Effect of Pore Size and Distribution on the PEC Properties of Si-Based Porous Monolithic Water-Splitting Devices

Abstract

A potential approach to minimizing cabling losses in Photoelectrochemical (PEC) water splitting devices is adapting a wireless stand-alone configuration. With all components integrated into a single device, this configuration also helps in reducing system cost, size, and complexity. The issue with this structure, however, is that the proton transport distance between the electrodes is quite large, as ions need to travel around the cell to reach the opposite electrode. This leads to a pH gradient between the electrodes, resulting in high ohmic losses and risking cross-over between the product gases, which is a safety hazard. This problem can be eliminated by integrating pores into the device that serve as ionic shortcuts between the electrodes, resulting in a Porous Monolithic Photoelectrochemical (PMP) cell. In this thesis, the effect of pore size and distribution on the performance of PMP cells was analyzed in pursuit of finding a range of optimal pore patterns for this application. A theoretical method involving 2D COMSOL simulations of PMP cells is devised to evaluate losses associated with proton transport (Electrochemical) and the active area available for light absorption (Photovoltaic). It was found that optimal pore size and distribution for proton transport trends towards smaller pore dimensions (diameter and pitch). It was also found that for pore diameters between 20 – 80 µm, the PMP cell can retain up to 70% of the ideal (lossless) photocurrent, if the pH gradient can be suppressed to < 0.36 pH units. Moreover, two pore-processing techniques were compared, namely Deep Reactive Ion Etching (DRIE) and Pulsed Laser Drilling (PLD), to determine their suitability for this application. DRIE processed holes can be near perfectly cylindrical compared to PLD processed pores, which have rougher sidewalls and exhibit significantly more tapering, in comparison. However, DRIE requires lithographic patterning, which is a more expensive and tedious process that adds several steps to the fabrication process.