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Final Report Summary - CO2TOSYNGAS (Visible-light-driven CO2 reduction to SynGas using water as electron and proton donor over a Z-scheme photoelectrochemical cell)

In line with the objectives laid out in the Project Proposal, the Fellow has successfully prepared highly active and stable photocathodes for their application in a Z-scheme photoelectrochemical cell. However, since the photovoltaic landscape shifted diametrically since the time of the Grant Submission, the photosensitizer chosen for this objective changed from SiC to organolead halide perovskites, while the overall goal is still valid.

Lead-halide perovskites have triggered the latest breakthrough in photovoltaic technology. Despite the great promise shown by these materials, their instability towards water even in the presence of low amounts of moisture makes them, a priori, unsuitable for their direct use as light harvesters in aqueous solution for the production of hydrogen through water splitting. In this project, the fellow developed a simple method that enables their use in photoelectrocatalytic hydrogen evolution while immersed in an aqueous solution. Field’s metal, a fusible InBiSn alloy, is used to efficiently protect the perovskite from water while simultaneously allowing the photogenerated electrons to reach a Pt hydrogen evolution catalyst.
We employed a 7.7±1.5% PCE p-i-n configuration solar cell (Figure 1a) and adapted it simply by covering it with a low melting point eutectic metal to protect the perovskite from water in order to employ them as photocathodes for the hydrogen evolution reaction (Figure 1b). A layer of FM was used as a protecting and conducting layer, capable of shielding the perovskite from water and allowing the transport of the photogenerated electrons to the top of the device, where they could reach the hydrogen evolution catalyst and produce hydrogen. The average photocurrent density obtained at 0 V versus RHE was 6.9±1.8 mAcm-2, with a record device at 9.8 mAcm-2, and onset potentials as positive as 0.95±0.03 V versus RHE. This performance is superior to current benchmark systems, making these devices extremely interesting for application in overall tandem water splitting devices. The photocathodes retained 80% of their initial photocurrent for more than 1.5 h in aqueous solution under chopped light, and approximately 1 h under continuous illumination. This metal-encapsulation technique is simple and potentially also applicable to other types of unstable or photocorrodible materials.
We subsequently coupled the perovskite photocathode with BiVO4 photoanodes in an attempt to perform unbiased full water splitting by illuminating both materials (which were decorated with Pt as catalysts) in a tandem configuration. Since the performance of the perovskite photocathode is much higher than that of the BiVO4 photoanode (Figure 3), the efficiency of the tandem system is limited by the latter. With this system we were able to perform unbiased full water splitting with an efficiency of ca. 2% (Figure 4).
In order to increase the efficiency, the BiVO4 was replaced by another perovskite in a photoanode configuration. NiMo and NiFe deposited on Ni foam were used as catalysts for hydrogen evolution and oxygen evolution catalysts, respectively. This ongoing work is yet to be optimized but has already shown an efficiency of 5.5% in the unbiased full water splitting reaction under standard 1 sun conditions. The photocurrent obtained was stable for at least 1 h in pH 9 borate buffer (Figure 5).
Our work demonstrates that the high potential that perovskites have shown in the solar cell field can indeed be translated into artificial photosynthesis research. These findings will consequently also spur further attempts to bridge the optoelectronics and solar fuels communities, to find joint applications towards the ultimate goal of harnessing the power of the sun.

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