Electrochemical reduction of carbon dioxide (CO(2)) to useful chemical materials is of great significance to the virtuous cycle of CO(2). However, some problems such as high overpotential, high applied voltage, and high energy consumption exist in the course of the conventional electrochemical reduction process. This study presents a new CO(2) reduction technique for targeted production of formic acid in a microbial electrolysis cell (MEC) driven by a microbial fuel cell (MFC). The multiwalled carbon nanotubes (MWCNT) and cobalt tetra-amino phthalocyanine (CoTAPc) composite modified electrode was fabricated by the layer-by-layer (LBL) self-assembly technique. The new electrodes significantly decreased the overpotential of CO(2) reduction, and as cathode successfully reduced CO(2) to formic acid (production rate of up to 21.0 ± 0.2 mg·L(-1)·h(-1)) in an MEC driven by a single MFC. Compared with the electrode modified by CoTAPc alone, the MWCNT/CoTAPc composite modified electrode could increase the current and formic acid production rate by approximately 20% and 100%, respectively. The Faraday efficiency for formic acid production depended on the cathode potential. The MWCNT/CoTAPc composite electrode reached the maximum Faraday efficiency at the cathode potential of ca. -0.5 V vs Ag/AgCl. Increasing the number of electrode modification layers favored the current and formic acid production rate. The production of formic acid was stable in the MFC-MEC system after multiple batches of CO(2) electrolysis, and no significant change was observed on the performances of the modified electrode. The coupling of the catalytic electrode and the bioelectrochemical system realized the targeted reduction of CO(2) in the absence of external energy input, providing a new way for CO(2) capture and conversion.

The nonheme iron(IV) oxo complex [Fe(O)(tpenaH)] and its conjugate base [Fe(O)(tpena)] [tpena = N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate] have been prepared electrochemically in water by bulk electrolysis of solutions prepared from [Fe(μ-O)(tpenaH)](ClO) at potentials over 1.3 V (vs NHE) using inexpensive and commercially available carbon-based electrodes. Once generated, these iron(IV) oxo complexes persist at room temperature for minutes to half an hour over a wide range of pH values. They are capable of rapidly decomposing aliphatic and aromatic alcohols, alkanes, formic acid, phenols, and the xanthene dye rhodamine B. The oxidation of formic acid to carbon dioxide demonstrates the capacity for total mineralization of organic compounds. A radical hydrogen-atom-abstraction mechanism is proposed with a reactivity profile for the series that is reminiscent of oxidations by the hydroxyl radical. Facile regeneration of [Fe(O)(tpenaH)]/ [Fe(O)(tpena)] and catalytic turnover in the oxidation of cyclohexanol under continuous electrolysis demonstrates the potential of the application of [Fe(tpena)] as an electrocatalyst. The promiscuity of the electrochemically generated iron(IV) oxo complexes, in terms of the broad range of substrates examined, represents an important step toward the goal of cost-effective electrocatalytic water purification.

[FeIII(OH)­(tpena)]+ (tpena– = N,N,N′-tris­(2-pyridylmethyl)­ethylenediamine-N′-acetate) catalytically activates H2O2 with the concomitant formation of the active oxidants [FeIV(O)­(tpena)]+ and HO• in aqueous solutions at pH 8. A kinetic model is used to demonstrate that the activation of [FeIII(OH)­(tpena)]+ by H2O2 proceeds by the formation of [FeIII(OOH)­(tpena)]+. Two previously unreported reactions of [FeIII(OOH)­(tpena)]+, the first with another H2O2 molecule to afford [FeIII(OH)­(tpena)]+, O2 •–, and HO• and the second, and dominant, with [FeIII(OH)­(tpena)]+ to yield 2 equiv of [FeIV(O)­(tpena)]+ and H2O, are found to be the major pathways for the formation of HO• and [FeIV(O)­(tpena)]+, respectively. The production of HO• was quantified by a chemiluminescence method showing that [FeIV(O)­(tpena)]+ is produced in much larger yields than HO•. The generation of HO• compromises the stability of [FeIII(OH)­(tpena)]+ unless an external substrate is present that can outcompete [FeIII(OH)­(tpena)]+ for HO•. Significantly, we demonstrate that the reaction commonly assumed to occur in the decay of nonheme iron­(III)­hydroperoxides, homolytic O–O bond cleavage, is of minor significance for the generation of HO• and the iron­(IV)­oxo complex. The production of both a reactive high-valent iron–oxo species and HO• under mild, aqueous ambient conditions represents a significant contribution to the current state of the art for biomimetic nonheme chemistry in water.

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A mixed valence diiron(II/III) complex with the ligand 2,6-bis{bis[(2-pyridinylmethyl)amino]methyl}-phenol (bppH) has been covalently anchored onto graphene using a mild in situ microwave-assisted diazonium coupling through an aryl amino precursor and isoamyl nitrite. A dinuclear iron complex is then formed by complexation of the grafted bppHgraphene material with iron(II) in the presence of dioxygen. X-ray photoelectron spectroscopy (XPS), atomic force microscopy, cyclic voltammetry, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy and electron paramagnetic resonance (EPR) spectroscopy confirms the formation of the anchored ligand and derivative diiron complexes. Semi-quantitative XPS analysis shows an average bppH ligand bulk loading of 0.33 mmol/g, corresponding to a significant 20.7 wt% of the functionalized material consisting of grafted moieties. EPR measurements reveal the existence of a strong isotropic S = 1/2 spin center associated with the graphene lattice, together with a much weaker S = 5/2 signal, associated with the iron(III) center of the grafted complex. The grafted complex is redox-active with surface-confined Fe II Fe II → Fe II Fe III (+0.56 V vs. NHE), Fe II Fe III → Fe III Fe III (+0.73 V) and Fe III Fe III → Fe III Fe IV (+0.95 V) redox processes accessible, with an estimated surface coverage of 58 pmol cm-2 established from the electrochemical measurements.