Elucidating Mass Transport Regimes in Gas Diffusion Electrodes for CO2 Electroreduction

Gas diffusion electrodes (GDEs) have shown promising performance for the electrochemical reduction of CO2 (CO2R). In this study, a resolved, pore scale model of electrochemical reduction of CO2 within a liquid-filled catalyst layer is developed. Three CO2 mass transport regimes are identified in which the CO2 penetration depth is controlled by CO2 consumption in the electrolyte, CO2 conversion along the solid-electrolyte double-phase boundaries (DPBs), and CO2 conversion concentrated around the gas–solid–electrolyte triple-phase boundaries (TPBs). While it is possible for CO2R to be localized around the TPBs, in systems with submicron pore radii operating at –2 CO2R will be distributed across the DPBs within the catalyst layer. This validates the assumption of pore-scale uniformity implicit in popular, volume-averaged GDE models. The CO2 conversion efficiency depends strongly on the governing mass transport regime, and operational-phase diagrams are constructed to guide the catalyst layer design.</p

Simplified Models of the Bicarbonate Buffer for Scaled Simulations of CO2 Electrolyzers

Bicarbonate electrolytes are used in a range of chemical processes; however, resolved simulation of these electrolytes is difficult, as disparate reaction time scales lead to numerical stiffness and the formation of fine boundary layers. Based on several physically motivated approximations, we reduce the full set of chemical reactions within a bicarbonate electrolyte to a simpler subset, eliminating the numerical stiffness. We supported this simplification via a two-variable singular perturbation expansion and demonstrated that under neutral conditions (6 2 electrolyzer, the simplifications lead to negligible error. We also discuss two alternative simplifications, one valid at high pH and another valid at arbitrary pH. These simplifications reduce the condition number of the matrices resulting from spatiotemporal discretization by up to 10 orders of magnitude and enable three-dimensional (3D) simulation of CO2 electrolyzers containing carbonate solutions.</p

Tuning Material Properties of Alkaline Anion Exchange Membranes Through Crosslinking: A Review of Synthetic Strategies and Property Relationships

Alkaline anion exchange membranes (AAEMs) are an enabling component for next generation electrochemical applications, including alkaline fuel cells, alkaline water electrolyzers, CO2 electrochemical reduction, and flow batteries. While commercial systems, notably fuel cells, have traditionally relied on proton-exchange membranes (PEMs), hydroxide-ion conducting AAEMs hold promise as a way to reduce cost-per-device by enabling the use of less expensive non-platinum group electrodes and cheaper cell components. AAEMs have undergone significant material development over the past two decades resulting in substantial improvements in hydroxide conductivity, alkaline stability, and dimensional stability. Despite these advances, challenges still remain in the areas of durability, water management, high temperature performance, and selectivity. In this review we discuss crosslinking as a synthesis tool for tuning various AAEM material properties, such as water uptake, conductivity, alkaline stability, and selectivity, and we describe synthetic strategies for incorporating crosslinks during membrane fabrication

Electrolyzer energy dominates separation costs in state-of-the-art CO2 electrolyzers: Implications for single-pass CO2 utilization

In low-temperature CO2 electrolysis, a fundamental trade-off exists between maximizing electrolyzer performance and minimizing downstream CO2 recovery. By coupling a down-the-gas-channel electrolyzer model with a techno-economic analysis, we find that the optimal single-pass CO2 conversion for ethylene production is typically low—on the order of 5%–10%—although larger optima are found if the H2 faradic efficiency is very low. Similarly, strategies for eliminating carbonate crossover require more energy than downstream gas separation if they increase the cell potential by ∼0.2 V; however, when CAPEX are accounted for, this “break-even” voltage increases to ∼ 0.4 to 0.8 V for electricity prices varying from 6c/kWh to 1.5c/kWh. These findings are a consequence of the low energy requirements of industrial gas separation relative to electrochemical CO2 reduction. Under most circumstances, maintaining near-optimal electrolyzer performance is more important than reducing or eliminating downstream gas separations.</p

Direct Ink Writing of 3D Zn Structures as High‐Capacity Anodes for Rechargeable Alkaline Batteries

The relationship between structure and performance in alkaline Zn batteries is undeniable, where anode utilization, dendrite formation, shape change, and passivation issues are all addressable through anode morphology. While tailoring 3D hosts can improve the electrode performance, these practices are inherently limited by scaffolds that increase the mass or volume. Herein, a direct write strategy for producing template‐free metallic 3D Zn electrode architectures is discussed. Concentrated inks are customized to build designs with low electrical resistivity (5 × 10−4 Ω cm), submillimeter sizes (200 μm filaments), and high mechanical stability (Young's modulus of 0.1–0.5 GPa at relative densities of 0.28–0.46). A printed Zn lattice anode versus NiOOH cathode with an alkaline polymer gel electrolyte is then demonstrated. This Zn||NiOOH cell operates for over 650 cycles at high rates of 25 mA cm−2 with an average areal capacity of 11.89 mAh cm−2, a cumulative capacity of 7.8 Ah cm−2, and a volumetric capacity of 23.78 mAh cm−3. A thicker Zn anode achieves an ultrahigh areal capacity of 85.45 mAh cm−2 and a volumetric capacity of 81.45 mAh cm−3 without significant microstructural changes after 50 cycles