Capacitor solution for range and charge time?

What single element of the electric car most hinders its acceptance by consumers? It's the battery, and not without reason. Batteries are heavy -- the NiMH one in the Toyota Prius weighs 70 kg (picture shows Camry Hybrid battery), while the one powering Audi's first e-tron, shown at Frankfurt in 2009, weighs 470 kg and takes up an entire R8 engine bay.

They're also inefficient storers of energy. Current battery technologies impose range limitations on EVs, with most running out of juice in well less than 200 km -- and that's before they hit the freeway. When they run out, they take hours to recharge, rather than the couple of minutes it takes to refill a petrol or diesel tank, which then delivers many hundreds of kilometres' driving.

All in all, battery power still has its work cut out convincing buyers of its viability to petroleum products.

But the battery is not the only way to power an electric motor. There's also a technology called the capacitor, and it has much going for it as an alternative on-board power source for electric cars. The capacitor is light, potent and durable -- something even the most advanced batteries in production EVs and hybrids can't claim.

While battery technologies still consume the lion's share of the massive -- and growing -- EV research and development budget, capacitors have not gone unnoticed. Certainly not by Case Western Reserve University professor Gerhard Welsch, who's been beavering away on them since 2000.

Nor the US Department of Energy's Advanced Research Projects Agency -- Energy (ARPA-E), which has just given Welsch a US$2.25 million grant to continue his efforts to come up with a compact, lightweight, reliable, high-powered capacitor. ARPA-E's take on the matter is that capacitor technologies have loads of potential to surmount the problems and obstacles presented by batteries. They're much smaller and therefore much lighter, and they can hold a high voltage charge, meaning they can absorb energy faster than batteries, and discharge it in higher voltages, eliminating the need for an inverter to boost the voltage to the engine.

How do they work? It's all rather complicated, but it goes like this: like batteries, capacitors have two electrodes: a positive anode and a negative cathode. Where they differ is in how they store energy. Batteries store it electrochemically, separating their poles with an electrolytic conductor. Capacitors separate them with an insulating material and store incoming energy in an electric field around it.

Capacitors have a number of advantages over conventional battery technologies that offer plenty of potential in EV and hybrid automotive applications. They absorb and release charge more efficiently and they last longer, with a lifespan of ten years or more. They're less vulnerable to damage from impact, overcharging and climatic fluctuations. They're low- maintenance and because they're not made of toxic chemicals, the potential environmental benefits are considerable.

Where they fall down is in energy storage -- lithium-ion batteries can hold up to 25 times more energy per unit of weight than high-end ultracapacitors.

That's where Welsch comes in. He believes he has it within his grasp to close that gap considerably, with a predicted tenfold increase in energy density over what's around at the moment. And ARPA-E is putting its money where his mouth is.

"A capacitor is the equivalent of an electron pressure tank, and the trick is to make the dielectric film (the insulating wall of the pressure tank), impenetrable to electrons by making it strong and as perfect as possible," Welsch said. "Perfect is not possible, but we can make a material that's close."

His capacitor uses an anode coated in a titanium oxide alloy with superfine texture to maximise its surface area in relation to its total volume. The bigger this surface area, the more electrons it can hold, boosting energy density and storage capacity. The anode takes spine-like form with many branches to maximise the surface area. The titanium oxide layer stores the energy by placing an insulating barrier between positive and negative electrical charges. The cathode uses a layer of electrolyte and a metallic layer of carbon or titanium. The system also overcomes one of the major bugbears of capacitors: electron leakage through flawed or damaged dielectric layers. It's self-repairing.

Until now, auto developers have viewed capacitors as a complementary technology, good for jobs like boosting the efficiency of regenerative braking systems by absorbing energy faster, or providing useful bursts of power for stop-start systems, but not up to the job of primary power source for the drivetrain.

Welsch's grant is but a drop alongside the ocean of cash being poured into batteries. But this is a space worth watching: it could help provide the missing link between the cars we know now and the cars of the future.

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