Theory of Hybrid Solar Hydrogen Generation

Thermally assisted solar electrolysis consists of (i) light harvesting, (ii) spectral resolution of thermal (sub bandgap) and electronic (super-bandgap) radiation, the latter of which (iiia) drives photovoltaic or photoelectrochemical charge transfer V(iH2o) while the former (iiib) elevates water to temperature T, and pressure, p  

The spontaneity of the H2 generating water splitting reaction is given by the free energy of formation, AG°f, of water and with the Faraday constant, F, the potential for water electrolysis  

Reaction 15 is endothermic and the electrolyzed water will undergo self cooling unless external heat is supplied. The enthalpy balance and its related thermoneutral 

The water electrolysis rest potential is determined from extrapolation to ideal conditions. Variations of the concentration, c, and pressure, p, from ideality are respectively expressed by the activity (or fugacity for a gas), as a = yc (or yp for a gas), with the ideal state defined at 1 atmosphere for a pure liquid (or solid), and extrapolated from p = 0 or for a gas or infinite dilution for a dissolved species. The formal potential, measured under real conditions of c and p can deviate significantly from the (ideal thermodynamic) rest potential, as for example the activity of water, aw, at, or near, ambient conditions generally ranges from approximately 1 for dilute solutions to less than 0.1 for concentrated alkaline and acidic electrolytes.91 93 The potential for the dissociation of water decreases from 1.229 V at 25 °C in the liquid phase to 1.167 V at 100 °C in the gas phase. Above the boiling the point, pressure is used to express the variation of water activity. The variation of the electrochemical potential for water in the liquid and gas phases are given by  

Conditions of qsoiar t ph0t can be shown to place specific restrictions on the photoabsorber. When Hi20 Etneut, heat must flow to compensate for the self cooling which occurs at the electrolysis rate. That is, for an enthalpy balanced system any additional required heat must flow in a flux equivalent to iieai = imo, and at an average power dieat, such that  

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