Abstract:
Fuel cells and metal-air batteries are promising electrochemical technologies for supporting increased grid penetration of intermittent renewable power such as solar and wind. For devices that utilize ambient or pure O2 as a fuel for the discharge reaction at the cathode, the kinetic limitations of the oxygen reduction reaction (ORR) must be addressed to increase device capacity/efficiency and deliver electricity at a competitive levelized cost. Electrocatalysts that catalyze the ORR at low overpotential via an efficient 4 e� pathway that are prepared from abundant elements and have excellent durability are needed. Transition metal oxides (TMOs) represent one class of active and abundant electrocatalyst materials. The TMO mediated ORR occurs at the three phase boundary between O2 (gas), electrolyte (liquid) and the surface atoms of the TMO (solid). Both intrinsic and extrinsic modifications to an electrocatalyst are viable methods to increase electrocatalyst performance.
Manganese oxides (MnOx) are particularly attractive as TMOs due to their natural abundance, relatively low cost, and benign nature. Furthermore, the ability to tailor the size, morphology, stoichiometry, crystalline phase along with the fact that manganese can exist in numerous valences (+2, +3, +4, +6, and +7) provides a real opportunity to drastically improve MnOx mediated oxygen electrocatalysis. Recent studies into MnOx and MnOx hybrid structures have provided for a better understanding of the parameters that are important in achieving more effective ORR in aqueous alkaline electrolyte and have resulted in electrocatalysts that have activity rivaling the more expensive and rare Pt-based systems.
For example, Cu- and Ni-metal ion doping studies of α-MnO2 nanowire have revealed that the Mn(3+)/Mn(4+) couple is the mediator for the rate-limiting redox-driven O2/OH− exchange during ORR and that metal ion doping leads to increased activity. O2 adsorbs via an axial site (the eg orbital on the Mn3+ d4 ion) at the surface or at edge defects of the nanowire, while the increase in covalent nature of the nanowire with metal-ion doping leads to stabilization of O2 adsorbates and faster rates of reduction. Examining the activity for both Ni−α-MnO2 and Cu−α-MnO2 materials indicates that the extent of Mn(3+) at the surface of the electrocatalysts dictates the activity trends within the overall series. In an effort to also understand the role of electrical conductivity on the electrocatalytic process, single-nanowire resistance/conductivity measurements have also been obtained. In each case, modifications that have provided for an increase in nanowire conductivity have also led to an increase in electrocatalysis; however, the data also suggests that the ORR charge transfer resistance value, as determined by electrochemical impedance spectroscopy, is a better indicator of the cation-doping effect on ORR catalysis than the electrical resistance of the nanowire.
Carbon-catalyst blends are typically used in actual ORR application due to the requirements of high electrochemical activity and high electronic conductivity. Hence, in order to expand the utility of the MnOx nanomaterials, more conductive hybrid structures with graphene or semiconducting polymers, and the development of new low carbon content core/shell MnOx/C structures have also been examined, resulting in electrocatalysts with properties rivaling that of the commercial Pt/C benchmark. Several aspects of this work will be presented.
This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.
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