Nonprecious Metal Catalysts for Fuel Cell Applications: Electrochemical Dioxygen Activation by a Series of First Row Transition Metal Tris(2-pyridylmethyl)amine Complexes

Abstract

A series of divalent first row triflate complexes supported by the ligand tris(2-pyridylmethyl)amine (TPA) have been investigated as oxygen reduction catalysts for fuel cell applications. [(TPA)M(2+)](n+) (M = Mn, Fe, Co, Ni, and Cu) derivatives were synthesized and characterized by X-ray crystallography, cyclic voltammetry, NMR spectroscopy, magnetic susceptibility, IR spectroscopy, and conductance measurements. The stoichiometric and electrochemical O(2) reactivities of the series were examined. Rotating-ring disk electrode (RRDE) voltammetry was used to examine the catalytic activity of the complexes on a carbon support in acidic media, emulating fuel cell performance. The iron complex displayed a selectivity of 89% for four-electron conversion and demonstrated the fastest reaction kinetics, as determined by a kinetic current of 7.6 mA. Additionally, the Mn, Co, and Cu complexes all showed selective four-electron oxygen reduction (<28% H(2)O(2)) at onset potentials (~0.44 V vs RHE) comparable to state of the art molecular catalysts, while being straightforward to access synthetically and derived from nonprecious metals.

Enhanced Catalytic Four-Electron Dioxygen (O2) and Two-Electron Hydrogen Peroxide (H2O2) Reduction with a Copper(II) Complex Possessing a Pendant Ligand Pivalamido Group

“…12–15 Multicopper oxidases such as laccase (possessing four copper ions centers per functional unit) can also catalyzed the four-electron four-proton reduction of O 2 , (the “ORR”) at potentials approaching 1.2 V (vs RHE). 16–22 Thus, it has become of interest to study discrete coordination complexes with copper ion for such studies. In fact, certain Cu complexes have recently been reported to exhibit electrocatalytic activity for the four-electron four-proton reduction of O 2 .…”

Incorporation of Hydrogen‐Bonding Functionalities into the Second Coordination Sphere of Iron‐Based Water‐Oxidation Catalysts

“…Costas and co‐workers have also reported a family of cis ‐[Fe II (N 4 )] 2+ complexes that serve as efficient homogeneous water‐oxidation catalysts in which N 4 is a neutral, tetradentate ligand (Figure 1, a) 4. In a related experiment, Arnold et al have also demonstrated that [M II (tpa)] 2+ [M = Mn, Fe, Co, Cu; tpa = tris(2‐pyridylmethyl)amine] complexes also catalyze the reverse reaction, O 2 ‐reduction 5. Considering the 4 e – , 4H + transformation required for the OER, the coupling of proton‐ and electron‐transfer events is critical for effective proton management and can lead to improved reaction kinetics and lower thermodynamic barriers in the catalytic cycle.…”

Coordination Versatility and Amide Shift in Mononuclear FeII Complexes with the Asymmetrical Tripod [(6‐Bromo‐2‐pyridyl)methyl][(6‐pivaloylamido‐2‐pyridyl)methyl](2‐pyridylmethyl)amine (BrMPPA)

“…[35,36] The cyclic voltammogram exhibits a quasi-reversible wave at E 1/2 = 1.07 V versus SCE, close to the recently reported value measured for the parent [(TPA)Fe(CH 3 CN) 2 ] 2+ dication. [37] In the 19 F NMR spectrum recorded in CD 3 CN, a single signal at δ = -77.3 ppm is observed. This signal is sharp with Δν 1/2 = 11 Hz, which indicates that the triflate ions remain noncoordinated and rules out any possibility of dynamic exchange with any other potential ligand.…”