Electrolysis ofWater

Inert electrodes such as stainless steel or platinum, immersed in water (containing a small addition of ions to promote conductivity), will evolve hydrogen gas at the cathode and oxygen gas at the anode if ~1.9 volts is applied. The chemical potential energy associated with a molecule ofhydrogen is 1.23 eV, so the efficiency ofthe electrolyzer is stated as 65%. According to Turner [109], the efficiency of commercial electrolyzers is in the range 60-73%. (Actually, about 4% of commercial hydrogen production is by electrolysis, and about half that is by electrolysis of brine (NaCl plus water) with chlorine gas, the primary desired product, hydrogen sometimes being abandoned.)

If the electrolyzer is connected with a photovoltaic array (PV) of efficiency 12%, connecting enough PV cells in series to achieve a working voltage greater than 1.9 V, then the overall conversion efficiency, sunlight energy into chemical energy in the form ofhydrogen, is 0.65 x 0.12, which gives 7.8%. This is an off-the-shelf approach with commercial products available at present. A tandem cell is needed to reach the

Nanophysics of Solar and Renewable Energy, First Edition. Edward L. Wolf.

© 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

minimum 1.9 V, and this, as noted in Chapter 7, section titled, “Low-Cost Tandem Technology: Amorphous Silicon:H-Based Solar Cells,” is available in amorphous silicon tandem cells, providing 2.2 V.

We will see below an example of a monolithic PV-hydrogen electrolytic converter with efficiency 12.4%. For comparison, a high-temperature “solar thermal” water­splitting process is predicted to provide 24% efficiency [110] in hydrogen production but requires a large facility.

In general, we regard cells for electrolysis of water, fuel cells, batteries, Stirling engines, and electrical generators as off-the-shelf items, and will focus here on research topics that might eventually broaden the set of available devices.

9.3

Updated: October 27, 2015 — 12:10 pm