Membrane-less photoelectrochemical cells: product separation by hydrodynamic control

Abstract

A key step in order to realise photo-electrochemical (PEC) water splitting to produce hydrogen sustainably, is reactor design. Good engineering will minimise energy losses (both optical and ohmic) due to reactor construction, whilst ensuring the H 2 and O 2 produced are separated, and this can subsequently relax the requirements on the photo-absorber material and/or electrocatalysts. In this paper we show that separation of the products through hydrodynamic flow alone would negate the need for the conventionally used membrane, which has an associated ohmic drop and cost. This is demonstrated to be possible using a 'laminar flow between two parallel plates' reactor design and AR/Pe and AR are found to be the two key dimensionless numbers that predict product cross-over (where AR, Pe are Aspect Ratio and Péclet number respectively). Supersaturation was used as an indicator of bubble formation, which disrupts the laminar flow required for separation and it is shown that by increasing the reactor pressure, higher current densities can be tolerated before supersaturation occurs. Removal of the dissolved hydrogen and oxygen from electrolyte is discussed. A multi-physics model, which employs an optical transfer matrix method, is used to validate the previous conclusions. Experimental data for hematite and Pt deposited on FTO was used as the anode and cathode respectively. Parasitic optical losses and efficiency improvement with stacking are shown for the example reactor configuration. Additionally, the concept of stacking this reactor design in order to absorb light in multiple passes is introduced. This approach relaxes a classical constraint on photo-absorber materials: Large absorption length compared to small diffusion length of charge carriers in the semiconductor.

En route to a unified model for photo-electrochemical reactor optimisation. I - Photocurrent and H2 yield predictions

“…The electrolyte ow, light scattering and reection due to bubbles and other optical phenomena, pH and temperature changes were not included in the present model, which will be extended to include them, based on the studies of Singh, 15 Haussener 16 and Holmes-Gentle. 61…”

Scalable Triple Cation Mixed Halide Perovskite–BiVO4 Tandems for Bias‐Free Water Splitting

“…From this point of view, a lower bias-free photocurrent can even prove beneficial for commercial systems by preventing bubble formation. [127] In this case, the dissolved gas could be removed from the PEC cell and separated in a recirculating system, which can be powered by the additional driving force of up to 0.6 V of the tandems. To mitigate the effect of device degradation and resistive losses on the overall performance of such assemblies, a tiled design consisting of smaller wireless tandems could be constructed, where any faulty device could be simply replaced.…”

Mitigating voltage losses in photoelectrochemical cell scale-up

“…Here, we use two-dimensional (2-D) nite element modeling to quantify the different scale-up related loss mechanisms in a solar-water splitting device: substrate ohmic loss, electrolyte ohmic loss, and local pH-gradient related losses. We consider a membrane-less water splitting device, where the product (i.e., O 2 and H 2 ) separation is done using hydrodynamic control, 29,30 operated with buffered electrolyte at neutral pH. Although the use of neutral pH increases the potential drop in the cell (i.e., due to the evolving pH-gradient and the associated overpotentials), a device operating at neutral pH allows the use of abundant seawater and is safer to operate over large collection areas.…”

“…In such a conguration, hydrodynamic ow alone is sufficient to separate the products and ensure safe operation without the use of conventional membranes. 29,30 Since it has been shown that the voltage losses associated with scale-up are not due to additional potential drop within the light absorbers, 21 only dark electrochemical reactions are taken into consideration here. For our analysis to be valid, the resistance of the semiconductor layer itself should therefore be negligible and the deposition at large area should still result in homogenous lms.…”