Watersplitting Systems By Renewable Energy

Presently, more than 98 % of the hydrogen gas consumed by the industries are provided by reforming coal, naphtha, and natural gas, and will be unable to bear the future demand [7], It is strongly required to supply the hydrogen produced from water by renewable energy sources.

Table 1 shows the water-splitting methods (-lysis) by the different kinds of energies. Hydrogen produced by water electrolysis is the traditional way since M. Faraday, however it cannot be qualified as clean energy carrier because of its energy resources, unless the electric power is generated by renewable energy. Accordingly electrolysis should be combined with for instance, solar cell, solar thermal, etc.

It is noticed that the improvement of solar cells is remarkable, and the efficiencies of Si-single crystal cell and the poly crystalline cell reach 17 % and 12.5 %, respectively. The average cost of solar cell module is $3 per watt, which can be competitive with other conventional power sources [8],

Author has introduced the discovery of mechanolysis, a novel phenomenon of water splitting [9,10], which has been understood as a result of frictional electricity between the Teflon stirring rod and the Pyrex glass of the beaker, where pure water containing semiconductor powder is filled. Author [9] has pointed opt that the semiconductor must have the property of the hopping conductivity, and called tribolysis. There exists another type of mechanolysis, which may be due to the piezo electrolysis. This type is called piezolysis, but not discovered yet.

However, a giant piezoelectric effect has been found in the Pb-based complex pervoskite oxides. In particular, the morphotropic boundary relaxor and PbTio3 complex exhibits huge piezoelectric response, so that an effective piezolysis is expected.

Another big merit of mechanolysis system combined with wind power, relative to wind electric generation, is that hydrogen can be stored in a vessel.

Water vapor is split into its components at the temperatures higher than ~4200 k, then hydrogen will be given by separating the mixture gas. This method is called direct thermal water-splitting. As the high temperature technologies are so difficult that this may not be promising. However, thermal energy is useful to split water especially in thermochemical method [11] and in fermentation [12], Fermentation does not need very high temperature and is environmentally friendly, and is expected to be one of the aces of the frontiers.

Water-splitting by thermal energy is called pyrolysis.

The global surface is filled of sunshine, total amount of which is more than ten thousand times compared to the total consuming energies by mankind.

Table 1. Water-splitting methods by renewable energies

() means duplicate. *Piezolysis is not discovered yet. *** Bubble fusion is not confirmed yet, which is a continuance of water-sonolysis**.

1. Electrolysis

2. Mechanolysis

3. Pyrolysis

4. Photolysis

5. Chemolysis

6. Biolysis

(1) combined with renewable power systems"^

(1) tribolysis, piezolysis*

(1) thermochmical^

(2) direct thermal

(3) fermentation"-'

(1) photoelectrochemical

(2) photobiochemical

(3)(solar cell combined with electrolysis)0^

(1) (density gradient combined with electrolysis)"''

(2) (ion exchange membrane)^

(3) (thermochemical)6'1

(1) living systems

(2) cell free systems

[7]Bubble fusion: D-D fusion triggered by the implosion of cavitation bubble.***

Solar energy with the short wave length range and long wave range can be utilized by photolysis and by pyrolysis, respectively. As for photolysis, we have (1) the biological area based upon the photosynthesis, and (2) the electrochemical area such as photoelectrochemical with photo semiconductor, with dye and metal complex etc. [13],

Photoelectrochemical water-splitting is a combination of solar cell with electrolysis in a electrolyte , and has been actively studied. However, the selection of the photo semiconductors is so tightly limited that photoelectrochemical methods can hardly compete with the combined system of solar cell with electrolysis.

e- - donors biological solar. _> system* ^photons

H+ - donors physico-chemical regulations*

Figure 2. Hydrogen production model by living systems. *

: Including the genetic applications.

On the other hand, as is repeated so far, the biolysis has a bright future because of the biological system, which may be improved by the genetic evolution (Fig. 2 [14]).

Besides the subjects in Table 1, someone would list up radiolysis, which is the water-splitting system by radioactive rays. However, it belongs to a kind of photolysis, and has apprehensions that the produced hydrogen may carry the contaminated radioactivity.

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