Stand-alone Photoelectrochemical Tandem Devices for Solar-driven Overall Water Splitting

/ Stand-alone Photoelectrochemical Tandem Devices for Solar-driven Overall Water Splitting

Stand-alone Photoelectrochemical Tandem Devices for Solar-driven Overall Water Splitting

Master internship, PhD internship
- Genk | More than two weeks ago

Be part of imec's interdisciplinary pursuit for the development of a low-cost, durable, and efficient stand-alone solar driven water splitting system.

Rising temperatures, oceanacidification,extended droughts,shifting rainfallpatterns,frequent forest fires, and melting glaciers; globalover-reliance on exhaustible fossil fuels must shift in favor of carbon-freeenergy resources to mitigate climate change. Replacing fossil fuels with renewably sourced fuels and chemicals canaccelerate the efforts to achieve carbon neutrality without enormousinfrastructure modifications. Consequently, solarenergy has attracted rapidly growing scientific interest. However, consideringthe Sun’s diurnal (day/night), seasonal intermittency and complex economic andtechnological aspects, solar energy storage in the form of chemicalenergy is one of the most viable pathways. This motivates the developmentof sustainable fuels and chemical feedstocks utilizing solar power and abundantmolecules like water and CO 2 . To this end, solar water splitting isan attractive option due to the abundance of water as a chemical feedstock to producerenewable H 2 . In the near term, renewable H 2 maydecarbonize large-scale industrial processes such as Haber–Bosch, whichcurrently rely on fossil H 2 . In the long term, renewable H 2 isposed to be the foundation of the global energy economy. Notably, to hasten the adoption of renewable H 2 and create a carbon-neutral European Union (EU), in 2050, the EuropeanCommission issued “A hydrogen strategy for a climate-neutral Europe”.

Several systems have been proposed to store solarenergy as chemical energy in the form of hydrogen and oxygen. Indirectphotovoltaic (PV) -driven water splitting by means of electrolysers isconsidered as one of the most straightforward and mature technology for thisapplication. It excels with currently highest efficiencies of 30% but its highsystem costs may hinder the large-scale implementation to meet the world’senergy demand ( link ). An alternative to thissystem is to directly perform solar-driven water splitting on a semiconductorsurface, done via photocatalysis or photoelectrochemical (PEC) watersplitting. However, despite decades of work, the efficiency of thissystem is still very low, with the highest solar-to-hydrogen (STH)efficiency of only 1.1% reported to date. One of the main drawbacksof this system is that the oxidation and reduction reaction occurs at the samematerial or materials with a redox shuttle, requiring highly efficient chargeseparation and consecutive separation of product gases. Alternatively, PECwater splitting offers an attractive solution for producing H 2 and O 2 on two solid-state semiconductor surfaces. Thecritical components for a PEC system are the photoelectrodes with p-type andn-type semiconductors typically acting as photocathode for the H 2 evolution reaction (HER) and photoanode for the O 2 evolutionreaction (OER), respectively. However, this system has yet to entercommercialization due to limitations mainly governed by materialproperties and synthesis methods.

Cu 2 ZnSn(S,Se) 4 semiconductors with atunable bandgap (1.0-1.5 eV) are suitable for application as an efficient,low-cost, and environmentally friendly photocathode. Among differentphotoanodes, relatively stable bismuth vanadate (BiVO 4 ) witha tunable bandgap energy (2.1-2.4 eV) has achieved a remarkable photovoltage of~ 0.8–1.0 V and a photocurrent density of over 6 mA/cm 2 at 1.23 V RHE . Thus, coupling CZTS with BiVO 4 in atandem configuration may provide an excellent avenue for standalone solarwater-slitting systems.

In this project, the student will focus

on BiVO 4 utilizing solution processing methods by leveraging a well-developed spray pyrolysis deposition technique. BiVO 4 has a bandgap energy of 2.4eV with light response in the wavelength range of 300-520 nm and a suitablevalence band position for OER (i.e., the valence band is below the OERpotential). Theoretically, pristine BiVO 4 thin films can reach up to7.5 mAcm -2 maximum photocurrent density. However, the reportedvalues are still much lower. A common dilemma exists between carrier transportdistance (tens of nm) and the thickness needed to absorb the above-bandgapphotons (hundreds of nm) completely. Hence, there is an urgent need to boost the absorption efficiency or carrier transport of BiVO 4 .
Notably, a recent report on this work showed that the bandgap energy of BiVO 4 was successfully reducedby employing a treatment with H 2 S gas or an S-rich atmosphere. Furt