Anode catalysts

Until now, Pt-based noble metal catalysts remain the best choice for activating the MOR at the DMFC anode. When using a pure Pt catalyst, the MOR is not completely realized because of the formation and subsequent irreversible absorption of CO and CHO intermediate species, ^{Reference Beden, Kadirgan, Lamy and Leger37–Reference Prabhuram and Manoharan39} which severely impede the kinetics of methanol oxidation on Pt. To mitigate the effect of these poisonous species, the Pt catalyst is usually alloyed with second metals such as Ru, Sn, Mo, Co, Ni, etc. It has been proposed that these second metals alleviate the irreversible absorption of the poisoning intermediate species by means of the bifunctional mechanism and the ligand effect. ^{Reference Watanabe and Motoo40,Reference Goto, Li, Senoo, Noda, Kudo, Maesaka and Hatazawa41} The bifunctional mechanism involves the oxidation of CO/CHO intermediates adsorbed on Pt by OH_ads species formed on the second metal at a relatively lower anodic potential. ^{Reference Watanabe and Motoo40} The ligand effect involves a decrease in the adsorption energy of CO/CHO_ads species on Pt due to alloying with the second metal, thereby facilitating the oxidation of these poisonous species at relatively lower anodic potential. ^{Reference Goto, Li, Senoo, Noda, Kudo, Maesaka and Hatazawa41} Extensive studies of the MOR in both acid and alkaline media on various Pt-based bimetallic catalysts (PtRu, PtMo, PtW, and PtSn) have shown that PtRu catalysts exhibit the highest MOR activity ^{Reference Gotz and Wendt42–Reference Prabhuram and Manoharan44} as well as higher DMFC power density and stability compared with other bimetallic catalysts. Ruthenium dissolution, however, poses a big issue in PtRu catalyst systems. ^{Reference Gotz and Wendt42,Reference Zhou, Zhou, Li, Zhou, Song, Sun, Xin, Douvartzides, Goula and Tsiakaras45–Reference Lizcano-Valbuena, Bortholin, Neto, Paganin and Gonzalez49}

In analogy to common practice in PEMFCs, DMFCs often use supported PtRu catalysts, where nanoscale PtRu catalyst particles are supported on a high surface area carbon powder to increase mass activity and therefore bring down the cost compared to using an unsupported PtRu black catalyst. Lizcano-Valbuena et al. ^{Reference Lizcano-Valbuena, Bortholin, Neto, Paganin and Gonzalez49} compared various types of carbon-supported Pt-based alloys for the anode of the acid-based DMFC and showed that PtRu/C (75:25 by weight) provided the best performance among the various Pt-based alloys as depicted in Fig. 4.

Figure 4.

Voltage versus current density plot for a single cell of the DMFC using various types of carbon-supported Pt-based alloys. Reproduced with permission from The Electrochemical Society, Copyright 2001. ^{Reference Lizcano-Valbuena, Bortholin, Neto, Paganin and Gonzalez49}

Further research on reducing the loading level of PtRu on the supporting material while at the same time maintaining performance and stability is needed. To improve DMFC catalyst performance and stability, various types of carbon supports have been explored. ^{Reference Salgado, Paganin, Gonzalez, Montemor, Tacchini, Anson, Salvador, Ferreira, Figueiredo and Ferreira50–Reference Bong, Kim, Kim, Woo, Uhm, Lee and Kim54} PtRu supported on carbon nanofibres, carbon nanotubes, and graphenes have exhibited particularly intriguing DMFC performance and stability. ^{Reference Prabhuram, Zhao, Tang, Chen and Liang52,Reference Bong, Kim, Kim, Woo, Uhm, Lee and Kim54,Reference Kang, Lim, Peck, Kim, Jung, Hong, Jung and Shul55} Several researchers have also explored chemically modifying the carbon support itself as a means to improve catalyst performance and stability. ^{Reference Zhou, Neyerlin, Olson, Pylypenko, Bult, Dinh, Gennett, Shao and O'Hayre56–Reference Olson, Dameron, Wood, Pylpenko, Hurst, Christensen, Bult, Ginley, O’Hayre, Dinh and Gennett60} Of particular note, PtRu catalysts impregnated/deposited on nitrogen modified carbon supports have been shown to exhibit significantly higher performance and stability when compared to undoped counterparts. ^{Reference Corpuz, Olson, Joghee, Pylypenko, Dameron, Dinh, O'Neill, Hurst, Bender, Gennett, Pivovar, Richards and O'Hayre61–Reference Corpuz, Wood, Pylypenko, Demeron, Joghee, Olson, Bender, Dinh, Gennett and Richards63}

Cathode catalysts

In the cathode of a DMFC, as in a PEMFC, Pt metal remains the best catalytic material even though Pt suffers a nearly 400 mV loss due to the sluggish electrochemical kinetics of the oxygen reduction reaction (ORR) and mixed potential caused by methanol crossover. ^{Reference Ralph and Hogarth64–Reference Casalegno, Bresciani, Zago and Marchesi66} In commercial DMFC systems, carbon-supported Pt is used at the cathode to minimize the cost.

To improve the kinetics of the ORR and improve the tolerance against methanol crossover, various carbon-supported Pt-based alloys (Pt–M, where M is Fe, Au, Pd, Cr, and W₂C) have been developed for the cathode and these alloys show improved performance in DMFCs versus pure Pt/C catalysts. ^{Reference Li, Xin and Yan67–Reference Meng, Shen, Wei and Jiang71} It has been proposed that the formation of Pt-based alloys slightly decreases the Pt–Pt interatomic distance, facilitating cleavage of the strong O=O bond of oxygen to achieve the complete oxygen reduction that favors 4 electron transfer. ^{Reference Ralph and Hogarth64,Reference Meng, Shen, Wei and Jiang71} The stability of the PtCr and PtFe alloys has been studied in PEMFC cathodes, ^{Reference Antolini, Salgado and Gonzalez72–Reference Xiong and Manthiram74} and the findings suggest that the stability of these alloys depends on the degree of alloying as well as the catalyst particle size. For instance, Ballard Inc. has demonstrated enhanced initial performance and better durability (over 500 h) for PtCr catalysts compared to Pt catalysts in PEMFC cathodes (see Fig. 5). ^{Reference Ralph and Hogarth64}

Figure 5.

(a) Comparative performance of Pt and PtCr alloys in PEMFC cathodes operating at 80 °C using hydrogen/air, hydrogen/helox (helox is a mixture of helium and oxygen), and hydrogen/oxygen at 1.5/2 and 2/10 anode/cathode stoichiometry (308/308 kPa anode/cathode pressure). (b) Durability test for Pt and PtCr alloys under the same temperature and pressure conditions as (a) using hydrogen/air at 1.5/2 stoichiometry. Reproduced with permission from Johnson Matthey, Copyright 2002. ^{Reference Ralph and Hogarth64}

To lower cost, it is highly desirable to develop non-noble metal alternatives to Pt-based catalysts. Potential alternatives include pyrolysed Fe porphyrins and Ru-based chalcogenides, which show decent ORR performance (although still not competitive with Pt) and excellent methanol tolerance. ^{Reference Reeve, Christensen, Hamnett, Haydock and Roy75,Reference Sun, Wang and Savinell76} However, long-term stability is a potential issue with many of these catalysts and further breakthroughs are needed. In alkaline media, because of a wider range of stable metal alloys and inherently faster ORR kinetics, less expensive metals such as Pd, Ag, and their alloys have been successfully used as carbon-supported cathode catalysts with reasonable performance and comparable stability to Pt/C catalysts. ^{Reference Bunazawa and Yamazaki77–Reference Okajima, Nabekura, Kondoh and Sudoh80} Current intensive research on Pd and Ag and their alloys for the ORR in alkaline media provides further hope that these metals could provide a lower-cost alternative to Pt and Pt-based alloys for alkaline-based PEMFC and DMFC cathodes.

Electrolyte materials

The desired traits for a PEM in a DMFC application include: (i) stable high temperature (80–90 °C) operation capability, (ii) low methanol crossover (<10⁻⁶ mol min⁻¹ cm⁻¹) or low diffusion coefficient of the membrane (<5.6 × 10⁻⁶ cm² s⁻¹ at T = 25 °C), ^{Reference Arico, Srinivasan and Antonucci81} (iii) high ionic conductivity (>80 mS cm⁻¹), ^{Reference Arico, Srinivasan and Antonucci81} (iv) high chemical and mechanical durability particularly at T > 80 °C, (v) low ruthenium crossover, and (vi) low cost. The most commonly used cation exchange membrane (CEM) in DMFCs is Dupont's Nafion membrane, which does not satisfy all the above requirements due to disadvantages including high cost ($600–$1200 m⁻²), ^{Reference Agro, DeCarmine, DeFelice and Thoma82} high cost per unit power (300 $ kW⁻¹ at 240 mW cm⁻²), ^{Reference Howell83} high methanol crossover, ^{Reference Reeve84} and high ruthenium crossover from anode to cathode. ^{Reference Piela, Eickes, Brosha, Garzon and Zelenay85} Despite these disadvantages, however, alternative membranes generally do not match the performance and durability of Nafion, which therefore remains the membrane of choice for most commercial DMFC applications. In the search for better membranes, researchers have pursued the modification of Nafion-type membranes as well as the development of entirely new classes of membranes. ^{Reference Antonucci, Arico, Creti, Ramunni and Antonucci86–Reference Wang, Wainright, Savinell and Litt91} While many of these newer membranes are lower in cost than Nafion, they have generally not yet obtained the combination of high conductivity and high stability that makes Nafion so compelling. The characteristic properties of various CEM types along with their DMFC performance are provided in Table 2.

Table 2.

Properties of various types of CEMs for application in DMFC.

^a At higher fuel cell operating temperature (above 80 °C), Nafion type membranes are likely to lose water and mechanical strength, which may have a detrimental effect on the performance/durability of the fuel cell. Additives like SiO₂ and ZrP can be added to enhance the thermal stability of Nafion as they can help to retain water and mechanical strength even at higher fuel cell operating temperature (above 100 °C).

In contrast to CEMs, anion exchange membranes (AEMs) are solid PEMs that contain positive ionic groups (typically quaternary ammonium functional groups such as poly-N⁺Me₃) and mobile OH⁻ ions. They are also commonly referred to as alkaline membranes. Some of the commercially available AEMs for DMFC application include MORGANE-ADP and an ammonia-based AEM membrane from Tokuyama Co. Japan. Interestingly, these AEMs exhibit low methanol crossover because of the absence of electro-osmotic drag from the anode side: the OH⁻ ions migrate from cathode to anode in alkaline-based DMFCs, whereas in CEMs, protons migrate from the anode to the cathode and tend to drag methanol molecules with them. Unfortunately, however, the diffusion coefficient and ionic mobility of OH⁻ are typically less than H⁺ so AEMs tend to show lower conductivities. Most AEMs are also not as stable as their CEM counterparts at elevated operating temperatures. ^{Reference Yu and Scott94,Reference Yu, Krewer and Scott95} The development of AEMs with high ionic conductivity, good chemical stability, and low methanol permeability is an area of active research. ^{Reference Yu, Krewer and Scott95–Reference Hou, Sun, He, Sun, Jin, Liu and Xin105} The basic properties of several widely investigated AEMs are given in Table 3.

Table 3.

Basic properties of various types of AEMs for DMFC application.

MEA fabrication

DMFC performance and durability mainly rely on the structure of the MEA, which is the key component in the DMFC. In recent years, many attempts have been made to improve the fabrication process and structural parameters of the MEAs to maximize the number of triple-phase boundaries available to facilitate the electrochemical reaction. Conventional methods to fabricate the MEAs are classified into two groups. ^{Reference Lindermeir, Rosenthal, Kunz and Hoffmann106–Reference Zhang, Yin, Wang and Shao108} One is the catalyst-coated substrate (CCS) method and the other is the catalyst-coated membrane (CCM) method. In the CCS approach, the anode and cathode catalysts are coated on the GDL, which is made of carbon paper, felt, or cloth, and subsequently hot pressed by sandwiching the membrane electrolyte in-between the anode and cathode catalyst-coated GDLs to form the MEA. In this method, the catalyst layer coated on the GDL is not always effectively transferred to the membrane and some of the catalytic particles get buried into the GDL and are not effectively utilized for the electrochemical reaction during fuel cell operation. Therefore, this method may not be suitable for achieving high MEA performance.

In the CCM method, the anode and cathode catalysts are directly coated on the two sides of the membrane and the GDLs are subsequently sandwiched on either side. In this approach, hot-pressing of the GDL is usually not required. ^{Reference Zhang, Yin, Wang and Shao108} The catalyst thickness can be controlled and the interfacial contact between the membrane and the catalyst layer is found to be very intimate due to the direct coating of the catalyst layer on the membrane. This helps to achieve higher cell performance. ^{Reference Pak, You, Choi and Chang109} The DMFC performance of the MEAs fabricated by the CCS (or catalyst coated on diffusion backing) and CCM method is shown in Fig. 6. ^{Reference Lindermeir, Rosenthal, Kunz and Hoffmann106}

Figure 6.

Polarization curves of MEA1 (catalyst loading anode/cathode = 1/1 mgPt cm⁻²) and MEA2 (catalyst loading anode/cathode = 1/1 mgPt cm⁻²). Conditions: T = 80 °C, C_MeOH= 0.75M, O₂ operation, cathode/anode overpressure =0.1/0 bar. Reproduced with permission from Elsevier, Copyright 2004. ^{Reference Lindermeir, Rosenthal, Kunz and Hoffmann106}

An alternative approach called the decal transfer method (DTM) has also been adopted to achieve higher cell performance and mass production of MEAs. ^{Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110} In the DTM approach, the catalyst layers are deposited on Teflon decal substrates and are then subsequently transferred to a dry electrolyte membrane during the hot-pressing of the MEA. MEAs fabricated by the CCM and DTM methods have an improved catalyst/membrane interface, better catalyst utilization, ^{Reference Zhang, Yin, Wang and Shao108,Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110} and superior formation of the ionomer network. ^{Reference Tang, Wang, Pan, Jiang and Ruan107,Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110} These aspects are all generally beneficial in improving the performance and durability of a DMFC used with both acid- and alkaline-based membranes. ^{Reference Joghee, Pylypenko, Wood, Bender and O'Hayre48,Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110} The CCM and DTM MEA fabrication processes are compared in Fig. 7. ^{Reference You, Lee, Cho, Kim, Pak, Lee, Park and Park111}

Figure 7.

Schematic comparison of the CCM and DTM MEA fabrication processes. Reproduced with permission from International Journal of Hydrogen Energy, Copyright 2011. ^{Reference You, Lee, Cho, Kim, Pak, Lee, Park and Park111}

Methanol ICEs

Methanol is a simple chemical with excellent combustion properties that makes it an ideal fuel for ICE-driven vehicles. ¹⁸⁰ Methanol contains about half the energy density of gasoline. Despite its lower energy content, methanol has a higher octane rating than gasoline (∼100), which facilitates greater fuel/air compression ratios (10–11 to 1 versus 8–9 to 1 for gasoline engine) and thus higher efficiencies. ^{Reference Olah, Goeppert and Prakash15} In addition, methanol is a safer and more environmentally benign fuel. In fact, methanol vehicles produce lower pollutant emissions, including decreased hydrocarbon emissions as well as decreased levels of NO _x , SO₂, and particulates when compared to gasoline-powered vehicles.

In the late 1980s, automotive companies including General Motors, Ford, Chrysler, Volvo, and Mercedes developed the technology to transform existing engine models into methanol flexible fuel vehicles (FFVs) for the cost of a few hundred dollars per vehicle. ^{Reference Olah, Goeppert and Prakash15} This methanol FFV option is available on a number of car/truck models and allows a vehicle to run on any combination of methanol and gasoline up to M-85, a blend of 85% methanol and 15% unleaded gasoline. In 1997, the number of U.S. methanol FFVs reached 21,000 units. Approximately 15,000 of these vehicles were located in California, where over 100 methanol refueling stations were located. ¹⁸¹ Unfortunately, in the late 1990s, the attractiveness of methanol as a transportation fuel was diminished by the lower price point of gasoline (less than $1.00 per gallon at that time). ¹⁸² Since then, gasoline prices have soared, but the methanol FFV has not yet rebounded. In the year 2009, the U.S. Congress passed the Open Fuel Standard Act, mandating that 50% of the vehicles produced in the US market must be compatible with blends up to M-85 and E-85 fuel (a blend of 85% of ethanol and 15% gasoline) by 2012 and that 80% of the vehicles produced should have the fuel option by the year 2015. ¹⁶⁶ Recently, the Lotus car company unveiled a “tri-flex-fuel” vehicle, the Exige-270E, which can run on a mixture of gasoline and methanol as well as gasoline and ethanol. ¹⁸³

The chemical and physical properties of methanol raise some issues for use in ICEs. Methanol can corrode some of the metals used in modern automotive engines, including aluminum, zinc, and magnesium. ¹⁸⁴ It can also react with plastics, rubbers, and gaskets, causing them to soften and swell and resulting in eventual leaks or system malfunctions. Therefore, automotive systems should be specifically designed for pure and blended methanol. Cold-start represents another major issue using pure methanol. Cold-start problems can occur because methanol lacks the highly volatile compounds such as butane, isobutene, or propane generally found in gasoline, which help to provide ignitable vapors to gasoline engines even under the most frigid conditions. ^{Reference Cheng and Kung185}

To further increase efficiency and lower emissions, methanol fed fuel cell vehicles (FCVs), which are fully qualified as zero emission vehicles (ZEVs) ¹⁸⁶ are being examined as an alternative for automotive power. The application and status of methanol-based fuel cell technology for the transportation sector are further discussed below.

DMFC technology to replace automotive ICEs

Both hydrogen and methanol fed FCVs can potentially offer higher efficiency and lower emission levels than either gasoline or pure methanol/blended methanol-powered ICE vehicles. ^{Reference McNicol, Rand and Williams12,Reference Olah, Goeppert and Prakash15,Reference Armstrong187} Presently, automotive companies are investing most heavily in hydrogen fed PEMFCs for FCVs. PEMFCs have high power density and a low working temperature (about 70 °C) that allows for rapid start-up. The electric efficiency is usually 40–60% and the output power can be changed quickly to meet a variable load. As a result of these characteristics, PEMFCs are considered to be promising for transport applications and a large number of companies are actively engaged in their development. ^{Reference Schaller and Gruber188–Reference Folkesson, Andersson, Alvfors, Alaküla and Overgaard191} Despite these advantages, both on-board and stationary hydrogen storage and distribution present significant challenges. ^{Reference Ogden, Steinbugler and Kreutz9,Reference Armaroli and Balzani10} The primary aim of any on-board fuel storage system is to provide a safe system to deliver sufficient fuel to satisfy the driving range requirement of the vehicle. These conditions must be accomplished at low cost without adding excessive weight and compromising the interior volume. At standard temperature and pressure (i.e., in the absence of compression), hydrogen gas requires about 3000 times more space than gasoline for an equivalent amount of energy. ^{Reference Armaroli and Balzani10} Storing under pressure improves the energy density by volume, allowing the tank to be smaller but not lighter. Currently, compressed hydrogen is the preferred solution in most fuel cell powered prototype vehicles, but its usage is far from satisfactory. Currently, hydrogen is pressurized from ∼350 to 700 atm in tanks made of new composite light-weight materials, such as carbon fiber with aluminum/steel/polymer liners, but these materials are very expensive. ^{Reference Armaroli and Balzani10} Even at 700 atm, hydrogen has 4.6 times lower volumetric energy content than gasoline, and the gravimetric energy density comparison is even less favorable (due to the considerable weight of the tanks). Moreover, in contrast to gasoline tanks, a hydrogen tank cannot adopt a shape that best suits the vehicle because the compressed hydrogen tank must be cylindrical to ensure its integrity under high pressure. Hydrogen tanks can therefore pose spatial integration issues. An intensive research is underway in an effort to find a more safe/convenient way to store hydrogen in fuel cell powered vehicles. ^{Reference Davis, Edelstein, Evenson, Breacher and Cox192–Reference Lipman194}

In view of the challenges associated with on-board hydrogen storage for PEMFC-powered vehicles, some companies have shown interest in developing DMFC-based FCVs. ^{Reference Dillon, Srinivasan, Arico and Antonucci11,Reference Harris195–Reference Baldauf and Preidel202} As discussed previously, methanol can significantly mitigate on-board storage constraints. Moreover, it can be distributed using the existing gasoline distribution infrastructure with only minor modifications. ^{Reference Dillon, Srinivasan, Arico and Antonucci11,Reference McNicol, Rand and Williams12} In 1998, General Motors Corporation (GMC) with its German subsidiary, Opel, introduced a methanol fuel-cell powered car called “Zafira”. ¹⁸⁶ The car was four-seat equipped with 50 kW electric motor. However, in 2000, Opel unveiled an updated version of Zafira, which run on hydrogen. ¹⁸³ In the same year, Daimler-Chrysler (Germany) unveiled NECAR 5 powered by a PEMFC, where the hydrogen was produced by on-board methanol reformation. The NECAR 5 was the first vehicle in which an entire fuel cell system with a built-in methanol reformer was accommodated within the underbody of a compact car platform (see Fig. 16). The car could carry five passengers with their luggage and attained a top speed of 90 mph. ¹⁸⁶

Figure 16.

Mercedes-Benz A-class compact car powered by a methanol-fueled FC system (although notably not a DMFC). Reproduced with permission from Mercedes-Benz – Copyright: Daimler AG.

In the following year, Ballard Power Systems Inc. (BPSI, Canada) in collaboration with Daimler-Chrysler, developed a 6 kW DMFC system for a one-person vehicle. ^{Reference Dillon, Srinivasan, Arico and Antonucci11,186} In this system, both the anode and cathode catalysts were impregnated over an oxidized carbon substrate (to increase the wettability of the carbon substrate). The MEAs were fabricated by a conventional approach by hot pressing the anode and cathode layers to either side of a solid polymer electrolyte. ^{Reference Zhang, Colbow and Wilkinson196} The details of the stack assembly, design, and performance of this system were not revealed. In 2003, Yamaha (Japan) developed the FC06 prototype DMFC-powered motorcycle with an output power of 500 W. ²⁰³ In 2007, an improved version of the DMFC-powered two-wheeler (FC-Dii) was introduced with an output power of 1 kW. The cell stack provided a 30% system electrical efficiency in a light-weight, compact, and high power density package. ²⁰⁴

Among hybrid electric vehicles (HEVs), the plug-in HEV has already made an impact in the market. On-board batteries provide the primary power, enabling typical driving range of 30–60 km before lengthy (8+ hour) recharging is required. ^{Reference Chan7} Some companies have therefore examined hybrid DFMC/battery systems to extend range and improve HEV performance. ^{Reference Chan7,186} Toyota (Japan) developed a 25 kW DMFC system that works in conjunction with a downsized battery pack to power its RAV 4 SUV. In this hybrid system, batteries are constantly recharged by the DMFC. Toyota's design draws extra power from the batteries to supplement the fuel cell during acceleration. The batteries also enhance the vehicle by providing instant power, and facilitate the initial warm-up cycle required to bring the fuel cell up to power upon cold start. ¹⁸⁶ In the year 1999, Georgetown unveiled a hybrid DMFC-battery system to power a prototype 40-foot transit bus using a 100 kW methanol-fueled phosphoric acid fuel cell. In this system, the fuel cell provided the baseload motive power while the batteries provided surge power and enabled storage of energy provided by regenerative braking. The speed and range of this hybrid bus was comparable to that of standard diesel buses, and it could be refueled quite easily and quickly. Even compared to clean-burning natural gas powered buses, fuel cell/hybrid buses are expected to have less than one-tenth of the hydrocarbon emissions and only 2% of the CO emissions as well as virtually no emission of nitrogen oxides or particulate matter (PM). ¹⁸⁶

Julich Forschungszeatrum (Germany) developed a commercial electric vehicle (EV) powered by a hybridized DMFC system (comprised of 100 single cells with a power output of 1.3 kW) coupled to a bank of Li-ion batteries. ²⁰⁵ Under normal operation, the DMFC drives the electric motor, while the battery boosts the power during start-up or uphill acceleration. Interestingly, in this DMFC system, the Pt loading level was reduced by 50% compared to other state-of-the-art DMFC electrodes. Furthermore, to ensure a lighter stack, many of the metallic stack materials were replaced with graphite components. A collaboration aimed at scooters as a potential early-adopter market entry point for DMFC technology was formed by Neah Power and EKO (India) to develop a DMFC to charge battery-powered “EV 60” electric scooters. ²⁰⁶ This second-generation electric scooter is marketed in the USA, Europe, and Asia. Smart Fuel Cell AG developed the EFOY DMFC + Li-ion hybrid system for light-weight EVs weighing less than 500 kg as shown in Fig. 17. ^{Reference Steckmann207} The DMFC is equipped with an automatic control system that constantly monitors the battery's charge state. Whenever it drops below a predefined level, the DMFC automatically begins charging the battery. Once the battery is fully charged, the fuel cell automatically returns to standby. This eliminates unfavorable operation modes like partial load operation and increases the total efficiency ratio. The company has successfully proved its reliability and functionality in many off-grid and light electric vehicle applications. The EVOY fuel cartridges are available at almost 1000 sales points in Europe and the US.

Figure 17.

SFC developed EFOY DMFC powered Li-ion hybrid system for small electrical cars weighing less than 500 kg. Reproduced with permission from World Electric Vehicle Journal, Copyright 2009. ^{Reference Steckmann207}

In 2008, Oorja Protonics developed the first DMFC on-board charging system, the OorjaPack™, which was designed to power material handling vehicles like forklifts (Fig. 18). ^{Reference Malhotra208} The DMFC system helps to continuously replenish the energy depleted by the operation of the vehicle and maintains the battery state of charge during operation between 50 and 80%. Using the hybrid DMFC/battery system eliminates the need for extra batteries and eliminates the downtime associated with periodic battery charging or swap-out. The resulting savings of 30–45 min per day per vehicle results in increased operational efficiency as well as productivity savings.

Figure 18.

DMFC (Oorja Protonics™) for on-board battery charging for material handing vehicles. Reproduced with permission from Elsevier, Copyright 2012. ^{Reference Malhotra208}

As discussed above, while progress continues on the development of DMFC systems for small vehicles or scooters, most current efforts for large-scale passenger cars and trucks have focused on hydrogen fed PEMFCs.

Key challenges in DMFC technology

Of all the potential applications for DMFC, the portable electronics power market appears to be the most compelling in the short-to-intermediate term. While several portable devices equipped with DMFC can already be purchased in niche commercial markets, widespread commercialization into mass markets such as mobile phone and laptops still faces numerous technical challenges. One of the key challenges for DMFC is increasing the power density and reducing the size of the system. In the transportation sector, although using DMFC technology alleviates the on-board fuel storage issue, the cost, power density, and efficiency of the DMFC system are far from satisfactory when compared against H₂-based fuel cells. Perhaps the greatest of many critical challenges impeding DMFC commercialization at larger scales is system costs, which are driven by materials and manufacturing challenges.

Materials challenges

Methanol crossover

DMFCs experience significantly lower OCVs (∼0.7 to 0.8 V versus ∼1.2 V theoretical) and lower power density than H₂-PEMFCs. The lower OCV and performance in the lower current density region is mainly attributed to two factors, methanol crossover from the anode to cathode through the Nafion membrane and the poor kinetics of the MOR as discussed above. ^{Reference Qi and Kaufman211,Reference Wang, Wasmus and Savinell212} Methanol crossover creates a mixed potential in the cathode, and also reduces the methanol utilization in the anode, thereby reducing the voltage of the DMFC by as much as 400 mV. ^{Reference Scott, Taama, Argyropoulos and Sundmacher213} Methanol crossover occurs mainly due to the structure of the Nafion membrane. The Nafion consists of hydrophobic main chains and hydrophilic side chains containing ionic sulfonic acid (–SO₃H) groups, where the latter groups cluster together to form ionic channels. While the flow of water through ionic channels helps to carry the protons and thus permits high proton conductivity (desirable), it also permits the flow of methanol across the membrane from the anode to cathode. The formation of wider ionic channels facilitated by the aliphatic polymer structure of Nafion leads to high methanol permeability. Because of this limitation, researchers have modified the Nafion type membranes and also developed new types of polymeric membranes to meet the additional membrane criteria needed for high-performance DMFC applications. These criteria include reduction of methanol crossover, maintenance of optimum proton conductivity, and an increase in the chemical stability. ^{Reference Antonucci, Arico, Creti, Ramunni and Antonucci86–Reference Wang, Wainright, Savinell and Litt91,Reference Li, Zhang and Wang93}

Important progress has been made in addressing these basic needs. For example, researchers from the Loss Alamos National Laboratory developed a new set of membranes from the poly(arylene ether sulfone) family (branded as 6F and 6F-CN), which showed higher initial DMFC performance than Nafion (Fig. 19), ^{Reference Zelenay, Brosha, Davey, Eickes, Fields, Garzon, Neergat, Pivovar, Purdy, Ramsey, Rowley, Wilson and Zhu214} possibly due to reduced methanol crossover. Furthermore, such membranes showed only slight performance degradation even after 700 h of durability testing. Recently, blended membranes consisting of an acid polymer and a basic polymer with similar aromatic backbones have emerged as a promising technology to reduce methanol crossover and achieve higher DMFC power density compared to Nafion. ^{Reference Fu, Manthiram and Guiver215–Reference Lee, Li, Manthiram and Guiver218} The blended membrane approach involves the tethering of an N-heterocycle group to an aromatic polymer like poly(sulfone) (PSf) or poly(ether ether ketone) (PEEK) to obtain a polymer with basic (rather than acidic) character. This polymer is then blended with an aromatic acid polymer such as sulfonated poly(ether ether ketone) (SPEEK) or sulfonated poly(sulfone) SPf. As an example, a 60 µm thick blended SPEEK/poly(sulfone)-amino benzimidazole (PSf-ABlm) membrane has been shown to exhibit significantly lower methanol crossover than Nafion 115 (thickness 125 µm). ^{Reference Manthiram219} However, the proton conductivity of the blended membrane is lower than that of the Nafion membrane. Although the proton conductivities of blended membranes tend to be lower, the lower thickness of these membranes enables lower total ionic resistance, resulting in lower voltage loss and higher power density than cells based on Nafion 115 or Nafion 117 membranes. Because of the lower methanol crossover, blended membranes exhibit higher OCV and allow the operation of DMFC with higher methanol concentrations, thereby enhancing DMFC power density. ^{Reference Lee, Li, Manthiram and Guiver218} In addition, blended membranes exhibit excellent chemical, thermal, and mechanical stabilities and are much less expensive than Nafion-based membranes.

Figure 19.

DMFC performance obtained with four different membranes (Nafion 117, 6F-40, 6F-CN-35, and BPSH-35) at an ambient air pressure and at the temperature of 80 °C. Reproduced with permission from Los Alamos National laboratory – FY progress report, 2003. ^{Reference Zelenay, Brosha, Davey, Eickes, Fields, Garzon, Neergat, Pivovar, Purdy, Ramsey, Rowley, Wilson and Zhu214}

As an alternative to blended membranes, alkaline membranes also show lower methanol crossover and can offer improved electrochemical kinetics. Commercially available AEMs include MORGANE-ADP and an ammonia-based AEM membrane from Tokuyama Co. Japan. Recent literature reports show that DMFCs made with Tokuyama alkaline membrane (fed along with NaOH/KOH solution) exhibited higher OCV and comparable or slightly higher performance than acid-based DMFCs. ^{Reference Prakash, Krause, Viva, Natrayanan and Olah47,Reference Joghee, Pylypenko, Wood, Bender and O'Hayre48} The ionic mobility of OH⁻ is relatively low compared to that of H⁺; thus it is generally required to feed an alkaline liquid (KOH/NaOH solution) along with methanol to improve its OH⁻ ionic mobility during the DMFC operation. However, reaction of the OH⁻ ions with the CO₂ produced during the cell operation can result in the formation of carbonate and bicarbonate ions. ^{Reference McLean, Niet, Prince-Richard and Djilali220,Reference Cifrain and Kordesch221} Formation of these ions pose a detrimental effect to the performance of a cell by means of (i) blocking the active sites of the electrodes through the formation of metal carbonate crystals (most commonly as Na₂CO₃ or K₂CO₃), ^{Reference Cifrain and Kordesch221} (ii) depletion of the OH⁻ ions concentration at the anode, thereby decreasing the MOR kinetics, ^{Reference Kim, Kim, Hwang and Lee138} and (iii) incorporation of carbonate/bicarbonate ions into the membrane, thereby decreasing conductivity and creating an unfavorable pH gradient across the membrane. ^{Reference Wang, Li, Hu, Zhuang, Lu and Xu222} The development of AEMs with higher ionic conductivity and good chemical stability may open up promising avenues for lower-cost, higher-performance DMFCs.

Manufacturing challenges

MEA fabrication

DMFC power density and performance are largely dictated by the structure of the MEA. The most widely used fabrication techniques for the mass production of the MEAs, such as the CCM and DTM methods present certain advantages and disadvantages in this regard. Although the CCM method provides a good interfacial contact between the catalyst layer and the electrolyte membrane, this method is limited by membrane swelling and wrinkling problems, resulting in deformation or cracking of the catalyst layer due to rapid volume changes of the membrane. To minimize the dimensional changes caused by the standard CCM method, Park et al. ^{Reference Park, Li and Manthiram236} presoaked their membranes in ethylene glycol (to induce swelling) and subsequently sprayed the catalyst slurry on the preswollen membranes. MEAs fabricated by this method showed better fuel cell performance when compared to conventional MEAs due to the reduction in stress of the membrane and catalyst layer by the preswelling process. Importantly, the spraying process must be carried out in several steps to avoid sedimentation of the catalytic particles. Since this is time consuming, the spraying method may not be suitable for high catalyst loading. ^{Reference Zheng, Suominen and Tuominen237}

In the DTM method, the catalyst layers are deposited on Teflon decal substrates and are subsequently transferred to a dry membrane during the hot-pressing of the MEA. The catalyst layers in MEAs fabricated by DTM tend to exhibit relatively poor interfacial contact with the membrane. ^{Reference Prabhuram, Krishnan, Choi, Lim, Ha and Kim122} Because of this problem, DTM MEAs experience higher ohmic resistance during DMFC operation. Moreover, it is difficult to control the porosity and the thickness of the catalyst layer due to the dehydration of the membrane during the decal transfer process and possible sintering of the catalytic nanoparticles occur because of the heat load that is applied. ^{Reference Song, Liang, Zhou, Sun, Xin, Stergiopoulos and Tsiakaras238} In the DTM, the ionomer segregation (ionomer in the catalyst layers migrates and form a skin layer in the side of the catalyst layer, which is closer to the surface of the Teflon substrates due to hydrophobic interaction between ionomer and Teflon) usually occurs on one side during the high-temperature MEA hot-pressing process (160–210 °C). ^{Reference Xiong, Liu, Zhang and Zhu100,Reference Xie, Garzon, Zawodzinski and Smith239} The formation of ionomer segregation in the catalyst layer acts as a barrier for reactant mass transport during the fuel cell operation. ^{Reference Xiong, Liu, Zhang and Zhu100} Recently, researchers overcame the ionomer segregation problem by introducing buffer layers composed of carbon powder and Nafion ionomer in between the catalyst layers and the Teflon decal substrates during the MEA fabrication process. ^{Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110,Reference Park, Cho, Cho, Park, Jung, Ahn and Sung240} The overall fabrication process associated with conventional DTM is rather complex and attempts are being made to simplify it for large-scale MEA production. Cho et al. and Krishnan et al. ^{Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110,Reference Krishnan, Prabhuram, Hong, Kim, Yoon, Ha, Lim and Kim241} have demonstrated a simplified DTM process that involves transfer of the catalyst layer to the membrane at temperatures as low as 110–140 °C without any material modification or performance deterioration. The schematic diagram of the conventional high-temperature and low-temperature DTM is shown in Fig. 20.

Figure 20.

Schematic diagram of conventional high-temperature and low-temperature DTM. Decal-IC refers to right-side low-temperature decal process, where I and C indicate the outer ionomer and carbon layers, respectively. Decal-I corresponds to the same method as decal-IC except for the carbon layer coating. Reproduced with permission from Elsevier, Copyright 2009. ^{Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110}

Cho et al. ^{Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110} compared the DMFC performance obtained from MEAs made from CCM and low-temperature DTM (Decal-IC) using two different concentrations of methanol (0.5 and 1M) as shown in Fig. 21. It can be seen from figure that the low-temperature DTM MEA yielded higher power density (130 mW cm⁻²) than the CCM MEA (120 mW cm⁻²) when using 0.5M methanol. However, at 1M methanol, the low-temperature DTM MEA exhibited lower power density than the CCM MEA, because in the former, MEA methanol crossover appears to be serious. Further investigation is required to mitigate the methanol crossover problem in low-temperature DTM MEAs.

Figure 21.

Comparison of DMFC performance of the MEAs fabricated by CCM and low-temperature DTM (Decal-IC). Reproduced with permission from Elsevier, Copyright 2009. ^{Reference Cho, Kim, Prabhuram, Hwang, Ahn, Ha and Kim110}

Environmental impact

A transition to DMFC technologies will produce environmental and societal impacts based on the costs, emissions, recycling potential, and toxicity associated with methanol and methanol production. In addition, the displacement of other fuels associated with a transition to methanol will also have significant environmental and societal impacts. The degree of these impacts will depend primarily on two factors – the amount and type of fuel that is displaced, and the amount of methanol required for the adopted technologies. The environmental implications will be significantly broader if DMFC technologies requiring large amounts of methanol (like vehicles) are adopted versus technologies that use relatively less methanol (like consumer electronics).

Methods of methanol production

Methanol can be produced using a number of different feedstocks and techniques (see Refs. Reference Olah, Goeppert and Prakash15 and 245 for a more complete study of methanol production conditions and methods). According to the Methanol Institute, methanol is the most basic alcohol and “can be made from virtually anything that is, or ever was, a plant. This includes common fossil fuels – like natural gas and coal – and renewable resources like biomass, landfill gas, and even power plant emissions and CO₂ from the atmosphere.” ²⁴⁵ The feedstock and method of methanol production must be considered when evaluating the environmental impact of expanding methanol-based technologies and here we will discuss only commonly used production methods and chemical recycling of CO₂ to produce methanol.

Converting the methane (CH₄) found in natural gas to methanol (CH₃OH) is currently the most widely used and cost-effective method of methanol production. ^{Reference Olah, Goeppert and Prakash15,245,Reference Aasberg-Petersen, Nielsen, Dybkjær and Perregaard246} It is likely this trend will continue in the near- and mid-term as companies are building, expanding, or restarting natural gas-based methanol production plants (mostly along the gulf coast) because of the recent natural gas boom in the US. ²⁴⁷ The typical natural gas to methanol plant utilizes a two-step process to produce methanol. First, the natural gas feedstock is converted into a synthesis gas (syngas) that consists of carbon monoxide (CO), carbon dioxide (CO₂), and hydrogen (H₂). Syngas is typically produced through the catalytic reforming of methane and steam by the following reaction ²⁴⁵ :

(14) $$2{\rm{C}}{{\rm{H}}_4} &#x002B; 3{{\rm{H}}_2}{\rm{O}} \to {\rm{CO}} &#x002B; {\rm{C}}{{\rm{O}}_2} &#x002B; 7{{\rm{H}}_2}\left( {{\rm{Syngas}}} \right).$$

The ratio of the components in the syngas depends on several factors including the reaction temperature, pressure, and water to methane ratio (H₂O/CH₄). The reaction is endothermic and becomes more efficient with increases in temperature and/or decreases in pressure. ^{Reference Olah, Goeppert and Prakash15} The second step in the process is the catalytic synthesis of methanol from the syngas ²⁴⁵ :

(15) $${\rm{CO}} &#x002B; {\rm{C}}{{\rm{O}}_{\rm{2}}} &#x002B; 7{{\rm{H}}_{\rm{2}}} \to 2{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{OH}} &#x002B; 2{{\rm{H}}_{\rm{2}}} &#x002B; {{\rm{H}}_{\rm{2}}}{\rm{O}}.$$

This step is highly exothermic and most methanol production plants use the excess heat to generate some of the electricity used in the production process.

While the combination of steam reforming and catalytic synthesis from natural gas feedstock is the most common route to produce methanol, each of these steps can be accomplished through a number of different methods and utilizing a variety of feedstocks. Currently, methanol is made almost exclusively using a two-step process where syngas production is the first step, ^{Reference Olah, Goeppert and Prakash15} and the relative amounts of CO, CO₂, and H₂ in the syngas stream vary depending on the feedstock. The syngas composition can be expressed using S, a number that depends on the number of moles (n) of each species within the gas stream ^{Reference Olah, Goeppert and Prakash15} :

(16) $$S &#x003D; {{\left( {n{{\rm{H}}_2} - n{\rm{C}}{{\rm{O}}_2}} \right)} \over {\left( {n{\rm{CO}} &#x002B; n{\rm{C}}{{\rm{O}}_2}} \right)}}.$$

The ideal value for S is 2, which would produce a perfectly balanced second stage reaction – one possessing neither a deficiency (S < 2) nor an excess (S > 2) of hydrogen. The composition of the syngas can be manipulated by steam reforming the feedstock or by adding an external source of H₂ (for S < 2) or CO₂ (for S > 2) during the methanol-producing reaction. ^{Reference Olah, Goeppert and Prakash15}

From an emissions standpoint, values of S at or above 2 are better because they do not produce excess CO₂. In fact, in the case of methanol production from natural gas via the route detailed in Eqs. (14) and (15), the value of S is 3 and excess CO₂ (from a waste stream or the environment) can be added and consumed to produce a greater quantity of methanol. Conversely, syngas produced from coal via a gasification process yields an S value below 2. In this case, the excess CO₂ produced in the syngas reaction is either consumed by adding H₂ or separated (and released as part of an emission stream). ^{Reference Olah, Goeppert and Prakash15}

Bio-feedstocks have also been examined as a renewable source for methanol production. The S-value for syngas produced from bio-feedstocks varies depending on the feedstock composition but is typically below 2 (Ref. Reference Specht, Bandi, Baumgart, Murray, Gretz, Eliasson, Riemer and Wokaun248) and can be manipulated using the same methods used for fossil-fuel feedstocks. ^{Reference Olah, Goeppert and Prakash15,245,Reference Specht, Bandi, Baumgart, Murray, Gretz, Eliasson, Riemer and Wokaun248} When discussing production of methanol from bio-feedstocks, it is also important to consider the possibility of food crop displacement. Already biofuels have had a significant impact on food prices in some areas because they compete for the same agricultural resources. ^{Reference Mitchell17} This would likely also be the case if mass quantities of methanol were produced from biomass.

In addition to the methods of methanol production discussed here, there are many other methods under investigation (see Refs. Reference Olah, Goeppert and Prakash15 and 245 for a more complete list) that have the potential to increase production efficiency and/or decrease emissions associated with production. Unfortunately, methanol production, like most industrial processes, is optimized for cost savings rather than for minimizing environmental impacts – a situation that is likely to change only with enactment of stricter emission regulations.

One further method of methanol production is worth noting – that of production by chemically recycling CO₂. This process requires H₂, which can be generated through electrolysis of water using any energy source. For a fully renewable process, a renewable energy source like geothermal, solar, or wind energy can be used to produce the necessary H₂. The second step in the process is the reaction of H₂ with CO₂ (from an industrial waste stream or the environment) to produce methanol. ^{Reference Olah, Goeppert and Prakash15,249}

Carbon Recycling International opened the first commercial plant to produce renewable methanol in 2011. Named after Nobel Laureate George Olah, the plant is located beside the Svartsengi Power Station in Iceland and uses power and waste CO₂ from the geothermal power plant to produce renewable methanol. The plant produces around 2 million liters of methanol per year but is currently under expansion to increase production capacity to 5 million liters per year. The production process is completely renewable and the sole byproduct is oxygen. ²⁴⁹ From an environmental standpoint, renewable methanol is the best methanol production option.

Environmental toxicity of methanol

Produced by a number of natural and human-related activities, methanol is already a part of the earth's environment. Methanol exhibits a low toxicity, and effects related to environmental exposure are negligible under normal circumstances. ²⁵⁰ Methanol is not known to accumulate in soil or water sources because it rapidly degrades through both photo-oxidation and biodegradation in aerobic and anaerobic environments. ^{Reference Olah, Goeppert and Prakash15,251}

As with any comparable fuel source, an increase in methanol use increases the chances for an accidental release of methanol into the environment during production, transportation, or storage. Although a fuel leak of any kind is never desirable, methanol possesses a few important advantages that set it apart from traditional fossil fuels. Compared to gasoline or diesel fuel, which contain many toxic and carcinogenic compounds that degrade very slowly in the environment, methanol exhibits low toxicity and rapid degradation – both qualities present a significant advantage in a spill. ^{Reference Olah, Goeppert and Prakash15,251}

In addition, methanol is completely miscible in water, ²⁵² which would allow for rapid dilution and dissipation to nontoxic levels in the case of a large spill into a water source. The remaining dilute methanol would then quickly be degraded to CO₂ and water by microorganisms in the environment. ^{Reference Olah, Goeppert and Prakash15,251} As has been evidenced by several oil spills in recent history, the immiscible nature of oil in water causes widespread and long-lived contamination, which is often devastating to the local environment. ^253–257 Based on these advantages, methanol is viewed as a less toxic and more environmentally friendly fuel.

Environmental and societal considerations for DMFC vehicles

Emissions from the transportation sector are an area of global concern because typical transportation emissions contain several harmful substances including VOCs, PM, sulfur dioxide (SO₂), carbon monoxide (CO), and various nitrogen oxides (NO _x ). Due to the hazardous nature of these emissions many local, ²⁵⁸ national, ^259–263 and international governing bodies have set forth regulations with increasingly stringent requirements for transportation emissions. These policies have driven interest in alternative fuels and technologies, and DMFC vehicles are only one of many alternatives under exploration.

On the emissions front, a DMFC vehicle is predicted to be far superior to the typical gasoline or diesel ICE vehicle because DMFC vehicles are expected to be ZEVs ¹⁸⁶ – or vehicles that do not emit any pollutants from the tailpipe during operation. Other alternatives that also reduce emissions compared to typical ICE vehicles include hydrogen-powered FCVs (H₂-FC), EVs, and hybrids. Both H₂-FC vehicles and EVs are also considered ZEVs, ²⁶⁴ putting them on par with DMFC vehicles on the direct (tailpipe) emissions front. Hybrids, with their combination of battery power and internal combustion produce approximately 80% less tailpipe pollutants than a comparable ICE vehicle. ²⁶⁵

Despite the clear emissions advantage of DMFC vehicles and other alternative technologies, the gradual changes to emission standards has left the door open for options that are technologically easier to achieve and still produce some emission reductions. One such option – that of using methanol in an ICE – either through blending methanol with gasoline for FFVs or using pure methanol fuel (M100) would provide a relatively easy method of reducing emissions.

Methanol forms no PM upon combustion and emits lower VOCs and NO _x than gasoline. According to the Environmental Protection Agency (EPA), hydrocarbon emissions can be reduced by up to 80% when M100 is substituted for gasoline in the ICE, however actual emissions would vary based on engine design. ²⁶⁶ In reality, despite the fact that methanol combustion is inherently cleaner, it is unlikely that it would yield such a high emissions reduction compared to gasoline due to emission control technologies which have already made significant emission reductions and are constantly being developed, allowing traditional fuels to compete with alternative fuels like methanol. ^{Reference Olah, Goeppert and Prakash15}

While emission reductions in the transportation sector are an important part of the equation, infrastructure will also play a significant role in any fuel or technology transition. The transportation sector relies on a vast infrastructure – one that is currently built around gasoline and diesel fuel. Adoption of any alternative fuel or technology will likely depend on the ease with which it integrates into the existing fuel infrastructure system, and here, methanol and DMFC vehicles have an advantage.

Because methanol is a liquid fuel it can utilize the same onboard storage (gas tanks) and fueling systems used in the current gasoline- and diesel-based transportation sector, with slight modifications. Fueling stations would also require modifications, namely the conversion of existing double-walled underground gasoline or diesel fuel storage tanks and installation of new piping and dispenser pumps compatible with methanol. Based on cost estimates, ²⁶⁷ 10% of the nearly 200,000 gasoline fueling stations in the US could be converted to supply methanol for around $1 billion. ¹⁸⁶ Compared to the more than $10 billion invested every year to maintain the US current fueling system, and the investment of over $12 billion by the oil industry to introduce reformulated gasoline in the US, ¹⁸⁶ the investment for conversion to methanol is well within the scope of possibility.

While it is unlikely that fueling infrastructure would be upgraded specifically for DMFC vehicles, a more likely scenario is the introduction of a methanol-based FFV which could tip the balance and initiate an infrastructure transition to accommodate methanol-based fuel. ^{Reference Olah, Goeppert and Prakash15} Therefore, the possible near-term transition to methanol-ICE FFVs could help eventually facilitate a future transition to DMFC vehicles. This is a significant advantage that DMFC vehicles hold over H₂-fed FCVs. Due to the necessary high pressures or low temperatures needed for H₂ storage, infrastructure development for this technology would be much more expensive than a similar transition to methanol. Indeed, there are currently only 12 hydrogen fueling stations in the US (most located in California), ²⁶⁸ while the number of methanol fueling stations in the US (again mostly in California) is an order of magnitude higher. ²⁶⁶

It is important to note that many of the methanol fueling stations in the US were established as part of a methanol flex fuel (mostly M85 or 85% methanol) program in California in the 1990s. At the time methanol was introduced, petroleum prices were rapidly falling, which effectively eliminated the economic incentive to switch to methanol. Despite the ultimate failure of methanol to enter the fuel market on a broader scale, the program demonstrated that methanol is a viable, and cleaner (than gasoline) transportation fuel. ^{Reference Bromberg and Cheng18} It is likely that the success of methanol as a transportation fuel will continue to be tied to petroleum prices. In our opinion, the recent steep price hikes and falls in the oil and gasoline markets which have led to a high degree of price uncertainty coupled with the surge in natural gas (most common feedstock for methanol) in the US may be enough to tip the scale to favor methanol.

Airline regulation of methanol for consumer electronics

Methanol is a toxic, flammable liquid and its use in DMFC-based consumer electronics aboard airlines is subject to regulation. The international standard for carry-on allowances for methanol fuel cartridges on airlines was set by the Dangerous Goods Panel (DGP) of the International Civil Aviation Organization (ICAO) and went into effect on January 1, 2007. Prior to rulemaking, the DGP analyzed several safety tests performed on micro fuel cells and fuel cell cartridges for consumer electronics. The tests provided data on how these devices performed at high altitude and under a number of different mechanical and electrical loading conditions. ²⁶⁹ The global standard, detailed in Part 8 of the ICAO Technical Instructions for the Safe Transport of Dangerous Goods by Air, ²⁷⁰ was developed from analysis of these tests and allows passengers to carry and use an approved fuel cell with an installed methanol cartridge and up to two spare cartridges. Each cartridge may contain up to 200 mL of methanol. ²⁷¹

Many countries around the world have incorporated the international standard into their passenger allowance regulations including Canada, China, Japan, the United Kingdom, and the United States, ²⁷² and no countries or airlines have filed variations to the rule with the ICAO. ²⁷⁰ In the US, the Department of Transportation (DoT) issued a ruling to match the international standard for methanol micro fuel cells and cartridges in 2008, ²⁷² however, the 200 mL maximum volume allowance per cartridge exceeds the 100 mL maximum allowance limit on liquids established by the Transportation Security Administration for carry-on bags. ²⁷³ Until now, the discrepancy between the two regulations in the US has not caused any significant problems, although it is likely that broad adoption of DMFC-powered consumer electronics would force reconsideration of the liquid allowance standards.

Policy and DMFC

Until now, methanol and DMFC have not had a significant market impact because economics favor existing technologies (batteries and oil/gasoline). Policies are often used to shift economics to favor development or adoption of new or different technologies, and there are several different policy options that might promote a shift toward methanol and DMFC-based technologies. Many countries institute policies that would promote specific technologies, while others prefer to enact policies that would enable a range of technologies within a specific area (i.e., clean energy).

Policy options that would impact methanol and DMFC technologies include, but are not limited to, reducing subsidies for fossil fuels (existing global fossil fuel subsidies are approximately USD $550 billion annually), ²⁷⁴ federal funding for research and development (broadly for clean energy technologies or specifically for methanol/DMFC technologies), increasingly strict industrial and vehicular emission regulations, establishment of a tax credit for CO₂ or other emission mitigations/reductions, and enactment of a carbon, emissions, or energy tax.

Most developed countries already provide some funding for research and development of clean energy technologies, however, the amount and specific allocation varies drastically from country to country and even from year to year as budgets and technologies change and evolve. Similarly, most developed countries have also established emission regulations, many of which mandate significantly decreasing emissions over a ten- to fifty-year time-span. ^259–263 While emission regulations do not specifically drive methanol or DMFC technologies, the ability to reduce emissions through methanol production (renewable methanol) and the fact that DMFC vehicles provide emission reductions compared to conventional gasoline-ICE vehicles means emission regulations can benefit these methanol-based technologies.

Changing the tax code is the other broadly used policy instrument that could promote a shift toward DMFC technologies. This can be accomplished in two ways – either through a tax incentive or through establishment of a new tax. Both routes have been utilized in governments around the world, and already there are some tax credits for mitigating or reducing emissions (like those given for hybrids, FFVs, EV, etc.). There has also been significant interest in – and in some places progress toward – the enactment of a carbon tax in recent years, ^275–280 however, wide-spread public support is not yet sufficient to drive this policy shift on a global scale. It is possible that with increasing climate change, a carbon tax or other tax on energy or emissions may gain the necessary support for enactment, which would also help in driving DMFC and other clean-energy technologies forward.