by William J. Cromie

of hydrogen, the cycle should be repeatable, the fuel inexhaustible.

The catch is that the energy required to extract hydrogen from water, is greater than the energy produced by burning hydrogen. But if the input energy for splitting water into oxygen and hydrogen by hydrolysis could be solar, the hydrogen could effectively concentrate and store sunlight for later use.

Several possible technologies exist for doing this, but the big payoff is to harvest solar energy as plants do. Besides splitting water, such an ability would enable photo-chemists to convert nitrogen to ammonia; incorporate carbon dioxide in fuels such as methanol; and turn other abundant, low-cost substances into useful chemicals, textiles, and even food. These photoconversion reactions also decompose sulfur oxides and other pollutants, as well as decompose un­wanted organic compounds and bacteria.

Commercial applications of such processes lie two decades away, photochemists say; much of the present research is still basic. But in university and industrial laboratories,

scores of groups pursue this modern alchemy, and even the most cautious photochemists sound enthusiastic.

Redesigning photosynthesis A major part of the photochemistry effort

involves water splitting. Initially, this looked easy. "After all, plants do it all the t ime/ ' observes Richard Kandel, a Department of Energy research manager.

In plants, chlorophyll absorbs enough solar energy to break the chemical bonds of a water molecule. The oxygen atoms in the water give up electrons, form pairs, and be­come oxygen gas (oxidation reaction). Hydro­gen nuclei, or protons, gain electrons from the oxidation reaction and become reduced to hydrogen gas (reduction reaction). The plant then combines the hydrogen with carbon from carbon dioxide to form carbo­hydrates. "Th i s , " remarks Kandel, "is the sort of thing we'd like to do in a beaker."

The most direct way involves irradiating chlorophyll with light in the presence of a catalyst which transfers electrons from the green pigment to the water. A number of researchers have done this, but the efficiencies achieved are very low. Francis K. Fong of Purdue University, for example, irradiated

chlorophyll on a plat inum electrode im­mersed in water. He generated a weak electric current along with hydrogen and oxygen in amounts too small to be easily measured. Others who have tried have been unable to produce the reaction Fong describes, and there is general agreement among photo­chemists that chlorophyll possesses an in­herently low efficiency for splitting water.

"Chlorophyll's efficiency for plant photo­synthesis is respectable but not astonishing," says Mark S. Wrighton , Massachusetts In­stitute of Technology photochemist. "Field efficiency, considering such factors as yearly average and best crops, is about 1 percent." Practical artificial systems, he adds, require efficiencies of at least 10 percent to compete with fossil and nuclear fuels.

Starting from scratch An alternative approach—using synthetic

rather than natural materials —currently attracts more attention. Melvin Calvin of the University of California at Berkeley sub­stitutes various pigmented molecules for chlorophyll. Excitation of a pigment creates electrons and what are called holes—vacancies left by electrons and having a nominal posi­tive charge. An artificial membrane separates electrons and holes so that oxidation occurs on one side and reduction on the other. Calvin, who won the 1961 Nobel Prize in chemistry for his research into natural pho­tosynthesis, is a t tempting to find a com­bination of membranes and reactants that will provide a simple, efficient substitute for chlorophyll.

Another method under study involves irradiation of organic molecules. The ab­sorbed solar energy is stored in the form of strained chemical bonds in the altered organic molecules. Charles R. Kutal and his associ­ates at the University of Georgia convert one hydrocarbon to another by this means, adding about one electron volt per molecule of additional energy. When a catalyst re­converts the strained molecule to its original form, the extra energy is released as heat or chemical energy.

To make this work, molecules must be ex­cited (pushed uphil l thermodynamically) by visible light frequencies corresponding to an energy somewhat greater than one volt. Since the Georgia group ' s target material (norbornadiene) absorbs only shorter ultra­violet wavelengths, it can not use solar energy directly. Kutal a t tempts to get around this by adding substances sensitive to visible light. Other systems use molecules that absorb visible light directly. Dyes also are used,

MOSAIC Seotember/October 1981 9

ydrogen burns cleanly. Properly oxidized, it produces only heat and water. Since water is also a source

but these sustain photoconversion only until the dye fades.

Excited metal complexes

Other approaches involve combinations of metals and organic molecules that absorb light efficiently and lead to excited mole­cules that are powerful redox reagents. David G. Whi t t en and Thomas J. Meyer of the University of North Carolina, for exam­ple, experiment with a combination of ruthenium ions and bipyridine. Ruthenium ions, colorless relatives of platinum, turn orange when complexed with bipyridine, making the combination an efficient ab­sorber of energy at visible wavelengths. Sunlight-excited electrons move from the metal into the organic envelope where they can reduce water to hydrogen. At the same time, enough positive charge is created at the metal to oxidize water and produce oxygen.

Excited electrons lose their extra energy unless they are quickly captured and put to work. In the Whit ten-Meyer system, the excited state lifetime is only about one micro­second, but in this molecular system that's long enough. Electron transfer approaches 100 percent, producing an efficient rate of conversion of solar radiation into chemical energy. The photochemists still face the problem, however , of generating hydrogen and oxygen before electrons recombine, or back-react with the holes. This is tricky, so researchers such as Whi t ten and Meyer adopted the strategy of working on hydrogen and oxygen generation separately in the hope of finding catalytic reactions that com­pete successfully with recombination. They then will a t tempt to combine the separate hydrogen and oxygen cells into a single reactor.

Harry B. Gray and his g roup at the Cali­fornia Insti tute of Technology work with metal compounds that absorb strongly in the visible region. Rhodium complexes that Gray has studied possess excited-state life­times as long as 10 microseconds. Acidic solutions prepared from some of these com­plexes yield molecular hydrogen when irradi­ated with visible light. However, the systems produce no oxygen, and both reduction and oxidation must occur for photoconversion to sustain itself.

Gray's g r o u p also investigates the photo-activity of molybdenum and tungsten com­plexed with chlorine, bromine, or other ha-lides. These metals are much less expensive than rhodium, ruthenium, and other mem­bers of the p la t inum group. Gray hopes that the molybdenum and tungsten complexes

Cromie is a frequent contributor to Mosaic.

can be induced to give up two electrons for each visible light photon captured, rather than one, as in other systems.

Semiconductor systems

Research such as that of Gray, Whitten, Meyer, and Calvin advances knowledge of basic photochemistry by exploring new reactions and materials, and making new molecules and structures. Some of these may be incorporated into commercial systems; some may not. But all of them contribute to understanding photochemistry.

"Assuming that practical applications lie some 20 years ahead, it would be undesir­able to concentrate on any single system, technique, or photosensitive material," MIT's Mark Wrighton notes. " N o w is the time to identify new systems and mechanisms, to elucidate basic principles fully, and to ex­plore new photoreactions." Even at this stage, however, he says researchers agree that practical devices " m u s t contain some sort of interface or barrier between a system's oxidation and reduction products to prevent back reactions." If the products are free to drift around in solution, they will recombine.

Membranes and related structures pro­vide such barriers, but to date the most im­pressive interface systems use junctions between solid semiconductors and liquid

solutions. Simple immersion of semicon­ductor electrodes in an electrolyte solution releases hydrogen at the dark electrode, the cathode, and oxygen at the light-absorbing anode. Electrons moving through a wire connecting the electrodes produce an elec­tric current which is the primary product in wet photovoltaic systems. Such liquid-junction devices produce electricity with an efficiency that rivals solid-state solar cells, but they must be durable and economic to become competitive.

In these photoelectrochemical systems, electrons, freed from their bonds by sun­light, move by wire to the cathode. Holes^ carriers of positive charges—move to the anode surface, where they oxidize water and produce oxygen gas. Oxidation is mirrored by reduction at the cathode, where protons, generated during oxidation, combine with electrons to generate hydrogen. The elec­trodes are separated so that the gases will not recombine.

Wrighton developed a cell in which both semiconductors absorb light. On the hydro­gen side, light strikes a silicon cathode, ex­citing electrons which move to the electrode surface. There a plat inum catalyst captures them before they give up their extra energy. At the anode, which consists of strontium titanium oxide, sunlight generates holes.

10 MOSAIC September/October 1981

These move to the anode surface where an oxidation reaction produces oxygen and protons . The protons then combine with the electrons held by the platinum to generate hydrogen gas. Wrighton claims an efficiency of 3 percent for this cell, and he believes such devices have the potential to exceed 10 percent.

Michael Gratzel's team at Ecole Poly-technique Federale in Lausanne, Switzerland, uses semiconductors in the form of micro­spheres dispersed in the electrolyte solution. (A sphere can serve as cathode or anode; when different materials are sandwiched together, it can perform both functions.) According to a report by the researchers, their reactor shows no signs of deterioration after running continuously for days, and it produces about five molecules of hydrogen for every hundred photons of light absorbed. (An ideal system would produce a molecule of hydrogen for every two photons.)

A major problem with systems using microspheres involves the difficulty of sep­arating hydrogen and oxygen from each other and from the solution. Gratzel hopes to solve this and to develop a commercially viable water-splitting system by the end of the decade.

Allen J. Bard at the University of Texas in Aust in has done most of the U.S. work with semiconductor particles. "We 've ex­perimented with titanium dioxide coated with plat inum for hydrogen evolution and

with ruthenium oxide for oxygen evolution," he explains. This system absorbs only 4 to 5 percent of the visible solar light; a com­mercial device should triple or quadruple that. In addition, Bard says, platinum and ru thenium are too expensive to be practical. "No one has yet found the right combination of inexpensive materials, long-term stability, and high efficiency needed to make a major impact on the energy economy," he notes.

"The major scientific hurdle associated with semiconductor/ l iquid-junct ions de­vices involves oxidation and decomposition of the photoanode ," Mark Wrighton ex­plains. Holes rising to the surface of the electrodes can oxidize them instead of water. This decomposes the semiconductors or, in the case of silicon, forms insulating coatings of silicon dioxide. Experimenters countered this by adding to the solution reagents that quickly scavange or fill the holes, promoting the oxidation of the reagent rather than the electrode. Several additives are available to stabilize one or another semiconductor material. Some investigators experiment with thin metal and polymer coatings to protect silicon semiconductors.

Generating electricity Researchers also are attempting to use the

electricity generated by water-splitting photo­cells. Several investigators are seeking systems that will convert solar energy to electricity efficiently.

Wrighton found that ferrocene—a soluble iron compound—prevents an insulating covering from forming on silicon. He devel­oped a cell with silicon semiconductors and an electrolyte of ethanol rather than water. The protected electrodes not only surpass unprotected silicon in ability to produce electric current, he says, "but the improve­ment in durability is substantial." The system converts sunlight to electricity with an ef­ficiency of about 2 percent, Wrighton reports. He believes it will be possible to raise that to 10 percent in the future.

Researchers at Bell Laboratories in Murray Hill, New Jersey, discovered a novel solution to the degradation problem in photovoltaic cells. " W e irradiate the cathode instead of the anode—the negative instead of the posi­tive electrode," explains Adam Heller of Bell. "Visible light drives electrons rather than holes to the surface, so no degradation occurs. The more light striking the electrode, the more stable it becomes." The photocathode, made from a crystal of indium phosphide, sits in an electrolyte of vanadium ions and hydrochloric acid. Heller and Barry Miller claims this cell converts 11.5 percent of the sunlight reaching it into electricity.

So high a conversion efficiency would make wet photochemical systems competitive

MOSAIC September/October 1981 11

with commercially available solid-state solar cells. The latter dry sandwiches of semicon­ductor materials have efficiencies of 12 to 15 percent and are very costly. If photo­chemical technology can produce the wet cells inexpensively, the electricity they gen­erate could be used for commercially prac­tical water splitting.

The new Bell Labs wet cell does not solve the cost problem however, indium—a silver-white metal—is expensive and scarce. Heller's group, therefore, has begun experiments with thin films of polycrystalline materials that are cheaper than indium and much less costly than single-crystal silicon. Using an electrode made of a gallium arsenide film on a graphite base, for instance, gives the Bell researchers a relatively inexpensive wet photovoltaic system with a reported efficiency of 7.S percent.

Battery assistance The Bell team also modified its indium

phosphide cell to split water for hydrogen. "We obtained a whopping efficiency—about 12 percent," boasts Heller. The cell, how­ever, requires a battery or other external electric source in addition to sunlight. Ap­proximately two volts are needed to excite the electrons to the level where they will generate hydrogen rapidly. The battery sup­plies about 75 percent of the energy; sunlight provides the rest. The 12 percent efficiency represents the amount of power saved, or how effectively sunlight is used to decrease electric input into the system.

That 's not the best way to reach the goal if the game is water splitting. But, "We don't want to be in that game," Heller declares. "Oxygen is cheap and abundant; making it together with hydrogen wastes effort and money. . . .We are more interested in a system that will yield hydrogen and a valu­able chemical like chlorine."

An ingredient in drugs, dyes, and many industrial chemicals, chlorine now is made by electrolyzing sea water or melted sodium chloride—energy-expensive processes. The Bell Labs researchers hope to develop a cell that will photoelectrolyze sea water, yielding hydrogen from the water and chlorine from the salt. At present they are testing their indium cell for this. "The first step is to develop a good laboratory system for extract­ing elements such as hydrogen, chlorine, and bromine from sea water," says Heller. "Then we will concentrate on an inexpensive system that will work in the field."

John Bockris and his team at Texas A&M University also use battery assistance to split water. They employ a photoanode of lan­thanum and chromium on a titanium base, together with a platinum cathode. Electrons

ejected from the anode diffuse through the water and collect on the platinum, where hydrogen evolves. Holes left behind at the anode produce oxygen. The external source of power provides a maximum of 0.5 volt. Discounting the contribution of the outside source, Bockris claims an efficiency of "about 4 percent ."

The Texas A&M group also produces hy­drogen and chlorine from simulated sea water, using a lanthanum ruthenate anode and a plat inum cathode. " W e get enough chlorine to smell ," Bockris says, "bu t we convert only about 1 percent of the absorbed solar energy into chemical energy."

Bockris believes that practical photocon-version systems will be developed in ten years and that the first application will in­volve manufacture of hydrogen and chlorine. Other researchers envision a bank of cells to extract from sea-water chemicals such as bromine and magnesium, now produced by energy-expensive electrolysis.

A promising system At the industrial end of the photochemistry

spectrum, Texas Instruments of Dallas has under development a system intended to supply electricity and heat to individual homes. In it, silicon solar cells in the form of tiny spheres absorb sunlight and use the ex­cited electrons and holes to split hydrobromic acid into hydrogen gas and bromine. A powdered metal alloy such as calcium-nickel absorbs the hydrogen for storage in a non-

explosive form. The hydrogen and the bro­mine then recombine in a fuel cell. The cell generates electricity and converts the hy­drogen gas and bromine back into hydro­bromic acid for recycling. Since the sun heats the acid solution, the system also produces usable heat.

"We developed a technique to make single-crystal silicon spheres 10 to 15 mils [0.25 to 0.375 millimeters] in diameter at a small fraction of the cost of an equivalent area prepared by conventional crystal-growing methods ," explains E. L. "Pete" Johnson, director of Texas Instruments ' solar project laboratory. The spheres are cast into large glass sheets which Johnson says can be made as large as three square meters. The sheets sit in the acid, which serves as an efficient interconnection, "drawing the full current that each cell can deliver," Johnson notes.

He claims the sheets convert 13 percent of the absorbed solar energy into electricity. The efficiency of an actual working system would be less, however, because of losses in recovering the hydrogen from storage and in fuel-cell operation. " W e need to do much more development, particularly in the area of scaling up for a manufacturable sys tem," he adds. "A successful system should produce 90 percent of the energy required in a typical home at a cost competitive with conventional electric energy." Johnson believes the system will be ready for commercial sale in "less than ten years."

Once photochemists master efficient

12 MOSAIC September/October 1981

splitting of sea water and generation of elec­tricity, they will be in a better position to attempt the more difficult conversion of carbon dioxide and nitrogen into fuels and chemicals such as methanol, ethylene glycol and ammonia. "At present, we can't do such reactions easily even if batteries or other electric sources supply electrons/' Allen Bard points out. " W e have more problems to work out with this than we do with water splitting."

Splitting pollutants While plenty of photochemical reactions

are more difficult than water splitting, many are easier. "Water splitting and nitrogen reduction represent reactions driven ther-modynamically uphill by sunl ight ," Bard says, "but we also can use solar energy to accelerate downhill thermodynamic reactions, to act as a catalyst." He cites photochemical treatment of wastes as an example.

Acetic acid, a common product of fer­mentation, decomposes to carbon dioxide and methane, but not fast enough to be useful. The reaction, however, proceeds at a rapid rate when titanium dioxide-platinum electrodes are added and exposed to light.

"We can also use photoelectrochemical means to decompose pollutants like sulfur oxides from stack gases and to oxidize cyanide from sources such as the steel industry," adds Bard.

John Bockris envisions semiconductor microelectrodes being used to convert various organic acids and household wastes to fuels and other products. Twenty years from now, he predicts, a community might consolidate its wastes as a brew or solution which could be reprocessed with solar concentrators and photoelectrochemical reactors.

Allen Bard adds water treatment to the list of future applications. Contaminated water is becoming a bigger and bigger prob­lem, he notes, and organic materials have been found to combine with chlorine to pro­duce compounds suspected of causing cancer. But "it should be possible to decompose the organic matter and clean up stream bac­teria photochemically," he says. "The germi­cidal properties of irradiated semiconductor particles are high. I would bet that systems to purify water and treat waste will find application sooner than those for splitting water."

Richard Kandel, who heads solar photo-conversion research at the Department of Energy, believes that stable, efficient systems for splitting water and other uses will be developed by the end of the decade. "There are so many good people working in this field, we should identify chemical cycles to convert sunlight to storable electrical and chemical energy efficiently by 1990," he C - ^ ^ ^ " T U « nAAn ^ ^ 4-U-.1- t - k ^ r I A M I I k n l U i c i a D i D . i n c U U U D a m u i a i u i c j w i n LJ\.

based on semiconductor/ l iquid-junct ion devices. The next step will be to substitute cheaper materials for plat inum, ruthenium, rhodium, and other expensive metals. I am confident that once we have efficient reac­tions based on plat inum and ruthenium, chemists will come up with cheaper substi­tutes in an additional ten years or less."

Taking advantage of an inexhaustible sup­ply of hydrogen—sea water—will require the development and mass product ion of inex­pensive electrode materials. Photochemists do not view this as an insurmountable hurdle to exploiting sunlight. Mark Wrighton typifies their attitude when he says: "Given the recent rate of progress, the influx of new and capable scientists to this field, and the multitude of exciting research possibilities, the prospects are bright for photochemical solar energy in the early part of the 21st century." •

The National Science Foundation supports its part of the research discussed in this article principally through its Chemical Dynamics Program.

MOSAIC September/October 1981 13