Asymmetric gas diffusion layers for improved water management in PGM-free electrodes.

Proton-exchange-membrane fuel cells (PEMFCs) offer a long-term, carbon-emission free solution to the energy needs of the transportation sector. However, high cost continues to limit PEMFC commercialization. Replacing expensive platinum group metal (PGM) catalysts with PGM-free catalysts could reduce cost, but the low active site density of PGM-free catalysts necessitates the use of thick electrodes that suffer from substantial mass transport losses.

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

Bipolar membranes in electrochemical CO conversion cells enable different reaction environments in the CO-reduction and O-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO utilizations. In practice, however, even minor unwanted ion crossover limits stability to short time periods.

Understanding Photovoltage Enhancement in Metal-Insulator Semiconductor Photoelectrodes with Metal Nanoparticles.

A metal-insulator-semiconductor (MIS) structure holds great potential to promote photoelectrochemical (PEC) reactions, such as water splitting and CO reduction, for the storage of solar energy in chemical bonds. The semiconductor absorbs photons, creating electron-hole pairs; the insulator facilitates charge separation; and the metal collects the desired charge and facilitates its use in the electrochemical reaction. Despite these attractive features, MIS photoelectrodes are significantly limited by their photovoltage, a combination of the voltage generated from photon absorption minus the potential drop across the insulator.

Parameter-Fitting-Free Continuum Modeling of Electric Double Layer in Aqueous Electrolyte.

Electric double layers (EDLs) play fundamental roles in various electrochemical processes. Despite the extensive history of EDL modeling, there remain challenges in the accurate prediction of its structure without expensive computation. Herein, we propose a predictive multiscale continuum model of EDL that eliminates the need for parameter fitting.

Pure-Water-Fed Forward-Bias Bipolar Membrane CO Electrolyzer.

Coupling renewable electricity to reduce carbon dioxide (CO) electrochemically into carbon feedstocks offers a promising pathway to produce chemical fuels sustainably. While there has been success in developing materials and theory for CO reduction, the widespread deployment of CO electrolyzers has been hindered by challenges in the reactor design and operational stability due to CO crossover and (bi)carbonate salt precipitation. Herein, we design asymmetrical bipolar membranes assembled into a zero-gap CO electrolyzer fed with pure water, solving both challenges.

Colloidal Stability of PFSA-Ionomer Dispersions Part II: Determination of Suspension pH Using Single-Ion Potential Energies.

Perfluorosulfonic acid (PFSA) ionomers serve a vital role in the performance and stability of fuel-cell catalyst layers. These properties, in turn, depend on the colloidal processing of precursor inks. To understand the colloidal structure of fuel-cell catalyst layers, we explore the aggregation of PFSA ionomers dissolved in water/alcohol solutions and relate the predicted aggregation to experimental measurements of solution pH.

Colloidal Stability of PFSA-Ionomer Dispersions. Part I. Single-Ion Electrostatic Interaction Potential Energies.

Charged colloidal particles neutralized by a single counterion are increasingly important for many emerging technologies. Attention here is paid specifically to hydrogen fuel cells and water electrolyzers whose catalyst layers are manufactured from a perfluorinated sulfonic acid polymer (PFSA) suspended in aqueous/alcohol solutions. Partially dissolved PFSA aggregates, known collectively as ionomers, are stabilized by the electrostatic repulsion of overlapping diffuse double layers consisting of only protons dissociated from the suspended polymer.

Engineering Bipolar Interfaces for Water Electrolysis Using Earth-Abundant Anodes.

Developing efficient and low-cost water electrolyzers for clean hydrogen production to reduce the carbon footprint of traditional hard-to-decarbonize sectors is a grand challenge toward tackling climate change. Bipolar-based water electrolysis combines the benefits of kinetically more favorable half-reactions and relatively inexpensive cell components compared to incumbent technologies, yet it has been shown to have limited performance. Here, we develop and test a bipolar-interface water electrolyzer (BPIWE) by combining an alkaline anode porous transport electrode with an acidic catalyst-coated membrane.

Ionomer-free and recyclable porous-transport electrode for high-performing proton-exchange-membrane water electrolysis.

Clean hydrogen production requires large-scale deployment of water-electrolysis technologies, particularly proton-exchange-membrane water electrolyzers (PEMWEs). However, as iridium-based electrocatalysts remain the only practical option for PEMWEs, their low abundance will become a bottleneck for a sustainable hydrogen economy. Herein, we propose high-performing and durable ionomer-free porous transport electrodes (PTEs) with facile recycling features enabling Ir thrifting and reclamation.

Theory and Simulation of Metal-Insulator-Semiconductor (MIS) Photoelectrodes.

A metal-insulator-semiconductor (MIS) structure is an attractive photoelectrode-catalyst architecture for promoting photoelectrochemical reactions, such as the formation of H by proton reduction. The metal catalyzes the generation of H using electrons generated by photon absorption and charge separation in the semiconductor. The insulator layer between the metal and the semiconductor protects the latter element from photo-corrosion and, also, significantly impacts the photovoltage at the metal surface.

Gas Mass-Transport Coefficients in Ionomer Membranes Using a Microelectrode.

Gas permeability, the product of gas diffusivity and Henry's gas-absorption constant, of ionomer membranes is an important transport parameter in fuel cell and electrolyzer research as it governs gas crossover between electrodes and perhaps in the catalyst layers as well. During transient operation, it is important to divide the gas permeability into its constituent properties as they are individually important. Although transient microelectrode measurements have been used previously to separate the gas permeability into these two parameters, inconsistencies remain in the interpretation of the experimental techniques.

Anion-exchange membranes with internal microchannels for water control in CO electrolysis.

Electrochemical reduction of carbon dioxide (COR) poses substantial promise to convert abundant feedstocks (water and CO) to value-added chemicals and fuels using solely renewable energy. However, recent membrane-electrode assembly (MEA) devices that have been demonstrated to achieve high rates of COR are limited by water management within the cell, due to both consumption of water by the COR reaction and electro-osmotic fluxes that transport water from the cathode to the anode. Additionally, crossover of potassium (K) ions poses concern at high current densities where saturation and precipitation of the salt ions can degrade cell performance.

Influence of Mesoscale Interactions on Proton, Water, and Electrokinetic Transport in Solvent-Filled Membranes: Theory and Simulation.

Transport of protons and water through water-filled, phase-separated cation-exchange membranes occurs through a network of interconnected nanoscale hydrophilic aqueous domains. This paper uses numerical simulations and theory to explore the role of the mesoscale network on water, proton, and electrokinetic transport in perfluorinated sulfonic acid (PFSA) membranes, pertinent to electrochemical energy-conversion devices. Concentrated-solution theory describes microscale transport.

Impact of Platinum Primary Particle Loading on Fuel Cell Performance: Insights from Catalyst/Ionomer Ink Interactions.

A variety of electrochemical energy conversion technologies, including fuel cells, rely on solution-processing techniques (via inks) to form their catalyst layers (CLs). The CLs are heterogeneous structures, often with uneven ion-conducting polymer (ionomer) coverage and underutilized catalysts. Various platinum-supported-on-carbon colloidal catalyst particles are used, but little is known about how or why changing the primary particle loading (PPL, or the weight fraction of platinum of the carbon-platinum catalyst particles) impacts performance.

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.

Engineering Catalyst-Electrolyte Microenvironments to Optimize the Activity and Selectivity for the Electrochemical Reduction of CO on Cu and Ag.

The electrochemical reduction of carbon dioxide (COR) driven by renewably generated electricity (e.g., solar and wind) offers a promising means for reusing the CO released during the production of cement, steel, and aluminum as well as the production of ammonia and methanol.

Linking Perfluorosulfonic Acid Ionomer Chemistry and High-Current Density Performance in Fuel-Cell Electrodes.

Transport phenomena are key in controlling the performance of electrochemical energy-conversion technologies and can be highly complex, involving multiple length scales and materials/phases. Material designs optimized for one reactant species transport however may inhibit other transport processes. We explore such trade-offs in the context of polymer-electrolyte fuel-cell electrodes, where ionomer thin films provide the necessary proton conductivity but retard oxygen transport to the Pt reaction site and cause interfacial resistance due to sulfonate/Pt interactions.

Pathway to Complete Energy Sector Decarbonization with Available Iridium Resources using Ultralow Loaded Water Electrolyzers.

We present ultralow Ir-loaded (ULL) proton exchange membrane water electrolyzer (PEMWE) cells that can produce enough hydrogen to largely decarbonize the global natural gas, transportation, and electrical storage sectors by 2050, using only half of the annual global Ir production for PEMWE deployment. This represents a significant improvement in PEMWE's global potential, enabled by careful control of the anode catalyst layer (CL), including its mesostructure and catalyst dispersion. Using commercially relevant membranes (Nafion 117), cell materials, electrocatalysts, and fabrication techniques, we achieve at peak a 250× improvement in Ir mass activity over commercial PEMWEs.

Understanding Multi-Ion Transport Mechanisms in Bipolar Membranes.

Bipolar membranes (BPMs) have the potential to become critical components in electrochemical devices for a variety of electrolysis and electrosynthesis applications. Because they can operate under large pH gradients, BPMs enable favorable environments for electrocatalysis at the individual electrodes. Critical to the implementation of BPMs in these devices is understanding the kinetics of water dissociation that occurs within the BPM as well as the co- and counter-ion crossover through the BPM, which both present significant obstacles to developing efficient and stable BPM-electrolyzers.

Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical Devices: Insights into Structure-Property Relationships.

Rapid improvements in polymer-electrolyte fuel-cell (PEFC) performance have been driven by the development of commercially available ion-conducting polymers (ionomers) that are employed as membranes and catalyst binders in membrane-electrode assemblies. Commercially available ionomers are based on a perfluorinated chemistry comprised of a polytetrafluoroethylene (PTFE) matrix that imparts low gas permeability and high mechanical strength but introduces significant mass-transport losses in the electrodes. These transport losses currently limit PEFC performance, especially for low Pt loadings.

Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application.

In situ electrochemical diagnostics designed to probe ionomer interactions with platinum and carbon were applied to relate ionomer coverage and conformation, gleaned from anion adsorption data, with O transport resistance for low-loaded (0.05 mg cm) platinum-supported Vulcan carbon (Pt/Vu)-based electrodes in a polymer electrolyte fuel cell. Coupling the in situ diagnostic data with ex situ characterization of catalyst inks and electrode structures, the effect of ink composition is explained by both ink-level interactions that dictate the electrode microstructure during fabrication and the resulting local ionomer distribution near catalyst sites.

Bilateral Orbital Compartment Syndrome Preceding Cerebellar Herniation in Neuropsychiatric Systemic Lupus Erythematosus.

Orbital compartment syndrome poses a significant risk for vision loss if not promptly treated. The authors report a patient with neuropsychiatric systemic lupus erythematosus presenting with bilateral orbital compartment syndrome and rapid decompensation leading to uncal and tonsillar herniation. While extremely rare, bilateral orbital compartment syndrome should prompt consideration of systemic etiologies including neuropsychiatric systemic lupus erythematosus.