Unlocking catalyst potentials in zero-gap membrane-electrode assemblies

Technical Summary: Unlocking Catalyst Potentials in Zero-Gap Membrane-Electrode Assemblies

Problem Statement
Practical electrochemical energy systems, particularly zero-gap membrane-electrode assemblies (MEAs), operate through highly coupled reaction-transport processes. A critical limitation in developing these systems is the inability to resolve internal electrochemical states from the overall cell voltage alone. Standard two-electrode measurements conflate electrode overpotentials, transmembrane potentials, and concentration overpotentials into a single lumped response. This "blind spot" prevents the operando separation of catalytic activity, catalyst utilization, and degradation under working conditions. While three-electrode measurements using reference electrodes (REs) are standard in H- or flow-cell configurations with bulk liquid electrolytes, they cannot faithfully reproduce the tightly coupled, spatially heterogeneous environments of practical MEAs. Existing attempts to embed REs in MEAs face two persistent challenges: the instability of miniaturized pseudo-REs under dynamic chemical environments and the difficulty of maintaining stable ionic connections between external REs and the reactive regions.

Methodology
The authors establish a new operando electrochemical diagnosis framework designed specifically for zero-gap MEAs. The methodology consists of two primary innovations:

1. Stabilized Miniaturized Reference Electrodes (REs): To address the instability of embedded REs, the authors developed a polyelectrolyte-stabilized Ag|AgBr reference electrode. Unlike conventional REs that rely on internal liquid electrolytes (which are difficult to miniaturize without drift), this design utilizes a thin layer of positively charged Sustainion polymer (balanced with Br⁻ counterions) cast over an electroplated AgBr layer on an Ag foil. The positive charge of the polymer mimics an internal electrolyte by providing a controlled Br⁻ environment via Donnan exclusion, stabilizing the Ag|AgBr redox couple. The intimate electrostatic contact between the positively charged polymer and the negatively charged Ag|AgBr surface prevents delamination, a common issue with negatively charged polymers like Nafion.
2. Controlled Ionic Bridges: The study identifies that stable REs alone are insufficient; the ionic pathway connecting the REs to the active area must also be controlled. The authors systematically investigated the hydration state of the ion-exchange membrane (IEM) acting as an "ionic bridge" between the active electrode area and the REs embedded in non-active regions. They defined three scenarios: dry (loss of ionic transport), wet (instability due to electrolyte invasion), and ideal (stable, moderately hydrated). By optimizing gasket design and RE positioning (e.g., near electrolyte outlets for liquid-fed electrodes or gas inlets for gas-fed electrodes), they established a stable ionic connection that prevents short-circuiting and electrolyte flooding while maintaining ion transport.

Key Contributions and Results
Using this framework, the authors validated the technique across acidic, neutral, and basic environments for both water and CO₂ electrolysis at current densities up to 500 mA cm⁻². Key findings include:

• Stability and Accuracy: The coated Ag|AgBr REs demonstrated significantly reduced voltage drift (< 5 mV over 2 hours) compared to bare counterparts, even under cyclic bias and wide pH ranges (0–14). The technique successfully decoupled component-specific potentials, showing linear correlations between membrane potentials and ohmic losses calculated via electrochemical impedance spectroscopy (EIS).
• Hidden Transport-Driven Collapse: When screening Ag-based catalyst layers (CLs) for CO₂-to-CO conversion, the framework revealed a non-linear relationship between catalyst loading and intrinsic activity. While increasing loading initially increased the electrochemical surface area (ECSA), loadings exceeding ~1.7 mg cm⁻² led to a dramatic collapse in intrinsic activity (dropping from ~5.2 to ~1.4 mA cm⁻² ECSA). This was attributed to mass transport limitations and restricted reactant accessibility caused by the densification of the CLs under MEA compression, a phenomenon obscured in conventional two-electrode measurements.
• Early Degradation Detection: The framework enabled the detection of component-specific degradation before it manifested in overall cell voltage. In an accelerated degradation test using a Cu anode, the anode potential showed immediate deterioration (increasing from 2.1 to 2.5 V) while the overall cell voltage initially appeared to improve due to concurrent changes in the cathode signal. This highlights how conventional metrics can mask component-specific failures.

Significance
The paper claims that this work restores electrochemical observability to practical membrane-electrode assemblies. By providing a broadly applicable diagnostic framework that links catalyst design, MEA cell architecture, and device-level performance, the authors argue that this approach accelerates the development of practical electrochemical energy systems. The ability to directly and independently characterize catalyst layers under industrially relevant conditions allows for the identification of early failure signatures and the separation of electrode- and membrane-driven losses. This facilitates the transition from empirical optimization to physics-informed design and operation strategies for fuel cells and electrolysers.