Unexpected Correlation Between Boron Chain Condensation and Hydrogen Evolution Reaction (HER) Activity in Highly Active Vanadium Borides: Enabling Predictions.

: Transition-metal borides (TMBs) have recently attracted attention as excellent hydrogen evolution (HER) electrocatalysts in bulk crystalline materials. Herein, we show for the first time that VB and V3 B4 have high electrocatalytic HER activity. Furthermore, we show that the HER activity (in 0.5 m H2 SO4 ) increases with increasing boron chain condensation in vanadium borides: Using a -23 mV overpotential decrement derived from -0.296 mV (for VB at -10 mA cm-2 current density) and -0.273 mV (for V3 B4 ) we accurately predict the overpotential of VB2 (-0.204 mV) as well as that of unstudied V2 B3 (-0.250 mV) and hypothetical "V5 B8 " (-0.227 mV). We then derived an exponential equation that predicts the overpotentials of known and hypothetical Vx By phases containing at least a boron chain. These results provide a direct correlation between crystal structure and HER activity, thus paving the way for the design of even better electrocatalytic materials through structure-activity relationships.