The Role of Surface Coatings in Improving the Performance of Fuel Cells and Electrolyzers

Fuel cells and electrolyzers are cornerstone technologies in the global transition to clean energy. Fuel cells convert chemical energy from hydrogen or other fuels into electricity with water as the primary byproduct, while electrolyzers use electricity to split water into hydrogen and oxygen, enabling energy storage and decarbonization of industrial processes. The efficiency, durability, and cost-effectiveness of these devices depend heavily on the materials used in their electrodes, catalysts, and bipolar plates. Among the most effective strategies to enhance performance is the application of surface coatings. By modifying the properties of component surfaces, coatings can improve catalytic activity, corrosion resistance, electrical conductivity, and overall stability. This article explores the critical role surface coatings play in advancing fuel cell and electrolyzer technology, detailing the mechanisms, materials, and future directions of this rapidly evolving field.

Fundamentals of Fuel Cells and Electrolyzers

How Fuel Cells Work

A fuel cell consists of an anode, a cathode, and an electrolyte membrane. Hydrogen gas is fed to the anode, where it is split into protons and electrons. The protons travel through the electrolyte membrane to the cathode, while electrons flow through an external circuit, generating electricity. At the cathode, oxygen combines with the protons and electrons to form water. The efficiency of this process is limited by the kinetics of the oxygen reduction reaction (ORR) at the cathode and the hydrogen oxidation reaction (HOR) at the anode. Surface coatings are used to accelerate these reactions and protect the electrodes from degradation.

How Electrolyzers Work

Electrolyzers operate in reverse. An electric current is applied to water, splitting it into hydrogen and oxygen. In a proton exchange membrane (PEM) electrolyzer, water is oxidized at the anode to produce oxygen, protons, and electrons. Protons cross the membrane to the cathode, where they combine with electrons to form hydrogen gas. The oxygen evolution reaction (OER) is particularly sluggish and requires catalysts. Surface coatings on the anode and cathode can lower the overpotentials needed for these electrochemical reactions, reducing energy consumption.

Why Surface Coatings Matter

Surface coatings offer a powerful lever to tune the physical and chemical properties of electrodes, bipolar plates, and other components without altering the bulk material. They address several critical challenges:

  • Corrosion Protection: In acidic or alkaline environments, electrodes degrade rapidly. Coatings act as barriers, preventing direct contact with corrosive electrolytes.
  • Enhanced Catalytic Activity: Coatings can expose more active sites, modify electronic structures, or provide synergistic effects that improve reaction rates.
  • Improved Conductivity: Many coatings, especially metallic or carbon-based, reduce electrical resistance at interfaces, boosting overall efficiency.
  • Stability and Lifespan: By resisting dissolution, sintering, and poisoning, coatings extend the operational life of expensive catalysts and bipolar plates.
  • Selectivity: In electrolyzers, coatings can suppress unwanted side reactions, such as hydrogen peroxide formation or parasitic oxygen reduction.

Types of Surface Coatings and Their Applications

Metallic Coatings

Noble metals like platinum, iridium, and ruthenium are widely used as coatings due to their exceptional catalytic activity for ORR and OER. Platinum coatings on carbon supports are standard in PEM fuel cells, but cost and scarcity drive the search for less expensive alternatives. Thin layers of platinum (often only a few atomic layers) deposited on non-noble metal cores, such as nickel or cobalt, provide similar activity with dramatically reduced mass loading. Iridium and ruthenium oxide coatings are preferred for the OER in PEM electrolyzers because of their corrosion resistance under highly oxidizing conditions. Non-noble metal coatings, such as nickel, could substitute in alkaline environments but require careful corrosion protection.

Oxide Coatings

Transition metal oxides offer a balance of activity, stability, and cost. Titanium dioxide (TiO₂) coatings are applied to bipolar plates in PEM electrolyzers to protect against corrosion and maintain low contact resistance. Doping TiO₂ with nitrogen or fluorine can further enhance electrical conductivity. Cerium oxide (CeO₂) coatings are investigated for their oxygen storage capacity, which can buffer oxygen activity during transient operation. Perovskite oxides (e.g., La₁₋ₓSrₓMnO₃) and spinel oxides (e.g., Co₃O₄) are explored as active coatings for oxygen electrodes, with performance comparable to noble metals under certain conditions.

Carbon-Based Coatings

Carbon is an excellent conductor and can be applied as a coating to protect metal components from corrosion while maintaining low electrical resistance. Carbon-based coatings, especially diamond-like carbon (DLC) and graphite, are used on stainless steel bipolar plates in both fuel cells and electrolyzers. They prevent metal ion contamination of the membrane and catalyst layers. DLC coatings also provide hydrophobic properties that improve water management in PEM fuel cells. Nitrogen-doped carbon coatings have shown promise as metal-free catalysts for ORR, offering an alternative to platinum.

Polymer Coatings

Conductive polymers, such as polyaniline (PANI) and polypyrrole (PPy), serve as coatings that combine electrical conductivity with flexibility and ease of processing. They can be electrodeposited onto electrode surfaces to enhance charge transfer and protect against corrosion. In some designs, polymer coatings also act as binders for catalyst particles, improving adhesion and preventing detachment. However, polymer coatings typically have lower stability under high temperatures or strong oxidants, limiting their use to lower-temperature systems.

Composite and Multilayer Coatings

To combine the advantages of different materials, composite and multilayer coatings are being developed. For example, a metal oxide layer covered with a thin noble metal shell can offer both corrosion resistance and high catalytic activity. Alternating layers of carbon and ceramics can tune thermal expansion and mechanical stress. Atomic layer deposition (ALD) enables precise control over the thickness and composition of each layer, resulting in coatings that are both functional and robust.

Application Methods for Surface Coatings

Physical Vapor Deposition (PVD)

PVD techniques, including sputtering and evaporation, are used to deposit thin films of metals or metal oxides onto substrates. These methods produce dense, uniform coatings with excellent adhesion. Sputtered platinum or iridium films are common in research-scale electrodes. PVD can be scaled for industrial production, though costs remain high for noble metals.

Chemical Vapor Deposition (CVD)

CVD reacts gaseous precursors to form solid coatings on the substrate surface. This technique is suitable for oxide and semiconductor coatings. For example, ALD (a variant of CVD) deposits films with atomic precision, making it ideal for creating ultra-thin catalyst layers that maximize utilization of expensive materials. ALD has been used to coat porous electrode structures with uniform layers of platinum, iridium oxide, or alumina for corrosion protection.

Electrodeposition

Electrodeposition uses an electric current to reduce metal ions from solution onto a conductive substrate. It is cost-effective, fast, and can coat complex geometries. Nickel-cobalt alloys are electrodeposited as protective coatings on bipolar plates. Pulse electrodeposition produces nanostructured coatings with high surface area.

Sol-Gel and Dip-Coating

Sol-gel processes allow application of oxide coatings from liquid precursors, followed by thermal treatment. This method is simple and can incorporate dopants easily. Sol-gel titanium dioxide coatings are applied to corrosion-prone stainless steel components. Dip-coating is practical for large flat surfaces like bipolar plates.

Thermal Spray

Thermal spray processes, such as plasma spraying, deposit thick coatings (50–500 μm) of metal or ceramic powders. These coatings are used primarily for protection against wear and corrosion rather than for catalytic activity. They are applied to bipolar plates in high-temperature fuel cells like solid oxide fuel cells (SOFCs).

Benefits of Surface Coatings in Fuel Cells

Enhanced Durability

Corrosion of carbon supports and metal catalyst particles is a major degradation mechanism in fuel cells. Surface coatings protect these components from acidic environments and high humidity. For instance, coating carbon black supports with a thin layer of titanium oxide prevents carbon corrosion during start-up/shutdown cycles. Coated bipolar plates avoid metal ion leaching that contaminates the membrane and reduces ionic conductivity.

Improved Catalytic Activity

By increasing the electrochemical active surface area (ECSA) through nanostructured coatings, catalyst utilization improves. Platinum coatings on nanowire arrays or on porous carbon substrates exhibit higher mass activity than conventional nanoparticle catalysts. Core-shell structures, where a thin platinum shell covers a nickel or cobalt core, are particularly effective. The shell can be deposited by galvanic displacement, achieving near-atomic dispersion.

Reduced Poisoning

Fuel cell catalysts are susceptible to poisoning by carbon monoxide (CO), sulfur compounds, and ammonia present in reformed hydrogen. Certain coatings, such as platinum-ruthenium alloys, have lower CO adsorption energy, allowing operation on reformate gas with higher impurity tolerance. Polymer coatings can also act as selective membranes, blocking poison molecules while allowing hydrogen to reach the catalyst.

Benefits of Surface Coatings in Electrolyzers

Lower Overpotentials

The OER at the anode is the rate-limiting step in PEM electrolysis. Iridium oxide coatings with high surface area reduce the overpotential by several hundred millivolts compared to bare metal substrates. Similarly, nickel-iron layered double hydroxide (LDH) coatings on nickel foam in alkaline electrolyzers dramatically improve OER kinetics.

Corrosion Resistance

Anodes in PEM electrolyzers operate under high anodic potential (>1.6 V) and acidic conditions, causing severe corrosion of most metals. Protective coatings of iridium, ruthenium oxide, or conductive ceramics (titanium suboxides) prevent dissolution and maintain performance over thousands of hours.

Enhanced Stability

Coating the cathode (hydrogen evolution reaction side) with molybdenum disulfide or nickel-molybdenum alloys can increase its resistance to deactivation. In alkaline electrolyzers, nickel-based coatings with added cobalt or iron show improved stability against formation of hydrides or passive layers.

Current Research and Future Directions

Nanostructured Coatings

Nanotechnology is enabling coatings with unprecedented properties. Nanostructured coatings, such as nanowires, nanoflakes, and mesoporous films, provide high surface area and expose active facets that are more catalytic. For fuel cells, platinum nanowires deposited on carbon cloth outperform commercial Pt/C catalysts in both activity and durability. For electrolyzers, mesoporous iridium oxide films have shown higher mass activity than dense films.

Two-Dimensional Materials

Graphene and transition metal dichalcogenides (TMDs) like MoS₂ and WS₂ are being explored as coatings for both protection and catalysis. Graphene coatings on metal substrates provide exceptional corrosion resistance and electrical conductivity. MoS₂ coatings are promising for the hydrogen evolution reaction (HER) as low-cost alternatives to platinum. Layered double hydroxides (LDHs), especially NiFe-LDH, are top-performing OER catalysts when coated onto conductive supports.

MXenes

MXenes (transition metal carbides and nitrides) are a class of two-dimensional materials with metallic conductivity and hydrophilicity. They have been tested as coatings for supercapacitors and are now being applied to electrochemical devices. Initial studies show MXene coatings can improve corrosion resistance of stainless steel bipolar plates and serve as catalyst supports for platinum, enhancing both activity and stability.

Machine Learning and High-Throughput Screening

Researchers are using machine learning to accelerate the discovery of optimal coating compositions. High-throughput computational screening can predict the catalytic activity and stability of thousands of possible coating materials, guiding experimental efforts toward the most promising candidates. This approach has identified new ternary and quaternary alloys that may outperform traditional noble metal coatings.

Challenges in Implementation

Cost: Noble metal coatings, especially platinum and iridium, are expensive. Reducing the thickness to atomic layers helps but requires precise manufacturing. Non-noble alternatives often lack long-term stability under operating conditions. Developing low-cost, durable coatings remains a top priority.

Scalability: Many advanced coating techniques (ALD, sputtering) are slow and costly for mass production. Transferring laboratory breakthroughs to industrial continuous processes is challenging. Roll-to-roll processing and electrodeposition are more scalable but may sacrifice uniformity.

Adhesion and Compatibility: Coatings must adhere strongly to the substrate under thermal and mechanical cycling. Mismatch in thermal expansion coefficients can cause delamination. Additionally, coatings must be compatible with the electrolyte and other components – for example, coating materials must not leach into the membrane and degrade performance.

Performance Monitoring: In situ characterization of coatings is difficult. Developing operando techniques (e.g., X-ray absorption spectroscopy, Raman microscopy) to monitor coating degradation during operation would help optimize design and accelerate development.

Conclusion

Surface coatings are not merely an add-on but a fundamental enabler of high-performance fuel cells and electrolyzers. From noble metal thin films to advanced polymers and two-dimensional materials, coatings address corrosion, catalysis, conductivity, and durability – all essential for commercial viability. The field is rapidly evolving with nanostructuring, multilayering, and high-throughput screening driving innovation. Continued investment in coating research will lower the cost and increase the lifetime of these devices, accelerating the adoption of hydrogen as a clean energy carrier. As global efforts expand in green hydrogen production and fuel cell vehicles, surface coatings will remain at the center of technological progress. For further reading, explore resources from the U.S. Department of Energy Fuel Cell Technologies Office, ScienceDirect's overview of surface coatings, and recent reviews in Energy & Environmental Science.