Project summary and concepts
Leaves and algae can split water into oxygen and hydrogen (in the form of reducing equivalents) at ambient conditions exploiting sun light. Prof. James Barber, one of the key participants of the SOLHYDROMICS project, was the recipient of the international Italgas prize in 2005 for his studies on Photosystem II, the enzyme that governs this process. In photosynthesis, the reducing equivalents derived from H2O splitting are used to reduce CO2 giving rise to the various organic compounds of living organisms including those which provide fuel (biomass, sugars, vegetable oils as well as being the origin of the fossil fuels). However, in certain types of photosynthetic organisms and under some conditions a specific enzyme, hydrogenase, can by-pass the CO2 fixation process and can lead to non-negligible H2 formation. The main goal of SOLHYDROMICS will be the development of an artificial device (see Fig. B.1.1.1) capable of splitting water to produce hydrogen at ambient temperature composed of:
-) an anode exposed to sunlight carrying Photosystem II or a PSII-like chemical mimic. Initially, PSII from microalgae known as cyanobacteria will be isolated with high water splitting activity, and immobilised for attachment to the electrically conducting membrane. In this way the generation of electrons and protons from water at the anodic surface will use the natural light harvesting system, charge separation machinery and water oxidation site of PSII. In the longer term synthetic metal-clusters will be explored which can bring about light-driven directional charge separation, thus mimicking the natural photosynthetic reaction centre, and use the oxidising potential of the “hole” to split water on a specifically tailored electrochemically active catalyst.
-) a cathode will carry a hydrogenase or an artificial hydrogenase catalyst in order to recombine protons and electrons into molecular hydrogen. Here again, the initial studies will involve immobilizing the natural enzymes, including those with low sensitivity to oxygen. Also, as for the water splitting site on the anodic side of the membrane, the longer term goal will be to synthesis a catalytic site which mimics hydrogenase activity in order to produce hydrogen gas.
-) a membrane enabling transport of both electrons and protons via e.g. carbon nanotubes or TiO2 connecting the two electrodes and ion-exchange resins like e.g. Nafion or SPEEK, respectively.
Fig. B.1.1.1 Scheme of the SOLHYDROMICS device concept, based on the original idea by Dutton & Moser (personal communication to J. Barber).
The membrane will have to be tailored to provide the minimum transfer resistance so as to achieve maximum conversion efficiencies. Previous experiences of some of the partners showed that Nafion can play a role both as an enzyme immobilisation medium over electrode materials and as solvated proton conductor (like in PEM fuel cells). This will offer a wonderful opportunity to design a system that can readily catch the protons where they are generated and drive them by diffusion towards the cathodic electrode where H2 will form, due to the injection of excited electrons derived from the chlorophyll /dye dependent photoactivated water splitting reaction.
The main technical and scientific objectives of the SOLHYDROMICS project are:
-) development of an innovative device capable of using sunlight to produce hydrogen from water splitting in a most cost effective way with routes based on photovoltaics coupled with electrochemically driven catalysis. An ambitious efficiency target is 10% conversion of solar energy into pure hydrogen which is considered to be feasible based on preliminary calculations by the proponents.
-) the device must be robust with long operational times. A target of 1 week continuous operation is envisaged for a first generation SOLHYDROMICS prototype based on natural enzymes, whilst a 1 month continuous operation is targeted for the second generation one based on stable enzyme mimics. Subsequent technology improvements, beyond the proposed project should lead to a lifetime of years.
-) to disclose wide potential application opportunities, the above targets must be reached without using expensive noble metals or materials and via assembling techniques amenable for mass production.
-) the potential of this artificial solar energy conversion route will also be assessed in comparison to intensive micro-algal growth systems aimed at producing hydrogen or vegetable oils as fuels.