Hydrogen gas generation utilizing a bromide electrolyte, a boron phosphide semiconductor and radiant energy

Radiant energy in conjunction with a boron phosphide semiconducting electrode to at least partially power an electrolytic cell is used in the generation of hydrogen, utilizing a bromide, preferably hydrogen bromide, as the essential electrolyte component in the electrolytic cell to solve overvoltage and corrosion problems associated with the use of conventional electrolytes in similar environments. The use of the bromide electrolyte results in the broadening of the selection of semiconductor electrodes which can be used in the process and apparatus of the present invention enabling the boron phosphide semiconducting electrode to be used with superior anticorrosive and radiant energy gathering results over conventional systems. The boron phosphide semiconductors employed can be either boron phosphide alone or multilayered structures with other semiconducting material. The hydrogen generated from such systems can be used to power a fuel cell.

CROSS REFERENCE TO RELATED APPLICATIONS 
Reference is made to U.S. patent application Ser. No. 956,761 filed by the 
same inventor Nov. 1, 1978, having the same assignee, which demonstrates a 
method useful for generating hydrogen bromide which can be used as an 
electrolyte in the present invention. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of art to which this invention pertains is electrolytic processes 
for producing hydrogen gas. 
2. Description of the Prior Art 
The use of solar energy to power electrolytic cells has received widespread 
attention in view of recent energy resource depletion and environmental 
pollution awareness. The production of hydrogen from electrolytic cells 
and the use of solar energy to power such cells has been recognized by the 
prior art as a marriage of two arts which has great potential in the 
solution of both these problems. While much work has been done on 
improving the efficiency of such systems, more work is needed in view of 
the low energy levels involved in extracting useful energy from the sun 
(i.e., low extractable voltages from sunlight per square foot of 
collection apparatus) and in view of the overvoltage and corrosion 
problems associated with the use of conventional electrolytes in this 
environment. The range of semiconductor material useful to gather this 
potentially great source of energy in this environment has also been 
limited because of the corrosive effects of conventional electrolytes on 
such semiconductors. For example, in an article by Frank and Bard (Journal 
of the American Chemical Society, Volume 99, July 1977, pgs. 4667-4675) 
the problem of corrosion of the electrode surfaces in photo-assisted 
electrolysis systems is described. 
What is needed is an electrolyte system useful in basically conventional 
electrolytic cells which are at least partially radiant energy powered and 
which will produce hydrogen to power a fuel cell while solving the 
inefficient overpotential and corrosion problems associated with the use 
of conventional electrolyte systems. What is also needed is a system which 
will expand the use of available semiconductor material which can be used 
in such systems to provide more flexibility in establishing photoelectric 
processes with greater efficiency. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, electrolytic processes for 
producing hydrogen gas useful to power a fuel cell have been invented 
which utilize bromides, and especially hydrogen bromide as the essential 
electrolyte, in conjunction with at least one boron phosphide 
semiconducting electrode, thereby solving the overpotential and corrosion 
problems associated with the use of conventional electrolytes in this 
environment in maximizing the efficiency of such photoelectrolytic 
processes. 
The foregoing and other objects, features and advantages of the present 
invention will become more apparent in light of the following detailed 
description of preferred embodiments thereof as discussed and illustrated 
in the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
As described above, much work has been done in combining solar energy with 
that body of electrolysis art which produces hydrogen for running, for 
example, a fuel cell, the marriage of the two arts providing a great 
source of electrical energy with limitless potential. However, such 
combination has been difficult because of the small amounts of energy 
which can be extracted from the sun without cumbersome equipment and the 
corrosion, overpotential, and other problems associated with the use of 
conventional electrolytes. The use of bromide compounds, and especially 
hydrogen bromide, as electrolytes in such a cell environment provides 
surprising advantages. The lower potentials at which a hydrogen bromide 
cell can be run, for example, compared to the higher cell potentials 
necessary for the dissociation of water or chloride compounds, both 
increases the life of the components of the cell and makes available a 
broader range of semiconductor material than is presently usable in such 
cells. Furthermore, the dissociation products from other halogen 
electrolytes such as hydrogen iodide or hydrogen fluoride, exist as solids 
or much more corrosive gases under normal conditions of atmosphere and 
pressure. This presents a myriad of problems of precipitation and special 
handling in both the electrolytic cell and the fuel cell. And a hydrogen 
bromide electrolyte provides more energy storage per pound than, for 
example, hydrogen iodide in an electrolytic cell environment. 
While the present invention has been described in terms of producing 
hydrogen for use in a fuel cell, the bromine produced also has fuel cell 
utility. Note in this regard, an article by Glass et al, "Performance of 
Hydrogen-Bromine Fuel Cells", Advances in Chemistry Series, Vol. 47, 1964, 
A.C.S. Applied Publications, which describes the various advantages of 
such a system. 
The bromide cell of the present invention can also be run at reduced 
pressures and concentrations such that the photoelectrolytic cell can be 
used with voltages equivalent to the use of such things as hydrogen iodide 
but with the advantages of dealing with the liquid bromine produced, thus 
eliminating the problems associated with a product which exists as a solid 
such as iodine under normal solution conditions. Also, the large optical 
absorption coefficients of even dilute solutions of other halogen 
dissociation products such as iodine would have a severely adverse effect 
on the efficiency of a system which utilizes radiant energy such as light 
as a power source as in the disclosed invention. 
Another advantage of the bromide electrolyte system is that conventional 
electrolytic cells for the dissociation of water can be readily adapted to 
a bromide electrolyte system with little or no modification. Chloride or 
fluoride electrolyte systems, for example, which are more corrosive than 
even conventional water dissociation systems and solid iodine dissociation 
products would all clearly require greater modification. There are also 
very definite advantages of the bromide system in the elimination of the 
overvoltages associated with chloride and especially water dissociation 
products. Note the Glass et al article mentioned supra at page 204 and 
U.S. Pat. No. 4,021,323 at column 7. 
While solar energy is the preferred source of radiation in the process and 
apparatus of the invention, other radiant energy sources can be used such 
as laser radiation or light emitting solid state diodes, the only 
requirement being that the radiant energy be of proper wavelength and 
sufficient intensity to evolve hydrogen gas in the particular cell being 
irradiated. The proper wavelength required relates to the particular 
semiconductor being used. The wavelength must be short enough to at least 
match the characteristic band gap wavelength of the particular 
semiconductor used. The semiconductor will not absorb radiant wavelengths 
longer than its band gap radiation characteristic. In fact, one of the 
advantages of the invention is the elimination of the corrosion and 
oxidation problems of conventional electrolytes which attack many 
semiconductor materials, thus enabling a broader range of semiconductor 
material to be used. With a broader range of semiconducting material thus 
available, a broader range of light wavelength can be used to more 
efficiently power the system. Also, while it is preferred to run the 
electrolysis solely powered by radiant energy, such as light, great 
advantage can be obtained by combining the light powered system with an 
external power source such as a battery. This is of particular value in 
instances where the semiconductor-radiation combination produces 
insufficient photovoltage to meet the threshold voltage required to run 
the cell. Note the Nernst equation, infra. For example, for a 48% solution 
of HBr, 0.6 volt would be required to run the cell, thus any 
semiconductor-radiation combination producing less than that voltage with 
such solution would require an external power source. Even with sufficient 
voltage supplied by the radiation source the external power source could 
also be used to increase the rate of hydrogen gas evolution, although at a 
cost in efficiency of the system. In any case, the amount of voltage 
supplied from this external power source must be less than that required 
to electrolyze the bromide compound in the absence of the light irradiated 
semiconductor electrode in order to have an energy efficient system. In 
such a situation, the power recovered from the recombination of, for 
example, hydrogen and bromine in a fuel cell would be approximately equal 
to the sum of the solar input power and external voltage supplied. 
As stated above, with the system of the present invention a broader range 
of semiconductor material is available for solar collection because of the 
solving of overvoltage and corrosion problems associated with other 
conventional electrolytes in similar systems, and in the particular 
embodiment of this invention the use of boron phosphide has been found to 
produce several advantages. First of all, in many conventional systems 
because of the corrosion problems associated with environments similar to 
that of the present invention (note the Frank and Bard article supra) 
boron phosphide would not be available as a viable solar collection 
source. However, with the system of the present invention such is not the 
case. And boron phosphide has advantages over, for example, silicon in 
that it has a higher photovoltage, lessening the need for and amount of 
external voltage necessary to run the cells of the present invention. It 
also has advantages over conventional titanium dioxide semiconductors 
which can only collect radiant energy at wavelengths below about 4,000 
angstroms in that the BP can collect radiant energy at wavelengths up to 
about 6,200 angstroms. In the bromide system of the present invention the 
BP also has a corrosion resistance at least comparable to TiO.sub.2 and 
about the same as Si. 
While commercial BP may be used in the process, in the preferred embodiment 
of the invention the BP was formed by a conventional pyrolysis method, 
e.g., reacting diborane and phosphine in a standard cold wall reactor with 
Rf susceptor substrate heaters. It is essential to the present invention 
that at least one boron phosphide semiconductor be used as the photoanode, 
but optionally the BP can be used as the photocathode as well. As the 
photoanode an n-type BP is preferably employed doped with suitable n-type 
dopants such as silicon. 
The BP semiconductors can be formed on conventional forming substrates by 
vapor deposition, cathode sputtering, etc. (including epitaxially grown). 
The boron phosphide can also be produced and used in the single crystal 
state, in the polycrystalline state, or in the amorphous state. In a 
preferred embodiment the BP layer is formed, followed by, for example, 
vapor depositing an ohmic contact such as aluminum or gold onto the formed 
BP. Alternatively, after formation of the BP layer, a semiconducting layer 
such as gallium arsenide or silicon can be vapor deposited, cathode 
sputtered, etc., onto the preformed BP followed by the deposition and 
formation of the ohmic contact. The formation steps are conventional and 
within the purview of one skilled in this art. 
As stated above according to the present invention at least the photoanode 
comprises boron phosphide. The photocathode can also comprise a p-type 
boron phosphide material similar to the n-type boron phosphide of the 
anode or the cathode can be a conventional metal electrode such as 
platinum or titanium. If the p-type boron phosphide is used it can be 
commercially purchased or formed in the same manner as the n-type boron 
phosphide described above, the p-type doping produced by the by-product 
reactants in the BP formation or any other conventional p-type doping. In 
the present invention, Hall effect measurements were used to classify the 
BP material as either n-type or p-type doped. 
The Nernst equation which governs the cell potential relationship required 
for electrolysis in this process can be described as follows: 
EQU E=E.degree.+0.059 log P.sub.H.sbsb.2 +0.059 log C.sub.Br.sbsb.2 -0.059 log 
C.sub.HBr 
wherein 
E.degree.=standard cell potential for cell components (e.g. for HBr 
electrolysis 1.06 volt), 
P.sub.H.sbsb.2 =partial pressure of hydrogen produced in the cell, 
C.sub.Br.sbsb.2 =molar concentration of bromine liquid produced in the 
cell, 
C.sub.HBr =molar concentration of hydrogen bromide or other bromide in the 
cell, 
E=the threshold voltage or cell potential to be overcome by the 
photovoltage. This is the voltage at which current begins to flow in the 
cell and significant amounts of hydrogen and bromine begin to evolve. 
The preferred parameters for efficient operation of the cell of the present 
invention are: 
P.sub.H.sbsb.2 =0.05 psi 
C.sub.BR.sbsb.2 =0.1% 
C.sub.HBr =48%. 
A cell with such parameters can be efficiently run at temperatures between 
about 0.degree. and 100.degree. C. Percents as recited throughout the 
disclosure are percents by weight. 
The particular bromide electrolyte system of the invention and the 
advantages inherent in its use because of the cell potential, lack of 
oxidation-corrosion problems, and elimination of overpotential problems of 
conventional cells allow many different cell arrangements to be used in 
the performance of the invention. One arrangement can comprise a standard 
cell arrangement with the entire cell subject to radiation from a light 
source. Other arrangements can comprise cells with one metal electrode and 
one semiconductor electrode where the semiconductor can be irradiated 
either from the solution side or dry side of the cell. 
As stated above, the key component in the electrolytic solution is the 
bromide compound present in the solution in amounts up to about 50% by 
weight, with a concentration of about 48% by weight preferred. This 
provides the hydrogen (and bromine if desired) to run the ultimate fuel 
cell which the photoelectrolytic cell is intended to produce. While water 
is the preferred solvent for the electrolyte and hydrogen bromide the 
preferred electrolyte the system is readily adaptable to other solvents 
and bromide containing electrolytes. For example, alcohols or amines may 
be used as solvents for the system and such bromide electrolytes as KBr, 
NaBr, LiBr, CsBr and SrBr.sub.2 may be used either individually, as 
mixtures or admixtures with the HBr. If alcohol or amine solvents are 
employed it is preferred to add at least small amounts of water to the 
system especially if a bromide other than HBr is used as the bromide 
electrolyte. The concentration of the hydrogen bromide may be any 
concentration up to the saturation point of the solution, provided the 
cell potential does not reach the corrosion potential for the 
semiconductor being used. The system may also be run at any operable 
pressure with up to 1 atmosphere being preferred. 
As mentioned above, the source of energy to run the cell can be any radiant 
energy source with wavelengths shorter than the band gap radiation 
characteristic of the semiconductor used. For example, for the boron 
phosphide semiconductor of the present invention any light source with 
wavelengths less than 6,200 angstroms could run the system. 
Reference is now made to the various figures for details of the cell 
configuration. In FIG. 1 a conventional electrolytic cell housing 1 
comprising an n-type BP semiconducting anode 2 and a p-type BP 
semiconducting cathode 3 are connected through external circuit 4. The 
electrolyte solution 5 is a 48% solution of hydrogen bromide and water 
separated by a hydrogen ion permeable membrane 6 such as Nafion.RTM. (E. 
I. Dupond de Nemours and Co.), thin quartz, polyvinyl chloride, or 
polytetrafluoroethylene, which allow free hydrogen ion transport in the 
system. Upon activation with light or other radiant energy 7 current is 
conducted through the external circuit 4 upon dissociation of the hydrogen 
bromide resulting in the production of hydrogen gas 8 in the p-electrode 
chamber and liquid bromine 9 in the n-electrode chamber. 
In FIG. 2, a dry side irradiation cell arrangement is depicted wherein the 
cell housing 10 contains a metal electrode 11, such as platinum or 
titanium, connected by external circuit 12 to the semiconductor electrode 
13 containing a tin oxide outer layer 14. When light or other radiant 
energy 15 impinges on semiconductor 13, the hydrogen bromide electrolyte 
solution 16 dissociates, causing the migration of the hydrogen ions to the 
platinum or titanium electrode 11 and bromide ions to semiconductor 
electrode 13 resulting in the evolution of hydrogen gas 17 at electrode 11 
and liquid bromine 18 at electrode 13. 
FIG. 3 demonstrates another solution side radiation apparatus. Housing 19 
encloses the hydrogen bromide and water electrolyte solution 20 which is 
subjected to light or other radiation 21. When the radiation impinges 
semiconductor surface 22, charge transfer across the 
electrolyte-semiconductor interface takes place, discharging one of the 
ions in the solution and hydrogen gas 23 is evolved at the platinum 
electrode 24 and liquid bromine 25 at electrode 22. The transfer of charge 
across the electrolyte-semiconductor interface results in an imbalance of 
charge in the semiconductor and a driving voltage for current flow through 
an external circuit 26 to electrode 24 immersed in the electrolyte. As 
summarized in FIG. 4 in the generation of electrical power from the 
chemical reaction of H.sub.2 and Br.sub.2, the fuel cell generates HBr 
which is recycled through the system. During periods of high solar 
radiation the solar generated H.sub.2 and Br.sub.2 can be stored for 
utilization in the generation of electrical power during periods of little 
or no solar radiation. 
EXAMPLE I 
A 48% weight solution of a hydrogen bromide in water was placed in an 
electrolyte cell comprised of an n-type boron phosphide anode and a 
platinum cathode. The n-type boron phosphide semiconductor was comprised 
of a composite of a boron phosphide layer on gallium arsenide, with 
aluminum ohmic contacts. An external power source of 0.1 amp per 
centimeter squared was impressed across the electrodes. The system was run 
at 50.degree. C. and subjected to a simulated solar distribution of solar 
light of an intensity of about three times that of a normal sun (produced 
by a mercury xenon lamp operated at about 900 watts input power). The 
system was run for over a hundred hours, producing bromine liquid at the 
n-type electrode and hydrogen gas at the platinum electrode. No corrosion 
of the semiconductors was detected. 
Although this invention has been shown and described with respect to a 
preferred embodiment thereof, it should be understood by those skilled in 
the art that various changes and omissions in the form and detail thereof 
may be made therein without departing from the spirit and scope of the 
invention.