For every metal system (e.g. Au-Ni), corresponding surface minimum adsorption energies were selected from the calculated adsorption energy data set based on whether the corresponding structure had a bulk electrochemical stability below 0.5 eV/atom as calculated using the Materials Project PourbaixDiagram module , assuming a pH of 8.5, a voltage of -1.2 V , and every metal ion at a concentration of 0.6 mM (corresponding approximately to the upper end of experimental metal ion concentration). The minimum hydrogen binding energy on every facet was taken, then the value closest to -0.24 eV (representing approximate thermodynamic neutrality after free energy correction ) was chosen to represent the overall binding energy of the chemical system. Experimental activities were calculated by smoothing observed cumulative hydrogen evolution over time with a Gaussian filter and taking the maximum rate for every trial (see Figure ). After averaging experimental values observed under identical input conditions, we used the maximum rate across every experiment within a metal system to represent chemical activity (see Fig. ). . The photosensitizer is charged by a sacrificial oxidant and then reduces a catalyst, resulting in the formation of hydrogen from water. (d) Time series data for the cumulative evolution of hydrogen over time for a single well in an experiment. The slope of the red line denotes the maximum rate observed. This maximum rate is aggregated across multiple experiments with the same metal combination and the resulting value is used in place of exchange current density as the y-axis of our volcano plot. (e) The observed maximum HER rates for various experiments containing only Au metal salt. Rates from experiments that ran concurrently in different wells are plotted using the same color and shape. Rates were averaged across all experiments that used the same metal salts at the same concentrations (average values are depicted as black bars), and the highest average rate is used to represent the metal system on the volcano plot.