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Prospects and challenges of PEM fuel cells
Prospects of PEM fuel cell generally relies upon the consistency with which it can perform. Now, that takes into account the overall efficiency of the system. PEM fuel cells, in its course of evolution, if commercialized completely, can prove to be an excellent alternative for commercial vehicles along with battery-powered vehicles. Similarly, stand-alone systems can make a way, by replacing diesel-powered generation sets for domestic applications. The development of the technology, along with creating a robust infrastructure with refueling stations, hydrogen storage, transportation, could well be an opportunity starting from the present and leading to the future, to offset conventional systems. For instance, ‘Fuel Cells and Hydrogen Joint Undertaking-EU’ has mentioned one of the pivotal aims is addressing emissions, as it is projected to have the highest growth rate in the automotive fuel cell market causing energy transition and account for 24% of final demand by 2050 in the EU [3-7,15].
One of the major challenges with fuel cells is improving its energy efficiency which currently is estimated to be around (40-60%), at the cell level i.e. feed with pure H2 and obtain a direct current, from the general estimation, made using the value of Gibbs free enthalpy (ΔG) and the heating vale or enthalpy (ΔH): the maximum or ideal efficiency is the ratio stated by the Eqn 1.2 Mench 2008 : 0= (1.2).
However, the performance is known to be affected from the start of the operation, and further during the stretch of the operational period the performance declines. The challenges concerning the PEM fuel cell can be classified into 2 categories ;
Challenges related to the technology; The losses in a typical PEM fuel cell are described under the following categories Figure e ;
(a) Activation; where a certain proportion of energy is needed to allow the occurrence of electrochemical reaction. This phenomenon produces a non-linear voltage drop called activation polarization. These losses occur on both anode and cathode catalysts, though anode overpotentials are usually very low with hydrogen as fuel.
(b) Ohmic; The resistance to (i) the flow of electrons through the electrically conductive fuel cell components and (ii) to the flow of ions through the membrane causes a voltage drop, which can be expressed as Ohmic drop.
(c) Mass transport; The consumption of reactant gases at the catalyst layers leads to concentration gradients and thus changes the partial pressure of the reactants, which affects the fuel cell voltage.
The above 3 regions of cell performance are valid for any operating range of applied current Figure e. The fuel cells are prone to severe degradations of its parts like MEA and its related components. Degradations are largely caused in its due course of operational hours, depending on the operating conditions like pressure, temperature, flow rates, and humidity of reactants to name a few. Wearing out and weakening of parts is normally observed throughout the operation .
Microporous layer (MPL)
To improve the performance of the fuel cell dealing with water flooding issues, the MPL is added to the GDL to increase the functionality and durability of the layer. The MPL can be well described as either a simple composite of carbon powder and a hydrophobic agent (PTFE) or a combination of C particles, graphitized resins, and PTFE, most of them with a tailored hydrophobicity depending on the targeted application. MPL is a layer but is not distinct in nature (sprayed or coated). The MPL comprises minimal carbon powder and hydrophobic agent agglomerates, with pore sizes in several orders of magnitude lower than the substrate (2-200 nm) whose surface is shown in Figure 1.2 d. Generally, MPL is manufactured by normally following three steps; (i) MPL slurry preparation, (ii) MPL deposition, and (iii) sintering [27- 30] .
In the first two stages, the carbon or graphite particles are mixed mechanically with hydrophobic agents dispersed in water or sometimes organic additives, and this slurry is deposited onto the MPS surface. The deposition technique may vary consisting of either; brushing, blading, spraying, or dipping. However, recently after having understood the several advantages of the MPL, this layer is prepared separately in some facilities which are further assembled onto the existing substrate [18,27]. However, the former technique has shown better adaptability compared to the latter one. Post completion of the MPL deposition, the entire GDL sheet is subjected to heat-treatment, normally at 240-250°C, for around 40-50 minutes under an inert atmosphere. This is done to remove any unnecessary moisture. The fully dried out GDL is now sintered at a higher temperature (350-400°C), for 40-60 minutes, to ensure that the hydrophobic agent is dispersed completely throughout the MPL [18,27].
The MPL provides the GDLs with several advantages over the non-layered ones ; (i) restricted but uniform gas distribution to the catalyst layer, (ii) improved interfacial interactions and mechanical compatibility between the catalyst layer and the substrate, (iii) reduced water accumulation by having strong hydrophobicity [18,25-30]. Though MPL gives advantages, it also generates an additional layer ‘intersection’ or ‘penetration’ layer which can often be complex due to the deployment technique.
Intersection region between MPS and MPL
The smaller agglomerates of the MPL can easily occupy the larger pores on offer from the MPS, leading to the formation of the intermediate layer. This surface penetration is usually caused by (i) specification of the carbon powder, (ii) presence of hydrophobic agent, (iii) deposition of MPL slurry, and (iv) manufacturing process. This mixing causes, the pore size distribution to significantly get compromised (lowering), and thus may pose a greater resistance to the flow of gas. No definite range of pore size and thickness of this intersection is known. However, often by using cross-sectional images using SEM, attempts have been made to at least identify the region.
Hydrophobic and hydrophilic treatments in GDLs
The GDLs are finely treated with a certain amount of hydrophobicity to strike a balance between water retainment (hydrophilicity) and water evacuation. Various hydrophobic agents are known to be used in the GDLs.
Agents such as (Fluorinated) perfluoropolyether (PFPE), perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene that is (PTFE) have been used as reported by [31-34]. Though hydrophobic agents used are not only F- contained, graphitized carbon materials (by treatment above 1000°C) are hydrophobic. The transfer of protons, through the ionomeric membrane, depends to a large extent on the level of humidification. Primarily the proton transfer works adequately in the presence of enough water. On the contrary, excess water should be removed to allow effective gas transport and would minimize the ohmic and concentration losses and helping in better cell performance. The hydrophobicity is imparted to the GDL using spraying, brushing, dipping, or floating. This treatment is generally followed by drying and then is subjected to high-temperature sintering (350°C) for uniform distribution along the surface. Such a process may necessarily impact the morphological parameters of the GDL .
SGL nomenclature for the GDLs manufactured
SGL Germany designates GDLs by nomenclature given here and is shown in Figure 1.3 The –AA Grades; The AA grades are the ones that are not having PTFE treatment nor MPL.- AA grades can be categorized into older grades such as 24 AA, and the latest groups such as 28, 29, and 39 AA. The general thickness of the –AA grades falls between 190-200 µm with high porosity range (82-88%).
The – BA Grades; The BA grades are also without the MPL, identified as i.e. 24 BA and 34 BA. These 2 GDLs are loaded with 5% PTFE by weight. The overall thickness of the AA grade is similar to the BA grade for the 24 series at 190µm, 34 BA has a higher thickness at 270 µm, compared to 24 BA, the –BA has porosity (75-81%), a little lower than –AA grades due to PTFE deposition. The -BC Grades; Lately, a majority of the fuel cell applications use GDLs with the MPL. The overall porosity for the –BC grades (global value-covering the presence of 2 layers) is known to vary between 69-81%, with the older grades having lower values between 69-75%, the recent development has drastically improved the porosity levels like that in 38, 39, 28 and 29 BC ranging between 76-81%. The particular technique followed in improving the grade is not known commercially, but changes in surface morphology were observed through SEM images (Images not shown here).
Structural strength of GDLs and clamping action on the cell assembly
Gradually over the last few years, an increase in the investigations of the structural strength of the GDLs are undertaken. In a fuel cell assembly, generally, the clamping force applied on the fixing bolts to the point when it ensures there was no leakage of gas. However, following some consequences concerning strong clamping action like breaking of plates, deformities in the MEA, improvements in this are still underway to ascertain optimum applied torque. During fuel cell installation, the GDLs are stacked on either side of the catalyst – membrane layer seals included, completing the MEA. While assembling, the bipolar plates are usually clamped against the MEA, with the GDL-MPS surfaces exposed to the channels (ribs) directly. Understandably, the porous layers of the GDL which are sensitive to the compression, often suffer deformity. Hence it is essential to know the extent of (i) overall cell assembly pressure and (ii) the clamping pressure acting on the GDLs [59-63].
In the assembly, the bolts are clamped from one side, at a designated torque often specified by the manufacturer. Therefore, the overall clamping force Ftotal can be related to the torque applied Tapplied taking into account the bolt diameter (Db), friction factor (Kff) of the bolt, and the number of bolts (N) given by the common equation stated by Mortensen 2013  as Eqn 1.9: 28 = . ( ) . ( 1 ) (1.9).
Depending uponthe type of cell used in particular the bipolar plate area the clamping pressure exerted can be estimated based upon Ftotal and the given bipolar plate area BPArea, where Ptotal is the total pressure exerted in MPa giving Eqn 1.10 : = ( ) (1.10).
The wetting system: for permeability in presence of water
One of the other influential factors that affect gas transport in GDLs is the presence of liquid water. An attempt to measure permeability in the GDLs with the presence of liquid water was proposed. So for that, a wetting system, diagrammed in Figure 2.5, was used to force water flow through a GDL sample to wet the GDL significantly.
The wetting system in Figure 2.5 a, constructed from a commercial filtration system, consisted of a 300 mL upper tank and an empty 300 mL lower tank which was connected to a ‘vacuum pump ME 2C NT Vacuumbrand’ that drew suction and water was dripped through. In between the 2 tanks, a 40 mm diameter GDL sample was placed on a perforated tray pictured in Figure 2.5 b. For GDL samples with a microporous layer, the MPL was placed face down on the tray. When a vacuum was drawn, the water in the top tank would pass through the GDL, allowing water to fill in the void space of the GDL. To ensure the GDL would not be dried after water was passed through, around 50 mL is left to rest in the top chamber above the GDL sample, ensuring that the GDL retains water. The entire suction process would take 10-15 seconds depending on the type of GDL. The wetted GDLs were then transferred to the fundamental cell for directional gas transport measurements as given in Section 2.2.2.
The real bi-polar plates of 25 cm2
Three differently designed 25 cm2 bipolar plates for fuel cells (Electrochem) were deployed for estimating the equivalent permeability in the GDLs stacked between the bipolar plates: this permeability does not refer exactly to through-plane or in-plane transport but can be considered as an equivalent property of the GDL in such conditions.
The bipolar plates were made from graphite materials and are partly resin-filled. (Figure 2.6 a-c): The first cell has a single serpentine flow channel (1.3 mm wide and 1 mm deep) with a total length of 102.9 cm and 20 bends.
The second cell with square ribs (144 ribs) with a dimension of 1.66 x 1.66 mm2 is engraved out of a 25 cm2 surface area.
The third cell with parallel channels (0.6 mm wide, 0.6 mm deep) was referred to as a multi-serpentine flow channel. In this cell, there are 7 clusters with each cluster allocated with 5 channels. Two different rib patterns exist, rib type (a) 4.3 cm length present with 28 in numbers and rib type (b) 4.4 cm length with 6 in numbers.
From the plates mentioned above, each plate consisted of 2 ports. For gas transport measurement purposes, one of the ports on either of the plates was closed (Figure 2.6 d), thus allowing gas to flow from one inlet port in the first chamber then be forcefully transported across the GDL placed in between, and exit from the unlocked port in the second chamber.
Table of contents :
A. Preface to the introduction
B. General context and global energy scenario
(i) The context of the energy transition
(ii) The global emission rates
(iii) The shift to renewable energy sources
C. Opportunity for hydrogen and the emergence of fuel cells
(i) Recognition of hydrogen
(ii) Emergence of the fuel cells
D. About proton exchange membrane fuel cells (PEMFCs) and the technology
(i) The PEM Fuel cell
(ii) Components of PEM fuel cells
(iii) Prospects and challenges of PEM fuel cells
E. Identifying the scope of the work and objectives
(i) The objectives identified are explained here as per their significance.
(ii) The layout of the thesis
1.1 The gas diffusion layer (GDLs)
1.1.1 The role of GDLs
1.1.2 GDL Manufacturing
1.1.3 Macro porous substrate (MPS)
1.1.4 Microporous layer (MPL)
1.1.5 Hydrophobic and hydrophilic treatments in GDLs
1.1.6 SGL nomenclature for the GDLs manufactured
1.2 Characteristics of GDLs
1.2.1 Porosity of GDLs
1.2.2 Gas permeability in GDLs
1.2.3 Anisotropy and tortuosity in GDLs
1.2.4 Structural strength of GDLs and clamping action on the cell assembly
1.2.5 Degradations in GDLs
1.3 Synthesis of few selected works in gas transport measurement and compression in GDLs
1.4 Conclusion to the chapter
2.1 Preface to the chapter
2.2 Measurement devices and experimental benches
2.2.1 The measurement ‘fundamental’ cell
2.2.2 The diffusion bridge measurement bench
2.2.3 Alterations in the cell
2.2.4 The developments made in the original bench
2.2.5 The wetting system: for permeability in presence of water
2.2.6 The real bi-polar plates of 25 cm2
2.2.7 The real bi-polar plates of 100 cm2
2.2.8 The fuel cell performance measurement bench
2.3 The GDLs used
2.4 Characterisation techniques and computer-based analysis
2.4.1 Chronopotentiometry (CP)
2.4.2 Scanning Electron Microscopy (SEM) for surface imaging
2.4.3 Mercury Intrusion Porosimeter (MIP)
2.4.4 Autodesk Interface
2.5 Data analysis approach
2.5.1 Lab-based study of GDL properties
2.5.2 Gas flow velocity in the cells used
2.5.3 Permeability estimations
2.6 Conclusion to the chapter
3.1 Preface to the chapter
3.2 The results obtained using the fundamental cell
3.2.1 Working conditions
3.2.2 Through-plane (TP) permeability estimation
3.2.3 In-plane (IP) permeability estimation
3.2.4 Permeability estimation for the MPL
3.3 The permeability results obtained using the real bi-polar plates
3.3.1 Working conditions
3.3.2 The equivalent permeability in the real bipolar plates (BPs)
3.3.3 The significance of the inertial flow and comparison between the cell patterns used 8
3.3.4 Comparison between 25 cm2 BPs with the fundamental cell
3.3.5 Comparison of equivalent permeability between 25 cm2 multiple channels and 100 cm2 parallel flow channel.
3.4 Conclusion to the chapter
4.1 Preface to the chapter
4.2 Permeability estimations using mixed dry and humidified gases
4.2.1 Working conditions for mixed dry gases
4.2.2 Directional and equivalent permeability estimations using mixed dry gases in the fundamental and 25 cm2 multi-channel cell.
4.2.3 Equivalent permeability in 25cm2 multi-channel BP with mixed gases
4.2.4 On the values for mixed gases in general
4.2.5 The validation of the pressure drop trend by imposing usage of different gases .
4.2.6 Working conditions for humidified gases
4.2.7 Through-plane and in-plane permeability estimations using humidified gases .
4.3 Permeability estimations in the presence of liquid water
4.3.1 Working conditions
4.3.2 Measuring the GDL capacities
4.3.3 Through-plane and in-plane permeability in the presence of liquid water
4.3.4 Through-plane permeability by forcing water through the MPL side of GDL
4.3.5 Permeability estimations with condensation approach
4.3.6 Comparison between wet and dry GDLs: tortuosity
4.4 Conclusion to the study
5.1 Preface to the chapter
5.2 Gas transport in GDLs under selected applied torque levels
5.2.1 The working conditions
5.2.2 Equivalent permeability values at selected applied torque levels
5.3 Electrochemical performance of GDLs under compression
5.3.1 Working conditions
5.3.2 Performance curves
5.4 Physical changes in the GDL morphology
5.4.1 Thickness and surface morphology
5.4.2 Effect on the pore size distribution
5.5 Compression analysis of GDL
5.5.1 Young Modulus (E) for GDL
5.5.2 For the simulations
5.5.3 The results for the different simulations on Autodesk Inventor
5.6 Conclusion to the chapter