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

Rising temperatures, ocean acidification, extended droughts, shifting rainfall patterns, frequent forest fires, and melting glaciers; global over-reliance on exhaustible fossil fuels must shift in favor of carbon-free energy resources to mitigate climate change. Replacing fossil fuels with renewably sourced fuels and chemicals can accelerate the efforts to achieve carbon neutrality without enormous infrastructure modifications. Consequently, solar energy has attracted rapidly growing scientific interest. However, considering the Sun’s diurnal (day/night), seasonal intermittency and complex economic and technological aspects, solar energy storage in the form of chemical energy is one of the most viable pathways. This motivates the development of sustainable fuels and chemical feedstocks utilizing solar power and abundant molecules like water and CO₂. To this end, solar water splitting is an attractive option due to the abundance of water as a chemical feedstock to produce renewable H₂. In the near term, renewable H₂ may decarbonize large-scale industrial processes such as Haber–Bosch, which currently rely on fossil H₂. In the long term, renewable H₂ is posed to be the foundation of the global energy economy. Notably, to hasten the adoption of renewable H₂ and create a carbon-neutral European Union (EU), in 2050, the European Commission issued “A hydrogen strategy for a climate-neutral Europe”.

Several systems have been proposed to store solar energy as chemical energy in the form of hydrogen and oxygen. Indirect photovoltaic (PV) -driven water splitting by means of electrolysers is considered as one of the most straightforward and mature technology for this application. It excels with currently highest efficiencies of 30% but its high system costs may hinder the large-scale implementation to meet the world’s energy demand (link). An alternative to this system is to directly perform solar-driven water splitting on a semiconductor surface, done via photocatalysis or photoelectrochemical (PEC) water splitting. However, despite decades of work, the efficiency of this system is still very low, with the highest solar-to-hydrogen (STH) efficiency of only 1.1% reported to date. One of the main drawbacks of this system is that the oxidation and reduction reaction occurs at the same material or materials with a redox shuttle, requiring highly efficient charge separation and consecutive separation of product gases. Alternatively, PEC water splitting offers an attractive solution for producing H₂ and O₂ on two solid-state semiconductor surfaces. The critical components for a PEC system are the photoelectrodes with p-type and n-type semiconductors typically acting as photocathode for the H₂ evolution reaction (HER) and photoanode for the O₂ evolution reaction (OER), respectively. However, this system has yet to enter commercialization due to limitations mainly governed by material properties and synthesis methods.

Cu₂ZnSn(S,Se)₄ semiconductors with a tunable bandgap (1.0-1.5 eV) are suitable for application as an efficient, low-cost, and environmentally friendly photocathode. Among different photoanodes, relatively stable bismuth vanadate (BiVO₄) with a 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/cm² at 1.23 V_RHE. Thus, coupling CZTS with BiVO₄ in a tandem configuration may provide an excellent avenue for standalone solar water-slitting systems.

In this project, the student will focus 

on BiVO₄ utilizing solution processing methods by leveraging a well-developed spray pyrolysis deposition technique. BiVO₄ has a bandgap energy of 2.4 eV with light response in the wavelength range of 300-520 nm and a suitable valence band position for OER (i.e., the valence band is below the OER potential). Theoretically, pristine BiVO₄ thin films can reach up to 7.5 mAcm^-2 maximum photocurrent density. However, the reported values are still much lower. A common dilemma exists between carrier transport distance (tens of nm) and the thickness needed to absorb the above-bandgap photons (hundreds of nm) completely. Hence, there is an urgent need to boost the absorption efficiency or carrier transport of BiVO₄. Notably, a recent report on this work showed that the bandgap energy of BiVO₄ was successfully reduced by employing a treatment with H₂S gas or an S-rich atmosphere. Furthermore, a cocatalyst will be deposited on top of the BiVO₄ layer to drive OER efficiently. For this, cobalt phosphate (CoPi) or nickel-iron oxyhydroxide (NiFeOOH), two of the most studied materials for electrocatalytic OER will be developed. In addition to conventional material characterization, emphasis will be placed on in-situ techniques to study the processes involved in the PEC reaction meticulously.

The student will test the activity of the thin film BiVO₄ photoanode for the OER in a photo electrochemistry lab (EnergyVille) equipped with a solar simulator, GC, and potentiostats. To elucidate the redox reaction involved in the system, further analysis will be done utilizing advanced PEC measurement setups, such as IPCE, EIS, and spectro-electrochemistry, which are available at Imec. Analysis of charge carrier kinetics and dynamics will be conducted using IMPS, which can distinguish the carrier dynamics in the bulk and the surface of the thin films (CZTS and BiVO₄).