All-Perovskite Scalable Photoelectrochemical System for Solar Hydrogen Generation
Rising to the challenges of practical solar hydrogen production
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A minimum of 10% solar-to-hydrogen (STH) efficiency is required to develop a viable and practical photoelectrochemical (PEC) system to produce green hydrogen, for which selecting an efficient material is the first criteria.
So far, intrinsically stable metal oxides have been studied as photoelectrode materials for the PEC cell, but in spite of steady developments for the last two decades, the obtained efficiencies are far from the practical efficiency target. On the other hand, photovoltaic (PV) grade materials such as silicon, perovskites, chalcogenides and III-V class are well established in solar cell industries for their high efficiency. But these materials are usually expensive and unstable, especially in water where PEC water splitting cells should operate.
Our group, based at the Ulsan National Institute of Science and Technology (UNIST) in South Korea, has previously studied the challenges associated with practical solar hydrogen production. Our recent article, published in Nature Energy, is our first attempt to develop a scalable PEC system to produce green hydrogen.
Efficient materials for scalable photoelectrochemical systems
Unlike other PV-grade materials, metal-halide perovskites (MHP) have a unique characteristic combination of high STH efficiency but with a low cost and could become an alternative photoelectrode material if their stability issue is properly addressed. The MHP materials have excellent optoelectronic properties and a tunable bandgap, which are desired to provide the necessary photocurrent and photovoltage to split water and produce oxygen and hydrogen in a single PEC cell. But a solution was still required to the remaining challenge – stability in humid conditions and under ultraviolet (UV) light. We therefore aimed to address these issues by stabilizing MHP materials using metal-encapsulation or metal-protection to prepare photoelectrodes for PEC cells. Another challenge for practical application we looked at is scalability – to maintain the high efficiency of a laboratory cell (< 1 cm2) at a practical, large scale (1 m2).
Translating a lab concept to a practical scale
In this study, we selected the most advanced MHP material in terms of efficiency and stability (FAPbI3) and encapsulated it with a thick nickel foil (30 mm) deposited with an NiFeOOH catalyst to protect the MHP in water and promote the oxygen evolution reaction for water splitting (Figure 1a). We optimized this photoanode using different metal foils and studied the catalyst–electrolyte interactions in-depth. Small area FAPbI3 photoanodes (0.25 cm2) were tested using a photoanode connected to a solar cell (PEC-PV) in a single reactor system. We then scaled up the device to a practical, large area PEC system by using a module-based design (Figure 1b) whereby a 7.68 cm2 device was selected as the basic mini-module and repeated horizontally and vertically to fabricate a large-size device.
The key findings of the paper were:
- The lab-scale device achieved 9.89% STH efficiency and demonstrated long-term stability.
- Our system enabled multiple components to be integrated into a single PEC device.
- We obtained 8.9% efficiency at 30.8 cm2 using multi-cells and 8.5% efficiency in 123 cm2, demonstrating only a minimal loss of efficiency as a result of upscaling.
- Long-term stability was also demonstrated in the upscale devices.
A system offering unique advantages and characteristics
Our lab-scale system demonstrated a high STH efficiency and long-term stability, key properties for any system intended for practical use. The combination of multiple components to enable us to scale up the system not only did not significantly impact performance but reduced the use of extra PV components, minimizing system complexity and reducing the cost.
The ability to replicate a similar performance of a small area photoanode over a large area is crucial for developing large solar energy conversion systems. Importantly, our system demonstrated its ability to do this and could therefore lead toward the practical application of our PEC technology for green hydrogen production in outdoor conditions.
There are, however, some limitations to the study and current system that should be considered. MHP is degraded easily in electrolyte, so focus must be given to protecting it with suitable encapsulation before applying it to photoelectrochemical water splitting. Also, leaching of lead from MHP is of environmental concern. These systems were tested under laboratory conditions and will therefore require further testing under real-world environmental conditions.
Improvements still possible to enhance system performance
While our results are promising, we plan to seek further improvements in the efficiency and stability of the PEC system by integrating photoelectrodes, e.g., photoanode–photocathode, together and selecting more efficient and durable catalysts. We will also seek to address the limitations mentioned above.
Reference: Hansora D, Yoo JW, Mehrotra R, et al. All-perovskite-based unassisted photoelectrochemical water splitting system for efficient, stable and scalable solar hydrogen production. Nat Energy. Published online January 23, 2024:1-13. doi:10.1038/s41560-023-01438-x
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