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CHAPTER 1
The Challenge of Water Splitting in View of Photosynthetic Reality and of Research Trends
H. TRIBUTSCH
Bio-Mimetics in Energy Systems Program, Carinthia University for Applied Sciences, Europastrasse 4, 9524 Villach, Austria Email: helmut.tributsch@alice.it
1.1 Introduction
For half a century and two generations, physical chemistry and electrochemical science have been confronted with the challenge of artificial water splitting using solar light. The dream of a sustainable energy technology based on a cheap photocatalyst providing hydrogen from water splitting has stimulated a lot of research initiatives and has seen the rise of several new research disciplines: semiconductor photoelectrochemistry, interfacial science, photocatalysis, nano-material science, dye solar cell development and research on quantum size phenomena. It became clear that the real challenge is not liberating hydrogen from water. For this much-investigated process, cheap catalytic electrodes such as Ni–Mo alloys or NiMoNx catalysts, on carbon supports, were identified, which can successfully replace the best performing Pt, since they keep the overpotential loss below 100 mV.
The big challenge turned out to be the evolution of oxygen, which involves the transfer of four electrons from water. A lot of progress in understanding basic mechanisms has been achieved over the years and many interesting new materials and catalysts were discovered. While scientists rightly feel that significant advances have been achieved and fully justified repeated funding initiatives towards solar energy utilisation, there are, however, some critical problems: few practically thinking analysts feel that an economically feasible photocatalytic water splitting technology is close to application. As outlined in Chapter 10, it is the much lower cost of hydrogen from fossil fuel, which is an important handicap, while the engineering aspects of solar water electrolysis are quite advanced.
Major technical obstacles towards cost reduction are the long-term instability of wet solar energy converting interfaces, the instability of catalysts, too high overpotentials for oxygen evolution, the formation of aggressive radical intermediates, theoretical problems in describing, understanding and tailoring multi-electron transfer, practical geometries for water splitting technology, the difficulty in handling high proton turnover rates and the cost of applied (noble) metal catalysts.
Facing a progressive deterioration of the environment our industrial civilisation requires convincing scientific and technical progress towards sustainability in energy technology. Here, the solar water splitting strategy remains an unequalled option, which, through photosynthesis, has also shaped and safeguarded the climate on earth.
To better understand the difficulties, and in order to learn what evolution has solved more elegantly and practically with photosynthesis and its fuel circuits than modern research, this analysis aims at a comparative evaluation. The challenges of energy conversion handled in nature should be compared with both past and present scientific and technical efforts aimed at solving the same problems. The aim is to identify key areas, into which extension of our knowledge is desirable or even unavoidable. An analysis of why such challenges have not yet been addressed before will be included. Figure 1.1 compares a simplified scheme for the solar water splitting strategy in nature (plants plus animals, left) with a scheme for a programmed technical solar hydrogen economy (centre). To the right a more general presentation of nature's energy-material strategy is given, which also emphasises its elegance in dealing with sustainable materials.
Nature has implemented a series of physical chemical technologies which cannot, at present, be reproduced technically: electronic processes are used for photon energy harvesting and fuel cell operation, because this is unavoidable, but proton gradients and proton currents are preferred for energy storage and production of mechanical and chemical energy (ATP). Nature is also, evidently, much more efficient in dealing with catalysis and, by fixing carbon dioxide, also provides materials for living activities. How the entire biological energy-material strategy could be approached bio-mimetically for human civilisation has been discussed elsewhere. For the simplified artificial solar hydrogen energy technology (Figure 1.1, centre) the key challenge is the cost of sustainable hydrogen from water splitting. It is at present approximately five times more expensive than hydrogen from natural gas. Making the process of photo-induced water splitting more efficient and cost-effective is here the key challenge.
1.2 The Evolution of Natural Photosynthetic Water Splitting: The Most Remarkable Facts
Oxygen-evolving photosynthetic-bacteria started to exist on earth around 3.2–3.5 billion years ago. Before their arrival, anoxygenic photosynthesis already existed. Here, molecules with greater negative oxidation potentials than water were used as electron donors. For major phyla of photosynthetic bacteria like Cyanobacteria it is accepted that at the very beginning of water splitting photosynthesis worked more or less as at present. The evolutionary discoveries for sustainable energy conversion were so well functioning that they were conserved and no simpler or more elegant water splitting mechanism has been discovered since. What were these pioneering inventions which evolution introduced? Let us first, for comparison, look at a modern artificial water splitting system: a photovoltaic cell supplying electricity via two wires for water electrolysis into oxygen and hydrogen. Photo-generated electrons and holes are separated by a thermodynamic potential gradient, permanently imprinted into a silicon or related junction, fabricated at high temperature. In photo-electrochemical cells the junction is present in the semiconductor interface bordering the liquid. From the generated electricity, electrons are extracted from water at a catalytically optimised electrode interface.
What nature solved differently is well known: materials are not fabricated at high but at ambient temperature via dynamic self-organisation processes. No permanent thermodynamic potential gradient is used for the separation of photo-generated charges. Charge separation occurs kinetically, via specially tailored molecular mechanisms, and occurs at 3 ps in purple bacteria, strangely speeding up to a shorter time of 0.9 ps at 1 K. When the excited species is transferring an electron the singlet state is, among other temporary changes, converted into a triplet state via a simultaneous inversion of a nuclear spin (Figure 1.2, left). A back reaction of the electron transferred is strongly suppressed. When a far from equilibrium thermodynamic kinetic model for the electron transfer was calculated considering autocatalytic feedback, a negative effective activation energy, a very fast rate and a negligible reverse reaction resulted. The activated complex behaves as a transient 'dissipative structure' (term used by Prigogine's group to describe self-organised order built up at the expense of energy dissipation and entropy generation). It obtains the energy not from entropy fluctuations, but from the reaction itself. The feedback is assumed to occur via the protein environment (mechanism explained in Figure 1.2, right). Solar energy conversion here follows the principle of kinetic charge separation: not a thermodynamic potential gradient, but rectifying molecular-electronic mechanisms are responsible for charge separation.
Also dye solar cells, based on nano-material photo-electrochemistry, work due to kinetic charge separation. They only function well with a poorly reversible redox system such as I-/I3-. The power output P of such kinetic solar cells, as well as the primary solar energy conversion process in photosynthesis, follow different laws, as compared with classical solar cells with inbuilt potential gradient. The mechanism has been derived from the principle of least action. It turns out that the power, P, of such kinetic solar cells is proportional to the rate constant w and the chemical affinity A of the system (eqn (1.1)). The latter (A) describes the distance from equilibrium and increases as the distance from equilibrium increases. Similarly, the rate constant (w) also increases as the back reaction is suppressed. This confirms that the kinetic solar cell only works well, when electron transfer is highly rectified and operation is pushed far from equilibrium.
PE ≈ wA (1.1)
This will also be true for primary solar energy conversion in photosynthesis and is not the case for classical water splitting photoelectrochemical cells. There is a big advantage from such a biological strategy. By designing molecularly rectified electron transfer it is also possible to operate solar cells with soft materials produced at ambient temperature. They do not have to be stable against internal ion movement, which would gradually degrade any imprinted thermodynamic potential gradient for charge separation.
Another remarkable difference between the biological and the technical approach is the nature of the electrochemical oxidation process of water. We are, in photosynthesis, definitively dealing with a multi-electron process of water oxidation. For the oxidation of water to molecular oxygen ideally a minimum of four times 1.23 eV of energy would have to be supplied. However, when a first electron is extracted approximately 2.8 eV would already be required for generating an OH radical. Only an efficient catalyst can avoid such an activation barrier by properly binding and handling intermediates. The best technical electrodes for water oxidation, such as RuO2, typically require, at 12 mAcm2 of current density an overpotential of 250 to 350 mV. In contrast, an overpotential of only 60 mV is estimated for photosynthetic water splitting at a comparable current density for water oxidation (Figure 1.3) and should attract our special attention (see below). What could nature have accomplished to bypass limitations, which science and technology have experienced on the basis of electrochemical research and what is exactly the experimental evidence for such extraordinary electrochemical behaviour in photosynthesis? According to the author this is the most central question within the water splitting initiative. It should give an answer to both mysteries, that of high oxygen evolution efficiency and that of the use of the abundant transition metal manganese for catalysis.
1.2.1 The Missing Overpotential in Photosynthesis: What Is the Evidence?
The photooxidation process of water in photosynthesis is initiated by electron transfer from chlorophyll P680 to Plastquinone QA, via several intermediates, which generates the cationic radical P6801. This abstracts all together step-by-step, via a tyrosine YZ, four electrons from the manganese containing catalyst, whereby O2 is evolved during the last step.
As pointed out in detail by Watanabe et al. there is a paradox linked with the oxygen evolution capacity of the oxygen evolution system: The redox potential of the Chla/Chla+ couple has been extensively measured. Six independent measurements, for example, confined a redox potential value in the following range (eqn (1.2)):
E0(Chla/Chla+) = +0.807 [+ or -] 0.045 V vs. NHE (1.2)
Dimerisation of chlorophyll is known to shift the potential even more negative so that complex formation cannot easily be used as an argument for a more positive shift of the redox potential. However, the reversible potential of the H2O/O2 couple and thus for oxygen evolution is (eqn (1.3)):
E0 = +1.230 V - 0.059 pH (1.3)
which, for a pH1/46 or pH1/45 solution in the thylakoid internal liquid, would yield a value of (eqn (1.4)):
E0 (pH: 6) = +0.876 V and E0 (pH: 5) = +0.935 V (1.4)
respectively. The values obtained are surprisingly close to that of the Chla/Chla+ couple and do not account for electrochemical energy losses. Geometrical estimations within the photosynthetic system have shown that under typical conditions of photosynthetic solar energy conversion, the manganese centres carry effective oxidation current densities of 12 mAcm-2. Under such conditions even the best artificial catalysts for oxygen evolution (e.g. RuO2) require an overpotential of 250–350 mV. But the overpotential loss in photosynthetic oxygen evolution is estimated to only 60 mV.
Up to 300 mV of oxidation potential are missing, when photosynthetic oxygen evolution is compared to classical electrochemical experience, not speaking of the use of the abundant transition metal manganese for catalysis.
To understand how nature could accomplish electrochemical processes, which require a significantly smaller overpotential than typical technical electrochemical processes, some general considerations on energy limitations for water splitting are helpful. In laboratory experiments on water splitting with platinum black and RuO2 electrodes it was shown that more than 90% of photovoltaic energy can be converted into chemical energy of hydrogen.
These experiments demonstrate that by combining efficient photovoltaic devices with efficient water electrolysis using noble metal catalysts overall efficiencies for solar energy conversion in the range between 10 and 30% are technically possible. In this mentioned experiment (Figure 1.4), which combined a 20% efficient laboratory tandem solar cell with electrolysis via noble metal electrodes at 90% efficiency, an 18% total efficiency for hydrogen generation was confirmed in the laboratory. This appears to be a very high efficiency and it is a quite positive prospect for technical solar electrolysis of water, when comparing it to the 3% energy conversion efficiency into biomass for productive C4 plants during their growth period, and 0.5% biomass efficiency for a sugar cane field with three harvests over the year. But nature in photosynthesis follows not only the aim of energy conversion, it also turns over materials, provides energy for living activities themselves and deals with many issues of survival. Our technology has the advantage of being able to design water splitting mechanisms for energy conversion only, and could therefore become much more efficient than a living organism. However, we would have to learn to do that without expensive photovoltaic technology and without noble metal electrodes. We need therefore to explore principles, which allow implementation of cheap photocatalytic water splitting. Here biomimetic strategies could help us. For this reason, we should learn to understand the principles, involved in the natural energy conversion process. Water electrolysis itself can approach 100% efficiency, when the energy is adequately provided. This is possible because the energy conversion efficiency for water splitting is determined by the ratio of enthalpy turnover (enthalpy change) for water splitting ΔHhyd and for catalyst (manganese complex) function ΔHM (see Section 1.3.1 below). The question now arises, how such a ratio of enthalpies could be manipulated to yield an electrochemical efficiency higher than empirically known from technical electrochemistry and the properties of the best technical catalyst for water splitting, RuO2 electrodes.
1.3 How Can Photosynthetic Water Oxidation Be More Efficient Than Technical?
The oxygen evolution reaction in photosynthesis (photosystem II + water oxidation system) is, because of its relevance for life and as a model for regenerative energy conversion, one of the most intensely studied processes in science. But it is also one of the most intriguing ones, to which numerous theoretical studies have been dedicated. After the approximate nature of the manganese complex became known, many artificial manganese complexes have been synthesised to mimic oxygen evolution. However, the efforts were essentially in vain. Theoretical calculations even suggested so high energy barriers for oxygen evolution that equilibrium structures of manganese clusters were considered not to be able to oxidise water. Distorted non-equilibrium clusters were consequently invoked. In spite of these apparent complications the oxygen evolution reaction from water is, in photosynthetic literature, typically considered to be preceded by a stepwise oxidation of the manganese complex. This essentially occurs via manganese states. The three to four residues within the surrounding amino acids are expected to essentially stabilise the cluster and to adjust charges. Various concepts on the function of Mn4O5Ca clusters and their interaction with water have been developed. Basically, the interaction of the nanostructured Mn4O5Ca cluster with water should be understood as an ordinary electrochemical oxidation reaction of water. It should, however, not be overlooked that manganese compounds have never shown catalytic activity in any relevant scientific-technological process. It was for this reason, and because classical (Marcus) electron transfer theory is not allowing to consider multi-electron transfer without intermediates, that self-organised multi-electron transfer catalysis has been suggested as a possible mechanism of oxidative water splitting. Why is it necessary and why must irreversible thermodynamics be invoked?
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Excerpted from "Advances in Photoelectrochemical Water Splitting"
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