Nanostructured manganese oxide catalysts for water oxidation

The storage of solar energy into chemical bonds, such as hydrogen or hydrocarbon fuels, through system mimicking photosynthesis is attracting intensive attention at the moment as a potential pathway to provide cheap and affordable source of renewable energy. The obstacle for the practical application of the artificial photosynthesis system is the low efficiency of the solar energy conversion. The development of active and efficient catalysts based on earth abundant elements is critically important for improving the efficiency of solar driven systems, especially in respect of the water oxidation process. These catalysts can be semiconductors capable of converting light into chemical bonds either by harvesting solar energy in their own right or combined with light harvesting materials, as in Photosystem II (PSII). Manganese, which has been identified as one of the key elements in water oxidation catalysis in PSII, is of great interest in the development of catalytic water oxidation systems. The activity of catalysts is highly determined by the fabrication technology and post treatment. In this work, a novel low-cost method for the synthesis of manganese oxide films was developed by electrodeposition from an ionic liquid at high temperature. By varying the acidity of the deposition electrolytes, the chemical composition of the obtained films was controlled from birnessite (manganese dioxide), Mn₂O₃ to hausmannite (Mn₃O₄), and the micro-structure varies from a porous to a dense morphology. Films composed of birnessite and Mn₂O₃ exhibits highly catalytic performance in water oxidation, while the Mn₃O₄ exhibits low activity. In addition, a facile heat treatment of the as-grown manganese oxide was proven to remarkably improve the water oxidation performance. Investigation into the effects of the heat treatment reveals that the dehydration process removes structural water and hydroxyl groups, and the growth of reduced Mn species (Mnᴵᴵ or Mnᴵᴵᴵ) in the heat treated MnOₓ contributes to the higher catalytic water oxidation activity. The light-harvesting and conversion performance on manganese oxide were also investigated. Although the manganese oxide exhibits good harvesting of visible light, previous studies have shown that the solar energy conversion efficiency is very low. This work developed a buffered organic/inorganic electrolyte for solar water splitting, in which the manganese oxide nanomaterial is capable of effectively using solar energy to promote water oxidation with photocurrents as high as 4.5 4.5 mA cm⁻² at η =540 mV. Long-term stability is another important property for practical water splitting. It is still a big challenge for MnOx to maintain high performance in long-term testing, as its activity decays with time. This research shows that the doping of nickel or iron into the manganese structure improves the water oxidation activity and long-term stability