Publications

14 Results

Search results

Jump to search filters

Understanding the Effects of Cationic Dopants on α-MnO2 Oxygen Reduction Reaction Electrocatalysis

Journal of Physical Chemistry C

Lambert, Timothy N.; Vigil, Julian A.; White, Suzanne E.; Delker, Collin J.; Davis, Danae J.; Kelly, Maria; Brumbach, Michael T.; Rodriguez, Mark A.; Swartzentruber, Brian

Nickel-doped α-MnO2 nanowires (Ni-α-MnO2) were prepared with 3.4% or 4.9% Ni using a hydrothermal method. A comparison of the electrocatalytic data for the oxygen reduction reaction (ORR) in alkaline electrolyte versus that obtained with α-MnO2 or Cu-α-MnO2 is provided. In general, Ni-α-MnO2 (e.g., Ni-4.9%) had higher n values (n = 3.6), faster kinetics (k = 0.015 cm s-1), and lower charge transfer resistance (RCT = 2264 Ω at half-wave) values than MnO2 (n = 3.0, k = 0.006 cm s-1, RCT = 6104 Ω at half-wave) or Cu-α-MnO2 (Cu-2.9%, n = 3.5, k = 0.015 cm s-1, RCT = 3412 Ω at half-wave), and the overall activity for Ni-α-MnO2 trended with increasing Ni content, i.e., Ni-4.9% > Ni-3.4%. As observed for Cu-α-MnO2, the increase in ORR activity correlates with the amount of Mn3+ at the surface of the Ni-α-MnO2 nanowire. Examining the activity for both Ni-α-MnO2 and Cu-α-MnO2 materials indicates that the Mn3+ at the surface of the electrocatalysts dictates the activity trends within the overall series. Single nanowire resistance measurements conducted on 47 nanowire devices (15 of α-MnO2, 16 of Cu-α-MnO2-2.9%, and 16 of Ni-α-MnO2-4.9%) demonstrated that Cu-doping leads to a slightly lower resistance value than Ni-doping, although both were considerably improved relative to the undoped α-MnO2. The data also suggest 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. (Figure Presented).

More Details

Role of Cu-Ion doping in Cu-α-MnO2 nanowire electrocatalysts for the oxygen reduction reaction

Journal of Physical Chemistry C

Davis, Danae J.; Lambert, Timothy N.; Vigil, Julian A.; Rodriguez, Marko A.; Brumbach, Michael T.; Coker, Eric N.; Limmer, S.J.

The role of Cu-ion doping in α-MnO2 electrocatalysts for the oxygen reduction reaction in alkaline electrolyte was investigated. Cu-doped α-MnO2 nanowires (Cu-α-MnO2) were prepared with varying amounts (up to ∼3%) of Cu2+ using a hydrothermal method. The electrocatalytic data indicate that Cu-α-MnO2 nanowires have up to 74% higher terminal current densities, 2.5 times enhanced kinetic rate constants, and 66% lower charge transfer resistances that trend with Cu content, exceeding values attained by α-MnO2 alone. The observed improvement in catalytic behavior correlates with an increase in Mn3+ content at the surface of the Cu-α-MnO2 nanowires. The Mn3+/Mn4+ couple is the mediator for the rate-limiting redox-driven O2/OH- exchange. O2 adsorbs via an axial site (the eg orbital on the Mn3+ d4 ion) at the surface or at edge defects of the nanowire, and the increase in covalent nature of the nanowire with Cu-ion doping leads to stabilization of O2 adsorbates and faster rates of reduction. A smaller crystallite size (roughly half) for Cu-α-MnO2 leading to a higher density of (catalytic) edge defect sites was also observed. This work is applicable to other manganese oxide electrocatalysts and shows for the first time there is a correlation for manganese oxides between electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline electrolyte and an increase in Mn3+ character at the surface of the oxide. © 2014 American Chemical Society.

More Details

Silver-graphene nanoribbon composite catalyst for the oxygen reduction reaction in alkaline electrolyte

Electroanalysis

Davis, Danae J.; Raji, Abdul R.; Lambert, Timothy N.; Vigil, Julian A.; Li, Lei; Nan, Kewang; Tour, James M.

Silver-graphene nanoribbons (Ag-GNRs) were prepared from the chemical unzipping of multiwalled carbon nanotubes (MWCNTs) by reaction with Na/K alloy, Ag(O2CCH3) and then CH3OH. Ag-GNRs exhibited improved electrocatalytic ability for the oxygen reduction reaction (ORR) in 0.1M KOH as compared to the underlying GNR substrate alone and displayed an earlier onset and higher currents than a commercial Ag/Carbon (Ag/C) catalyst. The Ag-GNR hybrid demonstrates an outstanding tolerance to CH3OH crossover which exceeds that of the commercial benchmark, 20% Pt/C. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

More Details
14 Results
14 Results