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Metal hydride fuel cells are a subclass of alkaline fuel cells that are in the research and development phase. A notable feature is their ability to chemically bond and store hydrogen within the cell. This feature is shared with direct borohydride fuel cells, although the two differ in that MHFCs are refueled with pure hydrogen. Though the absorption characteristic of metal hydrides (around 2%) is far lower than sodium-borohydrides and other "light" metal hydrides (around 10.8%), prototypes have been claimed to demonstrate a number of interesting characteristics: * The ability to be recharged with electrical energy (similar to NiMH batteries); * Low operating temperatures (down to −20°C); * Fast kinetics; * Extended shelf life; * Fast "cold start" properties; * Ability to operate for limited periods of time with no external hydrogen source, enabling "hot swapping" of fuel canisters. Metal hydrides fuel cells are being researched by ECD Ovonics, as well as by the Japanese National Institute of Advanced Industrial Science and Technology (AIST). Though similar, the two MHFC concepts use different catalysts.Thus far, neither research project has produced a demonstrable model outside of a laboratory – only publications and patents – and significant efficiency hurdles have yet to be overcome. The Ovonics and AIST metal hydride fuel cells claim current densities of 250 mA/cm² and 20 mA/cm², respectively, versus typical PEMFC performance at 1 A/cm².