Photoelectrochemical cells employing chalcogenophosphate (MPX3) photoelectrodes are described where M is selected from the group of transition metal series of elements beginning with scandium (atomic number 21) through germanium (atomic number 32) yttrium (atomic number 39) through antimony (atomic number 51) and lanthanum (atomic number 57) through polonium (atomic number 84); P is phosphorus; and X is selected from the chalogenide series consisting of sulfur, selenium, and tellurium. These compounds have bandgaps in the desirable range from 2.0 eV to 2.2 eV for the photoelectrolysis of water and are stable when used as photoelectrodes for the same

Byvik, C. E.

Reichman, B.

English

NASA Technical Reports Server

United States Patent 1191 
Reichman et al. 
11 11 4,439,301 
[45] Mar. 27, 1984 
[54] PHOTOELECI’ROCHEMICAL CELLS 
INCLUDING CHALCOGENOPHOSPHATE 
PHOTOELECI’RODES 
[75] Inventors: 
[73] Assignee: 
[21] Appl. No.: 
[22] Filed: 
Benjamin Reichman, Birmingham, 
Mich.; Charles E. Byvik, Hampton, 
Va. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Admiitration, Washington, D.C. 
433,196 
Oct. 7, 1982 
[51] 
[52] 
Int. (3 .3  .......................... C25B 1/02; HOlM 6/36 
U.S. Cl. ............................. 204/278; 204DIG. 3; 
204DIG. 4; 204/129; 204/280; 429/111; 
423/303 
[58] Field of Search ......... 429/111; 204/242, DIG. 3, 
204DIG. 4,278,280, 129; 136/252; 357/30; 
423/303 
1561 References Cited 
PUBLICATIONS 
C. E. Byvik et al., “Photoelectrochemical Response of 
Some Layered Chalcogenophosphate Compounds 
(MPX3),” J.  Electrochem SOC.. vol. 129, pp. 237-238, 
(Jan. 1982). 
C. E. Byvik et al., “Layered Transition Metal Thio- 
phosphates as Photoelectrodes in Photoelectrochemical 
Cells,” Solar Energy Mafls., vol. 7, pp. 213-213, (Sep.- 
/Oct. 1982). 
Primary Examiner-Aaron Weisstuch 
Attorney, Agent, or Firm-Howard J. Osborn; John R. 
Manning; William H. King 
I 
1571 ABSTRACT 
Photoelectrochemical cells employing chalcogeno- 
phosphate (MPX3) photoelectrodes are disclosed, 
where M is selected from the group of transition metal 
series of elements beginning with scandium (atomic 
number 21) through germanium (atomic number 32) 
yttrium (atomic number 39) through antimony (atomic 
number 51) and lanthanum (atomic number 57) through 
polonium (atomic number 84); P is phosphorus; and X is 
selected from the chalogenide series consisting of sulfur, 
selenium, and tellurium. These compounds have band- 
gaps in the desirable range of 2.0 eV to 2.2 eV for the 
photoelectrolysis of water and are stable when used as 
photoelectrodes for the same. 
6 Claims, 4 Drawing Figures 
TO STORAGE 
20 
‘ T O  STORAGE 
20 
https://ntrs.nasa.gov/search.jsp?R=19840014951 2020-03-22T08:43:49+00:00Z
U.S. Patent MU. 27,1984 Sheet 1 of 3 4,439,301 
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e 
Sheet 2 of 3 4,439,381 
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36 
3 
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U.S. Patent MW 27,1984 Sheet 3 of 3 4,439,301 
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2 .o 
NiPS, 
- 1.0 0 I .o 
VOLTS vs SCE 
FIG. 4 
4.439,30 1 
1 
PHOTOELECTROCHEMICAL CELLS 
INCLUDING CHALCOGENOPHOSPHATE 
PHOTOELECTRODES 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). 
BACKGROUND OF THE INVENTION 
Due to the sharp increase in the price of oil, attention 
has been focused in recent years on developing alterna- 
tive energy sources, such as solar energy. The sun pro- 
vides the earth with ample energy, but problems have 
been encountered in converting the sun’s photo energy 
into other useful forms of energy, such as electrical 
energy or storable chemical energy. 
One device which has been utilized to convert solar 
energy into electrical energy is the photovoltaic cell 
which employs semiconductors absorbing electromag- 
netic radiation in the visible and near IR region of the 
sun’s spectrum. For a semiconductor to absorb light, the 
light must be at least of some minimum energy, referred 
to as the bandgap of the semiconductor and being char- 
acteristic of the particular semiconductor and its crys- 
talline structure. Thus, all light of an energy greater 
than or equal to the bandgap is available for absorption 
5 
10 
15 
20 
25 
30 
and energy conversion, whereas all light of an energy 
less than the bandgap is not. The bandgap also defines 
the maximum potential output voltage of the semicon- 
ductor when used in an energy conversion device. It is 
therefore preferable that the bandgap of the semicon- 35 
ductor be relatively small so that a high percentage of 
the solar spectrum will be available for absorption, but 
not so small that the output voltage of the cell will be 
too low to be useful. The peak of the solar spectrum 
occurs about 2.5 eV with the curve dropping sharply as 
energy increases and tapering off slowly as energy de- 
creases. Based on this, the most potentially efficient 
semicondutors for photovoltaic cells have bandgaps of 
approximately 1.5 eV. 
One of the major problems of solar energy as a power 
source and photovoltaic cells as a conversion device is 
the inconsistency of sunlight, which is not available for 
power generation at night or on cloudy days. Photovol- 
taic cells only produce electricity and only when the 
sun is shining. Electricity may be converted to chemical 
energy and stored in batteries, but the batteries are 
bulky and expensive. Therefore, it is desirable to have 
an energy conversion device that is capable of convert- 
ing solar energy to a storable form of energy so that 
energy will still be available during no or low sunlight 
periods. The photoelectrolysis cell is such an energy 
conversion device. Sunlight is used as the power source 
to drive an endothermic chemical reaction involving 
40 
45 
50 
55 
two redox couples, such as the photoelectrolysis of 
water to produce hydrogen and oxygen. The photoelec- 60 
trolysis products may either be stored, or fed directly 
into fuel cells or other similar devices where the chemi- 
cal energy is converted to electrical or mechanical en- 
ergy. When a semiconductor is to be used as a photo- 
electrode in a photoelectrolysis cell, the electrolysis 65 
potential of the electrolyte and the overpotentials of the 
electrodes must also be taken into account when deter- 
mining the optimum handgap for the semiconductor. 
2 
Considering these factors, the optimum bandgap for a 
semiconductor in a photoelectrolysis cell is between 
1.75eV and 2.5 eV. For a more detailed discussion of the 
theoretical principals of photoelectrolysis cells see Deb 
and Wallace, and Noufi and Warren, Proceedings of the 
Society of Photo-Optical Instrumentation Engineem, Vol. 
248, pp 38-57, and 80-87 (1981). 
Previous materials for use as photoelectrolysis elec- 
trodes have primarily consisted of metal oxides or com- 
pound semiconductors. The compound semiconductors 
have desirable bandgaps, but tend to degrade when used 
as photoelectrodes in photoelectrolysis cells. The metal 
oxides. conversely, are generally stable but have band- 
gaps on the order of 3.0 eV or greater and thus are less 
efficient because a large portion of the solar spectrum is 
unavailable for absorption. 
Thus, there is need in the art for semiconductors with 
small bandgaps on the order of 1.75 eV to 2.5 eV which 
are stable when used as photoelectrodes in photoelec- 
trolysis cells. 
Accordingly, it is an object of the present invention 
to provide a photoelectrochemical cell employing a 
class of efficient photoelectrodes which do not degrade 
when used in photoelectrolysis cells. 
it is another object of the present invention to provide 
a photoelectrochemical cell employing a class of photo- 
electrodes with bandgaps on the order of 1.75 eV to 2.5 
eV. 
A further object of the present invention is provide a 
class of efficient and stable photoelectrodes which are 
made from relatively inexpensive starting materials. 
SUMMARY OF THE INVENTION 
According to the present invention, the foregoing 
and additional objects are attained by providing a 
photoelectrode employing a transition meial chal- 
cogenophosphate (MPX3) as the photosensihve mate- 
rial, where M is selected from the group of transition 
metal series of elements beginning with scandium 
(atomic number 2 1) through germanhm (atomic num- 
ber 32), yttrium (atomic number 39) through antimony 
(atomic number 51), and lanthanum (atomic number 57) 
through polonium (atomic number 84); P is phosphorus; 
and X is selected from the chalcogenide series consist- 
ing of sulfur, selenium, and tellurium. These compounds 
have desirable bandgaps on the order of 2.0 eV to 2.2 
eV and are stable when used in photoelectrolysis cells 
having acidic electrolytes. 
BRIEF DESCRIPTION OF THE DRAWINGS 
A more complete appreciation of the invention and 
many of the attendant advantages thereof will be 
readily apparent as the same becomes better understood 
by reference to the accompanying drawings, wherein: 
FIG. 1 is a schematic diagram of a photoelectrolysis 
cell employing the photoelectrode of the present inven- 
tion; 
FIG. 2 is a cross-sectional view of the photoelectrode 
taken on line 11-11 of FIG. 1; 
FIG. 3 is a schematic diagram of a photoelectrolysis 
cell connected to storage systems and an electricity 
generating system; and 
FIG. 4 depicts the curves for photocurrent versus 
applied voltage for three representative MPX3 elec- 
trodes. 
2 
4,439,301 
3 
DETAILED DESCRIPTION O F  THE 
INVENTION 
In FIG. 1, the reference numeral 5 indicates generally 
a photoelectrolysis cell. Photoelectrolysis cell 5 is com- 
prised of a photoelectrode 10, a counterelectrode 12, 
and an electrolyte solution 18 in a container 16. Gas 
collectors 14, made of a transparent material and posi- 
tioned over the electrodes 10 and 12, are connected to 
separate storage systems, one system for the gas 
evolved at the anode and other system for the gas 
evolved at the cathode. The electrodes 10 and 12 are 
connected by wires 20 to the appropriate terminals of a 
variable DC power supply 22. Both the photoelectrode 
and the counterelectrode connections are sealed where 
they pass through container 16 with a sealant such as 
silicone as at 19. If water is to be electrolysed, the elec- 
trolyte solution 18 is comprised mainly of water with an 
acid added to make the solution acidic, so as not to 
degrade the surface of the photoelectrode. 
FIG. 2 depicts a cross-section of the photoelectrode 
of FIG. 1 represented generally by the reference num- 
ber 10. Photoelectrode 10 is comprised of a crystal 31 of 
MPX3 with an ohmic contact 32 on the back and a wire 
38 connected thereto with a silver paste 34, wherein M 
is selected from the group of transition metal series of 
elements beginning with scandium (atomic number 2 1) 
through germanium (atomic number 32), yttrium 
(atomic number 39) through antimony (atomic number 
51), and lanthanum (atomic number 57) through polo- 
nium (atomic number 84); P is phosphorous; and X is 
selected from the chalcogenide series consisting of sul- 
fur, selenium, and tellurium. The photoelectrode is 
sealed wih a silicone adhesive 36 such that the back and 
the edges of the MPX3 crystal are insulated from the 
electrolyte solution 18 and the tube 40 is sealed from 
leakage of the electrolyte solution. This sealing is im- 
portant because the MPX3 crystal will tend to degrade 
if the edges are exposed to the electrolyte. 
5 
10 
15 
20 
25 
30 
35 
In practice, a cell is arranged as in FIG. 1, with the 40 
photoelectrode 10 being made of an MPX3 crystal as in 
FIG. 2 and the counter electrode 12 being either an 
inert electrode such as platinum or graphite or another 
photoelectrode. The variable DC power supply 22 is 
included to maximize the efficiency of the cell. The 45 
voltage or bias, required to be applied by the power 
supply will vary depending upon the electrode combi- 
nation used-in some arrangements no bias will be 
necessary-and is determined by plotting a graph of net 
energy output (that is, the potential energy of the gas 50 
evolved minus the energy put into the system through 
the variable DC power supply) versus bias. The peak of 
that curve is the proper bias for maximum efficiency. 
After the bias has been determined for the cell, the 
variable DC power source 22 is set for that voltage and 55 
the cell is arranged so that the photoelectrode 10 is 
exposed to sunlight (represented by hv in FIG. 1). Gas 
will then begin to be evolved at the electrodes, hydro- 
gen at the cathode and oxygen at the anode for the 
electrolysis of water. Whether the photoelectrode is 60 
anodic or cathodic will depend upon whether it is an 
n-type or p-type semiconductor; n-type semiconductors 
being anodic and p-type being cathodic. If the counter 
electrode 12 is also a photoelectrode, making a so-called 
heterocell, the counter electrode must be of the oppo- 65 
site type from the other electrode (i.e., if the counter 
electrode is made of a p-type semiconductor, the other 
electrode must be made of an n-type semiconductor). 
4 
This arrangement is usually more efficient because often 
no external bias is necessary due to the increased cell 
voltage from having two active electrodes. 
Referring now to FIG. 3, the oxygen evolved at 
anode 50 is trapped by gas collector 54 and fed through 
appropriate piping 58 into storage tank 62. In a like 
manner, the hydrogen evolved at cathode 52 is trapped 
by gas collector 56 and fed through piping 60 into a 
storage tank 64. As electrical power is needed, the oxy- 
gen and hydrogen are fed from the storage tanks 62 and 
64 through piping 66 and 68 respectively into an energy 
conversion device 70, which may be, for instance, a fuel 
cell or a gas turbine in connection with a generator. The 
electricity thus generated may then be transmitted by 
power lines 72 to external loads 74 to be used as desired. 
Table I is a tabulation of some of the physical charac- 
teristics of some representative MPX3 electrodes. All 
figures are for layered, single crystal electrodes in a 
solution at a pH of 2. As can be seen, some MPX3 crys- 
tals are n-type semiconductors and others are p-type. 
Thus, it is possible to construct a heterocell, as de- 
scribed above, using MPX3 crystals for both electrodes, 
so that both electrodes will have the advantages of 
stability and small bandgaps. 
TABLE I 
MPX3 V, (Volts vs. SCE') Conductivity Type 
SnPS3 0 
FePS3 0 
NiPSl +0.2 
n 
P 
D 
*Standard Calomel Electrode 
The photoelectrochemical response of three repre- 
sentative MPX3 electrodes, SnPS3, NiPS3, and FePS3, 
in a solution at pH=2 is shown in FIG. 4 which shows 
a plot of photocurrent versus applied potential. All 
three electrodes show low dark current clear photore- 
sponse. That is, the curves clearly indicate the absorp- 
tion of photo energy by the photoelectrodes. The Fe- 
and Ni-based crystals show photocathodic current typi- 
cal of p-type semiconductors. The Sn-based crystal has 
a photoanodic response typical of n-type semiconduc- 
tors. At the most extreme potentials shown in FIG. 4 
(cathodic potential for p-type and anodic potential for 
n-type materials, for example, 2.0 volts for SnPS3), gas 
was clearly seen to evolve from the electrode surface, 
presumably hydrogen for the iron-based p-type materi- 
als and oxygen for the tin-based n-type. 
The photocurrent (about 2mA/cm2) at these extreme 
potentials remained constant to within about ten per- 
cent for times up to at least one hour for electrodes in 
acid solutions, indicating stability of the crystals. These 
materials were very unstable in basic solutions, how- 
ever, as observed by a rapid decrease in the photocur- 
rent and visible deterioration of electrode surface. The 
onset potential of the photocurrent for the p-type mate- 
rials at pH=2 as measured by chopped light was 0 V 
and +0.2 V versus SCE for the Fe- and Ni-based crys- 
tals, respectively. These potentials are more positive 
than the reversible potential for the reduction of water, 
indicating the possibility of net gain in energy conver- 
sion. However, the change in the photocurrent with 
changing potential is slow, probably because of a high 
surface recombination of photogenerated carriers. The 
onset potential of the photocurrent for the n-type mate- 
rial SnPS3 was found to be 0 V versus SCE in pH=2 
solutions. This is about 850 mV more negative than the 
reversible potential for oxygen evolution at this pH, 
5 
4,439,30 1 
6 
indicating a net gain in energy conversion. These elec- an acidic electrolyte solution surrounding said photo- 
trodes also showed a slow rise in photocurrent with 
increasing anodic potentials. Spectral response experi- an external electrical circuit connected to the two 
ments indicate that the bandgap of these materials is 
about 2 eV and is indirect. Indirect bandgap semicon- 5 2. The device of claim 1 where one face of the lay- 
ductors are intrinsically less efficient than direct band- ered Mpx3 Photoelectrode is exposed to said electro- 
gap materials but also have less stringent manufacturing lyte solution, and the other face is coated with an elec- trically conductive material, which is in electrical requirements and therefore are usually less costly. Im- contact with an electrical circuit, and the photoelec- 
proved photocurrents can be expected from improved 10 trode is insulated such that only the aforementioned 
face of the MPX3 and not its edges or any other part of growth conditions for the MPX3 crystals. 
Although the operation of the present invention has the photoelectrode is in contact with the electrolyte 
been described in connection with a specific embodi- solution. 
ment thereof, it is not SO limited and the specific strut- 3. A device for converting photo energy to another 
tural features described herein are to be considered a~ 15 form of energy according to claim 1 wherein the exter- 
illustrative only and not exhaustive. It is recognized, for nal circuit includes a variable DC power Source for 
example, that the application of the MPX3 electrode is adjusting the bias of the photoelectrode. 
not limited to photoelectrolytic cells but may also have 4. A device for converting photo energy to another 
application in photovoltaic cells, where solar energy is form of energy according to claim 1 wherein the photo- 
converted directly into electrical energy, and other 20 energy is converted to storable chemical energy by 
photoelectrochemical cells. means of photoelectrolysis of the electrolyte solution by 
the Photoelectrode. 
form Of energy according to wherein the 
25 counter electrode is also made from a layered transition 
metal chalcogenophosphate (MPX3) according to claim 
1. 
other form of energy: 
electrode and said counter electrode; and 
electrodes. 
Obviously many modifications and variations of the 
teachings. It is therefore to be understood that within 
the scope of the appended claims the invention may be 
practiced otherwise than as specifically claimed. 
Patent of the United States is: 
other form of energy: 
present invention are possible in light of the above 5* A device for converting photo energy to another 
what is new and desired to be secured by Letters 6. In a device for converting photo energy into an- 
1. In a device for converting photo energy into an- 30 
a photoelectrode made from a layered transition 
metal chalcogenophosphate oMPX3) selected from 
SnPS3, FePS3 and NiPS3; 
a counter electrode; 35 
a photoelectrode made from a layered transition 
metal chalcogenophosphate (MPX3) selected from 
SnPS3, FePS3 and NiPS3; and 
a contact attached to the photoelectrode for connect- 
ing the photoelectrode to an electrical circuit. * * * * *  
45 
50 
55 
60 
65 


Photoelectrochemical cells including chalcogenophosphate photoelectrodes

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