The Active Layer (AL) is optimized for electrochemical reaction, while the Gas Diffusion Layer (GDL) is optimized for gas transport and electrolyte management. The AL typically combines ORR/OER catalysts, conductive carbon, and approximately 10–30% PTFE binder, with controlled partial wetting to form catalyst–oxygen–electrolyte three-phase interfaces. The GDL uses porous conductive carbon with a higher PTFE content—typically 30–70%—to remain strongly hydrophobic, prevent flooding, and maintain oxygen access from the air side.
The central design principle is a wettability gradient: the AL must be sufficiently wetted for ionic contact but remain gas-permeable, whereas the GDL must repel electrolyte while allowing rapid oxygen transport. Fabrication must preserve this gradient through homogeneous mixing, uniform coating, controlled porosity, and carefully applied pressing.
How the Active Layer and GDL Differ
Active Layer: The Reaction Zone
The AL faces the alkaline electrolyte and contains the primary ORR and OER catalyst, a conductive additive such as carbon black or carbon nanotubes, and a polymer binder, commonly PTFE.
Its structure must provide simultaneous access to:
- Catalyst sites for oxygen reactions.
- Electronic conduction through the carbon network and current collector.
- Ionic conduction through electrolyte penetration.
- Gas transport for oxygen during discharge and evolved oxygen during charging.
The AL therefore requires partial wetting, not complete hydrophilicity. Excessive wetting can block gas pathways, while excessive hydrophobicity can isolate catalyst sites from the electrolyte.
Gas Diffusion Layer: The Transport and Barrier Zone
The GDL faces ambient air and supplies oxygen to the AL. It generally consists of conductive porous carbon combined with a higher fraction of PTFE.
Its principal functions are to:
- Maintain open pathways for oxygen diffusion.
- Resist penetration by the alkaline electrolyte.
- Limit electrolyte evaporation and uncontrolled drying.
- Provide mechanical support for the active layer.
- Release oxygen generated during charging without structural failure.
The GDL must remain porous and strongly hydrophobic. A dense or poorly connected pore structure can reduce oxygen transport even if the material is chemically stable.
Material Composition Requirements
Catalyst and Conductive Additives in the AL
The AL requires a homogeneous distribution of catalyst and conductive agent. The catalyst provides ORR/OER activity, while carbon black or CNTs establish an electronic percolation network.
The conductive phase must be distributed sufficiently well to avoid isolated catalyst regions. However, excessive carbon can increase parasitic corrosion risk, particularly because peroxide intermediates generated during ORR can attack carbon under alkaline operating conditions.
Binder Content in the AL
A PTFE content of approximately 10–30% is commonly used as a starting range for the AL. The binder provides cohesion and contributes hydrophobicity, but too much PTFE can cover catalyst sites, reduce ionic access, and obstruct gas transport.
The appropriate value depends on catalyst type, carbon morphology, solvent system, coating thickness, and target porosity. The stated range should therefore be treated as a research formulation window, not a universal specification.
Carbon and PTFE in the GDL
The GDL generally uses conductive carbon as the porous structural framework and a higher PTFE fraction—approximately 30–70%—to establish the required hydrophobicity.
The higher binder content helps prevent electrolyte flooding and supports dimensional stability. It can also reduce pore volume or narrow pore throats if excessive, so the GDL must be engineered for both hydrophobicity and gas permeability.
Current Collector and Layer Integration
A nickel grid or related corrosion-resistant current collector may support the electrode and provide electronic conduction. The AL must make reliable electrical contact with this collector without sealing the pore network.
The AL and GDL should form a mechanically coherent multilayer structure, but their individual pore and wettability characteristics must remain distinct. Over-integration can destroy the intended separation between reaction and gas-transport functions.
Processing Requirements for the Active Layer
Homogeneous Slurry Mixing
The AL slurry must achieve uniform dispersion of catalyst, conductive carbon, and PTFE. Poor mixing produces catalyst-rich and binder-rich regions, causing local differences in conductivity, wettability, and reaction rate.
Mixing conditions should be controlled consistently because carbon morphology and PTFE distribution strongly affect the final pore structure. The objective is not simply a smooth slurry; it is a reproducible network after drying and pressing.
Uniform Coating
The AL should be applied with controlled thickness and areal loading. Nonuniform coating can create regions with excessive resistance or insufficient catalyst inventory.
Coating must also maintain electrical contact with the current collector while avoiding complete blockage of its openings. The resulting surface should support electrolyte access without becoming electrolyte-filled and gas-impermeable.
Drying and Binder Distribution
Drying conditions influence solvent removal, binder migration, cracking, and pore formation. Rapid or uneven drying can produce gradients through the electrode thickness.
The dried AL should retain interconnected electronic and gas pathways. Excessive binder migration toward the surface can create a dense barrier and reduce the availability of catalyst sites.
Processing Requirements for the GDL
Controlled Hydrophobic Treatment
PTFE must be distributed throughout the GDL sufficiently to repel electrolyte, but not so unevenly that some regions flood while others become gas-blocking.
The final layer should present a stable hydrophobic barrier toward the electrolyte while preserving interconnected pores for oxygen movement. This balance is central to preventing both flooding and loss of electrolyte contact.
Porosity and Pore-Size Control
The GDL must remain open enough for rapid oxygen transport. Its pore structure should prevent liquid electrolyte from penetrating deeply while allowing gaseous oxygen to reach the AL.
A useful design often involves a wettability and pore-size gradient, with the electrolyte-facing region supporting controlled interfacial contact and the air-facing region providing stronger hydrophobicity and gas access.
Layer Bonding
The GDL must adhere securely to the AL and current collector. Weak bonding can cause delamination during cycling, while excessive bonding pressure can collapse pores and increase gas-transport resistance.
The objective is a mechanically stable interface that does not eliminate the functional porosity of either layer.
Why Pressing and Fabrication Control Matter
Pressing Must Preserve Porosity
Heated pressing can improve layer adhesion and reduce interfacial resistance. However, pressure and temperature must be controlled because excessive compaction can collapse pores, reduce gas permeability, and alter the intended wettability gradient.
Insufficient pressing can leave weak interfaces, poor electrical contact, and delamination pathways. The correct process is therefore a compromise between cohesion and pore preservation.
Mechanical Resistance During Charging
OER produces oxygen gas inside or near the porous electrode. Local pressure increases can stress the layered structure and cause cracking, rupture, or delamination.
The AL–GDL assembly must therefore be mechanically robust while retaining enough open volume to release gas. This is one reason precise heated pressing and controlled layer adhesion are important in laboratory cell fabrication.
Reproducibility Is a Core Requirement
Research comparisons are unreliable if electrode thickness, PTFE distribution, porosity, compression, or catalyst loading vary between samples.
Laboratory fabrication should control slurry composition, mixing history, coating method, drying profile, pressing conditions, and final thickness. These parameters can affect measured overpotential as strongly as the nominal catalyst identity.
Understanding the Trade-offs
More PTFE Improves Flooding Resistance but Can Reduce Activity
Increasing PTFE generally improves hydrophobicity and electrolyte resistance, especially in the GDL. In the AL, however, excessive PTFE can reduce electrolyte penetration, cover catalyst sites, and impede oxygen transport.
The AL and GDL should therefore not be assigned the same binder formulation. Their different PTFE ranges reflect different functional requirements.
Higher Compression Improves Contact but Restricts Gas Flow
Pressing can improve electrical and mechanical contact between layers. Too much compression reduces pore volume and may raise oxygen-transport resistance.
Compression should be optimized against the electrode’s pore structure rather than maximized for mechanical strength alone.
More Carbon Improves Conductivity but Increases Degradation Risk
Conductive carbon supports electron transport and creates a porous framework. It can also be vulnerable to oxidation or corrosion under operating conditions, particularly in the presence of ORR peroxide intermediates and the high potentials associated with OER.
Material selection should therefore consider both initial conductivity and long-term electrochemical stability.
Environmental Conditions Can Change Electrode Behavior
Secondary zinc–air cells are sensitive to water balance and atmospheric CO₂. Dry conditions can cause electrolyte loss and poor interfacial contact, while humid conditions can promote flooding.
CO₂ absorption in alkaline electrolyte can form carbonate species that increase viscosity, reduce ionic conductivity, and block electrode pores. Environmental control is therefore part of electrode evaluation, not merely a testing convenience.
Making the Right Choice for Your Goal
The most useful formulation is the one that preserves the required reaction, transport, and mechanical functions simultaneously.
- If your primary focus is ORR/OER activity: Prioritize a homogeneous AL containing well-dispersed catalyst and conductive carbon, with only enough PTFE—commonly about 10–30%—to provide cohesion and controlled partial wetting.
- If your primary focus is flooding resistance: Use a more hydrophobic, porous GDL with a higher PTFE content—commonly about 30–70%—while verifying that oxygen transport remains adequate.
- If your primary focus is cycle life: Optimize layer adhesion, pore retention, and resistance to gas-induced mechanical stress rather than maximizing catalyst loading alone.
- If your primary focus is reproducible research data: Standardize slurry mixing, coating, drying, pressing, thickness, porosity, and environmental test conditions across all electrodes.
A reliable bifunctional air electrode uses the AL for controlled three-phase electrochemistry and the GDL for durable, hydrophobic oxygen transport.
Summary Table:
| Parameter | Active Layer (AL) | Gas Diffusion Layer (GDL) |
|---|---|---|
| Primary Function | ORR/OER reaction zone | Oxygen transport and electrolyte barrier |
| Catalyst | ORR/OER catalyst (e.g., Pt, IrO2, MnO2) | None (conductive carbon) |
| Conductive Additive | Carbon black, CNTs | Porous conductive carbon |
| PTFE Binder Content | 10–30% | 30–70% |
| Wettability | Partial wetting (hydrophilic and hydrophobic) | Strongly hydrophobic |
| Porosity | Moderate, interconnected pores | High open porosity for gas diffusion |
| Processing Focus | Homogeneous slurry, uniform coating, controlled drying | PTFE distribution, pore-size control, layer bonding |
| Key Trade-off | High activity vs. gas transport | Flooding resistance vs. gas permeability |
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