Guiding Practical Pathways for Photo-Electrochemical Solar-Hydrogen Generation

Photo-electrochemical electrolysis of water provides a direct pathway for the conversion of solar energy into an energy-dense, storable and transportable fuel. The practical implementation of photo-electrochemical approaches requires focusing simultaneously on four key developmental concerns: device and system i) efficiency, ii) durability and reliability, iii) environmental sustainability, and iv) scalability with economic viability.

We developed a techno-economic model and coupled it to a life cycle assessment (LCA) in order to jointly assess efficiency, hydrogen cost, energy input, and lifetime/degradation of photo-electrochemical devices. The model we used accounted for optical losses in the concentrator, recombination and resistive losses in the photoabsorbers, overpotentials in the electrocatalysts (including mass transport limitations, relevant at high irradiation concentrations), as well as photoabsorber and electrocatalytic degradation. The sustainability assessment accounted for the energy input required for material mining and manufacturing of the device components, as well as operation. The cost assessment accounted for material and component cost and manufacturing. The model was used to investigate and compare a class of 16 devices: combinations of devices using i) concentrated or non-concentrated irradiation (irradiation concentration, C), ii) low quality Si-based or high quality III-V-based photoabsorbers, iii) earth abundant or rare electrocatalysts, and iv) integrated photoabsorber and electrocatalyst with the same or different absorber and electrolysis area (current concentration, F).  

The results predict that maximum efficiency does not guarantee minimum hydrogen price or minimum energy demand, as depicted in figure 1 for a selection of devices. Devices using large irradiation concentration and III-V-based photoabsorbers showed low hydrogen prices and best efficiencies while non-concentrating devices using Si-based photoabsorbers and rare electrocatalysts showed slightly higher energy demand and hydrogen price. An increase in the component’s lifetime was generally beneficial to efficiency, cost, and energy demand. Degradation significantly influenced all three characteristics, and the calculations provided guidance for the most suitable exchange time of individual components – keeping overall device efficiency high while reducing energy and cost resulting from component exchange – during the lifetime of the complete device.

The coupled techno-economic-ecologic approach provides quantitative design guidelines for photo-electrochemical devices and can support decision-making for the implementation of large-scale, sustainable and competitive solar hydrogen production.