11.1.1 Operating principles of direct methanol fuel cells (DMFCs)

11.2.1 Historical development of DMFCs

In 1922, the German researcher Müller recognized that methanol was more reactive to produce hydrogen than other hydrocarbons due to the presence of oxygen in methanol. He started the studies on the electro-oxidation of methanol.⁵ By 1951, on the basis of Müller’s concept, Kordesch and Marko built a DMFC system with carbon electrodes and explored the possibility of using alcohols (ethanol and methanol) as fuels in the alkaline electrolyte of KOH.⁵ Methanol was reformed to produce hydrogen that was subsequently fed to the fuel cell system. After that, DMFC research made little progress during that decade. In the 1960s, interest in DMFC rose again, aiming at military applications. In 1963, Murray and Grimes at Allis-Chalmers Manufacturing Company developed the first DMFC stack with Pt-Pd anode and Ag cathode catalysts deposited on porous Ni sheet. The stack comprised of 40 unit cells and its maximum power reached 730 W at the voltage of 0.25 V in the 6 M KOH electrolyte and 6 M methanol at 50°C. In addition, they manufactured another DMFC stack that produced 400 W constantly over a period of 5 h at 40°C.⁵ These achievements opened up new hopes for DMFC research.

In 1965, Williams et al. at Shell Research Limited developed DMFCs based on acid electrolytes that do not react with the CO₂ produced from methanol oxidation at the anode.⁶ Among the different acids, aqueous sulfuric acid was selected due to its higher conductivity and better performance for the oxygen reduction reaction (ORR). Compared to alkaline electrolyte, the water produced at the cathode was easily removed by the air flow to avoid cathode flooding. PtRu and Pt were applied to the surface of a microporous polyvinyl chloride (PVC) substrate as the anode and the cathode, respectively. A gold layer was sputtered on the surface of the PVC to increase the electrical conductivity. A DMFC stack with 40 cells was constructed and it produced 300 W at the voltage of 12 V at 60°C. Although Shell found PtRu to be most effective as methanol oxidation electrocatalyst, the intermediates from methanol oxidation have poisoning effects on the anode catalyst. Also, Tarmy and Ciprios at ESSO Research and Engineering Company developed a DMFC/Air stack for the US Army Electronics Laboratories for a portable military communications system.⁶ The work started in 1962 and was completed in 1965. The stack used 3.7 M sulfuric acid electrolyte and 0.75 M methanol. The stack temperature and water removal were controlled by the air flow rate. The stack produced 100 W at the voltage of 7 V and demonstrated self-sustaining 60 W at the voltage of 6 V. However, the research on exploring more active catalyst in acid electrolyte was greatly depressed by the slow kinetics of methanol electro-oxidation.

Progress in DMFC technology then almost hibernated until DuPont’s Nafion™ solid-polymer electrolyte membrane was successfully used for the solid-polymer hydrogen/oxygen fuel cell (PEFC) in 1992.⁶ Around 1994, the Jet Propulsion Laboratory in California demonstrated a Nafion-based DMFC, and a new era of DMFC was initiated.⁷ The concepts and technology of the PEM fuel cell (PEMFC) were transplanted to the DMFC, and the DMFC and its components were constructed in a manner similar to that of the PEMFC. The OCP and cathode performance were much enhanced as the solid polymer electrolyte membrane reduced the methanol crossover from the anode to the cathode. The methanol fuel could be fed to the anode without getting mixed with the aqueous acid electrolyte.

Figure 11.2 shows the number of journal papers published which contain the phrase ‘direct methanol fuel cell’ in the title according to the Web of Science in the period 1992–2010. There are undoubtedly many more papers that may have direct relevance to DMFCs. So the data are not precise, but they give a sense of the high activity in the DMFC area.

11.2.2 Technical challenges presented by DMFCs

Although the DMFC has theoretically high energy density and favorable electrode reaction, in practice DMFC performance is limited by the following factors: the slow kinetics of methanol oxidation due to the poisoning of the catalyst by the intermediates produced, high methanol crossover from the anode to the cathode through the electrolyte membrane, poisoning of the cathode catalyst by methanol, obstruction to methanol flow at the anode by carbon dioxide formation, and obstruction to oxygen flow due to cathode flooding.⁸

The slow kinetics of both the anode and cathode reactions reduce the cell voltage by 0.4–0.6 V at practical current densities. This has a serious impact on the voltage efficiency of DMFC, which is consequently reduced by up to 50%. During methanol electro-oxidation, various intermediates such as COH_ads, HCO_ads and HCOO_ads are produced and strongly adsorbed on the catalyst surface. The poisoning effect seriously decreases the overall amount of active reaction sites available for methanol oxidation, thus slowing the oxidation reaction. Consequently, the methanol molecules could not be further absorbed on the catalyst surface and be oxidized. Meanwhile, the intermediates absorbed on the catalyst surface could not be oxidized to CO₂ and hence reduce the fuel efficiency due to incomplete oxidation. Developing highly active electrocatalysts that inhibit the poisoning is still a challenge facing DMFC research.

Nafion membranes are the commonly used electrolyte membrane and separator in DMFC due to their high proton conductivity and chemical stability. However, methanol permeates through the membrane, dragged by hydronium ions and concentration gradient between anode and cathode, which is called methanol crossover. Methanol reacts directly with oxygen at the cathode as illustrated in Fig. 11.3. Electrons are brought directly from the anode to the cathode along with methanol, resulting in an internal short-circuiting and consequently a loss of current as illustrated in Fig. 11.4.⁹ This causes not only depolarization effects on the cathode by forming mixed potentials, but also a decrease in the fuel utilization efficiency. Besides, the Pt catalyst at the cathode catalyst is poisoned by intermediates of methanol oxidation in a similar way to the anode. To mitigate the detrimental effect of methanol crossover, low concentration methanol (2–5 wt %) is usually fed to the anode, which reduces the energy density of DMFCs and makes water management more complicated.

The other major efficiency loss in the electrodes of DMFC is related to mass transport. At the anode, most of the carbon dioxide formed during methanol oxidation is in the gas phase and accumulates in the catalyst layer, diffusion layer and flow field channels.¹⁰ There is thus a two-phase flow inside the feed channels due to the small solubility of CO₂ in methanol solution. The high amount of CO₂ bubbles makes them coalesce and form slugs (Fig. 11.5), which prevent the methanol from diffusing to the catalyst surface and thus result in insufficient supply of methanol. These effects can decrease the voltage of DMFC by 100 mV or more at practical current densities.

11.3.1 Methanol oxidation reaction

Compared with the hydrogen oxidation reaction (HOR) at the anode in a PEMFC, the mechanism of methanol oxidation reaction (MOR) is much more complicated because it involves six electrons to transfer and more intermediates. Figure 11.6 shows a proposed reaction network for MOR, involving all possible intermediates and reaction paths,¹² where * denotes a catalytically active site. It needs to be mentioned that formaldehyde and formic acid are two stable intermediates and can be treated as byproducts. To guide the design of catalysts for MOR, the favorite pathways, that is, the minimal energy paths, and the rate determining step (RDS) need to be fully understood. Intensive research in this area has led to the conclusion that the favorite pathway depends on the catalyst, the electrolyte and the reaction conditions (e.g., the potential). However, the mechanisms of the catalysis of MOR are still far from fully understood.

The mechanisms of MOR can be classified into two types:¹² (1) an indirect mechanism that goes through a CO intermediate and its subsequent oxidation to CO₂ and (2) a direct mechanism where methanol is oxidized to CO₂ without the formation of the CO intermediate. Density-functional theory (DFT) calculations have shown that the catalyst performance for the direct and indirect mechanisms can be characterized by two reactivity descriptors: the adsorption free energies of OH* (G_OH*) and CO* (G_CO*) on the active sites. For a catalyst preferred for indirect mechanism, for example Pt, the performance can be enhanced by decreasing the G_CO*.

11.3.2 Pt-based catalysts

As shown in Fig. 11.6, MOR involves the reactions of deprotonation and combination of intermediates with OH* to finally produce CO₂. Pt is one of the most efficient catalysts for deprotonation reactions in acidic media and thus was used for MOR in DMFC at first. It is either unsupported (Pt black, 10–20 nm) or supported on high surface area carbon black (Pt/C, 3–5 nm Pt nanoparticles on 50 nm carbon black). The main advantage of using Pt/C is the high efficacy of Pt usage due to the high dispersion of Pt nanoparticles. Because methanol solution is a liquid phase under DMFC operating conditions, the MOR is subjected to larger mass transport resistance than the HOR at the anode in PEMFC. The advantage of using Pt black is to lower the mass transport resistance with a thinner anode catalyst layer. Using a Pt/C with high metal loading is a trade-off between the efficiency and mass transport resistance. This is applicable for other non-platinum catalysts as well. To date, 60wt. % metal loading is the highest for commercial Pt/C used in DMFC.

The mechanisms of MOR on Pt in acidic media have been investigated intensively. It has been concluded that it mainly takes indirect paths which proceed via the formation of CO. The RDS is the combination of CO* with OH* because the CO* is too strongly adsorbed on Pt which inhibits the consequential desorption and further adsorption. This combination rate is significantly lower at potentials less than 0.6 V (vs SHE) over Pt(111), where CO is strongly chemisorbed and inhibits oxidation. Besides, the production of OH* from the dissociation of water on Pt only takes place at a remarkable rate when the potential is higher than 0.45–0.5 V (vs SHE). Both cause insufficient activity of Pt toward MOR and are not commercially useful for DMFC.

An intensive search for more active materials has been conducted over the years. They should have the ability to simultaneously adsorb methanol and water at low potentials, and thus allow for the oxidation of methanol at low overpotential.¹³ Because Pt is still one of the most efficient catalysts for the sequential deprotonation of methanol in an acidic medium, the search has focused on the addition of a secondary material, such as metals and oxides.

11.3.3 Other catalysts

To reduce the cost, Pt needs to be replaced by non-noble materials. To date, there are limited publications on non-platinum catalysts for MOR in acidic media because even Pt, the best deprotonation catalyst, still has low performance toward MOR. The performances of non-platinum catalysts, such as dispersed gold nanoparticles onto a conductive polymer (polyaniline) grafted multiwall carbon nanotube (MWNT-g-PANI) matrix⁵³ and metal-free carbon nitride nanotubes,⁵⁴ are much lower than that of Pt catalysts. Therefore, very limited comparison of MEA performances has been reported.⁴¹ For these low-cost catalysts, a trade-off must be made between performance, durability and cost for practical use.

11.4.1 ORR with methanol tolerance

The ORR is known to have sluggish reaction kinetics. Even the state-of-the-art Pt catalysts toward ORR lead to a loss of ~ 400 mV at the cathode in PEMFC. This loss becomes even higher at the cathode of a DMFC due to the poisoning of and ‘mixed potential’⁵⁵ on the Pt catalysts by the methanol, which crosses over from the anode to the cathode through the Nafion membrane. The methanol oxidation on the cathode catalyst (Pt catalysts) produces CO intermediates, which strongly adsorbs on the Pt surface to block the active sites for ORR, known as poisoning. Besides, the MOR current at the cathode needs neutralization from the ORR current. A certain value of overpotential required to generate this amount of ORR current is known as ‘mixed potential’. It is thus apparent that a good cathode catalyst should be more selective toward the ORR to minimize the ‘mixed potential’ and should not be poisoned by the CO intermediate from MOR (termed methanol tolerance). The requirement of both good catalytic activity toward ORR and high methanol tolerance makes it much more challenging to design a cathode catalyst for DMFC than for PEMFC.

First of all, a comprehensive understanding of the mechanisms of ORR is needed. Despite the significant advances over the years, ORR promoted on the surface of a catalyst in acidic media is still not fully understood. The ORR involves three intermediates: O*, OOH* and OH*, where the * denotes a catalytically active site. The possible pathways for ORR are indicated in Fig. 11.8. Of all the pathways, (1) is the least probable because of the higher energy barrier for O₂ dissociation. H₂O and H₂O₂ are the products, respectively, following the four-electron and two-electron mechanisms. Two types of four-electron mechanism have been proposed in the literature: mono-site mechanisms following the pathways 2, 3, 6, and 7 and dual-site mechanisms following the pathways 2, 4, 6, and 7.⁵⁶ The two-electron mechanism follows pathways 2 and 5 to produce H₂ O₂. For maximum current efficiency, the four-electron mechanisms are desired and the two-electron mechanisms need to be suppressed.

It has been concluded that the RDS involves the removal of O* and OH* for the mono site mechanisms, and the rupture of the O–O bond for the dual site mechanism. The enhancement of ORR catalysts can be approached by effectively increasing the rate of the RDS in various ways such as altering the surface chemistry, electronic structure, and lattice parameter. All these approaches should not increase the activity toward MOR and the bonding between the catalysts and the intermediate of CO.

11.4.2 Pt-based catalysts

11.4.3 Pd-based catalysts

In addition to the poor methanol tolerance, Pt is expensive and there is not enough mining capacity to support a widespread commercialization of the fuel cell technology. There is enormous interest in developing alternative less expensive and high methanol-tolerant catalysts as well as in lowering the Pt loading to realize commercially viable DMFCs.^58,76–82

Pd has the best ORR performance except Pt in acidic media among all pure metals. It has lower cost, higher mining capacity and lower activity toward MOR in comparison with Pt, making it a very attractive cathode catalyst for DMFC. The target for designing Pd-based catalysts is to improve its ORR catalytic activity without increasing the activity toward MOR, as well as to improve its durability. Similar to Pt, the activity of Pd toward ORR can be enhanced by inhibiting oxide formation and adjusting the d-band center and shape. Alloying of palladium with other inexpensive metals like Co, Mo, Ti, Cu, etc., offers higher ORR activity and additional cost reduction. Figure 11.9 shows that the Pd₇₀Co₃₀ and Pd₇₀Co₂₀Mo₁₀ catalysts maintain the Tafel slope at high potential while Pd has a reduced Tafel slope as pure Pt does. The enhancement in activity is attributed to the suppression of Pd oxide formation. It has been found that Pd is inclined to segregate onto the surface while Co is rich inside. The Pd surface forms a protective layer, while Co modifies the electronic properties of Pd, making the ‘d-band center’ mimic the d-band of the Pt or Pt-M with high ORR performance. Similarly, PdCu/C is another binary Pd-based catalyst with promoted catalytic activity toward ORR with higher MOR tolerance.

11.4.4 Other catalysts

Besides the most promising Pd-based catalysts,^86–88 alternative electrocatalysts with good methanol tolerance such as non-platinum-based metal combinations, ^89–92 metal oxides, ^93–95 metal carbides, ⁹⁶ metal porphyrins, chalcogenides ^97,98 (ruthenium-based chalcogenides, ^99,100 chalcoginides such as RuSe, ^{58,96,99–101} and pyrolyzed transition metal N ₄ chelates ^{59,60,63,64,101}), enzymes, 102 inorganic and organometallic complexes including porphyrins ^102–105 have been investigated over the years for ORR to reduce the cost.

Although tremendous progress in improving the activity and stability of non-platinum cathode catalysts such as heat treated macrocyclic compounds of transition metals^106–109 and ruthenium-based chalcogenides^{86–88,110–114} has been achieved, they still exhibit lower catalytic activity toward ORR and lower stability than Pt catalysts in the absence of methanol. However, when methanol is present, the decrease in performance with the methanol-tolerant catalysts is much lower than that with Pt electrodes. Overall, their activity and stability need further improvement.

11.5.1 Perfluorosulfonic acid (PFSA) membranes

Much effort has been devoted to the research and development of perfluorosulfonic acid (PFSA) membranes such as Nafion (Dupont), Aciplex (Asahi Chemical company), Flemion (Asahi Glass Company) and XUS (Dow Chemical) due to their high proton conductivity and good chemical stability resulting in good performance and durability. The chemical structures of some PFSA membranes are shown in Fig. 11.13. Nafion is the most investigated PFSA membrane and widely used as the electrolyte membrane in PEMFC and DMFC. The proton and water transport properties of Nafion could be well explained by the cluster-network model shown in Fig. 11.14 based on the small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) studies. The hydrophobic polytetrafluoroethylene (PTFE) backbone of PFSA polymers provides thermal and chemical stability and forms a hydrophobic crystal region, while the perfluorinated side chains terminating with the hydrophilic sulfonic acid groups (− SO₃H) form a hydrophilic region (clusters and channels). The clusters formed by the absorbed water and side chains are around 4 nm wide, which are periodically arranged among the hydrophobic region. The distance between the clusters is around 5.0 nm, and the clusters are connected by channels with a diameter of 1 nm.

Despite the obvious advantages of the perfluorinated membranes, there are a few disadvantages when applying them in DMFCs:

Among these disadvantages, methanol crossover in perfluorinated membranes is the critical issue with respect to achieving higher fuel and power efficiency. It is also an unavoidable issue as long as the electrolyte membrane requires water for proton conduction. Generally, two major strategies are employed in the literature to minimize methanol crossover. The first one is to modify the existing perfluorinated membrane to offer higher proton conductivity and lower methanol crossover. The second one is to design and develop novel non-fluoropolymer materials.

11.5.2 Modification of Nafion® membranes

11.5.3 Polytetrafluoroethylene (PTFE)-reinforced composite membranes

PTFE is selected as a reinforcing material for its excellent chemical stability and good mechanical strength. The reinforced membrane comprising of expanded PTFE porous sheet and a perfluorinated ionomer was first developed by W.L. Gore & Associates in the 1990s, which has been commercialized as Gore-select® and has been widely used for PEMFC systems. The PTFE-reinforced membrane contains much lower amount of the expensive perfluorinated ionomer than the traditional perfluorinated membranes such as the Nafion series membrane, thus the cost of the composite membranes is much lower. The composite membranes also offer other advantages such as excellent mechanical and chemical stability and thinner thickness. Lin et al.¹²⁶ found both methanol diffusion and methanol electro-osmosis crossover in PTFE/Nafion composite membrane were reduced, thus giving better cell performance. But it has higher proton resistance compared to the Nafion membrane. Zirconium phosphate was then introduced to modify PTFE/Nafion membrane to further reduce methanol crossover rate and enhance proton conductivity.¹²⁷

11.5.4 Sulfonated aromatic polymer membranes

Among the various polymer materials being investigated, sulfonated aromatic polymers show promising features for possible applications in DMFC. For example, sulfonated polyimide (SPI),¹²⁸ sulfonated polyphenylene (SPPO),¹²⁹ sulfonated polysulfone (SPSf),¹³⁰ sulfonated poly(ethersulfone) (SPES)¹³¹ and sulfonated poly(ether ether ketone) (SPEEK)¹³² are actively being investigated as candidates to substitute for the PFSA membranes in DMFC. The structures of some of these sulfonated polymer materials are shown in Fig. 11.15. The aromatic polymers are non-fluorinated polymers with high glass transition temperature, high thermal stability, good mechanical properties, excellent resistance to hydrolysis and oxidation and lower cost.

Sulfonated aromatic polymers could be synthesized through polymerization reactions from different monomers bearing sulfonic acid groups. With this strategy, several PEM with controllable chemical structures (degree of sulfonation and sulfonation sites) and well-refined microstructure have been investigated, for example, sulfonated polyimide (SPI),¹²⁸ sulfonated polyphenylene (SPPO),¹²⁹ sulfonated poly(aryl ether)s (SPAEs)¹³³ and sulfonated poly(ether ketone) (SPEK).¹³⁴ By changing the monomers used in the polymerization reactions, the properties of the product polymers could be tuned, and some fundamental understanding of the structure-property-performance relationships have been achieved. Introduction of sulfonic acid groups to these polymers are performed in several ways as follows: (1) direct sulfonation in concentrated sulfuric acid or chlorosulfonic acid, (2) radiation-grafting of monomers onto the polymer backbone followed by sulfonation, (3) synthesis from monomers bearing sulfonic acid groups, and (4) lithiation-sulfonation-oxidation. The properties of these sulfonated polymers are mainly controlled by the degree of sulfonation (DS). The DS could be controlled by the reaction time, temperature and amount of sulfonating agent used in the reaction. Table 11.1 summarizes the IEC, DS, and water uptake at 80°C for SPEEK membranes.¹³²

Usually, the sulfonated aromatic polymers show lower methanol cross-over than the fluorinated membranes due to their more rigid backbone compared to the PTFE backbone in the Nafion type membranes.¹³² Using sulfonated polyetherketone (SPEEKK) as an example, based on the SAXS data and a cubic hydrophilic channel system in a hydrophobic matrix, a model for the microstructure has been proposed and compared with that of Nafion membrane (Fig. 11.16).¹³⁵ The water-filled channels in SPEEKK are narrower compared to those in Nafion. They are less separated and they have larger hydrophilic/hydrophobic interface (larger average separation of neighboring sulfonic acid functional groups). The stronger confinement of the water in the narrow channels of the aromatic polymers leads to a significantly lower dielectric constant of the water of hydration, leading to much lower methanol crossover than in Nafion membrane. In addition, the pK_a value of the sulfonic acid in sulfonated PEEK is around − 1, which is much higher than that of the Nafion membrane (pK_a ~ −6), leading to lower proton conductivity and usually lower fuel cell performance. In order to maximize the proton conductivity, a high degree of sulfonation (DS) is desired, which often leads to an increase in membrane swelling and degradation in mechanical stability.

The proton conductivity, water swelling, methanol permeability and DMFC performance of these membranes have been investigated.

11.5.5 Cross-linked polymer membranes

Various strategies have been investigated in order to reduce the swelling of the sulfonated polymers with high DS. Covalent/ionic cross-linking is found to be an effective way to control the dimensional stability and suppress methanol crossover of the PEM without sacrificing the proton conductivity too much. Recently, covalent cross-linked sulfonated aromatic polymers with high DS have been reported.^136–138 For example, Vona et al.¹³⁸ studied the properties of the covalent cross-linked SPEEK with high DS (Fig. 11.17), and the resulting membrane showed much better swelling stability and lower methanol permeability compared to the uncross-linked polymers. Usually, the thermal stability of these covalently cross-linked polymers is quite high (230–270°C). However, the polymers with covalent cross-linking become brittle after drying, which is a critical problem for fuel cell applications. The brittleness is possibly caused by the inflexibility of the covalent networks. Because of this reason, more and more attention is being paid to the development of ionomer networks containing ionic cross-links that are more flexible.

Ionic cross-linked membranes were first investigated with anhydrous polymeric proton conductors like polybenzimidazole (PBI).^139,140 Unlike the polymers containing sulfonic acid groups, the nitrogen sites on the heterocycles in PBI act as proton donors and acceptors, which facilitates the proton transfer through a Grotthuss-type mechanism.¹⁴¹ Due to the slow rate of the proton transfer facilitated by the Grotthuss-type mechanism at low temperature, usually the operation temperature of these kinds of membranes is higher than that of sulfonated polymers. In 1995, Wainright et al.^142,143 first proposed a blend of PBI and phosphoric acid as the electrolyte for PEMFCs and DMFCs. The intrinsic property of phosphoric acid and ease in preparing the blends with a large variety of polymers drew more and more attention after this investigation. In this type of acid-doped PBI membranes, nitrogen sites on the heterocycles exhibit basic properties, and they are also termed as acid–base blend membranes. The strong acid–base interactions and the hydrophobicity of the basic polymers could promote proton conduction and block methanol permeation through the membrane. Based on this approach, Schauer’s group¹⁴⁴ has studied the acid–base blend membranes consisting of SPPO/PBI, which showed good thermal stability and flexibility as PEM in fuel cells. Kerres’ group^145,146 reported several ionic cross-linked blend membrane systems based on acid–base interactions as shown in Fig. 11.18 (e.g., sulfonated polysulfone (SPSf)/aminated PSf and SPEEK/PBI). All the blend membranes exhibit good flexibility, thermal stability and lower methanol permeability in DMFCs. However, the dimensional stability at T > 70°C is inadequate with some of the blend membranes.

Our group^147,148 reported recently that blend membranes based on acid–base interactions between the sulfonic acid groups of SPEEK and the N-heterocycle groups tethered to polysulfone show suppressed methanol crossover and enhanced performance in DMFC. This blend membrane concept is based on industrially available, inexpensive polymer precursors like poly (ether ether ketone) (PEEK) and polysulfone (PSf) that are compatible with each other due to similar aromatic backbones. A series of heterocycle-tethered basic polymers such as polysulfone-bearing 4-nitro-benzimidazole (PSf-NBIm), polysulfone-bearing ¹H-Perimidine (PSf-PImd), and polysulfone-bearing 5-amino-benzotriazole (PSf-BTraz)) have been synthesized and characterized. Blend membranes consisting of sulfonated poly (ether ether ketone) (SPEEK) and different basic polymers have been investigated by measuring proton conductivity, liquid uptake, ion-exchange capacity, and electrochemical performance and methanol crossover in DMFC.^149,150

The synthesis scheme for polysulfone-bearing 4-nitro-benzimidazole (PSf-NBIm), and 5-amino-benzotriazole (PSf-BTraz)) are shown in Figs. 11.19 and 11.20, respectively. Table 11.2 gives the ion-exchange capacity (IEC) values, proton conductivity values, water uptake and methanol uptake values measured under 100% RH at 65°C for various contents of PSf-NBIm in the SPEEK/PSf-NBIm blend membranes. It can be seen that the IEC values of the blend membranes are lower than that of plain SPEEK, indicating the occurrence of acid–base interactions in the blend membranes and the consequent reduction in the amount of H⁺ ions dissociating from the sulfonic acid groups. Moreover, the IEC value decreases as the PSf–NBIm content increases due to an increase in the degree of acid–base interaction, while the blend membrane with 2.5 and 5.0 wt% PSf–NBIm shows higher proton conductivity than plain SPEEK membrane. The increase in proton conductivity is due to the assistance of proton transfer by the nitro-benzimidazole groups in PSf–NBIm through the acid–base interactions as illustrated in Fig. 11.21. The sulfonic acid groups of SPEEK can protonate the nitrogen site of nitro-benzimidazole, facilitating the hopping of the proton bound to the other nitrogen of the nitro-benzimidazole unit to the oxygen of another sulfonate anion group.

Table 11.2 also compares the percent liquid uptake in water and in 1.0 M methanol solution for various PSf-NBIm contents of the SPEEK/ PSf-NBIm blend membranes. The liquid uptake decreases with increasing PSf-NBIm content at a given temperature or methanol concentration. The lower liquid uptake of the SPEEK/PSf-NBIm blend membrane is believed to be due to the increase in both the hydrophobicity of PSf-NBIm and the acid–base interaction between and sulfonic acid and nitro-benzimidazole groups in the blend membrane. The lower liquid uptake could also help to lower the methanol crossover in DMFCs. Figure 11.22 compares the polarization curves of the SPEEK/PSf-NBIm blend membranes containing various PSf-NBIm contents (1.0-5.0 wt%) with those of plain SPEEK and Nafion 115 membranes at 65°C. The SPEEK/PSf-NBIm blend membranes show higher fuel cell performance than plain SPEEK membrane, which is consistent with the higher proton conductivity values seen in Table 11.2. More importantly, although the plain SPEEK membrane shows lower performance than Nafion 115 membrane due to the lower proton conductivity, the SPEEK/PSf-NBIm blend membranes with PSf-NBIm contents of 2.5 and 5.0 wt% exhibit higher performance than Nafion 115, confirming the assistance of PSf-NBIm in enhancing the proton conduction. The higher performance is also due to the lower methanol crossover in the blend membrane compared to that in Nafion 115 membrane.

11.6.1 Catalyst layer fabrication

The catalyst layer is the active region for the electrochemical reactions in a fuel cell. An effective catalyst layer should have transport channels for protons, electrons, reactants, and products (Fig. 11.23). The catalyst layer is made from the catalyst ink containing catalyst powder, binder and solvents. According to the type of binders, the following catalyst layers are commonly used in DMFC.

11.6.2 Methods for catalyst layer fabrication

11.6.3 MEA structure

11.6.4 MEA performance

DMFCs are viewed as a potential power source for various portable applications such as cellular phones, PDAs, laptop computers, military back power packs and auxiliary power units (APU) due to their high energy density, light weight, and simple system as well as their easy refueling and quick start-up. These significant advantages and huge market make the DMFC technology appealing. A lot of organizations and research groups are actively engaged in the development of high-performance single cells and stacks for portable device or vehicles applications.

The performance of DMFC single cell and stack are usually evaluated in term of maximum power density and current density at the specified working voltage. The operating conditions (methanol concentration, gas flow rate, cell temperature, back pressure), MEA components (catalyst type, catalyst loading, ionomer loading, thickness of membrane), electrode fabrication processes and polar plates are all factors influencing the performance of single cell and stack. It is very difficult to compare the MEA performances based on the reported data in the literature due to above factors. The MEA is usually tested in a single cell with an active area of 5–50 cm². A diluted methanol solution in the concentration range of 0.5–5.0 M is pumped to the anode and pressurized oxygen or air is fed to the cathode. The effects of methanol concentrations on the cell performance have been investigated by several groups. High concentration of methanol could facilitate the mass transport and reduce concentration polarization at the anode, but it does not necessarily contribute to the high cell performance due to the problems of high methanol crossover at high concentrations of methanol feed. High concentration of methanol can cause a significant reduction in cathode potential. The OCP drops by 50–150 mV, which is closely related to the thickness of the electrolyte membrane and methanol concentration. For the cathode, high back pressure is often adopted to increase the oxygen concentration and facilitate oxygen diffusion in the catalyst layer. Meanwhile, it can impede methanol crossover from the anode to the cathode. High gas flow rate can also improve cell performance by reducing cathode flooding to some extent. In practice, a gas compressor or blower will be needed to maintain high gas pressure and high flow rate, which will consume 10-30% of power produced by DMFC stack.

Cell temperature is an important consideration in DMFC operation. Elevated temperature can increase the kinetics of methanol oxidation and overcome the CO species poisoning. Representative data for DMFC performance at high temperature are shown in Table 11.3. The highest power density of MEA reported by Aricö et al.,¹⁶⁶ was around 450 and 290 mW/cm² for single cells operated on oxygen and air, respectively, at the temperature of 130°C. These performances were achieved with 2 mg/cm² Pt loadings on each electrode and 2 M methanol feed. Similar power density has also been achieved by Ren et al.¹⁶¹ at Los Alamos National Laboratory (LANL) based on MEA prepared with Nafion 112 membrane and 2.2 mg/cm² Pt-RuO_x catalyst and 2.3 mg/cm² Pt black. The power outputs of this MEA are almost 400 mW/cm² for the oxygen cathode at 130°C and about 250 mW/cm² for the air cathode at 110°C.

High performance of the DMFC is indispensable for portable applications at ambient temperature and pressure. The cell performance is much lower than that obtained at high cell temperature and pressurized conditions. Generally, the power density of DMFC varies between 10 and 40 mW/ cm² under passive operation mode and ambient temperature as shown in Table 11.4. It is still too low for portable device and vehicle applications. Moreover, the high catalyst loading (> 2 mg/cm²) is not acceptable for large and even for small scale applications.

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