A new method for optimal fabrication of carbon composite paper as gas diffusion layer used in proton exchange membrane of fuel cells

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Abstract

Carbon Papers (CPs) have been widely used as a Gas Diffusion Layer (GDL) for high performance fuel cells. Herein we report a novel method for production of GDL, without the need to carbonization and graphitization steps that is common steps in GDL production. CP is provided by a dry-laying of carbon fibers (CFs) and expanded graphite (EG) in the phenolic resin and the composites are compared with carbon paper of Toray Co., Ltd. The effect of paper thickness, aspect ratio of CF and EG value of the composite are investigated. The characterizations are performed by scanning electron microscope, maximum pore size, mean pore size, permeability, electrical conductivity, flexibility, and performance (I-V) curve. The results shown that in the optimized state of the composition in the manufactured composite, the values of mean pore size, permeability, electrical conductivity, and performance curve is reasonable and near the Toray paper, however the manufactured composite show a much higher flexibility than that of Toray paper in a qualitative test. In addition, due to removing the graphitization and carbonization steps for production of carbon paper, the produced carbon paper costs are much lower than Toray paper.

Introduction

In recent years, various researches have been done to utilize various energy sources such as sunlight, wind, fossil fuels, etc. in engineering applications [[1], [2], [3]]. The fuel cell is a power generation module that converts the chemical energy stored in the fuels and oxidants directly into power using a non-burning electrochemical reaction [4]. Among the various fuel cells, the proton exchange membrane fuel cell (PEMFC) possesses advantages such as non-polluting, non-erosion, high power, high energy conversion efficiency, fast activation and low working temperature. It is suitable for use as an energy source and power supply for electrical vehicles especially portable fields [[5], [6], [7], [8]].

A gas diffusion layer is an important component of the PEMFC. Being the path for the two-phase flow of fuel, air, and water, the gas diffusion layer is a critical component. The functions of this layer is that it supports the catalyst layer and provides important mass transfer channels for passing the reactant gases and is considered as an escape route for produced water in catalyst layer and conduct this water vapor or liquid into the bipolar plate channels [9,10].

Up to now, gas diffusion layer has been made of polymer based carbon fiber composites (carbon paper) [5,[11], [12], [13], [14], [15], [16]], carbon cloth [17], and metallic foams [18,19]. Most popular types of GDL is carbon paper containing polymer (such as phenolic resin) [20,21]. The aim of this research is fabrication of a new carbon paper containing a low price, high mechanical properties, and high performance. The performance of the PEMFC is strongly dependent to the size and distribution of porosity and the electrical conductivity of the carbon paper [22]. However, the weight of the above parameters still is a challenge between researchers. In this research the effect of these parameters will be investigated on the performance curve of PEMFC.

In the performance curve of PEMFC, there are three types of polarizations that decrease the performance. There are three major classifications of losses that result in a drop from the open circuit voltage: 1) activation polarization, 2) ohmic polarization, and 3) concentration polarization. The operating voltage of a fuel cell can be represented as the departure from the ideal voltage caused by these polarizations [[22], [23], [24]]:Vcell=Eηa,aηa,cηrηm,aηm,cwhere E is the open circuit potential of the cell, ηa, ηr and ηm represent activation, ohmic resistance and mass concentration polarization, respectively.

The Activation polarization, which dominates losses at low current density, is the voltage over potential required to overcome the activation energy of the electrochemical reaction on the catalytic surface, and is thus heuristically similar to the activation energy of purely chemical reactions [25].

At very high current densities, mass transport limitation of fuel or oxidizer to the corresponding electrode causes a sharp decline in the output voltage. This is referred to concentration polarization. This region of the polarization curve is related to facilitating species transport to the electrode surface can result in greatly improved performance at high current density and fuel utilization conditions [25,26].

ηr is the area-specific resistance of individual cell components, including the ionic resistance of the electrolyte, and the electric resistance of bipolar plates, cell interconnects, contact resistance between mating parts and any other cell components through which electrons flow. With proper cell design, ohmic polarization is typically dominated by electrolyte conductivity. Electrolyte conductivity is primarily a function of moisture content and temperature in PEM fuel cells [27].

The properties of GDL can be related to two types of polarizations. The electrical conductivity of GDL is related to ohmic polarization and porosity size and porosity distribution is related to concentration polarization. It seems that the decrease of pore size can lead to increasing contact points between GDL and bipolar plates, thereby decrease ohmic resistance and ohmic polarization. In other words, decreasing pore size can lead to decreasing concentration polarization. Because, by increasing the pore diameter, the passage path of reactant gases and produced water in cathode catalyst layer increases. The increasing pore size can inhibit the flooding phenomenon. Therefore, it is yet a challenge between researchers that the pore size of GDL leads to increasing performance or not. The ohmic polarization is dominated on medium current densities of operation and concentration polarization is attributed to the high current densities. However, it should be emphasized that the decreasing ohmic polarization is a more important issue than concentration polarization because, the current density domain of ohmic resistance is much wider [[8], [9], [10],16,21,22].

The aim of this article is investigation on this fact that which one of increasing porosity size and electrical conductivity of GDL can have a more influential on the improvement of the I-V or performance curve. In addition, in this research the production process is selected so that does not need to any high-cost process such as carbonization or graphitization. Moreover, the phenolic resin was mixed with carbon fiber in a constant mass ratio. In order to change porosity sizes and electrical conductivity of composite the expanded graphite was added into the composition.

Section snippets

Materials

The phenolic resin used in this research was novalac in powder form with 60 μm size and was purchased from Resitan Co., Ltd. The electrical conductivity of phenolic resin is ≈10−15 S/cm [28]. The expandable graphite and CF (T300) were purchased from Qingdao Yanxin Graphite and Toray Co., Ltd., respectively. In order to prepare the expanded graphite from the expandable graphite, the graphite was placed in furnace at 1000 °C for 2 min to expand up to 120 times. The Gas diffusion layer was

SEM

Fig. 3 shows the microstructures of sample 4. The composite has polymer as matrix and CF and EG as filler. It can be seen that the polymer has high wettability with carbon fibers, however the value of polymer has been decreased to make a high value of porosity and increase electrical conductivity. In fact, the innovation of this research was that by lowering the percentage of polymer, we create the pores for gas passage via the GDL. In this method in comparison to the common method in Toray

Conclusions

In this research a new method used for producing GDL papers without the need to carbonization and graphitization steps. The results are as follows:

  • 1-

    Unexpectedly, the aspect ratio has little effect on electrical conductivity of carbon paper. This would be related to high filler loading of CF and getting away from percolation threshold.

  • 2-

    The expanded graphite can increase the electrical conductivity of paper and as well as control porosity sizes and permeability, but the excess value of that can

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