Reports indicate that nanoporous carbon membranes are potential candidates for fuel cells and have shown higher performance than fuel cells containing other carbon-based materials. This is mainly due to their high porosity, pore size tenability, high conductivity and stability.
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Carbon-based materials are broadly used in various applications due to their wide availability, extraordinary physicochemical properties and low cost. Porous carbon membranes have potential applications in catalysis, water treatment, gas separation, optoelectronics and fuel cells.
Characteristics of atomic ordering, local chemical composition, morphology, pore architecture are relevant for applications of carbon membranes in energy conversion and storage devices.
In particular, a high degree of graphitization and hierarchal pore architecture is highly relevant since they provide fast electron mobility and fast mass transport through pores, accompanied by high surface reaction capacity.
Apart from these properties, porous carbon membranes have excellent mechanical strength, high chemical and thermal stability, good durability and can withstand high pressure.
How can Nanoporous Carbon Membranes be Synthesized?
Nanoporous carbon membranes can be synthesized through hard and soft templating, pyrolysis of organic polymers, chemical vapour deposition (CVD), physical vapour deposition (PVD) and electrochemical methods.
Disordered microporous structures of carbon membranes containing a pore width of less than two nanometers were reported to be obtained through physical and chemical vapour deposition methods.
In template synthesis, use of hard templates like zeolite, mesoporous silica, colloidal particles of silica and metal oxide nanocrystals are reported. Soft templates reported are supramolecular aggregates of surfactant or block copolymers.
Pyrolysis of different organic polymers and resins such as polyimide, poly vinyl chloride (PVC), poly vinylidene fluoride (PVF), hyper-crosslinked polystyrenes, phenolic resins, etc. are used for the synthesis of porous carbon membranes.
The membranes of porous carbon are of two types: unsupported monoliths and supported membranes. Unsupported membranes have shown property of high selectivity but they lack in mechanical strength, which limits their utilization in various engineering applications.
These drawbacks were reported to be rectified by providing support such as porous graphite and sintered stainless steel. However, these supported nanoporous carbon membranes lacked ease of fabrication and reproducibility.
Nanoporous Carbon Membranes for Fuel Cells
Fuel cells, which convert chemical energy into electrical energy, are a promising technology compared to fossil fuel-based energy conversion techniques.
Proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are the two types of fuel cells that have received much attention due to their low operation temperature, rich fuel sources and almost non-existent environmental pollution.
Fuels such as hydrogen, alcohols and natural gas are potential sources of fuel for fuel cells.
PEMFC and DMFC systems consist of a cathode for oxygen reduction and an anode for hydrogen or methanol oxidation. It involves an electrocatalyst that accelerates the electrochemical reactions, in which platinum is considered the most efficient catalyst.
To attain high conversion efficiency with a lower amount of platinum, support is needed to load platinum. This support should have properties of high surface area, high electrical conductivity, electrochemical stability and high porosity for efficient mass transportation.
Porous carbon membranes possess these properties and are considered a promising support for catalysts. Activated carbon and carbon black as support have reported lowering of performances caused due to corrosion of carbon.
Nanoporous carbon membranes with high surface area, tuned pore size, designed porosity, multiple length scale and different compositions have been reported to study as catalyst support for fuel cells. Different strategies for preparing nanoporous carbon membranes have been reported to explore their use as electrodes.
Parameters of Porous Carbon Membrane Affecting Fuel Cell Performances
Many parameters of porous carbon membranes influence the performance of fuel cells. These include pore size, morphology and structure.
The electrolyte diffusion as well as catalytic activity can be tuned by the pore size of carbon membranes.
Nanocasting methods have been studied to adjust the pore size of nanoporous carbon in the range from micropore (< 2 nm) to macropore regions (> 50 nm) simply by selecting the suitable hard template.
In a report by Du et al. (2007), platinum nanoparticles were deposited on a series of carbon aerogel samples having different pore sizes and studied their electrocatalytic activity for oxygen reduction reaction (ORR). Pt nanoparticles deposited over the carbon aerogel containing the mean mesopore size of 18.5 nm exhibited enhanced ORR.
In work by Fang et al. (2009), hierarchical nanostructured carbon materials obtained by using a hard template of silica showed high Pt nanoparticle loading and was studied as cathode catalyst in PEMFC. This cathode catalyst showed extremely enhanced catalytic activity in ORR compared to a catalyst of Pt nanoparticles loaded carbon black.
Atwa et al. (2021) reported the synthesis of a nanoporous carbon scaffold with monodispersed pores size in the range of 5 to 100 nm in diameter and showed pore size tunability.
The membrane was formed through a hard template and had properties of high porosity, low tortuosity and exceptional physical/chemical stability. Atomic layer deposition (ALD) was used to have a uniform deposition of Pt nanoparticles over the membrane.
High catalytic activity of Pt loaded nanoporous carbon scaffold was obtained in 30 cycles of ALD. This performance was reported higher than two commercial Pt-carbon catalysts, a gas diffusion electrode and a catalyst coated membrane, all tested under similar conditions.
Architecture, length/orientation of pores and external morphology are a few other parameters that influence the performance of fuel cells.
Future of Nanoporous Carbon Membrane Fuel Cells:
Researchers have shown nanoporous carbon membranes to be excellent support for catalysts. In addition, they have other necessary properties required for the membranes, as many works have reported higher performance in nanoporous carbon membranes fuel cells compared to other carbon materials.
More research must be undertaken to develop simple and economical procedures for manufacturing these membranes to ensure their commercialization.
Continue reading: Nanoporous Metals in Energy Technologies: An Overview.
References and Further Reading:
Wang, H., Min, S., Ma, C., Liu, Z., Zhang, W., Wang, Q., Li, D., Li, Y., Turner, S., Han, Y. and Zhu, H. (2017) Synthesis of single-crystal-like nanoporous carbon membranes and their application in overall water splitting. Nature communications, 8(1), pp.1-9. Available at: https://doi.org/10.1038/ncomms13592.
Park, H.B. and Lee, Y.M. (2005) Fabrication and characterization of nanoporous carbon/silica membranes. Advanced Materials, 17(4), pp.477-483. Available at: https://doi.org/10.1002/adma.200400944
Tao, Y., Endo, M., Inagaki, M. and Kaneko, K. (2011) Recent progress in the synthesis and applications of nanoporous carbon films. Journal of Materials Chemistry, 21(2), pp.313-323. Available at: https://doi.org/10.1039/C0JM01830A.
Shiflett, M.B., Pedrick, J.F., McLean, S.R., Subramoney, S. and Foley, H.C. (2000) Characterization of supported nanoporous carbon membranes. Advanced Materials, 12(1), pp.21-25.Available at: https://doi.org/10.1002/(SICI)1521-4095(200001)12:1%3C21::AID-ADMA21%3E3.0.CO;2-P.
Tang, J., Liu, J., Torad, N.L., Kimura, T. and Yamauchi, Y. (2014) Tailored design of functional nanoporous carbon materials toward fuel cell applications. Nano Today, 9(3), pp.305-323. Available at: https://doi.org/10.1016/j.nantod.2014.05.003.
Du, H., Gan, L., Li, B., Wu, P., Qiu, Y., Kang, F., Fu, R. and Zeng, Y. (2007) Influences of mesopore size on oxygen reduction reaction catalysis of Pt/carbon aerogels. The Journal of Physical Chemistry C, 111(5), pp.2040-2043. Available at: https://doi.org/10.1021/jp066374h.
Fang, B., Kim, J.H., Kim, M. and Yu, J.S. (2009) Ordered hierarchical nanostructured carbon as a highly efficient cathode catalyst support in proton exchange membrane fuel cell. Chemistry of Materials, 21(5), pp.789-796. Available at: https://doi.org/10.1021/cm801467y.
Atwa, M., Li, X., Wang, Z., Dull, S., Xu, S., Tong, X., Tang, R., Nishihara, H., Prinz, F. and Birss, V. (2021) Scalable nanoporous carbon films allow line-of-sight 3D atomic layer deposition of Pt: towards a new generation catalyst layer for PEM fuel cells. Materials Horizons, 8(9), pp.2451-2462. Available at: https://doi.org/10.1039/D1MH00268F.
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