ABSTRACT
Electrochemical reorganization of complex structures is directly related to catalytic reactivity; thus, the geometric changes of catalysts induced by electron transfer should be considered to scrutinize the reaction mechanism. Herein, we studied electron-induced reorganization patterns of six-coordinate Co complexes with neutral N-donor ligands. Upon two-electron transfer into a Co center enclosed within a bulky π-acceptor ligand, the catalytic site exhibited different reorganization patterns depending on the ligand characteristics. While a bipyridyl ligand released Co-bound solvent (CH3CN) to open a reaction site, a phenanthroline ligand caused Co-Narm (side "arm" of NNN-ligand) bond dissociation. The first electron transfer occurred in the Co(II/I) reduction step and the second electron entered the bulky π-acceptor, of which redox steps were assigned from cyclic voltammograms, magnetic moment measurements, and DFT calculations. In comparison, the Co complex of [NNNNCH3-Co(CH3CN)3](PF6)2 ([1-(CH3CN)3](PF6)2) showed a high H2 evolution reactivity (HER), whereas a series of Co complexes with bulky π-acceptors such as [NNNNCH3-Co(L)(CH3CN)](PF6)2 (L = phen ([2-CH3CN](PF6)2), bpy ([3-CH3CN](PF6)2), [NNNNCH3-Co(tpy)](PF6)2 ([4](PF6)2), and [NNNCH2-Co(phen)(CH3CN)](PF6)2 ([5-CH3CN](PF6)2)) suppressed the HER but rather enhanced the CO2 reduction reaction. The metal-ligand cooperative redox steps enabled the shift of Co(I) reactivity toward CO2 reduction. Additionally, the amine pendant attached to the NNNNCH3-ligand could stabilize the CO2 reduction intermediate through the hydrogen-bonding interaction with the Co-CO2H adduct.
ABSTRACT
We describe electrochemical reactivity of a pincer-type [NNN-Fe(tBuNC)3](ClO4)2 complex. Upon electron reduction, the Fe(i) species experienced disproportionation to Fe(0) and Fe(ii). An electron-reduced Fe center dissociated a tBuNC ligand to make an open coordination site, where a proton could be transferred. The low-spin Fe center, assisted by isocyanide and a pyridine-based NNN-pincer ligand, catalyzed efficiently the proton reduction reaction. Also, a Lewis basic amine site in the side 'arm' of the NNN-pincer ligand lowered the free energy for the protonation of an Fe center during the proton reduction process. DFT calculations provided insight into a plausible catalytic pathway.
ABSTRACT
Bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are necessary in the renewable energy systems. However, the kinetically slow and large energy-demanding procedures of oxygen electrocatalysis make the preparation of bifunctional catalysts difficult. In this work, we report a novel hierarchical GdFeO3 perovskite oxide of a spherelike nanostructure and surface modification with the group X heterometal oxides. The nanostructured GdFeO3 layer behaved as a bifunctional electrocatalyst in the oxygen electrocatalysis of OER and ORR. Moreover, the surface decoration with catalytically active PtOx + Ni/NiO nanoparticles enhanced the electrocatalytic performances substantially. Incorporation of mesoporous PtOx + Ni/NiO nanoparticles into the porous GdFeO3 nanostructure enlarged the electrochemically active surface area and provided the interconnected nanostructures to facilitate the OER/ORR. The nanostructures were visualized by scanning electron microscopy and transmission electron microscopy images, and the surface area and pore size of nanoparticles were analyzed from N2 adsorption/desorption isotherms. Tafel analysis indicates that surface modification effectively improves the kinetics of oxygen reactions and accordingly increases the electrocatalytic efficiency. Finally, the 2 wt % PtOx + NiO|GdFeO3 (x = 0, 1, and 2) electrode achieved the enhanced OER performance with an overpotential of 0.19 V at 10 mA/cm2 in an alkaline solution and a high turnover frequency of 0.28 s-1 at η = 0.5 V. Furthermore, the ORR activity is observed with an onset potential of 0.80 V and a half-wave potential (E1/2) of 0.40 V versus reversible hydrogen electrode.
