GM Electrochemists Suggest Ongoing Investment in Both Battery and Fuel Cell Research; Connecting the Science with Vehicle Engineering

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Projected incremental mass due to the energy storage and electricity generation system for (a) a 100- and (b) 300-mile range for a battery electric vehicle and a fuel cell electric vehicle. Credit: ACS, Wagner et al. Click to enlarge.

In a Perspective published in ACS’s Journal of Physical Chemistry Letters, researchers from the Electrochemical Energy Research Laboratory (EERL), General Motors Research & Development, suggest that given the strong societal need for full vehicle electrification and the respective technical challenges and commercial risk entailed, both Li-ion batteries and fuel cell systems for powering electric vehicles warrant continued strong development investment.

In their paper, Fred Wagner, a GM Technical Fellow and the Lab Group Manager for Advanced Electrodes of the EERL; Balasubramanian Lakshmanan, the Lab Group Manager for Materials-System Interface of EERL; and Mark Mathias, GM Technical Fellow and the Director of EERL, examine potential routes for improving the specific energy of battery storage, as well as for cost reduction on the fuel cell side. They stress the importance of the scientific developments being connected to vehicle developers who understand implementation realities. EERL is responsible for R&D for both battery and fuel cell systems.

The authors suggest that given the specific energy (Whnet/kg) characteristics of the technologies, full battery electric vehicles could be more suited for shorter range applications, with fuel cells used for longer distance vehicles.

..it is {the] fundamental dependence of the specific energy on the amount of electricity required...which determines the applicability of these systems in vehicles of various size and range. This allows batteries for small low-range pure electric vehicles but excludes them for larger-vehicle long-range applications. Previous studies comparing BEVs and FCEVs have reached similar conclusions that only limited-range BEVs are workable, with somewhat different crossover ranges between BEVs and FCEVs arising from different input assumptions for the two technologies.

...Li ion batteries provide a pathway for efficient use of renewable-sourced electricity in the transportation sector, but it is possible that fundamental physical limitations may prevent pure Li-ion-based BEVs from ever delivering the freedom of providing long trips, with intermittent quick refills, that consumers currently receive from their cars.

In addition to mass considerations, alternative powertrain feasibility is determined by packaging (i.e., volume) considerations. Once physical feasibility is established, commercialization is ultimately determined by cost. Packaging and high volume cost analyses of projected technologies are outside of the scope of this Perspective. Solely on the basis of the mass considerations above, we can conclude that battery-powered options are favored for small vehicles when short-range and long refueling times are acceptable. The fuel cell option is favored for large-vehicle, long-range options.

—Wagner et al.

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Estimated mass contributions of automotive Li ion battery technology as compared with USABC goals for advanced batteries for electric vehicles. Credit: ACS, Wagner et al. Click to enlarge.

Li-ion storage. With a focus on mass reduction, the authors estimate the kg/kWh total of various pack elements, concluding that about 50% of the mass is due to the cell materials, and ~70% of the cell material mass comes from the positive and negative electrode materials. Thus, they conclude, the primary mass reduction focus needs to be on improving the specific energy of the positive and negative active materials, in priority order.

New cathode materials with higher storage capacity and/or with substantially higher voltage—the latter also requiring development and implementation of electrolytes/solvents with improved oxidation resistance—are needed, according to the authors. One promising direction is the development of materials that enable the intercalation of more than one lithium ion per transition metal, they note. However, efforts to date have not yet yielded high-capacity durable materials ready for serious implementation efforts.

Replacing graphite anodes with other materials—such as silicon—that offer higher theoretical capacities is also an approach of interest, but the trade-off is swelling upon Li uptake, leading to durability challenges.

Using cell models of new positive and negative electrode material concepts such as those described above, we estimate that the USABC target of 200 Whtotal/kgpack is challenging but achievable. This will also require that the mass of the pack components be reduced by a factor of approximately 2 relative to state-of-the-art, an engineering and materials challenge that we consider achievable but outside of the scope of this paper.

—Wagner et al.

With respect to next-generation approaches, the Li-sulfur system, which relies on the use of low-cost sulfur on the positive electrode, offers promising specific energy because of the potential for storage of 2 moles of lithium for every mole of sulfur, the authors note. However, the sulfur is soluble in the electrolyte when not fully oxidized and is subject to migration and reduction on the negative electrode.

Metal-air systems—such as Zn-air or Li-air—are also of keen current interest.

Li-air cells pose fascinating scientific questions of the thermodynamics and the catalyzed kinetics of interactions of lithium with oxygen. For example, different catalysts may be needed for discharging and charging, and these catalysts must be able to coexist in the positive electrode without poisoning one another. Given the low levels of reversibility and low capacities at high current density demonstrated to date, Li-air cells will likely show impressive percentages of improvement in a number of metrics in the coming years. It is critical that enough attention also be given to fundamental engineering issues and to the absolute, not relative, metrics that reflect product requirements of a full-function BEV.

We are supportive of work on developing reversible Li-air and other advanced electrochemical energy storage systems while keeping the scientific development connected to vehicle developers who understand implementation realities.

—Wagner et al.

Fuel cells. Although fuel cell vehicles could offer the functionality of current automobiles “in an environmentally sustainable form”, the authors note that the issues of hydrogen supply and fuel cell system cost “remain significant.”

Although the platinum content on the anode constitutes a lower limit to the cost of mass-produced fuel cells, the authors state that anode Pt loadings can be low enough to have no real economic impact.

The cathode is another matter entirely. The kinetics of the oxygen reduction reaction (ORR) are notoriously slow (and therefore have received the attention of generations of generally frustrated, though recently more gratified, electrochemists), leading to the current necessity of using~0.4mgPt/cm2 geometric loadings of commercially optimized Pt/carbon black cathode catalysts, costing several thousands of dollars per vehicle.

—Wagner et al.

They outline five potential directions for improved ORR catalysts, with the potential for significant reduction of fuel cell cost:

  • Continuous-Layer Catalysts;
  • Pt Alloy and Dealloyed Catalysts;
  • Monolayer Catalysts;
  • Controlled Crystal Face Orientation Catalysts; and
  • Non-Pt Catalysts.

If the range problems of batteries could be solved, the pathways to acceptably low fuel cell Pt usage could be brought to fruition, and hydrogen and electrical infrastructure issues could be adequately addressed; the choice between these two technologies for electrification of the automobile would come down to matters of the overall system and lifetime operating costs.

Li ion batteries use intrinsically cheap materials but require a very large surface area of very finely controlled thin layers, interfaces, and separators and, by their nature, use monopolar design (current collectors coming out of the side of each cell). Li ion batteries are already mass produced for use in portable electronic devices; therefore, many of the opportunities for cost reduction through scale have already been taken, and therefore, cost reduction must be addressed through the development and implementation of improved materials.

Fuel cells utilize some intrinsically more-expensive materials, though as we have seen, pathways exist for drastic reductions in the amounts used. Due to the much higher current densities obtainable with the more conductive fuel cell electrolyte, the total geometric surface area of the cells is ~30-fold less. The ability to use bipolar construction, with cells stacked in series with the negative current collector of one cell serving also as the positive current collector of the adjacent cell, further simplifies the structures. However, fuel cell systems also require more complex balance of plant, including a hydrogen tank and an air compressor.

In summary, ongoing development work should be coupled to continuous reevaluation of the system-level physical feasibility and relative cost structures of BEV and FCEV systems,with the results informing future strategy setting.

—Wagner et al.

Resources

  • Frederick T. Wagner, Balasubramanian Lakshmanan and Mark F. Mathias (2010) Electrochemistry and the Future of the Automobile. J. Phys. Chem. Lett., doi: 10.1021/jz100553m


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