Doping for societal and environmental benefits
Facilitating the “green transportation” is a major and an inevitable solution to have a substantial reduction in the amount of air pollutants and the emission of greenhouse gas footprints. The emergence of proton exchange membrane fuel cell (PEMFC) is a revolution in green-energy science. PEMFC converts hydrogen and oxygen gases into electricity with water as the only by-product.
Owing to its high efficiency and zero emission, PEMFC attracts the greatest interest as a clean energy device beyond petrol and diesel.
Proton exchange membrane fuel cell has been a successful substitute for internal combustion engines in the automobiles in the developed countries' future plans. Hence, the capacity to install fuel cells in the transport sectors has been grown significantly in the recent years.
Cars and trucks powered with proton exchange membrane fuel cell stacks have been understood to be the definitive solution to replace traditional fuel driven automobiles owing to their advantages such as no pollution, low temperature start-up and high energy density.
Bipolar plates traditionally made of graphite or composite materials are one of the core components in the PEMFC stack. Such designs have been criticized for owing to their limitations such as lack of strength and toughness, excessive weight, and hefty processing cost.
However, bipolar plates made of stainless steel have demonstrated excellent and comprehensive performance, low cost, and varied options for automobile applications. Nevertheless, the bipolar plates made of stainless steel are prone to corrosion and passivation in the aggressive PEMFC working environment, that eventually lead either to the reduction in efficiency or to the premature failure.
To prevent the corrosion of bipolar plates, many non-coating and coating technical routes of stainless-steel bipolar plates have been demonstrated. These includes regulating the substrate component, plasma-based or thermal nitriding, electroplating, ion plating, chemical vapor deposition, and physical vapor deposition, etc. Alternatively stainless-steel bipolar plates are coated with metal nitrides, polymers (through surface polymerization), and carbonaceous coatings, etc.
Either technology reduce the challenges concerning corrosion of stainless steel without affecting the contact resistance to a significant extent. That is good news. However stainless-steel bipolar plates still face multiple challenges such as long-time stability under continuous operation, high cost and complexity in rolling them out into the mass production process.
Finding a suitable match that could meet the benchmark of graphite bipolar plates in terms of corrosion resistant, bringing down the hefty price in terms of manufacturing cost and easiness in managing weight are few but critical drawbacks of graphite BPP.
In contrary, carbon nanotubes comprise great functionalities such as mechanical strength, electrical conductivity, corrosive resistant, thermal stability, and durability altogether in a single system. Also carbon nanotubes improve fuel cell activity and give better proton exchange membrane fuel cell performance. Carbon nanotubes show excellent efficiency as they can be combined with other forms of carbon, for example, graphite to improve the mechanical and electrical properties of composite.
Bipolar plates for permeable exchange membrane fuel cells.
Figure curtesy: Yuan et.al. J. New Mater. Electrochem. Syst., 8 (2005), pp. 257-267
Proton exchange membrane fuel cell structure and components.
Figure curtesy: Haque et.al. Michigan Technological University ProQuest Dissertations Publishing, 2016. 10120296
Physical vapor deposition (PVD) based fuel cells
Plasma-assisted or ionized physical vapour deposition involves the condensation of vapour created from a solid source called target cathode, in the presence of a glow discharge plasma. Typical PVD processes are evaporative ion plating, reactive sputtering, and some plasma-based deposition techniques. Physical vapour deposition (PVD) is a very promising technology for improving the performance and longevity of both fuel cells and electrolysis. Metal-doped carbon-based targets can be used to coat stainless steel bipolar plates High Power Pulsed Magnetron Sputtering techniques.
HiPIMS is an established and industrial-friendly ionized physical vapour deposition technique (i-PVD) characterized by the deposition of uniform, extremely dense and well-adherent coatings. The utilization of HPPMS for the deposition of protective coatings for bipolar plates in PEMFC has been reported only very few times.
Pure amorphous carbon a-C layers are widely investigated owing to their superior performance, such as chemical inertness, mechanical hardness, optical transparency, and electrical conductivity. Normally a-C coatings contain mixed amounts of nanocrystalline structures and disordered structures caused by different hybridizations, such as sp3 and sp2. Hence, a method to deposit anti-corrosive while also a conductive a-C coating with excellent PEMFC functionality in an effective manner has remained always a challenge.
The material properties of a-C and metal substrates differ greatly. It is difficult to directly deposit a-C coatings on stainless-steel substrates. Therefore, the transition layer design and the material selection are critical issues for the high performance of the a-C coatings.
Inclusion of metal in the carbon-based bipolar plates can be achieved by combining pulsed magnetron sputtering technique with a similar technique (co-sputtering) another technique (ion implantation) to deposit the supporting catalyst with the main coating layers alternately or simultaneously. This can be done either using alloyed target (metal-doped carbon target) or using separate carbon and metal targets.
Metal elements doped in amorphous carbon coatings can form several types of nanostructures with carbon atoms, such as a solid solution and nanocrystalline embedded in a-C phases.
Doping of metal atoms into a-C coatings significantly affects the microstructure, morphology, and size of nanocrystalline formations and chemical nature of the of amorphous carbon coatings, which can be tapped to favour the fuel cell performance of PEMFC. Metal nitride doped carbonaceous materials grown at different N2-to-Ar ratios and an innovative coating based on Metal based conductive oxide layers exhibit excellent electrochemical performance.
Schematic illustrations of a single TiCx/a-C layer and of a multilayered TiCx/a-C
Figure Curtesy: ACS Appl Mater Interfaces, 10 (2018), pp. 19087-19096