New Study Reveals Key to Boosting PEM Fuel Cell Efficiency

Researchers at the University of Duisburg-Essen (UDE) Analyze How Polymer Membranes Affect PEM Fuel Cell Performance

For the first time, researchers at the University of Duisburg-Essen (UDE) have analyzed how the polymer membranes inside proton-exchange membrane (PEM) fuel cells influence performance. Their findings reveal a surprising insight: it is not the membrane's thickness but its mere presence that accounts for the majority of ohmic resistance. The contact interfaces between the membrane and the electrode already introduce a high baseline resistance, which thinner membranes do little to reduce.

PEM fuel cells are intended to drive decarbonization in both transportation and stationary power generation. The polymer electrolyte membrane at their core conducts protons, prevents fuel crossover, and thus determines the cell's efficiency and lifespan. Until now, however, its impact on individual loss mechanisms had been difficult to isolate, as too many overlapping processes occur in fully assembled fuel cells.

A team led by Dr. Fatih Özcan from UDE's Chair of Particle Technology has now addressed this challenge. Their study, "Electrochemical impedance spectroscopy-based screening of membrane effects via gas diffusion electrode half-cells for PEMFC performance optimization" (a method for assessing membrane effects using electrochemical impedance spectroscopy in gas diffusion electrode half-cells to optimize PEM fuel cell performance), was published in the journal Energy Advances and conducted in collaboration with the Center for Fuel Cell Technology (ZBT).

Half-Cell Instead of Full Cell: Isolating the Cathode

Rather than measuring a complete fuel cell, the researchers used a gas diffusion electrode half-cell (GDE half-cell), which limits observations to the cathode—the electrochemically limiting region of the cell. This approach eliminated interference from the anode and hydrogen crossover. The team employed 1-molar sulfuric acid as the electrolyte, with oxygen supplied directly through the gas diffusion layer.

Four commercial membranes of varying thickness and polymer chemistry were examined: 1. Aquivion Post Coat (4 µm, short-side-chain, SSC) 2. FumaPem (15 µm, SSC) 3. Nafion 211 (25 µm, long-side-chain, LSC) 4. Nafion 212 (50 µm, LSC).

A membrane-free electrode served as a baseline reference.

Combined Measurement Techniques Separate Overlapping Loss Mechanisms

The researchers integrated two electrochemical methods. Electrochemical impedance spectroscopy (EIS) captures the cell's electrical behavior across a broad frequency range but cannot distinctly separate overlapping processes. Distribution of relaxation times (DRT) analysis mathematically deconvolutes the spectrum into individual signal peaks, allowing four resistance components to be measured independently: ohmic resistance (Rₒₕₘ), localized proton transport resistance (Rₗₚₜ), charge transfer resistance (Rₒₜ), and mass transport resistance (Rₘₜ).

Measurements were taken at 0.6 V (ohmic-kinetic region) and 0.3 V (mass transport region). By linearly extrapolating the fitted resistance values to a hypothetical membrane thickness of zero—and comparing them to the actual membrane-free electrode—the team uncovered a nuanced picture: ohmic resistance, charge transfer, and mass transport each respond to different influencing factors.

Membrane Integration Dominates Ohmic Resistance

The key finding: Rₒₕₘ jumps from roughly 1.10 Ω cm² to about 1.84 Ω cm² when a membrane is introduced—but then remains nearly constant across all membrane thicknesses. The critical factor lies in the contact interfaces: the boundaries between the membrane and the catalyst layer, as well as between the membrane and the electrolyte, generate a baseline resistance that persists regardless of thickness and dominates the ohmic resistance value.

Charge Transfer Strongly Dependent on Membrane Thickness

Charge Transfer Resistance: A Different Pattern

The behavior of charge transfer resistance (Rct) follows a different trend. This parameter describes how quickly the electrochemical reaction occurs at the platinum particles. It increases continuously with membrane thickness—rising from 0.531 Ω cm² without a membrane to 0.691 Ω cm² with Aquivion post-coating, and even higher for Nafion 212. The analysis also reveals that the interface between the membrane and the catalyst layer alone already degrades reaction kinetics. This effect becomes more pronounced under high current flow. Distributed relaxation time (DRT) measurements confirm this: the corresponding signal shifts toward lower frequencies as membrane thickness increases, indicating slowed proton supply to the platinum particles.

Mass Transport: Polymer Chemistry Outweighs Thickness

When it comes to mass transport resistance (Rmt)—the resistance to oxygen and water transport through the catalyst layer—membrane thickness plays a secondary role. Here, polymer chemistry dominates. Long-side-chain (LSC) Nafion absorbs more water, swells more significantly, and thus blocks gas pathways within the catalyst layer. In contrast, short-side-chain (SSC) ionomers like Fumapem keep the pores open. The Aquivion post-coat produces virtually no measurable Rmt. In the half-cell, this resistance component is only detectable at 0.6 V—at 0.3 V, the high oxygen excess prevents measurable transport bottlenecks from external sources.

Implications for Membrane and Electrode Design

The study provides a method for separately measuring and evaluating membrane resistances under application-relevant conditions. For membrane development, the following key approaches emerge:

1. Polymer chemistry and ion exchange capacity (IEC) primarily govern mass transport. Optimization should focus on SSC ionomers with high IEC.
2. Reaction kinetics can be controlled via membrane thickness—thinner membranes reduce Rct.
3. Ohmic resistance, however, cannot be lowered simply by reducing membrane thickness; the critical factor is the quality of the interface between the catalyst layer and the membrane.

While the gas diffusion electrode (GDE) half-cell is not a substitute for full-cell testing, it enables rapid, reproducible screening of membrane candidates under cathode-relevant conditions. Follow-up studies aim to extend the method to humidified gases and elevated operating temperatures.