Local ionic transport enables selective PGM-free bipolar membrane electrode assembly.

Bipolar membranes in electrochemical CO2 conversion cells enable different reaction environments in the CO2-reduction and O2-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO2 utilizations. In practice, however, even minor unwanted ion crossover limits stability to short time periods. Here we report the vital role of managing ionic species to improve CO2 conversion efficiency while preventing acidification of the anodic compartment. Through transport modelling, we identify that an anion-exchange ionomer in the catalyst layer improves local bicarbonate availability and increasing the proton transference number in the bipolar membranes increases CO2 regeneration and limits K+ concentration in the cathode region. Through experiments, we show that a uniform local distribution of bicarbonate ions increases the accessibility of reverted CO2 to the catalyst surface, improving Faradaic efficiency and limiting current densities by twofold. Using these insights, we demonstrate a fully platinum-group-metal-free bipolar membrane electrode assembly CO2 conversion system exhibiting <1% CO2/cation crossover rates and 80-90% CO2-to-CO utilization efficiency over 150 h operation at 100 mA cm-2 without anolyte replenishment.

Visualization of CO2 electrolysis using optical coherence tomography.

Electrolysers offer an appealing technology for conversion of CO2 into high-value chemicals. However, there are few tools available to track the reactions that occur within electrolysers. Here we report an electrolysis optical coherence tomography platform to visualize the chemical reactions occurring in a CO2 electrolyser. This platform was designed to capture three-dimensional images and videos at high spatial and temporal resolutions. We recorded 12 h of footage of an electrolyser containing a porous electrode separated by a membrane, converting a continuous feed of liquid KHCO3 to reduce CO2 into CO at applied current densities of 50-800 mA cm-2. This platform visualized reactants, intermediates and products, and captured the strikingly dynamic movement of the cathode and membrane components during electrolysis. It also linked CO production to regions of the electrolyser in which CO2 was in direct contact with both membrane and catalyst layers. These results highlight how this platform can be used to track reactions in continuous flow electrochemical reactors.

Catalyst Aggregation Matters for Immobilized Molecular CO2RR Electrocatalysts.

Here, we detail how the catalytic behavior of immobilized molecular electrocatalysts for the CO2 reduction reaction (CO2RR) can be impacted by catalyst aggregation. Operando Raman spectroscopy was used to study the CO2RR mediated by a layer of cobalt phthalocyanine (CoPc) immobilized on the cathode of an electrochemical flow reactor. We demonstrate that during electrolysis, the oxidation state of CoPc in the catalyst layer is dependent upon the degree of catalyst aggregation. Our data indicate that immobilized molecular catalysts must be dispersed on conductive supports to mitigate the formation of aggregates and produce meaningful performance data. We leveraged insights from this mechanistic study to engineer an improved CO-forming immobilized molecular catalyst─cobalt octaethoxyphthalocyanine (EtO8-CoPc)─that exhibited high selectivity (FECO ≥ 95%), high partial current density (JCO ≥ 300 mA/cm2), and high durability (ΔFECO < 0.1%/h at 150 mA/cm2) in a flow cell. This work demonstrates how to accurately identify CO2RR active species of molecular catalysts using operando Raman spectroscopy and how to use this information to implement improved molecular electrocatalysts into flow cells. This work also shows that the active site of CoPc during CO2RR catalysis in a flow cell is the metal center.

Direct H2O2 Synthesis, without H2 Gas.

We report here the direct hydrogenation of O2 gas to form hydrogen peroxide (H2O2) using a membrane reactor without H2 gas. Hydrogen is sourced from water, and the reactor is driven by electricity. Hydrogenation chemistry is achieved using a hydrogen-permeable Pd foil that separates an electrolysis chamber that generates reactive H atoms, from a hydrogenation chamber where H atoms react with O2 to form H2O2. Our results show that the concentration of H2O2 can be increased ∼8 times (from 56.5 to 443 mg/L) by optimizing the ratio of methanol-to-water in the chemical chamber, and through catalyst design. We demonstrate that the concentration of H2O2 is acutely sensitive to the H2O2 decomposition rate. This decomposition rate can be minimized by using AuPd alloy catalysts instead of pure Pd. This study presents a new pathway to directly synthesize H2O2 using water electrolysis without ever using H2 gas.

Conversion of Reactive Carbon Solutions into CO at Low Voltage and High Carbon Efficiency.

Electrolyzers are now capable of reducing carbon dioxide (CO2) into products at high reaction rates but are often characterized by low energy efficiencies and low CO2 utilization efficiencies. We report here an electrolyzer that reduces 3.0 M KHCO3(aq) into CO(g) at a high rate (partial current density for CO of 220 mA cm-2) and a CO2 utilization efficiency of 40%, at a voltage of merely 2.3 V. These results were made possible by using: (i) a reactive carbon solution enriched in KHCO3 as the feedstock instead of gaseous CO2; (ii) a cation exchange membrane instead of an anion exchange membrane, which is common to the field; and (iii) the hydrogen oxidation reaction (HOR) at the anode instead of the oxygen evolution reaction (OER). The voltage reported here is the lowest reported for any CO2 to CO electrolyzer that operates at high current densities (i.e., a partial current density for CO greater than 200 mA cm-2) with a CO2 utilization efficiency of greater than 20%. This study highlights how the choice of feedstock, membrane, and anode chemistries affects the rate and efficiency at which CO2 is converted into products.

Continuum Modeling of Porous Electrodes for Electrochemical Synthesis.

Electrochemical synthesis possesses substantial promise to utilize renewable energy sources to power the conversion of abundant feedstocks to value-added commodity chemicals and fuels. Of the potential system architectures for these processes, only systems employing 3-D structured porous electrodes have the capacity to achieve the high rates of conversion necessary for industrial scale. However, the phenomena and environments in these systems are not well understood and are challenging to probe experimentally. Fortunately, continuum modeling is well-suited to rationalize the observed behavior in electrochemical synthesis, as well as to ultimately provide recommendations for guiding the design of next-generation devices and components. In this review, we begin by presenting an historical review of modeling of porous electrode systems, with the aim of showing how past knowledge of macroscale modeling can contribute to the rising challenge of electrochemical synthesis. We then present a detailed overview of the governing physics and assumptions required to simulate porous electrode systems for electrochemical synthesis. Leveraging the developed understanding of porous-electrode theory, we survey and discuss the present literature reports on simulating multiscale phenomena in porous electrodes in order to demonstrate their relevance to understanding and improving the performance of devices for electrochemical synthesis. Lastly, we provide our perspectives regarding future directions in the development of models that can most accurately describe and predict the performance of such devices and discuss the best potential applications of future models.

Electrolytic conversion of carbon capture solutions containing carbonic anhydrase.

The electrolysis of carbon capture solutions bypasses energy-intensive CO2 recovery steps that are often required to convert CO2 into value-added products. We report herein an electrochemical flow reactor that converts carbon capture solutions containing carbonic anhydrase enzymes into carbon-based products. Carbonic anhydrase enzymes benefit CO2 capture by increasing the rate of reaction between CO2 and weakly alkaline solutions by 20-fold. In this study, we reduced CO2-enriched bicarbonate solutions containing carbonic anhydrase ("enzymatic CO2 capture solutions") into CO at current densities of 100 mA cm-2. This result demonstrated how to electrolyse enzymatic CO2 capture solutions, but the selectivity for CO production was two-thirds less than bicarbonate solutions without carbonic anhydrase. This reduction in performance occurred because carbonic anhydrase deactivated the catalyst surface. A carbon microporous layer was found to suppress this deactivation.

Electrocatalysts Derived from Copper Complexes Transform CO into C2+ Products Effectively in a Flow Cell.

Electrochemical reactors that electrolytically convert CO2 into higher-value chemicals and fuels often pass a concentrated hydroxide electrolyte across the cathode. This strongly alkaline medium converts the majority of CO2 into unreactive HCO3 - and CO3 2- byproducts rather than into CO2 reduction reaction (CO2RR) products. The electrolysis of CO (instead of CO2 ) does not suffer from this undesirable reaction chemistry because CO does not react with OH- . Moreover, CO can be more readily reduced into products containing two or more carbon atoms (i. e., C2+ products) compared to CO2 . We demonstrate here that an electrocatalyst layer derived from copper phthalocyanine (CuPc) mediates this conversion effectively in a flow cell. This catalyst achieved a 25 % higher selectivity for acetate formation at 200 mA/cm2 than a known state-of-art oxide-derived Cu catalyst tested in the same flow cell. A gas diffusion electrode coated with CuPc electrolyzed CO into C2+ products at high rates of product formation (i. e., current densities ≥200 mA/cm2 ), and at high faradaic efficiencies for C2+ production (FEC2+ ; >70 % at 200 mA/cm2 ). While operando Raman spectroscopy did not reveal evidence of structural changes to the copper molecular complex, X-ray photoelectron spectroscopy suggests that the catalyst undergoes conversion to a metallic copper species during catalysis. Notwithstanding, the ligand environment about the metal still impacts catalysis, which we demonstrated through the study of a homologous CuPc bearing ethoxy substituents. These findings reveal new strategies for using metal complexes for the formation of carbon-neutral chemicals and fuels at industrially relevant conditions.

Quantification of the Effect of an External Magnetic Field on Water Oxidation with Cobalt Oxide Anodes.

Here, we quantify the effect of an external magnetic field (ß) on the oxygen evolution reaction (OER) for a cobalt oxide|fluorine-doped tin oxide coated glass (CoOx|FTO) anode. A bespoke apparatus enables us to precisely determine the relationship between magnetic flux density (ß) and OER activity at the surface of a CoOx|FTO anode. The apparatus includes a strong NdFeB magnet (ßmax = 450 ± 1 mT) capable of producing a magnetic field of 371 ± 1 mT at the surface of the anode. The distance between the magnet and the anode surface is controlled by a linear actuator, enabling submillimeter distance positioning of the magnet relative to the anode surface. We couple this apparatus with a finite element analysis magnetic model that was validated by Hall probe measurements to determine the value of ß at the anode surface. At the largest tested magnetic field strength of ß = 371 ± 1 mT, a 4.7% increase in current at 1.5 V vs the normal hydrogen electrode (NHE) and a change in the Tafel slope of 14.5 mV/dec were observed. We demonstrate through a series of OER measurements at sequential values of ß that the enhancement consists of two distinct regions. The possible use of this effect to improve the energy efficiency of commercial water electrolyzers is discussed, and major challenges pertaining to the accurate measurement of the phenomenon are demonstrated.