Producing and Storing Hydrogen

We know it is completely feasible to produce hydrogen using solar energy from the point of view of thermodynamics. The idea is to use thermochemical processing. 

Step I metal oxide reduced metal oxide + O2 Step II reduced metal oxide + H2O metal oxide + 2H2 

If we use FeO in step I, then the temperature must be 1600 K. However, step II is then spontaneous at a temperature of -1200 K. 

Several systems are being explored for this purpose. These include the reduction of ZnO (in some cases enhanced by the presence of C), the reduction of ceria, and the reduction of oxides containing Fe (both iron oxide and a range of ferrites). 

All that we need to do is to optimize the ceramics and, perhaps the more challenging step, optimize the geometry of the sample to allow repeated cycling (1) without degradation and (2) while allowing easy extraction of the hydrogen. So the usual process is to improve the design (tailor) of both the material (e.g., powder versus multilayer films or foams) and of the reactor itself. 

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The palladised cathode gave the greatest percentage of PCP removal due to its superior capability to produce and store hydrogen. The poor performance at the iron cathode was mainly a result of its instability under the HDH condition. Both instability and poisoning of catalyst by intermediates were responsible for the poorer performance of the Ni cathode. 

Because hydrogen fluoride is extremely reactive, special materials are necessary for its handling and storage. Glass reacts with HF to produce SiF which leads to pressure buildup and potential mptures. Anhydrous hydrogen fluoride is produced and stored ia mild steel equipment. Teflon or polyethylene are frequently used for aqueous solutions. 

Fig. 27.6. A typical facility where liquid hydrogen is produced and stored. (Courtesy Air Liquide Canada.)...

In the decomposer, deionized water reacts with the amalgam, which becomes the anode to a short-circuited cathode. The caustic soda produced is stored or evaporated, if higher concentration is required. The hydrogen gas is cooled by refrigeration to remove water vapor and traces of mercury. Some of these techniques are employed in different facilities to maximize the production of chlorine, minimize the consumption of NaCl, and also to prevent the buildup of impurities such as sulfate in the brine.26 The production of pure chlorine gas and pure 50% sodium hydroxide with no need for further concentration of the dilute solution is the advantage that the mercury cell possesses over other cells. However, the cell consumes more energy and requires a very pure brine solution with least metal contaminants and above all requires more concern about mercury releases into the environment.4... 

A National Academy of Sciences committee estimated that the transition to a hydrogen economy could take decades, since tough challenges remain. These include how to produce, store and distribute hydrogen in adequate quantities and at reasonable costs without releasing greenhouse gases. 

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