Multiwall carbon nanotubes tailored porous carbon fiber paper-based gas diffusion layer performance in polymer electrolyte membrane fuel cell
Introduction
Fuel cells are clean sources of renewable energy that can contribute to the solution of many current environmental problems through their high energy conversion efficiencies and low greenhouse gas emission compared with conventional internal combustion power generation [1,2]. There are several types of fuel cells have been developed, polymer electrolyte membrane fuel cell (PEMFC) and direct alcohol fuel cell are operating at ambient temperatures. In PEM fuel cell, membrane electrode assembly (MEA) is a polymer membrane flanked by two electrodes. The membrane acts as the ionic conductor between the two electrodes, i.e., an anode, where the fuel is oxidized and the cathode, where the oxidant is reduced. Usually, the electrodes are formed by a porous material with a coating of thin layer of electrocatalyst named Gas Diffusion Layer (GDL) [3,4]. The GDL is one of the critical components that comprise the macroporous substrate and a microporous layer (MPL). Also, GDL provides structural support to the catalyst, which is in the form of a powder. The main task of the GDL is to enable the distribution of reactant gases to the catalyst, provides the electrical connection between the catalyst layer and the current collector plate, and also transfers the heat generated by the catalyst away from MEA [[5], [6], [7]]. Generally; GDL is made from carbon cloth, carbon felt, and non-woven carbon fiber sheets, but non-woven sheet has been considered as a suitable macroporous substrate for the GDL. The non-woven porous carbon fiber sheet offers the desired value of permeability and electrical conductivity which make it an ideal electrode material for the fuel cell [[8], [9], [10], [11], [12]]. Furthermore, the in-situ and ex-situ properties of porous carbon fiber paper such as electronic conductivity, porosity, pore size distribution and hydrophobicity are important factors affecting the performance of fuel cell system [6,13].
On the other hand, reliable operation of a PEM fuel cell is also require proper water management to prevent water flooding in GDL, but still, it remains a challenge. Additionally, water management refers to the control of the water content inside the fuel cell [14,15]. The low content of water within the boundaries of the MEA causes the membrane to dry, consequently reducing an ion transport properties. On the other hand, an excessive amount of water mainly on the cathode side, known as cathode flooding, hinders the reaction on the catalyst surface [16]. Both high and low water content may produce the shutdown of the fuel cell system.
To enhanced liquid water management various efforts are made to develop novel carbon fiber substrates. Thomas et al. [17], reported a novel method for preparing hydrophobic GDLs via the electrochemical reduction of diazonium salts; it was prepared with homogenous hydrophobicity distributions in the absence of pore structure modifications. Nguyen et al. [18], introduced the direct fluorination of the GDL for providing homogenous hydrophobicity. Less liquid water content was observed in these novel GDLs compared to the conventional GDL. Forner-Cuenca et al. [19,20] was modified the carbon fiber GDL by wettability layer through local irradiation and subsequent grafting throughout the thickness. These water highways have improved functional performance due to optimized water management inside the cells. Chevalier et al. [21], developed novel hydrophobic electrospun fiber GDL (eGDL) which was fabricated via electrospinning and radiation grafting. It is reported that eGDLs with small fiber diameters (150–200 nm) and correspondingly smaller pore sizes, reduced liquid water accumulation under the flow field ribs. However, more liquid water is pinned onto the eGDL at the interface with flow field channels. The orienting of electrospun fiber alignment perpendicular to the flow field channel direction leads to improved eGDL-catalyst layer contact and prevents rib-channel membrane deformation. On the other hand, eGDLs facilitate significant membrane dry-out, even under highly humidified operating conditions at high current densities. Ko et al. [22] reported the development of a novel GDL through the dry deposition of hydrophobic silicone (i.e., hexamethyldisiloxane) nanolayer on carbon fibers by a dry deposition process. The GDL with the nanolayer exhibited an increased contact angle, decrease the contact angle hysteresis, and suppressed water condensation. It was found that optimum nanolayer thickness resulted in to much higher cell performance than the pristine GDL.
It is known that MPL is typically comprised of carbon black such as Vulcan XC 72 particles and hydrophobic agent polytetrafluoroethylene (PTFE). Moreover, to improve the water management of MPL by hydrophobic treatment such as PTFE produces water repellant properties to carbon fiber paper substrate. However, recent findings suggested that nano-carbon additive materials in the MPL can further enhance the performance of the fuel cell [23]. The MPL modified by adding carbon nanofibers and carbon nanotubes (CNTs) are exhibit the increase in porosity, pore volume, gas permeability and electrical conductance in the GDL, and higher power density as compared to conventional MPL based fuel cell [24,25]. However, recent findings suggest that carbon fiber based macroporous layer not being directly modified by nano additives such as multi-wall carbon nanotubes.
In our previous study, we have investigated the effect of natural graphite as a filler material in carbon fiber paper which enhances the power density up to 55% as compared to conventional carbon fiber paper [26]. However, hydrophobicity which can control the water flooding in the GDL cannot be solved by micron size natural graphite addition. It is well known that MWCNTs exhibits an excellent value of mechanical strength, electronic conductivity and it is hydrophobic in general unless it has been functionalized with polar moieties [27]. Therefore, in this work, we have modified the carbon fiber paper by incorporating carbon nanotubes by two approaches. In the first case, MWCNT's are added in different weight fraction in a matrix phase of carbon fiber composite paper and another case MWCNT's in-situ grown over the carbon fiber substrate by chemical vapor deposition technique (CVD) technique. The effect of MWCNTs inclusion in non-woven carbon fibers paper as GDL and its influence on the properties of PEM fuel cell was investigated.
Section snippets
Materials
Polyacrylonitrile based T-300 carbon fibers (Toray, Japan), Novolac type phenol-formaldehyde (Phenolic resin) obtained from M/s. Pheno organic Ltd., Delhi, The acrylic soluble emulsifier (ASE-60) and liquid ammonia were used for dispersing chopped carbon fibers in aqueous solution. The sodium dodecyl sulfate (SDS) was used as a surfactant for the dispersion of MWCNT [28], which was procured M/s. Scientific Industries, Delhi. The MWCNTs was synthesized in Laboratory by catalytic thermal chemical
Microstructure and contact angle
Fig. 1 illustrates the microstructure of pristine porous carbon fiber paper and MWCNTs decorated carbon fiber paper. Fig. 1(a) shows SEM image of non-woven carbon fiber paper in which the pristine carbon fibers randomly oriented in two directions. Fig. 1(b) shows MWCNTs loaded carbon fiber paper (1.0 wt%), MWCNTs were mixed with phenolic resin to form carbon composite paper. It is found that most of the MWCNTs-phenolic resin mixture coated on carbon fiber surface. As a result, the carbon fiber
Conclusion
In summary, it is found that incorporation of MWCNTs in non-woven carbon substrate as GDL have a positive effect on the fuel cell performance. The MWCNTs inclusion increases electrical conductivity, hydrophobicity, surface area, contact angle and optimal pore size distribution in GDL without compromising the mechanical properties. The electrical conductivity increases from 66 S/cm to 175 S/cm and also flexural modulus increases from 5 GPa to 20 GPa. The extent of increase in electrical
Acknowledgment
Authors are thankful to the Dr. D.K. Aswal,Director, CSIR-NPL and Head of Advanced Materials and Devices Metrology Division for their kind permission to publish the results. Authors would like Mr. Jai Tawale for providing the SEM characterization of carbon fiber composite porous paper.
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