11 Fuel Cell Materials
11.1 Electrolyte Layer
The electrolyte layer is essential for a fuel cell to work properly. In low temperature fuel cells, when the fuel in the fuel cell travels to the cat- alyst layer, the fuel molecule gets broken into protons (H) and electrons. The electrons travel to the external circuit to power the load, and the hydrogen proton (ions) travel through the electrolyte until it reaches the cathode to combine with oxygen to form water. In high temperature and alkaline fuel cells, the oxygen reacts at the cathode to produce either hydroxide (OH), a carbonate ion (CO3
2), or an oxygen ion (O2).
The ion travels through the electrolyte to react with hydrogen at the cathode. Depending upon the fuel cell type, the electrons are produced
Flow in Flow out Bipolar Plate Electricity Electrolyte H+ H+ H+ e− e− e− e− e− e− e− e−
e− e− e− e− e− e− e− Catalyst Layer Catalyst Layer Gas Diffusion Layer Backing Layer Heat Water Hydrogen O2− O2− Nafion 117 CF2 = CFOCF2CFOCF2CF2SO3H| CF3 Catalyst/ Diffusion Layer Single Cell Fuel Cell
Liquid molten Proton exchange Direct methanol Solid oxide Alkaline Phosphoric carbonate membrane fuel cell fuel cell fuel cell fuel cell acid fuel cell fuel cell Fuel cell/component (PEMFC) (DMFC) (SOFC) (AFC) (PAFC) (MCFC) Most Common Electrolyte Perflourosulfonic Perflourosulfonic Yttria stabilized Potassium Liquid phosphoric Liquid molten
acid membrane acid membrane zirconia (YSZ) hydroxide acid carbonate (Nafion by DuPont) (Nafion by DuPont) (8 mol% Y)
Electrolyte Thickness ~50 to 175 m ~50 to 175 m ~25 to 250 m N/A N/A 0.5 to 1 mm Ion Transferred H H O2 OH H CO32
Most Common Anode Pt Pt/Ru Nickel/YSZ Pt/Pa Pt Nickel Catalyst
Anode Catalyst Layer ~10 to 30 m ~10 to 30 m ~25 to 150 m N/A ~10 to 30m 0.20 to 1.5 mm Thickness
at either the cathode or anode. Regardless of the fuel cell type, the elec- trolyte must meet the following requirements:
■ High ionic conductivity
■ Present an adequate barrier to the reactants
■ Be chemically and mechanically stable
■ Low electronic conductivity
■ Ease of manufacturability/availability
■ Preferably low-cost
Finding a material that meets all of these requirements is tough. The toughest requirements are high ionic conductivity, and a material that is stable in both an oxidizing and reducing environment.
11.1.1 PEMFCs and DMFCs
The standard electrolyte material presently used in low-temperature fuel cells is a fully fluorinated Teflon-based material (perfluorosulfonic acid [PFSA]), produced by DuPont for space applications in the 1960s. This membrane is a PTFE-based structure, and is relatively strong and stable in both oxidative and reductive environments, and has high pro- tonic conductivity (0.2 S/cm) at typical PEMFC and DMFC operating temperatures. Figure 11-2 illustrates the chemical structure [21-23].
The DuPont electrolytes have the generic brand name Nafion, and the specific type used most often is number 117. The Nafion membranes, are stable against chemical attack in strong bases, strong oxidizing and reducing acids, H2O2, Cl2, H2, and O2 at temperatures up to 125C.
Similar materials have been developed for PEMFC and DMFC by Dupont, Gore and Associates, Asahi Glass, Asahi Chemical, and Pall, as illustrated in Table 11-2 [18].
The proton-conducting membrane usually consists of a PTFE-based polymer backbone, to which sulfonic acid groups are attached. The proton conducting membrane works well for fuel cell applications because the Hjumps from SO3site to SO3site throughout the mate-
rial. The Hemerges on the other side of the membrane. The membrane must remain hydrated to be proton-conductive. This limits the operat- ing temperature of PEM fuel cells to under the boiling point of water and makes water management a key issue in PEM fuel cell development. Figure 11-3 illustrates the SO3sites in the Nafion membrane.
CF2 = CFOCF2CFOCF2CF2SO3H
| CF3
Figure 11-2 The chemical struc- ture of Nafion.
TABLE11-2 Properties of Commercial Ion-Exchange Membranes [Adapted from 18] Conductivity Membrane IEC Thickness (S/cm) @ 30C Membrane chemistry (mequiv/g) (mm) and 100% RH Asahi Chemical K-101 Sulfonated 1.4 0.24 0.0114
polyarylene
Asahi Glass CMV Sulfonated 2.4 0.15 0.0051
polyarylene
Asahi Glass DMV Sulfonated — 0.15 0.0071
polyarylene
DuPont Nafion–117 Perfluorinated 0.9 0.2 0.0133 DuPont Nafion–901 Perfluorinated 1.1 0.4 0.01053
Ionac 61AZL386 — 2.3 0.5 0.0081
Ionac 61CZL386 — 2.7 0.6 0.0067
Pall RAI R-1010 Perfluorinated 1.2 0.1 0.0333
SO3− SO3− SO3− SO3− SO3− SO3−
Nafion membranes come in various thicknesses, and can be cut to any
size. Nafion membranes are available in 25.4 m (Nafion NRE-211),
50.8 m (Nafion NRE-212), 127 m (Nafion 115), 183 m (Nafion 117) and 254 m (Nafion NE-1110). It is a clear membrane that has to be carefully handled to avoid tears or defects. Figure 11-4 shows a PEM fuel cell with a Nafion membrane.
PFSA membranes, such as Nafion, have a low cell resistance (0.05 Q cm2) for a 100 m thick membrane with a voltage loss of only 50 mV at 1 A/cm2 [22,23]. There are also several disadvantages of PFSA membranes, such as material cost, supporting structure requirements, and temperature- related limitations. The plant components required to keep a PFSA membrane hydrated also adds considerable cost and complexity to the fuel cell system. The fuel cell efficiency increases at higher temperatures, but issues with the membrane, such as membrane dehydration, reduc- tion of ionic conductivity, decreased affinity for water, loss of mechani- cal strength via softening of the polymer backbone, and increased parasitic losses through high fuel permeation become worse. PFSA membranes must be kept hydrated to retain proton conductivity, but the operating temperature must be kept below the boiling point of water. The largest challenge in finding a replacement for PFSA membranes is low-cost materials.
Hydrocarbon alternative membranes may provide some advantages over PFSA membranes, such as cost, commercial availability, and high water uptakes over a wide temperature range, with the absorbed water restricted to the polar groups of polymer chains. Five main cate- gories of membranes are currently being researched: (1) perfluorinated, (2) partially fluorinated, (3) non-fluorinated (including hydrocarbon), (4) non-fluorinated (including hydrocarbon) composite, and (5) others. There is a wide range of material properties between the membranes
Figure 11-4 PEM fuel cell with Nafion 117 proton electrolyte layer and electrode. Clear Nafion 117 proton
electrolyte layer Catalyst/gas diffusion layer (electrode) Complete PEM fuel cell
in each category. Table 11-3 shows some examples of the polymer mem- branes currently being researched. Most membranes have degrada- tion temperatures ranging from 250 to 500C, water uptake from 2.5 to 27.5 H2O/SO3H, and conductance from 10 to 10 –S/cm [21-24].
11.1.2 PAFCs
Phosphoric acid fuel cells use a phosphoric acid electrolyte to tolerate the carbon dioxide in reactant gas streams and because of the low rate of elec- trolyte loss due to evaporation. Other reasons include high oxygen solu- bility, and good ionic conductivity at high temperatures. When phosphoric acid is above 150C, phosphoric acid is in the polymeric state as pyrophos- phoric acid (H4P2O7) [9]. Usually 100-percent phosphoric acid is used in
PAFCs, which have a solidification temperature of 42C [42]. The solidi- fication results in a volume increase, which can damage the porous elec- trode and matrix structures, and thus lower cell performance and shorten lifetime [9]. PAFC stacks must then be maintained above 45C at all times. The solidification temperature is dependent upon the phosphoric acid concentration. It is the highest for pure phosphoric acid solution, and then it decreases with decreasing concentration [13].
TABLE11-3 Examples of Alternate Polymer Membranes Being Researched for Low-Temperature Fuel Cells (Adapted from [21-24])
Chemical classification Membrane type Performance Perfluorinated ■ Perfluorosulfonic acid ■ Good proton
■ Perfluorocarboxylic acid conductivities and
■ Gore-Select resistance
■ Bis(perfluoroalkylsulfonyl) imide ■ Very durable membrane ( 60,000 hrs) Partially flourinated ■ a, , – trifluorostyrene ■ Less durable and
grafted onto poly lower performance (tetrafluoroethylene-ethylene) perfluorinated then with post sulfonation
■ Styrene grafted and sulfonated poly(vinylidenefluoride)
Non-flourinated ■ Methylbenzensulfonated ■ Good water absorption polybenzimidazoles [MBS-PBI] ■ Some types have good
■ Naphthalenicpolyimide conductivity; others
■ Sulfonatedpolyetherketone poor Non-fluorinated ■ Acid-doped polybenzimidazoles ■ Good proton
composite ■ Base-doped S-polybenzimidazoles conductivity.
■ Durability needs to be further tested.