Cs 3 Bi 2 I 9 nanodiscs with phase and Bi(III) state stability under reductive potential or illumination for H 2 generation from diluted aqueous HI

The increasingly popular, lead-free perovskite, Cs Bi I has a vulnerable Bi state under reductive potentials, due to the high standard reduction potential of Bi /Bi (0 < < 3). Contrary to this fundamental understanding, herein, ligand-coated Cs Bi I nanodiscs (NDs) demonstrate outstanding electrochemical stability with up to -1 V a saturated calomel electrode in aqueous 0.63 M (5% v/v) and 6.34 M (50% v/v) hydroiodic acid (HI), with a minor BiI fraction due to the unavoidable partial aqueous disintegration of the perovskite phase after 8 and 16 h, respectively. A dynamic equilibrium of saturated 0.005 M NDs maintains the common ion effect of I , and remarkably stabilizes ∼93% Bi in 0.63 M HI under a strong reductive potential. In comparison, the hexagonal phase of bulk Cs Bi I disintegrates considerably in the semi-aqueous media. Lowering the concentration of synthetic HI from the commonly used ∼50% v/v by elevating the pH from -0.8 to 0.2 helps in reducing the cost per unit of H production. Our Cs Bi I NDs with a hexagonal lattice have 4-6 (002) planes stacked along the -axis. With 0.005 M photostable NDs, 22.5 μmol h H is photochemically obtained within 8 h in a 6.34 M HI solution. Electrocatalytic H evolution occurs with a turnover frequency of 11.7 H per s at -533 mV and outstanding operational stability for more than 20 h.