Reassessing the intrinsic hydrogen evolution reaction activity of platinum using scanning electrochemical cell microscopy

While promising catalysts are constantly being discovered for the electrochemical hydrogen evolution reaction (HER), none have surpassed platinum's performance, despite its intrinsic activity being underestimated. A thorough assessment of intrinsic activity is therefore necessary to understand platinum's superior performance. Here, we use scanning electrochemical cell microscopy to overcome limitations in proton and hydrogen mass transport at multiple scales. Reliable HER current transients with steady-state limiting current densities far above the exchange current density are recorded (jl of 4 A/cm2 with a 440 nm pipette) in acid electrolyte. Furthermore, exchange current density analysis shows that platinum's intrinsic activity (230 G 34 mA/cm2) is over 200-fold higher than reported in rotating disc measurements (1 mA/cm2), 3-fold higher than the H2 pump method (75 mA/cm2), and almost twice that from micropolarization analysis (140 mA/cm2). These findings demonstrate the importance of mass transport in achieving high current-density electrocatalysis and reveal platinum's underestimated intrinsic activity.