In the sustainable production of non-fossil fuels, water oxidation is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochemical water oxidation at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at constant anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biological paragon, the water-splitting Mn 4 Ca complex of photosynthesis, with respect to mean Mn oxidation state (ca. +3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidation, we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di-m-oxo bridging between Mn ions is observed. However in the MnCat, the voltage-cycling protocol resulted in formation of Mn III sites and prevented formation of well-ordered and unreactive Mn IV O 2 . Structure-function relations in Mn-based wateroxidation catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technological use.
Broader contextThreatening global climate changes and unsecured supply of fossil fuels call for a global transition toward sustainable energyconversion systems. The storage of wind or solar energy by formation of energy-rich fuel molecules could play a central role in both transient storage of the intermittently provided energy and replacement of fossil fuels in the transportation sector. Whether hydrogen or a carbon-based fuel is the target, in any event the extraction of reducing equivalents and protons from water (that is, water oxidation) is pivotal. In search of water-oxidation catalysts that ultimately could play a role at a global scale, we and others are aiming at development of simple routes towards formation of Mn-based catalysts. Manganese excels by high availability and low toxicity; and in oxygenic photosynthesis, nature has demonstrated that a Mn-based catalyst can oxidize water efficiently. When aiming at an 'artificial leaf' with a Mn-based catalyst directly coupled to a solar-energy-converting material, the activity of the catalyst (per area) needs to cope with the incoming photon flux. Moreover the device design typically requires the use of benign synthesis and operation conditions, that is, temperatures close to room temperature and pH close to neutral. As an important first step, we report a simple protocol for e...