(157b) Designing Artificial Photosynthesis Cells Under Various pH Conditions

Using a photoelectrochemical cell (PEC) to produce solar fuels is sometimes called artificial photosynthesis, and it has attracted significant research interest [1-3].  The concept employs a photoactive component that absorbs light to produce a difference in potential.  By interacting with an electrolyte, this potential difference can drive water electrolysis, possibly augmented with other fuel-producing reactions. 

The design of such a system necessitates close interaction between experiment and modeling.  The coupled processes of light absorption, electronic transport, catalysis, and solution transport must all be taken into account simultaneously.  To this end, an integrated 1-D model is developed to predict device performance accurately.  For geometries of interest, we specify target materials properties, such as the exchange current densities of catalysts and the conductivity of the membrane.  For instance, in a microstructured device such as a forest of microwires embedded in a membrane, the membrane conductivity can be as low as 10^-4S/cm with minimal impact on device performance.  The targets indicate that new membranes designed to replace Nafion may still improve device performance despite having lower ionic conductivity, as long as the new membranes also have lower gas permeability.

Because corrosion remains a significant limitation for photoelectrochemical cells (PECs), there is some desire to operate closer to neutral pH conditions.  However, running under milder conditions presents a host of new problems that must be addressed.  Two problems that have been shown experimentally are ohmic drop and losses due to pH gradients [4].  This model offers insight into the origin of these problems and how to avoid them.  The choice of membrane and electrolyte composition affects the depletion of the electrolyte and, by extension, the limiting current behavior.  Additionally, the relative magnitude of losses due to ohmic drop, the dependence of open circuit potential on pH, and the diffusion for different choices of electrolyte is calculated.  Results show that a supporting electrolyte fails to eliminate the pH-dependent loss. 

To overcome this, one strategy is to recirculate the electrolyte between the anode and cathode chambers.  Experimental results are given for a membrane electrolysis cell operating with Nafion and aqueous potassium borate at 10 mA/cm² and a modest recirculation rate on the order of 60 mL/h.  The model is in agreement with several experimental findings, including the decrease in pH difference between anode and cathode as the recirculation rate is increased, the current efficiency, and the composition of the exit stream.  A maximum flow rate for each operating current density is identified in order to avoid flammable gas mixtures, and the optimum flow rate for overall hydrogen production efficiency is calculated.

References:

1. A. Listorti, J. Durrant, and J. Barber, Nat. Mater., 8, 929–930 (2009).

2. M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev., 110, 6446–6473 (2010).

3. S. Bensaid, G. Centi, E. Garrone, S. Perathoner, and G. Saracco, ChemSusChem, 5, 500–521 (2012).

4. E. A. Hernández-Pagán, N. M. Vargas-Barbosa, T. Wang, Y. Zhao, E. S. Smotkin, and T. E. Mallouk, Energ. Environ. Sci., 5, 7582 (2012).