Electrolytic Silicon and Iron Powders as Alternatives to Hydrogen as Energy Carrier and Store

The hydrogen technology is at present regarded as a potential solution to the problems resulting from using fossil fuels, particularly CO2 emission. However, the development of the hydrogen technology has encountered a number of difficulties, of which the need to reversibly store the hydrogen gas is a major challenge. Particularly, the reversible storage capacities achievable even in the best materials or devices are too low, for example, LaNi5H6 (< 1.5 wt%, ~300K) and high pressure or liquid hydrogen tank (< 4 wt%), but the high storage capacity in some others, e.g. NaAlH4 (> 7 wt%, >520K) and LiBH4 (>18.4 wt%, >553K), does not allow convenient reuse of the stored hydrogen.In fact, hydrogen gas is not an energy source because it does not exist in nature. In any energy application, the hydrogen gas only plays the roles of the energy store when the hydrogen gas is produced using another form of energy, such as electricity from the renewable or nuclear energy, and of the energy carrier when the gas is combusted in an internal combustion engine or fed into a fuel cell. These two roles of hydrogen can be well played by other pure elemental substances, such as silicon and iron. Like hydrogen, the energy stored in silicon and iron can be released through a chemical or an electrochemical reaction with oxygen. The products from these reactions, i.e. silicon and iron oxides, are the natural components of the Earth and will have zero environmental impact. One of the reasons why hydrogen has been so far the research and public focus is its high specific energy (energy per unit mass). Unfortunately, hydrogen is a gas under ambient conditions and the need for storage by any known method significantly reduces hydrogen's real specific energy. For example, the specific heat from the combustion of hydrogen gas in air is 122.8 kJ/g (425 degC), but it reduces to 30.7 kJ/g when hydrogen is stored at 25wt% (the theoretical maximum hydrogen storage capacity), but to a disappointing low value of 8.0 kJ/g when hydrogen is stored at 6.5 wt% (the targeted reversible hydrogen storage capacity of the US Department of Energy). On the contrast, iron and silicon are stable solids under ambient conditions and there is no storage problem. For combustion in air, the specific heat of silicon is 32.4 kJ/g and that of iron is 7.3 kJ/g. The other consideration for energy application is the energy density (energy per unit volume). Using the mass density of the three elements, it can be shown that, again for combustion in air, the heat density is only 8.6 kJ/cm3 for liquid hydrogen, but 75.5 kJ/cm3 for silicon and 57.5 kJ/cm3 for iron. Therefore, silicon and iron are thermodynamically better than hydrogen when storage is considered. On the technical side, the combustion of silicon and iron powders has long been proven in research, and it is now the time to develop a technique in which silicon and iron powders can be produced easily using renewable energy, particularly solar energy. This proposed research aims to experimentally demonstrate the thermodynamically predicted feasibility of using silicon and iron powders as the alternatives to hydrogen as the energy store and carrier. Particularly, it is intended to produce and regenerate the silicon and iron powders from their oxides using molten salt electrolysis under solar energy workable conditions (electricity and heat). The applicant and co-workers have already performed preliminary tests and produced successfully fine silicon and iron powders using the novel FFC Cambridge Process (co-invented by the applicant in the UK) at relatively high temperatures (800 degC ~ 900 degC). It is intended to lower the molten salt temperatures in this project (< 500 degC) so that solar heat can be used in the process. The products will be investigated by TG and DSC and tested for combustion in air. The optimal powder particle morphology and its correlation with the electrolysis conditions will be identified