The storage of solar energy in chemical bonds will depend on pH-universal catalysts that are not only impervious to acid, but actually thrive in it. Whereas other homogeneous water oxidation catalysts are less active in acid, we report a catalyst that maintained high electrocatalytic turnover frequency at pH values as low as 1.1 and 0.43 (k cat = 1501 AE 608 s À1 and 831 AE 254 s À1 , respectively). Moreover, current densities, related to catalytic reaction rates, ranged from 15 to 50 mA cm À2 mM À1 comparable to those reported for state-ofthe-art heterogeneous catalysts and 30 to 100 times greater than those measured for two prominent literature homogeneous catalysts at pH 1.1 and 0.43. The catalyst also exhibited excellent durability when a chemical oxidant was used (Ce IV , 7400 turnovers, TOF 0.88 s À1 ). Preliminary computational studies suggest that the unusual active-site sulfonate group acts a proton relay even in strong acid, as intended.

now generate hundreds of megawatts of electricity, [1] enabling solar energy to compete with fossil fuels in the near future. Nevertheless, one challenge facing this solar future is its intermittency. [2] Consequently, deployable systems which can store solar energy have been added to the growing lists of requirements for building ideal clean and renewable energy platforms. One promising approach is using solar energy to drive photoelectrochemical reactions for fuel production, such as generation of hydrogen (H 2 ) via water splitting. This method has been described as "artificial photosynthesis," akin to the process in nature which converts light into energy stored as sugars. [3][4][5] Thus, there remains a need to develop an economically viable water splitting cell composed of a light-absorbing semiconductor for solarto-electron conversion coupled to a viable electrocatalyst for a rapid chemical conversion process.Currently, limitations of constructing an efficient semiconductor−catalyst interface include instability, insufficient light absorptivity of the semiconductor materials, and poor charge separation efficiency at the semiconductor−catalyst interface. [6,7] It is known that functionalizing semiconductor surfaces with organic compounds can tune surface properties by adjusting charge distribution, density of surface states, surface dipole, and electric fields at the semiconductor−molecule interface. [8] However, only a handful of reports review the stability of semiconductor−molecule interfaces or their decomposition mechanisms. [7,[9][10][11] Herein, we tether organometallic catalysts onto semiconductor photoelectrodes to shed light on catalyst decomposition pathways during electrolysis. Moreover, the presence of functional molecules on the semiconductor surface can stabilize the interface by limiting growth of native oxides on unmodified surface sites. [12][13][14] Current and future applications of semiconductor devices demand higher performance systems that prevent the accumulation of insulating oxide layers which inhibit charge transport at the semiconductor−functional compound interface.The employed organometallic catalyst, Fe 2 (CO) 6 (μ-S-C 6 H 4 -p-OH) 2 , denoted herein as [FeFe], is a synthetic analog of the active site in a diiron hydrogenase

The storage of solar energy in chemical bonds will depend on pH-universal catalysts that are not only impervious to acid, but actually thrive in it. Whereas other homogeneous water oxidation catalysts are less active in acid, we report a catalyst that maintained high electrocatalytic turnover frequency at pH values as low as 1.1 and 0.43 (k cat = 1501 AE 608 s À1 and 831 AE 254 s À1 , respectively). Moreover, current densities, related to catalytic reaction rates, ranged from 15 to 50 mA cm À2 mM À1 comparable to those reported for state-ofthe-art heterogeneous catalysts and 30 to 100 times greater than those measured for two prominent literature homogeneous catalysts at pH 1.1 and 0.43. The catalyst also exhibited excellent durability when a chemical oxidant was used (Ce IV , 7400 turnovers, TOF 0.88 s À1 ). Preliminary computational studies suggest that the unusual active-site sulfonate group acts a proton relay even in strong acid, as intended.