantifying liquid products is proposed for high-rate CO2/CO electrolysis in three-compartment flow electrolyzers.

Electrochemically Generated Periodates As a Platform Oxidizer

Periodates represent one of the most powerful and versatile oxidizers in organic synthesis. Common ways to generate it electrochemically employ lead dioxide anodes. The disintegration of these electrodes cause contaminations which impede sensitive applications.[1] The purification of periodates makes this reagent usually too costly for pharmaceutical applications. With the use of boron-doped diamond anodes, a direct conversion of the inexpensive sodium and potassium iodide within an eight-electron oxidation process renders periodate. The novel process is scalable, heavy metal-free and can be conducted in batch as well as flow electrolyzers.[2] This approach can also be used for the regeneration of periodates from iodates and other iodine sources (waste streams). The process was implemented in a new synthetic approach to levetiracetam, which demonstrates the technical application and recycling of iodates in API synthesis.[3] This reagent can also be employed to make sulfoximines and degrade lignins to elaborated building blocks. [4] The reactor concepts for the efficient preparation of multi-kg quantities will be presented.

Engineering Aspects for the Design of a Bicarbonate Zero-Gap Flow Electrolyzer for the Conversion of CO2 to Formate.

CO2 electrolyzers require gaseous CO2 or saturated CO2 solutions to achieve high energy efficiency (EE) in flow reactors. However, CO2 capture and delivery to electrolyzers are in most cases responsible for the inefficiency of the technology. Recently, bicarbonate zero-gap flow electrolyzers have proven to convert CO2 directly from bicarbonate solutions, thus mimicking a CO2 capture medium, obtaining high Faradaic efficiency (FE) and partial current density (CD) toward carbon products. However, since bicarbonate electrolyzers use a bipolar membrane (BPM) as a separator, the cell voltage (VCell) is high, and the system becomes less efficient compared to analogous CO2 electrolyzers. Due to the role of the bicarbonate both as a carbon donor and proton donor (in contrast to gas-fed CO2 electrolyzers), optimization by using know-how from conventional gas-fed CO2 electrolyzers is not valid. In this study, we have investigated how different engineering aspects, widely studied for upscaling gas-fed CO2 electrolyzers, influence the performance of bicarbonate zero-gap flow electrolyzers when converting CO2 to formate. The temperature, flow rate, and concentration of the electrolyte are evaluated in terms of FE, productivity, VCell, and EE in a broad range of current densities (10-400 mA cm-2). A CD of 50 mA cm-2, room temperature, high flow rate (5 mL cm-2) of the electrolyte, and high carbon load (KHCO3 3 M) are proposed as potentially optimal parameters to benchmark a design to achieve the highest EE (27% is obtained this way), one of the most important criteria when upscaling and evaluating carbon capture and conversion technologies. On the other hand, at high CD (>300 mA cm-2), low flow rate (0.5 mL cm-2) has the highest interest for downstream processing (>40 g L-1 formate is obtained this way) at the cost of a low EE (<10%).

Pitfalls and Protocols: Evaluating Catalysts for CO2 Reduction in Electrolyzers Based on Gas Diffusion Electrodes

The evaluation of catalysts on gas diffusion electrodes (GDEs) have propelled the progress of electrochemical CO2 reduction reaction (CO2RR) at industry-relevant activities. However, high experimental complexities exist in GDE-based flow electrolyzers, whereby various experimental factors can influence the evaluation of catalytic CO2RR performances. Not accounting for these experimental factors could result in inconsistent conclusions and thus hinder rational catalyst developments. This Perspective highlights a range of experimental factors that can affect the performance metrics for electrocatalysts. Specifically, the product faradaic efficiency can be influenced by the overestimation of the effluent gas flow rate, unaccounted losses of products, and unintended alteration of microenvironments. In addition, cathodic voltage can be inaccurately determined due to the unaccounted dynamic changes in uncompensated resistance. By raising awareness of these potential pitfalls and establishing appropriate protocols, we foresee a more meaningful benchmarking of catalytic performances across the literature.

Is Hydroxide Just Hydroxide? Unidentical CO2 Hydration Conditions during Hydrogen Evolution and Carbon Dioxide Reduction in Zero-Gap Gas Diffusion Electrode Reactors

The implementation of gas diffusion electrodes is a prerequisite to achieving industrially relevant reaction rates in gas-phase electrochemical CO2 reduction (CO2RR). In the state-of-the-art anion exchange membrane flow electrolyzers, however, there is a substantial loss of reactants due to a nonelectrochemical CO2 consumption at the cathode and the transport of its products to the anode. Our detailed analysis of CO2 crossover in a zero-gap CO2-to-CO flow electrolyzer showed a change in the chemical nature of the transported ionic species through the membrane. With the increasing reaction rate, a continuous shift from HCO3– to CO32– conduction was found to be similar to pure carbonate conduction in the high current density region (>100 mA cm–2). As competing hydrogen evolution takes over the cathodic reaction in a CO2-rich environment, hydroxide conduction becomes more pronounced. This reveals an alteration in the chemical CO2 consumption, the so-called CO2 hydration (CO2 + OH– ↔ HCO3– + OH– ↔ CO32–), implying an unidentical environment for the hydroxide ions generated in CO2RR and hydrogen evolution reaction under a CO2 atmosphere. Our work draws attention to the incomplete description of CO2 hydration at the confined cathode/membrane interface in membrane electrode assembly-type zero-gap CO2 electrolyzers.

Catalyst Formation and In Operando Monitoring of the Electrocatalytic Activity in Flow Reactors.

Flow reactors are of increasing importance and have become crucial devices due to their wide application in chemical synthesis, electrochemical hydrogen evolution reaction (HER), or electrochemical waste water treatment. In many of these applications, catalyst materials such as transition-metal chalcogenides (TMCs) for the HER, provide the desired electrochemical reactivity for the HER. Generally, the flow electrolyzers' performance is evaluated as the overall output, but the decrease in activity of the electrolyzer is due to localized failure of the catalyst. Herein, we present a method for the spatially resolved (tens of micrometers) In Operando analysis of the catalytic activity under real operation conditions as well as the localized deposition of the catalyst in an operating model flow reactor. For these purposes, scanning electrochemical microscopy was applied for MoSx catalyst deposition and for localized tracking of the TMC activity with a resolution of 25 μm. This approach offers detailed information about the catalytic performance and should find broad application for the characterization and optimization of flow reactor catalysis under real operational conditions.

(Invited) Paired Flow Electrolyzers for Furanic Compounds Conversion to Valuable Chemicals

Paired electrolysis has been emerged as an electricity-powered platform for converting biorewable feedstock to higher-valued chemicals at both the cathode and anode. In this presentation, we explored paired electrolyzers of different architectures with remarkable performance and stability. We first designed three-electrode flow electrolyzers to pair electrocatalytic hydrogenation of 5-(hydroxymethyl)furfural (HMF) on oxide-derived silver electrode and TEMPO-mediated HMF oxidation on carbon cloth. The paired flow cell achieved a combined faradaic efficiency of 163% to desired 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5-furandicarboxylic acid (FDCA) with a cell potential of ~1.7 V, at the constant current of 10 mA. When the anodic reaction was replaced by hydrogen oxidation, a minimized cell voltage of only ~0.9 V was achieved. We then assmbled a membrane electrode assembly (MEA)-based two-electrode flow cell, which realized a minimized cell potential of only ~1.5 V for a continuous 24 hours paired electrolysis of HMF. Finally, a pH asymmetric architecture was designed to match the optimum reaction conditions and to replace HMF oxidation on a NiFeOOH anode catalyst without a redox mediator. Our recent development of electrochemical-chemical combined reactors for furanic compounds conversion will also be briefly presented.

The Green Electrochemical Synthesis of Periodates and Application in API Synthesis

Periodates represent one of the most powerful and versatile oxidizers for organic synthesis. Common ways to generate it electrochemically employ lead dioxide anodes. The disintegration of these electrodes cause contaminations which impede sensitive applications.[1] The purification of periodates makes this reagent usually too costly pharmaceutical applications. With the use of boron-doped diamond a direct conversion of the inexpensive sodium and potassium iodide within an eight-electron oxidation process renders periodate. The novel process is scalable, heavy metal-free and can be conducted in batch as well as flow electrolyzers.[2] This approach can also be used for the regeneration of periodates from iodates. The process was implemented in a new synthetic approach to levetiracetam, which demonstrates the technical application and recycling of iodates in API synthesis.[3] The reactor concepts for the efficient preparation of multi-kg quantities will be presented.

Binder-Focused Approaches to Improve the Stability of Cathodes for CO2 Electroreduction

While the use of flow electrolyzers has enabled high selectivity (>80%) and activity (>200 mA cm–2) in the reduction of CO2 to value-added chemicals, the durability of these systems is still insuff...

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Summary Electrochemical CO2 reduction is a promising approach for upgrading excessive CO2 into value-added chemicals, while the exquisite control of the catalyst atomic structures to obtain high C2+ alcohol selectivity has remained challenging due to the intrinsically favored ethylene pathways at Cu surface. Herein, we demonstrate a rational strategy to achieve ∼70% faradaic efficiency toward C2+ alcohols. We utilized a CO-rich environment to construct Cu catalysts with stepped sites that enabled high surface coverages of ∗CO intermediates and the bridge-bound ∗CO adsorption, which allowed to trigger CO2 reduction pathways toward the formation alcohols. Using this defect-site-rich Cu catalyst, we achieved C2+ alcohols with partial current densities of > 100 mA·cm−2 in both a flow-cell electrolyzer and a membrane electrode assembly (MEA) electrolyzer. A stable alcohol faradaic efficiency of ∼60% was also obtained, with ∼500 mg C2+ alcohol production per cm2 catalyst during a continuous 30-h operation.

Get into flow: Design of experiments as a key technique in the optimization of anodic dehydrogenative C,C cross-coupling reaction of phenols in flow electrolyzers

The optimization of electro-organic reactions poses a challenge due to the various parameters involved. Quite often those parameters are not independent from each other, leading the experimental scientist using linear approaches into an optimization loophole. We report a strategy for the optimization of the anodic oxidative dehydrogenative C,C cross-coupling reactions for the synthesis of biphenols based on Design of Experiments (DoE), which overcomes the drawbacks of linear optimization approaches. Using a fractional design, we increased the yield of a long-time investigated example reaction from 44% up to 85% and point out different suitable reaction conditions through to linear regression of the parameter space investigated.

Modulating Local CO2 Concentration as a General Strategy for Enhancing C−C Coupling in CO2 Electroreduction

Summary Flow electrolyzers based on gas-diffusion electrodes (GDEs) have been increasingly employed to advance toward industry-relevant electrochemical CO2 reduction reaction (CO2RR) performance, though fundamental understanding of the GDE system is still lacking. Here, we propose that regulating local CO2 concentration on copper (Cu) surfaces is an effective and general strategy to promote C−C coupling in CO2RR. Local CO2 concentration could influence the surface coverage of ∗CO2, ∗H, and ∗CO, which affects the reaction pathways toward multi-carbon (C2+) products. Guided by mass-transport modeling, we have identified three approaches to modulate the local CO2 concentration in GDE-based electrolyzers: (1) catalyst layer structure, (2) feed CO2 concentration, and (3) feed flow rate. Utilizing Cu2O nanoparticles as the model catalysts, modulation of local CO2 concentration enabled an optimized faradaic efficiency toward C2+ products of up to 75.5% at 300 mA cm−2 and C2+ partial current density of up to 342 mA cm−2 in 1.0 M KHCO3.

Electrohydrodimerization of biomass-derived furfural generates a jet fuel precursor

A catalyst-free electrohydrodimerization route is developed to convert biomass-derived furfural to a jet fuel precursor, hydrofuroin, with high yield and excellent Faradaic efficiency in both batch and flow electrolyzers.