(105e) Nanostructured Manganese Oxides As Efficient Oxygen Evolution Catalysts

Feng Jiao,* Venkata Bharat Ram Boppana, Seif Yusuf

Center for Catalytic Science & Technology, Department of Chemical Engineering, University of Delaware, Newark, DE, 19716 USA. Fax: +1-302-831-1048; Tel: +1-302-831-3679; E-mail: jiao@udel.edu

Solar energy harvesting is an important technological challenge, considering that the energy of sunlight that strikes the earth’s surface in an hour is sufficient to meet our energy demands for a year.^[1] Moreover, an economic and mobile energy storage media that does not significantly affect the current energy infrastructure is necessary to offset the diffuse and intermittent nature of sunlight. These challenges could be resolved by generating transportable solar fuels (like hydrogen or methanol) from abundant sources, e.g. H₂O and CO₂, utilizing sunlight as the primary energy source. Multiple approaches including photoelectrochemical and photocatalytic methods have been proposed and investigated in the past decades. Irrespective of the approach that is pursued, oxygen evolution from water is the critical reaction, because water is the only cheap, clean and abundant source that is capable of completing the redox cycle  for producing either hydrogen (from H₂O) or carbonaceous fuels (from CO₂) on a terawatt scale. Thus, an effective catalyst for oxygen evolution via water oxidation is the key to accomplish the challenge of efficient solar energy harvesting.^[1,2]

Here, we will demonstrate that the morphology and crystal structure have negligible effect on the photocatalytic properties of MnO₂ based oxygen evolution catalysts, while the turnover rate is proportional to its surface area (i.e. Mn sites available on the surface).^[3] In order to testify the hypothesis, a wide range of manganese oxides with various morphologies and polymorphs are synthesized and their structures are well characterized. The as-synthesized catalysts, such as α-MnO₂ nanotubes, α-MnO₂ nanowires, and β-MnO₂ nanowires, exhibit excellent activities in water oxidation driven by visible light. By calculating the TOFs per surface Mn site, the rates for all the different catalysts are similar (~0.001-0.0005 per second per surface Mn), indicating the negligible morphology and crystal structure effects. Based on this finding, one should expect that the highest activity would be obtained from the manganese oxide with the highest surface area.

To further enhance the turnover frequencies (TOFs) that limited by surface area, surface active site with a higher TOF rate compared with Mn⁴⁺ is required. Along this direction, we introduce K⁺ doped MnO₂ catalysts into oxygen evolution reaction. By doping MnO₂ with K⁺, we create Mn³⁺ sites on the surface of mixed manganese oxides. Our preliminary data show that more than one order higher oxygen evolution rates per surface Mn were observed. In order to explore the origin of the enhancement in oxygen evolution activity, detailed structural characterizations have been performed and the results indicate that Mn³⁺ sites generated by K⁺ doping may be responsible for the high TOFs.

References:

1.   Jiao, F. &Frei, H. Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts. Energy & Environmental Science 3, 1018-1027 (2010).

2.   Jiao, F. & Frei, H. Nanostructured manganese oxide clusters supported on mesoporous silica as efficient oxygen-evolving catalysts. Chemical Communications 46, 2920-2922 (2010).

3.   Boppana, V. B. R. & Jiao, F. Nanostructured MnO₂: an efficient and robust water oxidation catalyst. Chemical Communications 47, 8973-8975 (2011).