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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
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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