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Novel architectures for microporous layers without binding agent Published on April 2011 by Catherine Lepiller, PhD, for Pragma Industries My two previous notes were focused on porous layers in low temperature fuel cells, i.e. the gas diffusion layer (GDL), the microporous layer (MPL) and the catalyst layer (CL), which make up a crucial part of the fuel cell core and are involved in water management. In these three components with graded porosity and pore size distribution, water exists in liquid and vapor forms. For high performance and durable operation, water must be efficiently transported from the cathode CL, where it has been produced by the oxygen reduction reaction, to the external gas circuit. After years of contradictory interpretations of experimental results, the different interactions between porous layers have been eventually – almost? – elucidated. Meanwhile, the beneficial role of the MPL has been evidenced with help of both improved models and decisive experiments (check previous notes on water extraction here and mechanisms in microporous layers here). Like other porous components of a fuel cell, the MPL is carbon-based due to high electronic conduction and chemical resistance requirements. Usual carbon sources include several types of carbon blacks (furnace, acetylene, etc.), which are all characterized by rounded nanoparticles in the ten to hundred nanometers range, a very porous structure and high surface area. Another requirement for the MPL is a high level of hydrophobicity to form a good capillary barrier to liquid water. Anti-flooding properties are typically obtained by mixing the carbon powder with a fluorinated polymer like PTFE, FEP or PVDF. In the conventional process, the hydrophobic polymer is introduced as a dispersion/solution and the resulting ink is deposited onto the gas diffusion media by a spraying or printing method. However, a number of studies have started suggesting a stability issue with those polymers during long term operation, and especially with PTFE, which is the current standard. PTFE’s structural integrity in the bulk MPL (and GDL) is challenged under fuel cell’s warm and humid conditions, leading to a gradual loss of hydrophobicity as operating time increases. With that in mind, I picked up in recent literature a few recent experimental studies considering the architecture of the cell core without polymeric binding agent. |
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After the seminal work of G.G. Park et al. [1] about the alternative use of carbon nanomaterials such as nanofibers and nanotubes as co-components with carbon black for replacement of PTFE in the MPL, Kannan et al. from the US Arizona State University [2] have further explored the idea and successfully grown in situ multi-wall carbon nanotubes (CNTs) as novel MPLs onto plain carbon paper substrate by an initial surface modification step followed by chemical vapor deposition (CVD) of a carbon source and Fe-based catalyst at 800°C. Unfortunately, the thickness of the carbon nanotube-based layer was not mentioned by the authors; but accounting for the surface deposition technique used, it was likely much thinner than standard sprayed or printed 20-100 µm-thick MPLs. Structural investigation clarified the composite nature of the MPL as an intimate mixture of entangled CNTs and oblong-shaped carbon nanoparticles homogeneously covering the carbon paper fibers and forming a porous but robust 3-D network without reduction of the initial GDL’s macroporosity in the 30 µm range (Figure 1). A complete characterization of the layers also demonstrated their intrinsic highly hydrophobic properties (the contact angle to water droplet was 145°) thereby ruling out the need of additional hydrophobic agent. When evaluated in a single fuel cell at 80°C with a standard Pt/C catalyst-coated Nafion? 212 membrane, these in situ grown CNT-based GDLs exhibited performances fairly insensitive to gas relative humidity conditions under oxygen and air feeding (Figure 2): the good resistance to water flooding was explained by the high hydrophobicity of the pristine CNTs and their uniform distribution onto the surface of the carbon fibers retaining large pore diameters, hence allowing quicker removal of product water.
Daniel Shua’s team at the National University of Singapore employed a similar Fe-catalyzed CVD technique at 750°C for synthesizing an in situ layer of carbon nanotubes on carbon paper from acetylene gas carbon source and further sputtered a pure platinum layer to make a Pt/CNT-based cathode [3]. They could achieve a dense carbon layer with nanotube diameters of 20-30 nm characterized by strong coiling and twisting of the curled nanotubes together. Consistently with [2], the 3-4 µm-thick nano carbon layer exhibited high surface roughness and porosity. It was also composed of a mixture of crystalline multi-wall carbon nanotubes and amorphous carbon, which was mostly located close to the surface. Interestingly, the disordered surface of the in situ as-grown CNTs is thought to provide a stable interface between the carbon nanotubes and the pure Pt particles due to their pristine high oxidation level. This permitted direct sputter deposition without additional surface oxidation step. The surface roughness of the layer could give rise to a scaly Pt layer with homogeneously dispersed nanoparticles 2-3 nm in diameter instead of the continuous layer usually achieved by the sputtering method (Figure 3). A gas diffusion cathode with Pt loading controlled at 40 µg/cm² fabricated by this combined technique was implemented in a Nafion? 112-based fuel cell with a standard anode and run in hydrogen/oxygen configuration with fully humidified gases at 80°C and 2 atm pressure. Tang et al. did not vary the fuel cell testing conditions as in [1,2] but rather looked at catalyst and carbon support corrosion by in situ accelerated tests [3]: they found that compared with the conventional carbon black-based electrode, the Pt/CNT/carbon paper-based electrode shows a significant improvement in electrochemical activity and stability. The initial activity before oxidation was significant unlike Pt/carbon black electrode, and after 100 oxidation cycles, the activity loss was much less pronounced (Figure 4), which strongly suggests that the CNT support is more corrosion resistant than standard carbon black. The corrosion current measured under cyclic step potential between 0.6 and 1.8 V was also much lower, with a main part that may be readily attributed to the fraction of amorphous carbon at the surface. The most recent study published by the Singaporean team deals more specifically with the influence of CVD synthesis conditions on the microstructure of the CNT layers [4]. They show that sufficiently high acetylene gas flow rates during CNT growth can provide layers at a weight ratio of 1-1.5% on carbon paper with both high density and porosity due to enhanced lateral growth, thus yielding refined gas diffusion channels and larger surface area for Pt deposition. Fuel cell tests performed under identical conditions as in [3] exhibit a significant improvement in the whole current density range for the integrated Pt-sputtered/in situ grown CNT cathode-based MEA compared to those of the Pt-sputtered/carbon black and Pt-sputtered/commercial CNT cathode-based MEAs prepared by the conventional ink fabrication process. These results further demonstrate the better catalyst utilization and mass transport properties achieved by this novel dense while porous morphology allowing larger penetration depth of sputtered Pt nanoparticles. Above studies based on the chemical vapor deposition method, are quite convincing about the relevance of replacing carbon black by carbon nanotubes as integrated catalyst support/microporous layer. Nevertheless, an important question left to be raised is about cost and viable mass production for such a high temperature process. Some scientists have followed this pragmatic rationale and considered other fabrication techniques for carbon nanomaterial layers. For example, a feasibility study was conducted at the Dongshua University in China [5]. It was related to the development of carbon nanofibers using a combination of electrospinning of polyacrylonitrile (PAN) solution under high voltage and subsequent thermal treatments. Electrospinning is described as a versatile technique to process solutions or melts into a 3-D web of fibers with diameters in the nanometer to micrometer range, and stated as being both easy and cheap to implement at the industrial scale, which is undoubtedly a good starting point! |
| Figure 1. SEM images of GDL surface fabricated by using in situ CVD on carbon paper at: (a) low and (b) high magnifications. | |
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| Figure 2. Fuel cell polarization data at 80°C using H2/air at various RH conditions. (a) For cathode using Pt deposited on the in situ CNT/carbon paper GDL. (b) For cathode using Pt deposited on the standard wire-rod-coated GDL. | |
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| Figure 3. High Resolution TEM image showing Pt nanoparticles on a carbon nanotube tip. | |
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| Figure 4. Cyclic voltammograms of Pt/CNT catalysts before and after 10 and 100 oxidation cycles between 0.1 and 1.2 V. | |
| According to the process, carbon nanofibers 400-700 nm in diameter were produced as self-standing sheets after stabilisation at 250°C in air and carbonisation in nitrogen at 900°C of the room temperature as-spun PAN nanofiber sheets. These carbon nanofiber sheets (CNFSs) were then directly transferred onto plain carbon fiber paper by hot-pressing to serve as MPL with a uniform thickness of about 25 µm. Like previous CNT-based MPLs grown by chemical vapor deposition [2-4], the present nonwoven CNFSs have a porous morphology with open 3-D interconnected structure that clearly enhances reactant gases transport. Gas permeability of the GDL having a nanofiber sheet-based MPL is about twice that of a GDL with a conventional MPL made from slurry of carbon black powder and FEP suspension, in agreement with Park’s team results [1]. CNFSs are also highly hydrophobic with an average water contact angle of 136°, which allows using them as single component of an MPL. Fuel cell performances were also tested at 70°C under 100% RH hydrogen and air with the same type of Nafion? 212-based catalyst coated membrane as in [2] including a CNFS layer at the cathode side: a promising 23% power improvement was achieved as compared to the conventional structure. Yet, what is absent there is some durability testing to check the good initial behaviour of the carbon nanofiber sheets as operation goes on…
All previous results are clearly consistent about the great potential of using carbon nanotubes/nanofibers as free-standing MPLs without hydrophobic binding agent, and even as integrated catalyst and diffusion layer for low temperature fuel cell applications. These new micro-layer architectures are intrinsically hydrophobic and do not require mixing with a polymer like PTFE to achieve anti-flooding properties. They form also both highly porous and mechanically robust structures thanks to the three-dimensional network of nanotubes/nanofibers. Finally, despite high porosity, electric conductivity is comparable to standard MPLs with reported values about 60 S/cm [5]. Such favorable characteristics should push further the questioning of current hydrophobic polymer-based microporous layers and their insufficient reliability. As already noted above, an important point still to be addressed by ongoing research is about the proper fabrication method of these materials that would be compatible with future mass production and commercialization. [1] G.G. Park, Y.-J. Sun, S.-D. Yim, T.H. Yang, Y.-G. Yoon, W.-Y. Lee, K. Eguchi and C.-S. Kim, J. Power Sources, 163 (2006), 113. |
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