Efficient Oxygen Reduction Reaction (ORR) Catalysts Based on Single Iron Atoms Dispersed on a Hierarchically Structured Porous Carbon Framework

Single Fe atoms dispersed on hierarchically structured porous carbon (SA‐Fe‐HPC) frameworks are prepared by pyrolysis of unsubstituted phthalocyanine/iron phthalocyanine complexes confined within micropores of the porous carbon support. The single‐atom Fe catalysts have a well‐defined atomic dispersion of Fe atoms coordinated by N ligands on the 3D hierarchically porous carbon support. These SA‐Fe‐HPC catalysts are comparable to the commercial Pt/C electrode even in acidic electrolytes for oxygen reduction reaction (ORR) in terms of the ORR activity (E_1/2=0.81 V), but have better long‐term electrochemical stability (7 mV negative shift after 3000 potential cycles) and fuel selectivity. In alkaline media, the SA‐Fe‐HPC catalysts outperform the commercial Pt/C electrode in ORR activity (E_1/2=0.89 V), fuel selectivity, and long‐term stability (1 mV negative shift after 3000 potential cycles). Thus, these nSA‐Fe‐HPCs are promising non‐platinum‐group metal ORR catalysts for fuel‐cell technologies.     

Accelerating the oxygen reduction reaction via a bioinspired carbon‐supported zinc electrocatalyst

Oxygen utilization in electrochemical energy generation systems requires to overcome the slow kinetics of oxygen reduction reaction (ORR). Herein, we have outstretched an efficient strategy in order for developing a bioinspired Zn (N₄)/sulfur/graphitic carbon composite (Zn‐S‐Gc) with an effective performance for the ORR at low temperature. The catalyst composite was created by attaching the Zn (N₄) centers in the form of zinc phthalocyanine on the sulfur‐linked graphitic carbon surface. The most positive ORR onset potential of about 1.00 V versus a reversible hydrogen electrode (RHE) was obtained due to the unique structure of a new catalyst in KOH solution (pH = 13) at low temperature (T = 298 K). The catalyst was evaluated using the rotating‐disk electrode method in the potential range of ?0.02–1.18 V versus RHE. The number of transferred electrons as one of the most important parameters (n > 3.70) is almost constant in a wide range of low overpotentials (0.1–0.6 V), which indicates a more efficient four‐electron pathway from O₂ to H₂O on the catalyst surface. The estimated Tafel slope in an appropriate range is about ≈ ?133.3 mV/dec at a low current density and E_1/2 of the electrocatalyst displays a negative shift of only 11 mV after 10,000 cycles. The mean size of the catalyst centers is on the nanoscale (<50 nm).     

Bio‐inspired iron/sulfur/graphene nanocomposite and its use in the catalysis of the oxygen reduction reaction at room temperature in alkaline media on a glassy carbon electrode

This work demonstrates the performance of a bio‐inspired iron/sulfur/graphene nanocomposite as a non‐platinum electrocatalyst for the oxygen reduction reaction (ORR) in an alkaline medium. The catalyst shows the most positive ORR onset potential (1.1 V vs. RHE) according to its unique structure in the alkaline medium (KOH solution, pH = 13) at low temperature (T = 298 K). The catalyst is evaluated by the rotating‐disk electrode (RDE) method under various rotating speeds (0–2,000 rpm) in the potential range ?0.02–1.18 V vs. a rechargeable hydrogen electrode (RHE). The number of transferred electrons, as one of the most important parameters, is almost constant over a wide range of potentials (0.1–0.8 V), which indicates a more efficient four‐electron pathway from O₂ to H₂O on the FePc‐S‐Gr surface. The mean size of catalyst centers are in the nanoscale (<10 nm). The estimated Tafel slope in the appropriate range is about ?110 mV per decade at low current density, and E_1/2 of FePc‐S‐Gr displays a negative shift of only 7.1 mV after 10,000 cycles.     

Oxygen Reduction with a Bifunctional Iridium Dihydride Complex

The iridium dihydride [Ir(H)₂(HPNP)]⁺ (PNP=N(CH₂CH₂PtBu₂)₂) reacts with O₂ to give the unusual, square‐planar iridium(III) hydroxide [Ir(OH)(PNP)]⁺ and water. Regeneration of the dihydride with H₂ closes a quasi‐catalytic synthetic oxygen‐reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate‐limiting O₂ coordination followed by H‐transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent M? H bonds of a mononuclear metal complex.     

Electrochemical Preparation of N‐Doped Cobalt Oxide Nanoparticles with High Electrocatalytic Activity for the Oxygen‐Reduction Reaction

Nitrogen‐doped CoO (N‐CoO) nanoparticles with high electrocatalytic activity for the oxygen‐reduction reaction (ORR) were fabricated by electrochemical reduction of CoCl₂ in acetonitrile solution at cathodic potentials. The initially generated, highly reactive nitrogen‐doped Co nanoparticles were readily oxidized to N‐CoO nanoparticles in air. In contrast to their N‐free counterparts (CoO or Co₃O₄), N‐CoO nanoparticles with a N content of about 4.6 % exhibit remarkable ORR electrocatalytic activity, stability, and immunity to methanol crossover in an alkaline medium. The Co?N_x active sites in the CoO nanoparticles are held responsible for the high ORR activity. This work opens a new path for the preparation of nitrogen‐doped transition metal oxide nanomaterials, which are promising electrocatalysts for fuel cells.     

Electrochemical impedance spectroscopy analysis for oxygen reduction reaction in 3.5% NaCl solution

The electrochemical impedance spectroscopy (EIS) at different potentials has been used to study the oxygen reduction reaction (ORR) in 3.5% NaCl solution on glassy carbon (GC) electrode in this work. Results show that ORR consists of three two-electron reaction steps and both superoxide ion (O₂ ^–) and hydrogen peroxide (H₂O₂), which are produced by ORR, obstruct the diffusion of oxygen to the surface of the electrode and make the EIS results change into a transmissive finite diffusion process with the real part contraction and a reflective finite diffusion process from a semi-infinite diffusion process. The values of electron transfer resistance (R _t) and diffusion resistance (R _d) were calculated from EIS. O₂ ^– influenced strongly on the R _t values and induced a maximum at −0.45 V.     

Sodalite‐type Cu‐based Three‐dimensional Metal–Organic Framework for Efficient Oxygen Reduction Reaction

Inspired by copper‐based oxygen reduction biocatalysts, we have studied the electrocatalytic behavior of a Cu‐based MOF (Cu‐BTT) for oxygen reduction reaction (ORR) in alkaline medium. This catalyst reduces the oxygen at the onset (E_onset) and half‐wave potential (E^1/2) of 0. 940 V and 0.778 V, respectively. The high halfway potential supports the good activity of Cu‐BTT MOF. The high ORR catalytic activity can be interpreted by the presence of nitrogen‐rich ligand (tetrazole) and the generation of nascent copper(I) during the reaction. In addition to the excellent activity, Cu‐BTT MOF showed exceptional stability too, which was confirmed through chronoamperometry study, where current was unchanged up to 12 h. Further, the 4‐electrons transfer of ORR kinetics was confirmed by hydrodynamic voltammetry. The oxygen active center namely copper(I) generation during ORR has been understood by the reduction peak in cyclic voltammetry as well in the XPS analysis.     

Electrocatalytic Reduction of Oxygen at Multi‐Walled Carbon Nanotubes and Cobalt Porphyrin Modified Glassy Carbon Electrode

The multi‐walled carbon nanotubes (MWNTs) modified glassy carbon electrode exhibited electrocatalytic activity to the reduction of oxygen in 0.1 M HAc‐NaAc (pH 3.8) buffer solution. Further modification with cobalt porphyrin film on the MWNTs by adsorption, the resulted modified electrode showed more efficient catalytic activity to O₂ reduction. The reduction peak potential of O₂ is shifted much more positively to 0.12 V (vs. Ag/AgCl), and the peak current is increased greatly. Cyclic voltammetry (CV), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), were used to characterize the material and the modified film on electrode surface. Electrochemical experiments gave the total number of electron transfer for oxygen reduction as about 3, which indicated a co‐exist process of 2 electrons and 4 electrons for reduction of oxygen at this modified electrode. Meanwhile, the catalytic activities of the multilayer film (MWNTs/CoTMPyP)_n prepared by layer‐by‐layer method were investigated, and the results showed that the peak current of O₂ reduction increased and the peak potential shifted to a positive direction with the increase of layer numbers.     

On the Influence of Oxygen on the Degradation of Fe‐N‐C Catalysts

Fe‐N‐C catalysts containing atomic FeN_x sites are promising candidates as precious‐metal‐free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The durability of Fe‐N‐C catalysts in fuel cells has been extensively studied using accelerated stress tests (AST). Herein we reveal stronger degradation of the Fe‐N‐C structure and four‐times higher ORR activity loss when performing load cycling AST in O₂‐ vs. Ar‐saturated pH 1 electrolyte. Raman spectroscopy results show carbon corrosion after AST in O₂, even when cycling at low potentials, while no corrosion occurred after any load cycling AST in Ar. The load‐cycling AST in O₂ leads to loss of a significant fraction of FeN_x sites, as shown by energy dispersive X‐ray spectroscopy analyses, and to the formation of Fe oxides. The results support that the unexpected carbon corrosion occurring at such low potential in the presence of O₂ is due to reactive oxygen species produced between H₂O₂ and Fe sites via Fenton reactions.     

Antiperovskite Intermetallic Nanoparticles for Enhanced Oxygen Reduction

Antiperovskite Co₃InC_0.7N_0.3 nanomaterials with highly enhanced oxygen reduction reaction (ORR) performance were prepared by tuning nitrogen contents through a metal–organic framework (MOF)‐derived strategy. The nanomaterial surpasses all reported noble‐metal‐free antiperovskites and even most perovskites in terms of onset potential (0.957 V at J=0.1 mA cm^?2) and half‐wave potential (0.854 V). The OER and zinc–air battery performance demonstrate its multifunctional oxygen catalytic activities. DFT calculation was performed and for the first time, a 4 e^? dissociative ORR pathway on (200) facets of antiperovskite was revealed. Free energy studies showed that nitrogen substitution could strengthen the OH desorption as well as hydrogenation that accounts for the enhanced ORR performance. This work expands the scope for material design via tailoring the nitrogen contents for optimal reaction free energy and hence performance of the antiperovskite system.     

Concurrent oxygen reduction and methanol oxidation in sulfuric acid solutions on platinized platinum electrodes

Steady-state polarization curves are compared in solutions of 0.5 M H₂SO₄ + O₂ (saturated), 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH, and 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH + O₂ (saturated) on a Pt/Pt electrode. A considerable difference is found between the currents in mixed solutions and those expected based on the principle of additivity of currents in CH₃OH and O₂ individual solutions. The surface coverages with the CH₃OH and O₂ adsorption products are determined in the potential range of 0.2–0.9 V (RHE). Open-circuit potentials are measured in mixed solutions. The obtained results suggest that the direct heterogeneous interaction between methanol and oxygen occurs alongside with faradaic reactions. This is assumed to lead to a decrease in methanol electrooxidation currents at E ≥ 0.8 V and their increase at E ≤ 0.65 V.     

Mass‐Production of Mesoporous MnCo2O4 Spinels with Manganese(IV)‐ and Cobalt(II)‐Rich Surfaces for Superior Bifunctional Oxygen Electrocatalysis

A mesoporous MnCo₂O₄ electrode material is made for bifunctional oxygen electrocatalysis. The MnCo₂O₄ exhibits both Co₃O₄‐like activity for oxygen evolution reaction (OER) and Mn₂O₃‐like performance for oxygen reduction reaction (ORR). The potential difference between the ORR and OER of MnCo₂O₄ is as low as 0.83 V. By XANES and XPS investigation, the notable activity results from the preferred Mn^IV‐ and Co^II‐rich surface. The electrode material can be obtained on large‐scale with the precise chemical control of the components at relatively low temperature. The surface state engineering may open a new avenue to optimize the electrocatalysis performance of electrode materials. The prominent bifunctional activity shows that MnCo₂O₄ could be used in metal–air batteries and/or other energy devices.     

Electrocatalytic Reduction of Oxygen at Anthraquinonedisulfonate/Polypyrrole Composite Film Modified Electrodes and Its Application to the Electrochemical Oxidation of Azo Dye

We report here the electrocatalytic reduction of oxygen on thin anthraquindisulfonate (AQDS)/poplypyrrole (PPy) composite film modified electrodes and its application to the electrooxidation of azo dye‐amaranth. The polymer‐coated cathode exhibited good electrocatalytic activity towards oxygen reduction reaction (ORR), and allowed the formation of strong oxidant hydroxyl radical (^.OH) in the medium via Electro‐Fenton's reaction between cathodically generated H₂O₂ and added or regenerated Fe²⁺. The electrochemical behaviors of ORR in various pH solutions were described using cyclic voltammetry (CV), rotating disk electrode (RDE) and chronoamperometric (CA) techniques. The effect of solution pH on amaranth mineralization by the Fe²⁺/H₂O₂ and Fe³⁺/H₂O₂ electrooxidation systems was studied. In addition, the long‐term electrocatalytic activity and stability of the AQDS/PPy composite film during multiple experimental runs were also examined electrochemically.     

Enhanced Electrocatalytic Performance of Anthraquinonemonosulfonate‐Doped Polypyrrole Composite: Electroanalysis for the Specific Roles of Anthraquinone Derivative and Polypyrrole Layer on Oxygen Reduction Reaction

To analyze the specific roles of anthraquinone‐2‐sulfonate (AQS) and polypyrrole (PPy) layer on oxygen reduction reaction (ORR), the electrocatalytic reduction of oxygen was investigated on the AQS/PPy composite modified graphite electrode. Results show that the enhanced electrocatalytic performance is attributed to the excellent electrocatalytic activity of the immobilized AQS functional groups to mediate two‐electron reduction of O₂ to H₂O₂. The PPy layer may not participate in ORR, but it can further catalyze the two‐electron reduction of H₂O₂ to produce H₂O in the potential range more negative than that the two‐electron reduction of oxygen proceeds efficiently on the AQS sites.     

A novel binuclear copper complex incorporating a nalidixic acid derivative displaying a one‐dimensional coordination polymeric structure

The identification of the antibacterial action of nalidixic acid (nx) was central to the development of the quinolone antibacterial compounds. The ability of the nx naphthyridyl ring to interact with and inhibit some proteins has encouraged the investigation of similar structures in the search for more active compounds with less adverse effects. The possibility of structural modification by attachment of other biologically active moieties to the naphthyridyl ring of nx allowed the development of new active antimicrobial molecules. Hydrazone derivatives of nx can be synthesized easily based on the condensation of the hydrazide derivative of nx with the desired aldehyde or ketone. Only a few complexes with nx hydrazone derivatives have been described but for none were the crystal structures elucidated. The synthesis of a new one‐dimensional Cu^II coordination polymer, namely catena‐poly[[copper(II)‐di‐μ‐chlorido‐copper(II)‐{μ‐1‐ethyl‐N′‐[(1H‐imidazol‐4‐yl)methylidene]‐7‐methyl‐4‐oxo‐1,4‐dihydro‐1,8‐naphthyridine‐3‐carbohydrazidato}‐[dimethanolcopper(II)]‐{μ‐1‐ethyl‐N′‐[(1H‐imidazol‐3‐yl)methylidene]‐7‐methyl‐4‐oxo‐1,4‐dihydro‐1,8‐naphthyridine‐3‐carbohydrazidato}] dichloride methanol tetrasolvate], {[Cu₃(C₁₆H₁₅N₆O₂)₂Cl₂(CH₃OH)₂]Cl₂·4CH₃OH}_n, with the (1H‐imidazol‐4‐yl)methylidene carbohydrazide derivative of nalidixic acid (denoted h4imi), is presented and its structure is compared to the density functional theory (DFT) optimized structure of free h4imi. The title structure presents an octahedral Cu^II ion on an inversion centre alternating along a polymer chain with a square‐pyramidal Cu^II ion, with the two Cu^II centres bridged by two chloride ligands. Hydrogen bonds involving chloride counter‐ions and methanol solvent molecules mediate the three‐dimensional packing of the polymer. Comparison of the geometrical results from the structure analysis with those derived from a DFT study of the free ligand reveal the differences that arise upon coordination.     

Rotating ring-disk voltammetry: Diagnosis of catalytic activity of metallic copper catalysts toward CO<Subscript>2</Subscript> electroreduction

Using the rotating ring (platinum)—disk (glassy carbon) electrode methodology, electrocatalytic activity of the microstructured copper centers (imbedded within the polyvinylpyrrolidone polymer matrix and deposited onto the glassy carbon disk electrode) has been monitored during electroreduction of carbon dioxide both in acid (HClO₄) and neutral (KHCO₃) media as well as diagnosed (at Pt ring) with respect to formation of the electroactive products. Combination of the stripping-type and rotating ring-disk voltammetric approaches has led to the observation that, regardless the overlapping reduction phenomena, the reduction of carbon dioxide at copper catalyst is, indeed, operative and coexists with hydrogen evolution reaction. Using the fundamental concepts of surface electrochemistry and analytical voltammetry, the reaction products (thrown onto the platinum ring electrode) could be considered and identified as adsorbates (on Pt) under conditions of the stripping-type oxidation experiment. Judging from the potentials at which the stripping voltammetric peaks appear in neutral CO₂-saturated KHCO₃ (pH 6.8), formic acid or carbon monoxide seem to be the most likely reaction products or intermediates. The proposed methodology also permits correlation between the CO₂ electroreduction products and the potentials applied to the disk electrode. By performing the comparative stripping-type voltammetric experiments in acid medium (HClO₄ at pH 1) with the adsorbates of formic acid, ethanol and acetaldehyde (on Pt ring), it can be rationalized that, although C₂H₅OH or CH₃CHO are very likely CO₂-reduction electroactive products, formation of some HCOOH, CH₃OH or even CO cannot be excluded.     

A Phthalocyanine‐Based Layered Two‐Dimensional Conjugated Metal–Organic Framework as a Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction

Layered two‐dimensional (2D) conjugated metal–organic frameworks (MOFs) represent a family of rising electrocatalysts for the oxygen reduction reaction (ORR), due to the controllable architectures, excellent electrical conductivity, and highly exposed well‐defined molecular active sites. Herein, we report a copper phthalocyanine based 2D conjugated MOF with square‐planar cobalt bis(dihydroxy) complexes (Co‐O₄) as linkages (PcCu‐O₈‐Co) and layer‐stacked structures prepared via solvothermal synthesis. PcCu‐O₈‐Co 2D MOF mixed with carbon nanotubes exhibits excellent electrocatalytic ORR activity (E_1/2=0.83 V vs. RHE, n=3.93, and j_L=5.3 mA cm^?2) in alkaline media, which is the record value among the reported intrinsic MOF electrocatalysts. Supported by in situ Raman spectro‐electrochemistry and theoretical modeling as well as contrast catalytic tests, we identified the cobalt nodes as ORR active sites. Furthermore, when employed as a cathode electrocatalyst for zinc–air batteries, PcCu‐O₈‐Co delivers a maximum power density of 94 mW cm^?2, outperforming the state‐of‐the‐art Pt/C electrocatalysts (78.3 mW cm^?2).     

The electrochemical reduction reaction of dissolved oxygen on Q235 carbon steel in alkaline solution containing chloride ions

Cyclic voltammetry, electrochemical impedance spectroscopy, and rotating disk electrode voltammetry have been used to study the effect of chloride ions on the dissolved oxygen reduction reaction (ORR) on Q235 carbon steel electrode in a 0.02 M calcium hydroxide (Ca(OH)₂) solutions imitating the liquid phase in concrete pores. The results indicate that the cathodic process on Q235 carbon steel electrode in oxygen-saturated 0.02 M Ca(OH)₂ with different concentrations of chloride ions contain three reactions except hydrogen evolution: dissolved oxygen reduction, the reduction of Fe(III) to Fe(II), and then the reduction of Fe(II) to Fe. The peak potential of ORR shifts to the positive direction as the chloride ion concentration increases. The oxygen molecule adsorption can be inhibited by the chloride ion adsorption, and the rate of ORR decreases as the concentration of chloride ions increases. The mechanism of ORR is changed from 2e⁻ and 4e⁻ reactions, occurring simultaneously, to quietly 4e⁻ reaction with the increasing chloride ion concentration.     

A Molecular Copper Catalyst for Electrochemical Water Reduction with a Large Hydrogen‐Generation Rate Constant in Aqueous Solution

The copper complex [(bztpen)Cu](BF₄)₂ (bztpen=N‐benzyl‐N,N′,N′‐tris(pyridin‐2‐ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen‐generation rate constant (k_obs) of over 10000 s^?1. A turnover frequency (TOF) of 7000 h^?1 cm^?2 and a Faradaic efficiency of 96 % were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu]²⁺ in pH 2.5 buffer solution at ?0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton‐coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu]²⁺.     

Syntheses and structures of one CuI‐containing coordination polymer and two macrocyclic supramolecular complexes based on flexible 1,3,4‐oxadiazole‐containing ligands

Three coordination complexes with Cu^I centres have been prepared using the symmetrical flexible organic ligands 1,3‐bis{[5‐(quinolin‐2‐yl)‐1,3,4‐oxadiazol‐2‐yl]sulfanyl}propane (L1) and 1,4‐bis{[5‐(quinolin‐2‐yl)‐1,3,4‐oxadiazol‐2‐yl]sulfanyl}butane (L2). Crystallization of L1 with Cu(SO₃CF₃)₂ and of L2 with Cu(BF₄)₂ and Cu(ClO₄)₂ in a CH₂Cl₂/CH₃OH mixed‐solvent system at room temperature afforded the coordination complexes catena‐poly[[copper(I)‐μ‐1,3‐bis{[5‐(quinolin‐2‐yl)‐1,3,4‐oxadiazol‐2‐yl]sulfanyl}propane] methanesulfonate dichloromethane 0.6‐solvate], {[Cu(C₂₅H₁₈N₆O₂S₂)](CF₃SO₃)·0.6CH₂Cl₂}_n, (I), bis(μ‐1,4‐bis{[5‐(quinolin‐2‐yl)‐1,3,4‐oxadiazol‐2‐yl]sulfanyl}butane)dicopper(I) bis(tetrafluoridoborate)–dichloromethane–methanol (1/1.5/1), [Cu₂(C₂₆H₂₀N₆O₂S₂)₂](BF₄)₂·1.5CH₂Cl₂·CH₃OH, (II), and bis(μ‐1,4‐bis{[5‐(quinolin‐2‐yl)‐1,3,4‐oxadiazol‐2‐yl]sulfanyl}butane)dicopper(I) bis(perchlorate)–dichloromethane–methanol (1/2/1), [Cu₂(C₂₆H₂₀N₆O₂S₂)₂](ClO₄)₂·2CH₂Cl₂·CH₃OH, (III). Under the control of the dumbbell‐shaped CF₃SO₃⁻ anion, complex (I) forms a one‐dimensional chain and neighbouring chains form a spiral double chain. Under the control of the regular tetrahedron‐shaped BF₄⁻ and ClO₄⁻ anions, complexes (II) and (III) have been obtained as bimetallic rings, which further interact viaπ–π interactions to form two‐dimensional networks. The anions play a decisive role in determining the arrangement of these discrete molecular complexes in the solid state.