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44 otherwise be lost when an electric vehicle is braking and provide extra power to lessen the load on the batteries. Ultracapacitors will work with lithium- ion batteries in hybrid 48 V electric car systems to increase the lifetime of the battery pack by 50%, give 20% more range or allow designers to reduce the size of the battery.  They are even being used as the sole power source, with one maker of automated guided vehicles in factories developing systems powered by them. These are recharged from electrical contacts on the factory floor every 10-20 ft. Similarly, electric buses in China are using banks of ultracapacitors to power their motors, recharging in 20-30 s at each bus stop every 500 m. A new generation of carbon material is enabling new ultracapacitors to be designed with a higher energy density. Ultracapacitors currently have an energy density of around 5 kWh/kg, but the new designs are reaching 30 Wh/kg, which is approaching that of lead-acid batteries. One approach is to use 3D carbon nanotubes, which have surface area of 600 m 2 /kg. This means that one centimetre of aluminium electrode coated with the material provides a 5000x boost in surface area. The process is derived from the photovoltaic process used to apply anti-reflective layers on solar panels. It applies heat and particular chemicals to a small area on the substrate to create small bubbles on the surface, from which the nanotubes grow. The process can be performed on various substrates – copper, steel, even on carbon fibres – but for the initial ultracapacitors an aluminium substrate is used. The substrate is made in 30 cm wide rolls from which any type of ultracapacitor, either stacked or cylindrical, can be produced. Researchers are also looking at new materials to replace carbon as the electrodes. A new electrode design from a highly conductive, two-dimensional material called MXene is also boosting the performance of ultracapacitor designs. The MXene is combined with a hydrogel electrode design with more active sites that allow the ultracapacitor to store as much charge for its volume as a battery. To do this, researchers designed an electrode with multiple small openings to make each active site in the MXene readily accessible to ions to boost the energy capacity. These thin electrodes can charge up in just tens of milliseconds, which could lead to ultrafast energy storage devices than can be charged and discharged within seconds, but store much more energy than conventional ultracapacitors. Fuel cells and proton batteries Fuel cells are another form of energy storage and delivery for unmanned systems. A fuel cell generates electricity by the chemical reaction of hydrogen with oxygen. Every fuel cell has two electrodes as well as an electrolyte, which carries electrically charged particles from one electrode to the other, and a catalyst that speeds the reactions at the electrodes. The electrodes are separated by an insulating proton exchange membrane. One appeal of fuel cells is that they generate electricity with very little pollution, as the hydrogen and oxygen used in generating power combine to form water as a harmless by-product. A single fuel cell generates a voltage of only around 0.5 V, and the power output depends on the size of the cell, so in practice many fuel cells are assembled into a stack to provide the required power and voltage. A fuel cell needs to be refilled with hydrogen, however, and that needs either hydrogen from a pump (in a similar way to using gasoline or kerosene) or a local hydrogen generator. Some electric buses and cars already use fuel cells, but this need for refuelling limits their use for autonomous systems as the refuelling is more complex. To overcome that, researchers in Australia have developed a rechargeable ‘proton battery’. This uses a carbon electrode as a solid-state hydrogen store, coupled with a reversible fuel cell to produce the power.  During charging, the carbon in the electrode bonds with protons produced by splitting water with the help of electrons from the power supply. The protons are released again and pass back through the fuel cell to form water with oxygen from air.  April/May 2018 | Unmanned Systems Technology An ultracapacitor can be used to replace a battery with a redesigned charging system (Courtesy of NAWA Technologies) Hydrogen fuel cells are being used in electric vehicles (Courtesy of Nuvera)