Highly Efficient Chiral 2D/3D Perovskite Artificial Leaf for Spin-Dependent Photoelectrochemical Overall Water Splitting

The intricate nature of the multi-step oxygen evolution reaction (OER) continues to pose a challenge in achieving bias-free photoelectrochemical water-splitting systems. Several theoretical and experimental studies have suggested that OER efficiency can be significantly improved in chiral material-based photoanodes by utilizing the chirality-induced spin selectivity (CISS) phenomenon. The final product at the photoanode is the oxygen molecules, where the spin alignment of intermediate radicals is favorable to the generation of triplet ³O₂ (most stable molecular oxygen species). Considering the energy difference between a singlet ¹O₂ and triplet ³O₂ (the energy of a singlet ¹O₂ is approximately 100 kJ mol^-1 higher than that of triplet ³O₂), spin-dependent chemical reactions combined with the CISS phenomenon can play a crucial role in facilitating the OER process. Especially, by promptly aligning the spin distribution of photo-generated charges within the absorber, it could effectively prevent the formation of hydrogen peroxide, leading to a dramatic reduction in overpotential. In this regard, the incorporation of a chiral spin-filter layer on top of the absorber has been recognized as a promising strategy to achieve a significant enhancement on the PEC performance of the solar-driven water splitting system.<br/>Chiral 2D OIHPs have been conventionally employed as spin-filter layer with two distinct enantiomers, namely <i>R</i>- and <i>S</i>-methyl benzylamine (<i>R</i>- and <i>S</i>-MBA). Owing to the weak π-π interaction of the MBA organic cation, however, the coupling effect between the π-electron and p-orbital of the halide in the inorganic framework is relatively weak in chiral 2D OIHPs. To reinforce the coupling effect of chiral 2D OIHPs, we utilized naphthyl ethylamine (NEA) capable of stronger π–π stacking interaction between naphthalene rings. Moreover, the introduction of NEA organic cations can simplify the molecular arrangement of chiral 2D OIHPs, leading to enhanced film crystallinity and improved chiroptical properties. Herein, we demonstrated that the CISS phenomena can become a remarkable approach by adopting NEA-based chiral 2D OIHPs as a spin-filtering layer on the 3D OIHPs photoanode. Our chiral 2D OIHPs exhibited an impressive CISS efficiency of approximately 95%, indicating a remarkable achievement in spin controlling when compared to other chiral materials. Moreover, chiral 2D/3D OIHPs-based water-splitting device achieved enhanced oxygen evolution performance with a reduced overpotential of 0.2 V, high fill factor, and increased photocurrent of 22 mA cm^–2 at 1.2 V_RHE compared to a device without a spin-filtering layer. For unbiased water splitting, a perovskite-based photocathode decorated with a spatially decoupled hydrogen evolution reaction catalyst was fabricated. This decoupled geometry was adopted to enable the physical protection of the perovskite layer from the electrolyte, thus allowing excellent stability for over 100 h. To implement the bias-free water splitting performance, this perovskite photocathode was connected in parallel with spin-filtering layer adopted 2D/3D OIHPs-based photoanode. Overall solar-driven water splitting was achieved using all the OIHP photoelectrodes, resulting in a remarkable unassisted solar-to-hydrogen efficiency of ~13% and a continuous 100 h stable operation.