Electrodeposited MnO_<i>x</i>/PEDOT Composite Thin Films for the Oxygen Reduction Reaction

Manganese oxide (MnO_<i>x</i>) was anodically coelectrodeposited with poly­(3,4-ethylenedioxythiophene) (PEDOT) from an aqueous solution of Mn­(OAc)₂, 3,4-ethylenedioxythiophene, LiClO₄ and sodium dodecyl sulfate to yield a MnO_<i>x</i>/PEDOT composite thin film. The MnO_<i>x</i>/PEDOT film showed significant improvement over the MnO_<i>x</i> only and PEDOT only films for the oxygen reduction reaction, with a >0.2 V decrease in onset and half-wave overpotential and >1.5 times increase in current density. Furthermore, the MnO_<i>x</i>/PEDOT films were competitive with commercial benchmark 20% Pt/C, outperforming it in the half-wave ORR region and exhibiting better electrocatalytic selectivity for the oxygen reduction reaction upon methanol exposure. The high activity of the MnO_<i>x</i>/PEDOT composite is attributed to synergistic charge transfer capabilities, attained by coelectrodepositing MnO_<i>x</i> with a conductive polymer while simultaneously achieving intimate substrate contact

Nanoscale Carbon Modified α‑MnO₂ Nanowires: Highly Active and Stable Oxygen Reduction Electrocatalysts with Low Carbon Content

Carbon-coated α-MnO₂ nanowires (C-MnO₂ NWs) were prepared from α-MnO₂ NWs by a two-step sucrose coating and pyrolysis method. This method resulted in the formation of a thin, porous, low mass-percentage amorphous carbon coating (<5 nm, ≤1.2 wt % C) on the nanowire with an increase in single-nanowire electronic conductivity of roughly 5 orders of magnitude (α-MnO₂, 3.2 × 10^–6 S cm^–1; C-MnO₂, 0.52 S cm^–1) and an increase in surface Mn³⁺ (average oxidation state: α-MnO₂, 3.88; C-MnO₂, 3.66) while suppressing a phase change to Mn₃O₄ at high temperature. The enhanced physical and electronic properties of the C-MnO₂ NWsenriched surface Mn³⁺ and high conductivityare manifested in the electrocatalytic activity toward the oxygen reduction reaction (ORR), where a 13-fold increase in specific activity (α-MnO₂, 0.13 A m^–2; C-MnO₂, 1.70 A m^–2) and 6-fold decrease in charge transfer resistance (α-MnO₂, 6.2 kΩ; C-MnO₂, 0.9 kΩ) were observed relative to the precursor α-MnO₂ NWs. The C-MnO₂ NWs, composed of ∼99 wt % MnO₂ and ∼1 wt % carbon coating, also demonstrated an ORR onset potential within 20 mV of commercial 20% Pt/C and a chronoamperometric current/stability equal to or greater than 20% Pt/C at high overpotential (0.4 V vs RHE) and high temperature (60 °C) with no additional conductive carbon

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

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 <i>n</i> values (<i>n</i> = 3.6), faster kinetics (<i>k</i> = 0.015 cm s^–1), and lower charge transfer resistance (<i>R</i>_CT = 2264 Ω at half-wave) values than MnO₂ (<i>n</i> = 3.0, <i>k</i> = 0.006 cm s^–1, <i>R</i>_CT = 6104 Ω at half-wave) or Cu–α-MnO₂ (Cu-2.9%, <i>n</i> = 3.5, <i>k</i> = 0.015 cm s^–1, <i>R</i>_CT = 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