(609f) Membrane-Free Photo-Electrochemical Cell Design Involving Flow Field Hydrogen Separation

In order for solar fuel research to make an impact on the future of energy, it needs to transition from materials to devices. In the absence of a breakthrough in photo-absorber materials that fit all the requirements of water splitting, reactor designs that can operate efficiently with sub-optimal (but inexpensive materials) are crucial to realise this.

 

By applying research into membrane-free electrolysers to photo-electrochemical cells (PEC), the performance of such reactors can, theoretically, surpass many designs in literature. The hydrodynamics of the flowing electrolyte between two parallel plates can be controlled through judicious choice of Reynolds and Pechlet numbers for each of the dissolved product species so that convective forces dominate. This would allow product separation of the dissolved hydrogen and oxygen by fluid flow alone.

 

If the photo-absorber substrate is transparent (e.g. thin film on FTO, Fluorine-doped Tin Oxide), then PEC flow cells could be stacked perpendicular to the light source. This simple, compact, design would improve the overall solar to hydrogen efficiency as the light that is not captured by the photo-absorber on the first pass, is captured on the second or third thus creating a large photo-electrode surface area per unit volume.

 

A multi-physics model of the proposed cell arrangement has been developed and solved using MATLAB to implement a finite-difference numerical scheme. Using the model, it is possible to optimise fuel production, overall solar-to-hydrogen efficiency and pumping cost using a cost analysis of the various processes.

 

The results of the model show that there are multiple benefits of such a reactor design, not least in terms of reduced capital cost due to membrane free operation. It also crucially relaxes two key performance constraints of photo-absorbers, namely a) the photo-voltage required is reduced due to lowered ohmic losses in the electrode gap and b) the removal of the classical compromise between the optical absorption and excitation path length. This reactor design could thus help to realise the potential of sub-optimal semiconductor material such as ultra-thin hematite, which have a very high quantum efficiency but low optical absorbance.

 

We shall further touch on issues of bubble formation and high pressure operation to highlight elements for potential process intensification, also with respect to concurrent solar energy capture below the 1.23 eV threshold.

 

Experimental verification of the theoretical/model results will be demonstrated with a single membrane-free flow cell employing stable and inexpensive Fe based materials.