When Daimler unveiled its gull-winged Mercedes-Benz F 125! research vehicle at the recent Frankfurt Motor Show, the German automaker telegraphed its belief that large electric cars that travel long distances in 2025 will be powered by advanced hydrogen fuel cells, not next-generation batteries.
This sort of insight into the carmaker’s view of future auto tech abounds on its “long-term vision of the premium luxury sedan,” said Christian Mohrdieck, Director of Fuel Cell and Battery Drive Development. “By the end we had a rolling summary of the major future car technologies.” A team of top staffers at Daimler’s Research & Advanced Engineering in Sindelfingen, Germany, and Mercedes-Benz Design collaborated for two years on the concept sedan to commemorate the 125th anniversary of the automobile.
The team’s goal was to create a concept vehicle that looks two model generations ahead in technology, Mohrdieck said. “This meant that we were looking for a significant change—at least two digit percent-point improvements in all parameters. Not just an evolutionary but a revolutionary degree of change.”
“Take driving range,” he offered. “Today’s fuel cell vehicles go 400 to 500 km; the F 125! goes 1000 km.”
Fuel cells crucial
The continuing range limitations of battery power mean in this case that batteries only augment the car’s plug-in hybrid fuel-cell powerplant. “For the foreseeable future, battery vehicles will be used mostly for urban applications,” he said. “They do not make much sense for long-distance driving.”
That the company’s approach would opt for a green propulsion package that relies on the costly and yet-to-be-built distribution system for hydrogen fuel underscores its anticipation of shortcomings in next-generation batteries. Mohrdieck further defended the need for multibillion-dollar investments in nationwide hydrogen fueling infrastructures by noting that such fueling networks would cost no more than the grid infrastructures that will be needed to charge battery-only vehicles.
The F 125!’s high-power, clean, and fuel-efficient fuel-cell prototype will incorporate an improved PEM (proton exchange membrane) cell stack that uses significantly less platinum catalyst than today’s models, he said. “So far we haven’t seen any validated alternative to platinum catalysts, but we have reduced platinum loading by learning not to apply it to regions of the membrane that wouldn’t use it anyway.”
Mohrdieck noted several technical challenges with PEM fuel cells that engineers will need to have solved by 2025, such as minimizing humidification needs, a step that “requires changing the membrane,” as well as finding a means to boost the stacks’ operational lifetime by avoiding degradation from episodes of fuel starvation.
Alternative hydrogen storage
Rather than storing the hydrogen for the fuel cell as a cryogenic liquid or as a high-pressure gas, the F 125! will hold its hydrogen in a novel, high-capacity molecular sponge now in development that would itself sit compactly inside the vehicle’s structure. Whereas conventional tanks need to be cylindrical to withstand pressures as high as 700 bar (10.2 ksi), which wastes space, the F 125!'s storage compartment could be filled to only 30 bar (435 psi) or less, allowing engineers to integrate the fuel storage right into the body shell itself, even allowing the fuel tank to serve as a “structural” component.
One idea, Mohrdieck explained, is to use MOFs (metal organic frameworks), which involve ultraporous crystalline materials that chemically absorb and hold huge quantities of hydrogen in a small package—more than 24 million ft² (2.2 million m²) of internal surface area per pound. The MOF materials can be placed into many shapes, which assists space-efficient packaging.
Next battery in line
The hybrid’s fuel-cell stack will be augmented by a next-generation lithium-sulfur battery that is installed behind the rear seats. “Lithium-sulfur electrochemistry offers a lot more energy density. Much of industry and the German government views it as the next practical battery technology after lithium-ion,” he added. By 2025, the engineers reckon, lithium-sulfur batteries should have energy densities up to 350 W·h/kg (159 W·h/lb), a figure that represents roughly a doubling of current lithium-ion performance.
The vehicle’s battery pack, which has a 10-kW capacity, provides about 50 km (31 mi) of battery-only driving plus power assist for acceleration and hill climbing as well as braking energy recuperation. Of course, the real potential of the technology is difficult to assess at present because no lithium-sulfur battery packs yet exist, he said, but sample cell-level technology is under test.
The F 125! designers’ choice to include giant gull wing doors and an extra long wheelbase with minimum front and rear overhangs provides considerable open interior space, an effect that is further enhanced by the use of four wheel-rim electric motors, comprising what Daimler calls its e4MATIC system. The wheel hub motors provide four-wheel drive as well as flexible, optimal traction, he noted. The system combines active torque vectoring that assigns power to each individual wheel as needed and wheel-specific yaw-damping to improve handling stability at high speeds.
Mohrdieck admitted that all types of electric motors would be under consideration for use in the 2025 Mercedes sedan. “We’ve tried them all and each has its pros and cons.” If the wheel-rim motor approach were pursued, he said, “we’d need to address some handling issues due to unsprung weight as well as find very compact, high-power-density motors that can be exposed to constant abuse by the elements. We think that we can solve these issues in the 2025 time frame.”
Lightweight but safe body structure
The body shell of the four-seater, a judicious mix of advanced high-strength steels, aluminum alloys, carbon fiber weaves, and plastic composites, has a mass of only 551 lb (250 kg), which is some 40% less than that of today’s S-class sedan. At the same time, the stiff structure delivers a considerable improvement in safety performance.
To make structural costs affordable and boost demand, he concluded, the auto industry and its suppliers will need to find ways “to use cheaper materials and less of the more expensive materials.” Also key to cost-cutting will be to establish a truly competitive supplier network “with more than just a few suppliers” as well as “volume manufacturing, which enables development of cheaper production technology.”