Fuelling the future - coatings for automotive applications
Dr Kevin Cooke, R&D Technology Centre Manager for Miba Coating Group at Teer Coatings Ltd in Worcestershire, UK, talks to Melanie Rutherford about the development of coatings to meet the on-going needs of the automotive sector.
Carbon footprints and how to reduce them are at the forefront of the minds of consumers and manufacturers alike. And perhaps no sector is more aware of the need to reduce carbon emissions than automotive. Without doubt, the hydrogen economy will be a key facilitator in the decarbonisation of energy and transport networks, and it will require the serial production of devices to produce, store and exploit hydrogen as an energy vector, at a sustainable economic cost.
Polymer electrolyte membrane (PEM) fuel cells for transport applications must provide high power densities, in terms of both volume and weight, to replace internal combustion engine (ICE) power units in future fuel cell electric vehicles (FCEVs) or to act as range extenders in combination with on-board reforming for battery-driven vehicles. The bipolar plate (BPP) is a key component of the fuel cell stack and has multiple functions, including separation of the gases between cells, providing electrical conduction between the anode and cathode, ensuring even distribution of reaction gases via its flow field channel structure, acting as a solid structure for the stack, and facilitating water and heat management. Steels are the substrate of choice for automotive BPPs, and weight and cost considerations have driven the plate thickness down to 100μm or below. Within the stack, the environmental conditions for BPPs are very demanding, including high acidity, oxidising (cathode) and reducing (anode) atmospheres, high-temperature (especially for high-temperature PEM) and low-temperature cycling (when idle). Coatings are now essential to achieve the required low interfacial contact resistance, high corrosion resistance and a longevity in excess of 5,000 hours.
Thin film physical vapour deposition (PVD) coatings with controlled nano-composition, structure and topography have the potential to satisfy these demands. By divorcing the surface properties of BPP and electrode components from the bulk material, they can facilitate the use of lower cost, less highly alloyed substrates, helping to reduce overall costs while maintaining adequate performance.
The future of motoring
Fuel cell electric vehicles with a driving range and performance comparable to conventional internal combustion engine-powered models will provide the lowest carbon solution for medium and larger-size cars, especially over longer distances. These vehicle categories are responsible for 75% of the automotive sector’s CO2 emissions, therefore substituting ICE with FCEV is an attractive route for CO2 reduction.
The UK H2 Mobility project was established to evaluate the benefits of FCEVs to the UK and to develop a roadmap for the introduction of vehicles and hydrogen refuelling infrastructure. The project has predicted that by 2030, UK annual sales of FCEVs will exceed 300,000 and there will be more than 1.5 million of these vehicles on our roads. A typical 95kW automotive polymer electrolyte membrane fuel cell (PEMFC) stack might contain 315 cells, each with an active area of 300cm2. Therefore, to satisfy the UK’s predicted requirements, an annual production of almost 95 million bipolar plates will be needed.
To achieve the essential targets of low cost, high power density and light weight, selection of the correct material for these fuel cells is critical. Several possibilities, including machined graphite, carbonpolymer composite, stainless steel sheets and metallic foams have been considered. Metallic plates offer the highest power density, their thickness being less than half that of carbon composite plates or bulk-machined graphite. Metallic foams offer weight reduction, but at the expense of increased volume and, therefore, reduced power density.
Machining, stamping, die casting, investment casting, powder metal forging, electroforming, stretch forming and hydroforming have all been investigated as fabrication methods for metallic BPPs. The two most promising techniques for serial production of these plates are stamping and hydroforming of thin metallic foils. The stamping method combines high production rates with cost competitiveness, while hydroforming offers flexibility, higher drawing ratio, improved surface quality, reduced springback, improved dimensional retention, the ability to form complex-shape sheet metal parts, a shorter production cycle, lower cost and easier manufacturing. Clearly, the design and accurate dimensional tolerances achieved in the formed flow-field channels are critical to the functionality of the cell. However, the electrical conductivity (or, inversely, the interfacial contact resistance [ICR]) between the BPP and the polymeric gas distribution layer that separates it from the fuel cell membrane itself, are key to the efficient collection and distribution of the generated electric current within and through the cell. In a state-of-theart PEMFC, the interfacial contact resistance between the different materials in the cell contribute 55% of the total I2R (power) losses, dominating the cell’s ohmic loss contributions. It has previously been shown that stamped stainless steel BPP samples demonstrate higher electrical conductivity (low interfacial contact resistance) than similar hydroformed samples, and that manufacturing methods and their parameters determine the geometry and surface condition of the plates, which influences interfacial contact resistance. For both forming processes, however, the measured interfacial contact resistance values were significantly above the US Department of Energy’s 2015 target of 10mΩcm2. A surface coating on stainless steel is therefore essential to eliminate the relatively high interfacial contact resistance resulting from the passive oxide layer of the stainless steel, but that coating must also replace the corrosion-protective function of the natural passivating layer.
Figures courtesy of H Sun, K Cooke, G Eitzinger, P Hamilton, Pollet, Thin Solid Films, Vol. 528 (2013) 199–204
In addition to high intrinsic interfacial contact resistance, the main disadvantage of metals has been cited as their chemical instability in the corrosive internal environment of the fuel cell (low pH, high humidity and temperatures of around 80°C), which can result in the formation of electrically insulating surface layers and leaching of metal ions into the MEA. Bulk precious metals, such as gold, do remain chemically stable but are uneconomic for commercial applications. Previous studies have provided a review of thin film coatings that have been applied to base metals to reduce the effective interfacial contact resistance and enhance corrosion resistance. These include precious metals, conductive metal carbides, nitrides and oxides, carbon-based vacuum and plasma-deposited coatings, and conductive polymers.
A 10nm-thick gold coating has been suggested to improve interfacial contact resistance of stainless steel BPPs. However, consider the above example of 315 plates of 300cm2 active area that require coating on both sides. This would call for almost 11kg of gold to meet a projected annual requirement of 300,000 stacks, even if the coating process could be 100% efficient in the transfer of gold to the plates. Transition metal nitrides, including titanium nitride (TiN) and chromium nitride (CrN), offer metallic levels of electrical conductivity combined with relative chemical inertness, and have already been optimised as industrial coatings, exploiting high-rate reactive magnetron sputtering from metal targets (nitrogen gas is injected into the deposition atmosphere along with the inert argon sputtering gas). As such, coating technology originally developed for engineering applications (for example, coatings on drill bits and engine components) can be readily adapted to coat steel BPPs with sub-micron nitride layers, providing corrosion resistance and low contact resistance, although still significantly greater than that of gold. Conductive graphite-like carbon can also be deposited by magnetron sputtering, with contact resistance approaching the 10nm gold benchmark.
Recent R&D has resulted in a range of coatings that combine the best aspects of nitride and carbon-based coatings. Industrial magnetron PVD technology has many successful examples of scale-up to large area applications, whether as inline, air-to-air, load-locked coaters for architectural glass sheets, or multi-metre wide, roll-to-roll coaters for continuous webs. Similar technologies will be needed for the bipolar plate surfaces that will undoubtedly be essential components for the future of Britain’s automotive needs.