The Photodissociation of Water PLATINUM METAL CATALYSTS PROMOTE THE PRODUCTION OF HYDROGEN AND OXYGEN By Anthony Harriman Dab) Faraday Research Laboratory, The Royal Institution. Imndon

The development of artificial photosynthesis units capable of mnt'ert- ing solar energy into useful chemical products is still at u very pre- liminary stage, although some progress has been madr. during the lust .fau years. Most work concerns the photodissociation o,f water, and this paper considers the two basic strategies that are being followed. Both 0.f these require the use 0.f a catalyst and, to datp, platinum metal catalysts have proved to be the most efficient. With many laboratories actively investigating photodissociation, rapid progress should ensue.

Over the past twenty years there has been a source; water, carbon dioxide or nitrogen. Of steady growth in research effort directed these materials, the large scale consumption of towards the construction of a practical device carbon dioxide involves problems with con- capable of the collection and storage of solar servation of the atmosphere but both the energy. Considerable progress has been made photodissociation of water into hydrogen and during this time and, already, solar energy is oxygen and the photoreduction of nitrogen to used in many countries to heat water for ammonia appear to be promising systems for domestic purposes. Similarly, the space the conversion of solar energy into useful programme has resulted in the development of chemical products. So far, most work has been photovoltaic cells that are able to convert solar concerned with the photodissociation of water energy into electrical energy with efficiencies and the photoreduction of nitrogen has been around 15 per cent. Only cost limits the virtually ignored. widescale use of such cells. Solar energy can Two basic strategies are being followed in the also be converted into chemical potential design of systems for the photodissociation of although, here, development is still at a pre- water into hydrogen and oxygen. The first liminary stage and no practical system exists at approach involves systems that are present. The scope of this field is enormous but, homogeneous, or nearly so, in which a on practical grounds, it seems necessary that photosensitiser is used to promote electron the final chemical product is a gas. This is transfer between suitable electron mediators. because sunlight is a dilute form of energy and The oxidised and reduced forms of these large collection areas are required so that the mediators can be used to liberate oxygen and concentration of the product will always be hydrogen, respectively, from water, provided quite low. Filtration and distillation are too suitable catalysts are present. The second expensive to be off-set by the price of the approach is based upon the irradiation of product. Collection of a gas, provided it is inorganic semiconductors, either evolved at a specific site and not throughout the macroelectrodes or colloidal particles, with light entire collection area, should be straight- of energy higher than the band gap. The forward. In terms of cost and availability, the electrodhole pair which is formed upon such gas must be derived from a cheap, inorganic illumination can migrate separately to the

Platinum Metals Rev., 1983,27, (3),102-107 102 surface of the semiconductor where water complete, cyclic dissociation of water still reduction and oxidation can occur. Again, eludes us. In part, this failure is due to the poor catalysts are required to promote gas evolution performance of the catalysts employed and and, in all cases, the most efficient catalysts greatly improved catalysts must be developed involve the platinum group metals. before we can progress much further. Because of its low overpotential (v), platinum Homogeneous Systems is the most commonly used electrode material Here, a photosensitiser (S,), such as a water- for the liberation of hydrogen from water and it soluble metalloporphyrin, is used to reduce an has enjoyed considerable success as a catalyst in electron acceptor (A), such as methyl viologen, model hydrogen producing photosystems. With in aqueous solution. The oxidised form of the a macroelectrode in acidic solution, the Butler- sensitiser is reduced by an electron donor (D) so Volmer equation can be used to relate the over- that the sensitiser is recycled and the reduced potential to the net current density (i, measured form of the electron acceptor is used to liberate in NcmZ). hydrogen from water in the presence of a .. . I = 1CA.I. platinum catalyst, see Figure I. In a separate IAN- photosystem, the oxidised form of the donor (I - tr)zFq = i, { exp RT -“p- (D+) is used to oxidise a photosensitiser (SJ, RT such as tris(2,2’-bipyridyl)ruthenium (11), which where tr is the cathodic transfer coefficient is able to oxidise water to oxygen in the pre- (tr = 0.5 for hydrogen evolution) and z is the sence of a suitable catalyst, for example RuO, or charge transfer valence (z = I). The exchange IrOp current density (i,) is a known function of the The two separate photosystems have been concentrations of H+and H,. In acidic solution, well developed, a wide variety of systems are the reoxidation of evolved hydrogen is not too available, and quantum efficiencies for forma- important and the ratedetermining step is tion of hydrogen and oxygen of 60 per cent and discharge of an electron to an adsorbed proton. 20 per cent, respectively, have been obtained Therefore, the above equation can be simplified (I ,z). Tables I and I1 give summaries of the best to developed systems. In all cases listed there, the 0.gF1 i = i, exp - (ii) reversible redox couple D+/D has been replaced -RT -0.gFq with a sacrificial (or irreversible) couple which or i = I.‘ k,C,+ exp -RT (iii) undergoes destruction upon oxidation or reduc- tion. So far, it has not been possible to link where C,, refers to the concentration of protons together two such photosystems so that the in the bulk electrolyte. This simplified equation

Platinum Metals Rev., 1983,27, (3) 103 Table I Hydrogen-Evolving Photosystems I3uantun Electron Electron 3fficiencI Sensitiser acceptor donor 16 3H2 I Catalyst per cent bipy,Ru2+ methyl viologen E DTA platinum colloid 26 cysteine PtO, - proflavine E DTA platinum/asbestos - acridine yellow platinum colloid 32 ZnTMPyP4+ platinum colloid 60 none platinum colloid 7 ZnTSPP4- methyl viologen platinum colloid 2

Table II Oxygen-Evolving Photosystems Quantum efficiency Electron z so, Sensitiser acceptor Catalyst per cent

bipy,Ru2+ CO(NH,),CI~+ RuO, powder 1.2 RuO, colloid 20 coso, 8 RuOJzeolite 5 RuOj'TiO, 12 MnO, -

bi py,R u + s,o,2- RuOJTiO, 24 IrO, 14 bipy3Ru2+ Ti3+ RuO, colloid <1

ZnTMPvP4 zinc tetralN-methvi-4-~vridvll~or~hine ZnTSPP- zinc tesal4-sulphophenyl~porphine relates the net current density to the applied, or will be determined by the redox potential of the available, overpotential, the pH of the bulk electron mediator used, usually methyl viologen solution and the heterogeneous rate constant for which Eo= -0.44 V. Secondly, the limiting for the electron transfer process (kd. At high current density will be set by the rate of charg- overpotential, the currentholtage curve flattens ing the catalyst rather than by the rate of out due to rate determining diffusion of the discharge so that Equation (iv) will apply with electroactive species to the electrode surface. D and C referring to the reduced electron Here, the limiting current density (i,) is given acceptor. We can, therefore, express the by zFDC efficiency of a photosystem in terms of the (iv) 4. = 7 product of the rate of production of the reduced where D IS the diffusion coefficient of the relay and the rate at which this species is species in the medium, C is its concentration reoxidised on the catalyst surface. and 6 is the thickness of the diffusion layer. With methyl viologen as electron acceptor, In order to apply the above equations to our the available overpotential for Equation (iii) can proposed model photosystems, it is necessary to be expressed acknowledge two important points. First, the 1 = (0.44 - 0.059 PHI available overpotential for hydrogen production so that reaction is restricted to acidic solution.

Platinum Metals Rea., 1983,27, (3) 104 In order to obtain the maximum rate of mass colloidal platinum particles work extremely well transfer to the electrode surface, it is seen from in model photosystems, they appear to have Equation (iv) that the thickness of the diffusion limited potential for practical applications. layer should be kept as small as possible Similar arguments can be applied to the whereas the total surface area of the catalyst oxygen-evolving photosystems. From should be as large as possible. For a given electrochemical work, it is known that the most amount of platinum, high surface areas are suitable anodes for oxygen liberation are cons- obtained if the catalyst is in the form of tructed from RuO, and IrO, and both materials colloidal particles (radius r nm) rather than as a have been used in model photosystems. So far, planar sheet. For such particles, the most work has centred upon RuO, but it is bimolecular rate constant for interaction known now that this is not a particularly good between a reduced acceptor and a platinum redox catalyst in photochemical systems. particle can be written Although claims exist to the contrary, it is clear k=qnNDr (4 that RuO, is not a selective catalyst for oxygen where N refers to Avogadro's number. If the production but it can be used for hydrogen concentration of particles exceeds the steady- generation; its overpotential in acidic solution is state concentration of the reduced acceptor, not much higher than that of platinum. This then the pseudo-first order rate constant for can lead to short-circuits in the photochemistry. interaction becomes More importantly, in acidic solution the over- 0.027 D C, potential that can be applied to an RuO, k'= (4 r2 electrode is limited to low levels by a where C, is the molar concentration of decomposition process: platinum (3). Thus, smaller particles favour RuO, + 4H+ + 4e + RuO, + zH20(vii) faster mass transfer to the catalyst surface and, Eo= + 1.4 V therefore, allow higher current densities to be 0, + 4H+ + 4e + 2H,O (viii) achieved. The importance of using colloidal Eo= +1.23 V platinum catalysts cannot be overstressed. However, the dissolution of RuO, Equation These colloidal platinum particles are made (vii), can be inhibited almost completely by by the reduction of HStCl, in aqueous solu- supporting the material on an inert oxide such tion, with hydrogen, citrate or pulse radiolysis as TiO,. These mixed oxides function as effec- and an inert support such as polyvinyl alcohol tive oxygen-evolving catalysts and they can be is added to stabilise the colloid against floccula- obtained in particulate form. Using colloidal tion. According to Equation (vi), there are TiO, loaded with small (about 2 per cent w/w) advantages in preparing very small platinum deposits of RuO, Gratzel and his colleagues particles but the lower limit for hydrogen for- have reported that the oxidation of water to mation seems to be reached with r - 1.0nm. oxygen, via a powerful one electron oxidant Such particles do not scatter visible light and, (bipy3Ru3+), is a concerted process (4). This when properly supported, they are stable over finding is based upon the observation that the many months standing. However, they give rise reduction of bipy,Ru3+ and the formation of a to two major problems in that they cannot proton, Equation (viii), occur simultaneously. easily be recovered from the reaction solution At the moment, the TiO,/RuO, catalysts and they facilitate hydrogen production seem to be the most promising materials for use throughout the entire volume of the photolysis in oxygenevolving photosystems but little work cell. This latter point is extremely important has centred upon the use of IrOp More studies because, in any practical system, the hydrogen are needed before we can be satisfied with these should be evolved at a particular site where it catalysts but the oxygen-evolving photosystem can be collected. This can be achieved only with has a big advantage over the analogous a macroelectrode and, therefore, although the hydrogen-photosystem. This concerns the use

Platinum Metals Rev., 1983,27, (3) 105 of colloidal catalysts. We have seen earlier that, surface of the semiconductor, where recombina- because of their large surface area, colloidal tion occurs. However, given appropriate catalysts offer important benefits for maximis- thermodynamics, the electrons in the con- ing the rate of gas evolution but they do not duction band can be used to reduce water to facilitate collection of the gas. For oxygen hydrogen and the positive holes in the valence production, it may not be necessary to collect band can be used to oxidise water to oxygen. the gas and, provided it does not interfere too Catalysts are required for at least one of these much with the photochemistry, it can be processes, usually the reduction step, and the evolved throughout the entire volume of the most commonly employed materials are photolysis cell. Thus, colloidal catalysts seem to platinum and/or RuO, There is now great have a future in oxygen-evolving photosystems. activity in the design of suitable systems based Although it is important that more efficient upon colloidal semiconductors loaded with one oxygen-evolving catalysts are developed, the or more of the above catalysts. The problems oxygen-photosystem is severely limited by the associated with finding a useful system can be lack of suitable photosensitisers. As outlined in summarised as: (a) the energetics must be care- Table 11, only tris(z,2’-bipyridyl)ruthenium (11) fully balanced so that visible light can be used functions in this manner. Since this compound to photodissociate water, and (b) many semi- absorbs only a modest fraction of the solar conductors undergo photocorrosion. spectrum and it is costly, it is therefore essential It is clear from Figure 2 that the positions of that new photosensitisers are identified soon. both CB and VB are critical if the photodissociation of water is to be realised. Semiconductor Systems Very few known semiconductors have their The same type of considerations can be energy levels located at appropriate positions applied to the design of semiconductors for the while retaining a relatively small band-gap. photodissociation of water. Consider the case of The smallest band-gap that is consistent with a wide band-gap semiconductor, as shown in efficient electrodhole separation and water Figure 2. Irradiation, with light of energy dissociation is (2.2 & 0.2) eV, which greater than the band-gap, results in formation corresponds to a wavelength of about 500 nm. of an electrodhole pair (e-/h+) in which the Unfortunately, semiconductors with band-gaps electron resides in the conduction band and the in this region are usually unstable with respect positive hole remains in the valence band. Both to photocorrosion. Only wide band-gap the hole and the electron can migrate to the semiconductors, like TiO, (bg = 3.0 eV) or

Platinum Metals Rev., 1983,27, (3) 106 SrTiO, (bg = 3.2 eV), are completely stable Ac knowledgernent towards illumination, but these compounds We thank the Science and Engineering Research absorb only a very small fraction of the solar Council and the European Economic Community spectrum. However, recent work has shown for financial support of this work. that it might be possible to stabilise semi- References conductors against photocorrosion by loading J. R. Darwent, P. Douglas, A. Harriman, G. Porter the surface with highly efficient catalysts (5). and M. C. Richoux, Coord. Chem. Reu., 1982,44, (I),83 Thus, in the case of CdS (bg = 2.4 eV), which A. Harriman, G. Porter and P. Walters, 3. Chem. undergoes rapid anodic photodecomposition, Soc., Faraday Trans II, 1981,77,2373 loading with small amounts (about I per cent A. Henglein,J. Phys. Chem., 1979~83,(17), 2209 w/w) of RuO, results in a drastic improvement R. Humphry-Baker, J. Lilie and M. Gratzel, 3. in stability. These systems are being Am. Chem. SOC.,1982,104, (2), 422 K. Kalyanasundaram, E. Borgarello, D. investigated in many laboratories around the Duonghong and M. Gratzel, Angnu. Chem., Int. world and rapid progress should ensue. Ed. Engl., 198r,zo, (I I), 987

Multi-Megawatt Fuel Cell Produces Electricity PLATINUM CATALYSTS PROMOTE EFFICIENT ENERGY CONVERSION The Tokyo Electric Power Company electricity producers in the United States. (TEPCO) has recently commenced a year-long Although this demonstrator unit has not yet test of their 4.5MW demonstration fuel cell produced power, experience gained with it has power plant, manufactured by United already enabled improvements to be Technologies Corporation and incorporating incorporated into the design of an I IMW com- platinum catalysts supplied by Johnson mercial plant now being planned by U.T.C. Matthey, at the Goi power station complex in Ichihara City. Characteristics of Fuel Cells In a special bulletin the Fuel Cell Users The basic advantage of a fuel cell is its Group of the Electric Utility Industry, in the efficiency in converting chemical energy into United States of America, reported that the electricity and heat, which is maintained over a Japanese plant first produced electricity on wide range of operating loads, combined with April 8th, 1983 and at the time of the an ability to respond quickly to load changes. announcement had accumulated over 55 hours Low levels of emissions, quiet operation and a of operating time at 2.4MW. Testing will con- minimal requirement for water are features that tinue to verify operating conditions and perfor- make installation of fuel cell power plants mance characteristics, and to provide data that acceptable in environmentally sensitive areas will help to determine the potential of fuel cell close to the point of power requirement. technology for commercial power generation. The TEPCO generator system was installed The plant, which is designed to have an in less than two years, demonstrating an average output of 4.8MW direct current - advantage of modular construction that will converted to 4.5MW alternating current for enable capacity to be increased incrementally utility use - and a service life of twenty years, and rapidly in response to growing demands. In is a modified version of another United addition TEPCO have postulated siting Technologies demonstration fuel cell power modules in multi-storey buildings, so saving plant now in the final stages of preparation for space in areas with a high population density. operation by Consolidated Edison Company of On the basis of acculnulating experience with New York, Inc. in Manhattan, New York City. numerous kilowatt units and the two multi- The latter is an interim result of a research and megawatt demonstration plants, it seems most development programme initiated in the early probable that phosphoric acid fuel cells 1970s by a group of U.S. electric utilities and incorporating electrodes catalysed with the U.S. Government to explore the feasibility platinum group metals will make a significant of building and operating 27MW fuel cell contribution to the efficient commercial genera- generators, a size that would be suitable for the tion of both heat and electrical power over the needs of 80 per cent of municipal and rural next few decades.

Platinum Metals Rev., 1983,27, (3) 107