Carbon paper—often referred to as carbon fiber paper (CFP) or porous carbon substrate—has emerged as a critical material in modern battery systems, thanks to its unique combination of high electrical conductivity, structural flexibility, porous architecture, and chemical stability. Unlike traditional paper, carbon paper is manufactured from carbon fibers (typically polyacrylonitrile-based or pitch-based) bonded into a thin, porous sheet, offering properties that address key challenges in battery design: efficient ion/electron transport, mechanical support for active materials, and resistance to harsh electrochemical environments. As the demand for high-energy-density, long-cycle-life batteries grows—driven by electric vehicles (EVs), grid storage, and portable electronics—carbon paper's role in enabling next-generation battery chemistries has become increasingly indispensable. This article explores the core properties of carbon paper and its diverse applications across major battery technologies, highlighting its impact on performance and innovation.

Core Properties of Carbon Paper: Why It Excels in Batteries

Carbon paper's suitability for battery applications stems from four defining characteristics, each tailored to address critical battery performance needs:

1. Exceptional Electrical Conductivity

Carbon paper exhibits electrical conductivity ranging from 100–1,000 S/cm, significantly higher than polymer substrates (≤10⁻⁸ S/cm) and comparable to thin metal foils (e.g., copper: ~59,600 S/cm). This high conductivity ensures efficient electron transfer between active materials and external circuits, minimizing internal resistance—a key factor in battery power density. For example, carbon paper used in fuel cells maintains conductivity even at high temperatures (up to 200°C), outperforming metal collectors that may corrode or degrade under such conditions.

2. Controlled Porosity and High Specific Surface Area

Carbon paper features a hierarchical porous structure with porosity levels of 70–90% and pore sizes ranging from microscale (＜2 nm) to macroscale (＞50 nm). This architecture serves two critical functions: it maximizes the surface area available for active material loading (e.g., catalysts, sulfur, or silicon) and creates interconnected pathways for ion transport. A typical carbon paper has a specific surface area of 100–500 m²/g, allowing for higher active material loading (e.g., 5–20 mg/cm² for lithium-sulfur battery cathodes) compared to dense metal collectors (≤5 mg/cm²).

3. Mechanical Flexibility and Durability

Carbon paper is lightweight (density: 0.2–0.5 g/cm³) and flexible, making it compatible with flexible battery designs (e.g., wearable electronics) and resilient to the volume changes of active materials during cycling. For instance, silicon anodes expand by 300% during lithium insertion; carbon paper's porous structure acts as a buffer, absorbing mechanical stress and preventing electrode cracking. Additionally, carbon paper exhibits high tensile strength (10–30 MPa) and resistance to tearing, ensuring structural integrity over thousands of charge-discharge cycles.

4. Chemical Inertness

Carbon paper is chemically stable in the harsh environments of batteries, including acidic/alkaline electrolytes, high voltages (up to 5V), and extreme temperatures (-40°C to 200°C). Unlike metal collectors (e.g., aluminum, copper), it does not corrode, oxidize, or react with electrolytes—extending battery lifespan. For example, in sodium-ion batteries using aqueous electrolytes, carbon paper outperforms copper foil, which degrades rapidly in water-based solutions.

Key Applications of Carbon Paper in Battery Technologies

Carbon paper's unique properties make it adaptable to a wide range of battery chemistries, from mature technologies like lithium-ion to emerging systems like solid-state and sodium-ion batteries. Below are its most impactful applications:

1. Fuel Cells: Gas Diffusion Layers (GDLs)

Carbon paper is the gold standard for gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs)—used in EVs, backup power systems, and hydrogen infrastructure. As a GDL, carbon paper performs three critical roles:

Gas Distribution: Its porous structure evenly distributes reactant gases (hydrogen and oxygen) across the catalyst layer, ensuring uniform electrochemical reactions.

Water Management: It wicks away water produced during fuel cell operation, preventing "flooding" that blocks gas flow and reduces efficiency.

Electron Conduction: It conducts electrons from the catalyst layer to the external circuit, minimizing contact resistance.

2. Lithium-Sulfur (Li-S) Batteries: Cathode Host and Polysulfide Trapper

Li-S batteries offer 3x higher theoretical energy density (2,600 Wh/kg) than lithium-ion batteries, but suffer from "polysulfide shuttling"—the migration of soluble sulfur species from the cathode to the anode, causing capacity fade. Carbon paper addresses this challenge by serving as a multifunctional cathode host:

Active Material Support: Its high surface area and porosity accommodate sulfur loading (up to 80 wt% sulfur), maximizing energy density.

Polysulfide Trapping: Carbon's high affinity for sulfur species adsorbs polysulfides, preventing their diffusion to the anode.

3. Lithium-Ion Batteries (LIBs): High-Capacity Anode Substrates

Carbon paper is increasingly used as a substrate for high-capacity anode materials like silicon (Si) and tin (Sn), which suffer from severe volume expansion. By embedding Si nanoparticles into carbon paper's porous structure:

The carbon paper absorbs Si's 300% volume expansion, preventing electrode cracking.

Its conductivity compensates for Si's poor electrical conductivity (10⁻³ S/cm), reducing internal resistance.

Its high surface area allows for uniform Si dispersion, minimizing agglomeration.

A 2025 study from Stanford University's Battery Lab showed that Si-carbon paper anodes achieved a specific capacity of 1,800 mAh/g (5x higher than graphite) and retained 70% capacity after 1,000 cycles—outperforming Si-coated copper foil anodes (50% capacity retention after 500 cycles). These anodes are now being tested in next-generation LIBs for EVs, targeting 500 Wh/kg energy density (vs. 300 Wh/kg for current LIBs).

4. Sodium-Ion Batteries (SIBs): Low-Cost, Corrosion-Resistant Collectors

SIBs are emerging as a low-cost alternative to LIBs for grid storage, but their aqueous or high-salt electrolytes corrode traditional metal collectors (copper, aluminum). Carbon paper serves as a corrosion-resistant current collector for SIB electrodes, offering:

Compatibility with aqueous electrolytes, eliminating the need for expensive organic solvents.

High conductivity for sodium-ion transport, despite Na⁺'s larger size (compared to Li⁺).

5. Solid-State Batteries (SSBs): Interfacial Conductivity Enhancers

SSBs use solid electrolytes instead of liquid ones, offering improved safety and energy density—but face challenges with poor interfacial contact between electrodes and solid electrolytes. Carbon paper acts as an interfacial layer, enhancing contact and reducing resistance:

Its porous structure conforms to the solid electrolyte's surface, eliminating air gaps.

Its conductivity bridges the electrode and electrolyte, facilitating ion/electron transport.

Its chemical inertness prevents reactions with solid electrolytes (e.g., sulfide or oxide-based).

Modifications to Enhance Carbon Paper Performance

To tailor carbon paper for specific battery applications, researchers and manufacturers use three key modification techniques:

1. Doping with Heteroatoms

Doping carbon paper with nitrogen (N), phosphorus (P), or sulfur (S) introduces active sites that enhance catalytic activity and polysulfide adsorption. For example, N-doped carbon paper used in Li-S cathodes increases polysulfide trapping by 40%, while P-doped carbon paper improves oxygen reduction reaction (ORR) activity in fuel cells by 25%.

2. Coating with Functional Materials

Coating carbon paper with materials like graphene, carbon nanotubes (CNTs), or ceramic nanoparticles (e.g., Al₂O₃) boosts conductivity, mechanical strength, and electrolyte compatibility. Graphene-coated carbon paper for LIB anodes exhibits 20% higher conductivity than uncoated carbon paper, while Al₂O₃-coated carbon paper in SSBs reduces interfacial resistance by 30%.

3. Structural Optimization

Adjusting carbon paper's porosity, fiber diameter, and thickness optimizes its performance for specific batteries. For example, fuel cell GDLs require high porosity (85–90%) for gas diffusion, while Li-S cathodes use moderate porosity (70–75%) to balance sulfur loading and ion transport.

Carbon paper's exceptional conductivity, porosity, flexibility, and chemical stability make it a versatile enabler across battery technologies—from fuel cells powering EVs to Li-S batteries for grid storage. Its ability to address critical challenges (e.g., polysulfide shuttling, volume expansion, interfacial resistance) has positioned it as a key material in the transition to high-performance, sustainable energy storage. As modifications and innovations continue to enhance its properties, carbon paper will remain indispensable in unlocking the full potential of next-generation batteries, driving progress in electrification and renewable energy adoption.