The effect of oxygen content of carbon nanotubes on the catalytic activity of carbon-based iron phthalocyanine for oxygen reduction reaction

Enhanced dissimilatory nitrate reduction to ammonium and electron transfer mechanisms in bidirectional electron transfer biofilm constructed by iron phthalocyanine

Bidirectional electron transfer biofilms (BETB) could efficiently reduce nitrate without accumulating nitrite, representing a promising biological electrochemical denitrification technology. This study utilized iron phthalocyanine modified carbon felt (FePc-CF) to enrich electroactive bacteria, constructing a long-term stable FePc-BETB. Its nitrate removal rate reached 91%, far exceeding the traditional nitrate-reducing biocathode (45%) and Con-BETB (46%). The dissimilatory nitrate reduction to ammonium (DNRA) dominated nitrate reduction in FePc-BETB, consuming 35% of the total electrons. Additionally, FePc-BETB effectively reduced the accumulation of NO2--N and N2O. Electrochemical analysis demonstrated FePc-BETB exhibited stronger electrochemical activity and electron transfer capability. Mediated electron transfer (MET) enhanced by increased extracellular humic acid in FePc-BETB favored the electron supplement for nitrate removal. The relative abundance of nrfA, marker of the DNRA, increased significantly. This study provided new insights into regulating denitrification and DNRA pathways and treating nitrate wastewater lacking electron donors.

Catalytic Degradation of Lignin over Sulfonyl-Chloride-Modified Lignin-Based Porous Carbon-Supported Metal Phthalocyanine: Effect of Catalyst Concentrations

A sulfonyl-chloride-modified lignin-based porous carbon-supported metal phthalocyanine catalyst was prepared and used to replace the traditional Fenton's reagent for lignin degradation. The catalyst underwent a detailed characterization analysis in terms of functional group distributions, surface area, morphological structure, via FT-IR, XPS, BET, and SEM. The catalyst possessed a specific surface area of 638.98 m2/g and a pore volume of 0.291 cm3/g. The prepared catalyst was studied for its ability of oxidative degradation of lignin under different reaction conditions. By optimizing the reaction conditions, a maximum liquid product yield of 38.94% was obtained at 135 °C with 3.5 wt% of catalyst and 15 × 10-2 mol/L H2O2; at the same time, a maximum phenols selectivity of 32.58% was achieved. The compositions and properties of liquid products obtained from lignin degradation using different catalyst concentrations were studied comparatively via GC-MS, FT-IR, 1H-NMR, and EA. Furthermore, the structure changes of solid residues are also discussed.

Engineering bio-interfaces for the direct electron transfer of Myriococcum thermophilum cellobiose dehydrogenase: Towards a mediator-less biosupercapacitor/biofuel cell hybrid

Direct electron transfer (DET) of enzymes on electrode surfaces is highly desirable both for fundamental mechanistic studies and to achieve membrane- and mediator-less bioenergy harvesting. In this report, we describe the preparation and comprehensive structural and electrochemical characterization of a three-dimensional (3D) graphene-based carbon electrode, onto which the two-domain redox enzyme Myriococcum thermophilum cellobiose dehydrogenase (MtCDH) is immobilized. The electrode is prepared by an entirely novel method, which combines in a single step electrochemical reduction of graphene oxide (GO) and simultaneous electrodeposition of positively charged polyethylenimine (PEI), resulting in a well dispersed MtCDH surface. The resulting MtCDH bio-interface was characterized structurally in detail, optimized, and found to exhibit a DET maximum current density of 7.7 ± 0.9 μA cm^-2 and a half-lifetime of 48 h for glucose oxidation, attributed to favorable MtCDH surface orientation. A dual, entirely DET-based enzymatic biofuel cell (EBFC) was constructed with a MtCDH bioanode and a Myrothecium verrucaria bilirubin oxidase (MvBOD) biocathode. The EBFC delivers a maximum power density (P_max) of 7.6 ± 1.3 μW cm^-2, an open-circuit voltage (OCV) of 0.60 V, and an operational lifetime over seven days, which exceeds most reported CDH based DET-type EBFCs. A biosupercapacitor/EBFC hybrid was also constructed and found to register maximum power densities 62 and 43 times higher than single glucose/air and lactose/air EBFCs, respectively. This hybrid also shows excellent operational stability with self-charging/discharging over at least 500 cycles.

Creating Hybrid Coordination Environment in Fe-Based Single Atom Catalyst for Efficient Oxygen Reduction

Tailoring the local chemistry environment to optimize the geometric and electronic properties of single atom catalysts has received much attention recently. Yet, most efforts have been devoted to establishing the preferable binding between the solid support and the single metal atom. In this work, a hybrid coordination environment was created for Fe-based single atom catalysts, comprising inorganic anchoring site from the support and organic ligands from the precursor. Using N,S co-doped graphene oxide as the support, Fe phthalocyanine was selectively anchored by the N/S sites, creating the unique N/S-Fe-N₄ active sites as evidenced by extended X-ray absorption fine structure and Mössbauer spectrometry. Compared with other analogues with different metal centers or support, N/S-Fe-N₄ showed much improved activity in oxygen reduction reaction, delivering onset and half-wave potentials of 1.02 and 0.94 V. This was superior over the state-of-the-art 20 wt % Pt/C and the classic Fe-N₄ carbon catalysts. Density functional theory calculations revealed that the interaction between phthalocyanine ligands and heteroatom dopant from the support pushed electrons of Fe site to para-position, facilitating O₂ adsorption and activation. This work shows the exciting opportunities of creating a hybrid coordination environment in single atom catalysts and paves a new avenue of improving their catalytic performance.

Heterogeneous Structure Regulated by Selection Pressure on Bacterial Adhesion Optimized the Viability Stratification Structure of Electroactive Biofilms

As the core of microbial fuel cells (MFCs), the components and structure of electroactive biofilms (EABs) are essential for MFC performance. Bacterial adhesion plays a vital role in shaping the structure of EABs, but the effect of bacterial adhesion under selection pressure on EABs has not been systematically studied. Here, the response of the composition, structure, and electrochemical performance of EABs to the selective adhesion pressure due to the selective coordination of Fe(III) and Co(II) with thiol and the different affinities for bacteria on hybrid electrodes (Fe₁Co, Fe₄Co, and Fe₁₀Co) were comprehensively investigated. Compared with carbon cloth (CC), the appropriate selective adhesion pressure of Fe₄Co activated the dead inner core of EABs and optimized their viability stratification structure. Both the total viability and the viability of the inner core layer in the Fe₄Co EAB (0.67, 0.70 ± 0.01) were higher than those of the CC (0.46, 0.54 ± 0.01), Fe₁Co (0.50, 0.48 ± 0.03), and Fe₁₀Co (0.51, 0.51 ± 0.03). Moreover, a higher proportion of proteins was detected in the Fe₄Co EAB, enhancing the redox activity of extracellular polymeric substances. Fe₄Co enriched Geobacter and stimulated microbial metabolism. Electrochemical analysis revealed that the Fe₄Co EAB was the most electroactive EAB, with a maximum power density of 2032.4 mW m^-2, which was 1.7, 1.3, and 1.1 times that of the CC (1202.6 mW m^-2), Fe₁Co (1610.3 mW m^-2), and Fe₁₀Co (1824.4 mW m^-2) EABs, respectively. Our findings confirmed that highly active EABs could be formed by imposing selection pressure on bacterial adhesion.

Rational Design of a Highly Dispersed Fe-N-C Nanosheet with 1,10-Phenanthroline-2,9-Dicarboxylic Acid as a Preorganized Ligand: Boosted Electrochemiluminescence Detection of Tetracycline

In view of the shortcomings of the current coreactant electrochemiluminescence (ECL) and inspired by natural oxygen (O₂) reduction metalloenzymes, a novel ECL amplification strategy was established. A pyrolytic iron- and nitrogen-doped (Fe-N-C) nanosheet rich in singly ionized oxygen vacancy (V_O^•) defects was rationally designed by destroying the highly saturated coordination with a preorganized ligand 1,10-phenanthroline-2,9-dicarboxylic acid (PDA). Extraordinary catalytic activity for O₂ activation was obtained via screening a special pyrolysis temperature using spectroscopic and electrochemical methods. The high-spin ferric centers of highly dispersed FeC nanoclusters and abundant carbon and oxygen vacancy defects fully contributed to the inherent catalytic activity. ECL amplification was achieved by integrating the material with luminol to generate redox-active radicals in situ from dissolved O₂ and simultaneously shorten the transferring distance of radicals. Tetracycline (TC), which posed a growing threat to aquatic biodiversity and environmental safety, as a model antibiotic was successfully detected with a detection limit of 3.88 nM (S/N = 3), clarifying a promising application prospect of this new effective ECL amplification strategy in biological analysis and environmental monitoring.

Tailoring Surface Properties of Electrodes for Synchronous Enhanced Extracellular Electron Transfer and Enriched Exoelectrogens in Microbial Fuel Cells

An extracellular electron transfer (EET) process between an electroactive biofilm and an electrode is a crucial step for the performance of microbial fuel cells (MFCs), which is highly related to the enrichment of exoelectrogens and the electrocatalytic activity of the electrode. Herein, an efficient N- and Fe-abundant carbon cloth (CC) electrode with the comodification of iron porphyrin (FePor) and polyquaternium-7 (PQ) was synthesized using a facile solvent evaporation and immersion method and developed as an anode (named FePor-PQ) in MFCs. The surface structural characterizations confirmed the successful introduction of N and Fe atoms, whereas FePor-PQ achieved the N content of 9.59 at %, which may offer various active sites for EET. The introduction of PQ contributed to improving the surface hydrophilicity, providing the composite electrode good biocompatibility for bacterial attachment and colonization as well as substrate diffusion. Based on the advantages, the MFC with the FePor-PQ anode produced a maximum power density of 2165.7 mW m^-2, strikingly higher than those of CC (1124.0 mW m^-2), PQ (1668.8 mW m^-2), and FePor (1978.9 mW m^-2). Furthermore, with the EET mediated by the binding of flavins and c-type cytochromes on the outer membrane was enhanced prominently, the typical exoelectrogen Geobacter was enriched up to 55.84% in the FePor-PQ anode biofilm. This work reveals a synergistic effect from heteroatom coating and surface properties tailoring to boost both the EET efficiency and exoelectrogen enrichment for enhancing MFC performance, which also provides valuable insights for designing electrodes in other bio-electrochemical systems.

Recent advances in electrocatalysis with phthalocyanines

Applications of phthalocyanines (Pcs) in electrocatalysis-including the oxygen reduction reaction (ORR), the carbon dioxide reduction reaction (CO₂RR), the oxygen evolution reaction (OER), and the hydrogen evolution reaction (HER)-have attracted considerable attention recently. Pcs and their derivatives are more attractive than many other macrocycles as electrocatalysts since, although they are structurally related to natural porphyrin complexes, they offer the advantages of low cost, facile synthesis and good chemical stability. Moreover, their high tailorability and structural diversity mean Pcs have great potential for application in electrochemical devices. Here we review the structure and composition of Pcs, methods of synthesis of Pcs and their analogues, as well as applications of Pc-based heterogeneous electrocatalysts. Optimization strategies for Pc-based materials for electrocatalysis of ORR, CO₂RR, OER and HER are proposed, based on the mechanisms of the different electrochemical reactions. We also discuss the structure/composition-catalytic activity relationships for different Pc materials and Pc-based electrocatalysts in order to identify future practical applications. Finally, future opportunities and challenges in the use of molecular Pcs and Pc derivatives as electrocatalysts are discussed.

Study of the Interaction of an Iron Phthalocyanine Complex over Surface Modified Carbon Nanotubes

Carbon nanotubes (CNT) were prepared by a modified chemical vapor deposition (CVD) method. The synthesized carbon materials were treated with acidic and basic solutions in order to introduce certain surface functional groups, mainly containing oxygen (OCNT) or amine (ACNT) species. These modified CNTs (OCNT and ACNT) as well as the originally prepared CNT were reacted with a non-ionic Fe complex, Iron (II) Phthalocyanine, and three composites were obtained. The amount of metal complex introduced in each case and the interaction between the complex and the CNT materials were studied with the aid of various characterization techniques such as TGA, XRD, and XPS. The results obtained in these experiments all indicated that the interaction between the complex and the CNT was greatly affected by the functionalization of the latter.

Tailoring spatial structure of electroactive biofilm for enhanced activity and direct electron transfer on iron phthalocyanine modified anode in microbial fuel cells

Electroactive biofilm (EAB) has been considered as the core determining electricity generation in microbial fuel cells (MFCs), and its spatial structure regulation for enhanced activity and selectivity is of great concern. In this study, iron phthalocyanine (FePc) was introduced into a carbon cloth (CC) electrode, aiming at improving the affinity between the anode and outer membrane c-type cytochromes (OM c-Cyts) and achieving a highly active EAB. The FePc modified CC anode (FePc-CC) effectively improved the viability of EAB and enriched the Geobacter species up to 44.83% (FePc-CC) from 6.97% (CC). The FePc-CC anode achieved a much higher power density of 2419 mW m^-2 than the CC (560 mW m^-2) and a remarkable higher biomass loading of 2477.2 ± 84.5 μg cm^-2 than the CC (749.3 ± 31.3 μg cm^-2). As the charge transfer resistance was decreased by 58.6 times from 395.2 Ω (CC) to 6.74 Ω (FePc-CC), the interfacial reaction rate was accelerated and the direct electron transfer via OM c-Cyts was promoted. This work provides an effective method to improve the EAB activity by regulating its spatial structure, and opens the door toward the development of highly active EAB using metal phthalocyanines in MFCs.

Bifunctional Single Atom Electrocatalysts: Coordination-Performance Correlations and Reaction Pathways

Single atom catalysts (SACs) are ideal model systems in catalysis research. Here we employ SACs to address the fundamental catalytic challenge of generating well-defined active metal centers to elucidate their interactions with coordinating atoms, which define their catalytic performance. We introduce a soft-landing molecular strategy for tailored SACs based on metal phthalocyanines (MPcs, M = Ni, Co, Fe) on graphene oxide (GO) layers to generate well-defined model targets for mechanistic studies. The formation of electronic channels through π-π conjugation with the graphene sheets enhances the MPc-GO performance in both oxygen evolution and reduction reactions (OER and ORR). Density functional theory (DFT) calculations unravel that the outstanding ORR activity of FePc-GO among the series is due to the high affinity of Fe atoms toward O₂ species. Operando X-ray absorption spectroscopy and DFT studies demonstrate that the OER performance of the catalysts relates to thermodynamic or kinetic control at low- or high-potential ranges, respectively. We furthermore provide evidence that the participation of ligating N and C atoms around the metal centers provides a wider selection of active OER sites for both NiPc-GO and CoPc-GO. Our strategy promotes the understanding of coordination-activity relationships of high-performance SACs and their optimization for different processes through tailored combinations of metal centers and suitable ligand environments.

Iron phthalocyanine with coordination induced electronic localization to boost oxygen reduction reaction

Iron phthalocyanine (FePc) is a promising non-precious catalyst for the oxygen reduction reaction (ORR). Unfortunately, FePc with plane-symmetric FeN₄ site usually exhibits an unsatisfactory ORR activity due to its poor O₂ adsorption and activation. Here, we report an axial Fe-O coordination induced electronic localization strategy to improve its O₂ adsorption, activation and thus the ORR performance. Theoretical calculations indicate that the Fe-O coordination evokes the electronic localization among the axial direction of O-FeN₄ sites to enhance O₂ adsorption and activation. To realize this speculation, FePc is coordinated with an oxidized carbon. Synchrotron X-ray absorption and Mössbauer spectra validate Fe-O coordination between FePc and carbon. The obtained catalyst exhibits fast kinetics for O₂ adsorption and activation with an ultralow Tafel slope of 27.5 mV dec^-1 and a remarkable half-wave potential of 0.90 V. This work offers a new strategy to regulate catalytic sites for better performance.

Carbon nanotubes-based PdM bimetallic catalysts through N4-system for efficient ethanol oxidation and hydrogen evolution reaction

Transitional metal-nitrogen-carbon system is a promising candidate to replace the Pt-based electrocatalyst due to its superior activity, durability and cost effectiveness. In this study, we have designed a simple strategy to fabricate carbon nanotubes-supported binary-nitrogen-carbon catalyst via wet-chemical method. Palladium and transitional metals (M, i.e. manganese cobalt and copper) nanoparticles are anchored through four-nitrogen system onto carbon nanotubes (denoted as PdM-N4/CNTs). This material has been used as bifunctional electrocatalyst for electrochemical ethanol oxidation reaction and hydrogen evolution reaction for the first time. The N4-linked nanoparticles onto carbon nanotubes plays a crucial role in intrinsic catalytic activity for both reactions in 1 M KOH electrolyte. Among three PdM-N4/CNTs catalysts, the PdMn-N4/CNTs catalyst exhibits higher catalytic activity in terms of current density, mass activity and stability compared to the benchmark Pt/C. The robust electrocatalysis are inherited from the better attachment of PdMn through N4-system onto carbon nanotubes, comparatively smaller particles formation with better dispersion and higher electrical conductivity.