Polymer electrolyte membrane fuel cells, also known as proton exchange membrane fuel cells (PEMFCs) are believed to be the most promising type of fuel cell for electric vehicles. The PEM fuel cell was invented by Willard Thomas Grubb and Leonard Niedrach in the early 1960s where sulfonated polystyrene membranes were used for electrolytes. In 1966, sulfonated polystyrene membranes were replaced by Nafion ionomer which proved to be more durable and superior in performance.
Plug Power and Nuvera Fuel Cells are the two major PEM fuel cell manufacturers of the USA. General Motors and Ford Motor are also active in the emerging fuel cell market. Outside the USA, Ballard and Hydrogenics of Canada; Re-fire, Sino Hytec and Horizon of China; Elring Klinger and Power Cell of Europe are the major manufacturers of PEM fuel cell.
| Anode: 2H2 → 4H+ + 4e− Cathode: O2 + 4H+ + 4e− → 2H2O Cell Reaction: 2H2 + O2 → 2H2O + heat + energy |
The important part of PEMFCs, are membrane electrode assemblies, which include the electrodes, electrolyte, catalyst, and gas diffusion layers. The most commonly used membrane is Nafion, which works on liquid water humidification of the membrane to transport protons within 80 to 90 °C temperature limit.
The recent advances of membranes are phosphoric acid doped polybenzimidazole membrane that can reach up to 220 °C without using any water management.1 This higher operating temperature ensures better controllability, and more efficiency. Additionally, it reduces the catalyst’s sensitivity to carbon monoxide poisoning. It is cheaper than Nafion and exhibits higher strength and better mechanical properties. However, it is not commercially popular as processing and mixing with catalyst is complicated and acid leaching is a considerable issue.
Current research on catalysts for PEM fuel cells focus on improvement of the catalytic activity and reduction of the poisoning of PEM fuel cell catalysts by impurity gases. Researchers are trying to discover alternative catalysts that will replace platinum or at least reduce the cost. Recent studies showed that formation of alloy with other metals increases the catalytic activity of platinum.
Recently, a new class of oxygen reduction reaction (ORR) electrocatalysts have been introduced where palladium-cobalt alloy supported on nitrogen-doped reduced graphene oxide (Pd3Co/NG) nanocomposite has replaced the high cost platinum catalyst. In a laboratory trial, maximum 68 mW/cm2 power density was achieved using Pd3Co/NG as both anode and cathode with individual loading of 0.5 mg/cm2 at 60°C without any backpressure. Pd3Co/NG showed 1.6 times better performance than standard catalyst.2 When hydrogen gas is produced by steam reforming process, it also contains CO (1–3%), CO2 (19–25%), and N2 (25%). Reduced sensitivity of catalyst to impurities, especially carbon monoxide (CO) also improve the catalyst performance.
PEM fuel cells are an alternative of fossil fuel for automobiles. Since water is the only byproduct, PEM fuel cell has no environmental impact. However, most motor companies work on PEM fuel cells due to their light weight, outstanding dynamics and high power density. Although, many people argue that it is an unrealistic technology for automobiles; PEM fuel cell driven Toyota Mirai is not a dream today.3
A group of scientists who work on metal-organic frameworks (MOFs) would be the potential electrolytes and catalysts that will replace traditional polymer membranes and expensive platinum catalyst. MOFs are porous coordination polymers that consist of metal nodes connected by organic linkers. MOFs exhibit many exclusive properties like large surface area, adjustable pore sizes, thermal stability, and desirable electrochemical characteristics. At 25 °C temperature and 98% relative humid condition, MOFs exhibit 4.2 x 10−2 S/cm conductivity which is close to Nafion.4 Instead of the traditional single crystal MOFs, nowadays, researchers successfully produce thin film MOFs for industrial application.
Therefore, polymer membranes will be replaced by MOFs in near future. MOFs are ideal candidates to become catalysts due to their tunable pore size, large surface area and high-volume density. Future research on MOFs will be helpful to achieve proper mechanical strength, good conductivity and water stability that is required to play important role on PEM fuel cells.
References:
- Mack, F.; Aniol, K.; Ellwein, C.; Kerres, J.; Zeis. R. J. Mater. Chem. A, 2015, 3, 10864-10874.
- Chandran, P.; Ghosh A.; Ramaprabhu S. Sci. Rep. 2018, 8, 3591.
- The Future. Available Now. https://ssl.toyota.com/mirai/fcv.html (accessed Oct 22, 2018)
- Ramaswamy, P.; Wong, N.E.; Shimizu G.K.H. Chem. Soc. Rev., 2014, 43, 5913-5932.
- Cao, Y.; Li Z.; Wu, H.; Yin, Y.; Cao L.; He X.; Jiang, Z.; Electrochim. Acta, 2017, 240, 186-194.