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The increasingly popular, lead-free perovskite, Cs 3 Bi 2 I 9 has a vulnerable Bi 3+ state under reductive potentials, due to the high standard reduction potential of Bi 3+ /Bi δ + (0 < δ < 3). Contrary to this fundamental understanding, herein, ligand-coated Cs 3 Bi 2 I 9 nanodiscs (NDs) demonstrate outstanding electrochemical stability with up to -1 V versus 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 3 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 3+ in 0.63 M HI under a strong reductive potential. In comparison, the hexagonal phase of bulk Cs 3 Bi 2 I 9 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 2 production. Our Cs 3 Bi 2 I 9 NDs with a hexagonal lattice have 4-6 (002) planes stacked along the c -axis. With 0.005 M photostable NDs, 22.5 μmol h -1 H 2 is photochemically obtained within 8 h in a 6.34 M HI solution. Electrocatalytic H 2 evolution occurs with a turnover frequency of 11.7 H 2 per s at -533 mV and outstanding operational stability for more than 20 h.