Cathodic Oxygen Reduction Reaction Research Articles

Consolidating the Oxygen Reduction with Sub-Polarized Graphitic Fe-N4 Atomic Sites for an Efficient Flexible Zinc-Air Battery.

The effectuation of the Zn-air battery (ZAB) is appealing for active and durable catalysts to kinetically drive the sluggish cathodic oxygen reduction reaction (ORR). Atomic metal-Nx-C sites are widely witnessed with Pt-like activity, but their demetalations still severely restrict the durability in ORR. Here we have profiled an ordered hierarchical porous carbon supported Fe-N4 single-atom (FeNC) catalyst by a template derivation method for efficient ORR and flexible ZAB studies. The FeNC structure is observed with a sub-polarized graphitic Fe-N4 coordination with a shortened Fe-N bond for potentially consolidating the ORR, together with the hierarchical porous matrix for kinetical mass transfer. Resultantly, the optimized FeNC catalyst showcases Pt-beyond alkaline ORR activity (E1/2 = 0.95 V) with long-term durability for 100 h, delivering the flexible ZAB device with high power density (251 mW cm-2) and durable cycle life (80 h). This research underscores the criterion in rationalizing active and robust ORR catalysts through metal-nitrogen bond modulation.

In-Situ HF Forming Agents for Sustainable Manufacturing of Iron-Based Oxygen Reduction Reaction Electrocatalysis Synthesized Through Sacrificial Support Method.

Fe-Nx-Cs being suitable to replace scarce and overpriced platinum group metals (PGMs) for cathodic oxygen reduction reaction (ORR) are gaining significant importance in the fuel cell arena. Although the typical sacrificial support method (SSM) ensures the superior electrocatalytic activity of derived Fe-Nx-C, removing silica hard templates always remains a great challenge due to the hazardous use of highly toxic and not environmentally friendly hydrofluoric acid. Herein, strategic insight was given to modified SSM by exploiting the in-situ formation of HF, deriving from the decomposition of NH4HF2 and NaF, to dissolve silica templates, thus avoiding the direct use of HF. First, the suitable molar ratio between the etching agent and the silica was analyzed, revealing that NH4HF2 efficiently dissolved silica even in a stoichiometric amount, whereas an excess of NaF was required. However, both etching agents exhibited conformal removal of silica while dispersed active moieties within the highly porous architecture of derived electrocatalysts were left behind. Moreover, NH4HF2-washed counterparts demonstrated relatively higher performance both in acidic and alkaline media. Notably, with NH4HF2-washed Fe-Nx-C electrocatalyst, a remarkable onset potential of 970 mV (vs RHE) was achieved with nearly tetra-electronic ORR as the peroxide yield remained less than 10 % in the alkaline medium.

Porous Carbon‐Supported Catalysts for Proton Exchange Membrane Fuel Cells

AbstractProton exchange membrane fuel cells (PEMFCs) are crucial for the efficient utilization of hydrogen. Currently, their efficiency is mainly limited by the slow kinetics of the cathode oxygen reduction reaction (ORR) and the poisoning effect between ionomers and catalytic sites, particularly with Pt‐based catalysts. Recent works suggest that the emerging porous carbon‐supported catalysts hold promise in mitigating these challenges by ensuring fast kinetics while alleviating the poisoning. This review examines porous carbon‐supported catalysts for PEMFC cathodes, covering synthesis methods, structure and performance evaluation, and future prospects, with an emphasis on the influence of porous carbon support on PEMFC performance. On one hand, the rational design of pore structure in carbon support can help optimize the location of the active sites and enhance mass transfer. On the other hand, diverse pore structures provide a platform for gaining a deeper understanding of the mechanisms behind microscale mass transfer and reaction at the three‐phase boundaries. This review aims to inspire innovative strategies for the precise synthesis of porous carbon‐supported catalysts with various pore structures to further boost PEMFC performance.

Nickel-Nitrogen Doped MnO2 as Oxygen Reduction Reaction Catalyst for Aluminum Air Batteries.

Aluminum-air battery has the advantages of high energy density, low cost and environmental protection, and is considered as an ideal next-generation energy storage conversion system. However, the slow oxygen reduction reaction (ORR) in air cathode leads toitsunsatisfactory performance. Here, we report an electrode made of N and Ni co-doped MnO2nanotubes. In alkaline solution, Ni/N-MnO2has higher oxygen reduction activity than undoped MnO2, with an initial potential of 1.00 V and a half-wave potential of 0.75 V. This is because it has abundant defects, high specific surface area and sufficient Mn3+active sites, which promote the transfer of electrons and oxygen-containing intermediates.Density functional theory (DFT) calculations show that MnO2doped with N and Ni atoms reduces the reaction overpotential and improves the ORR kinetics. The peak power density and energy density of the Ni/N-MnO2air electrodeincreased by 34.03 mW·cm-2and 316.41 mWh·g-1, respectively. The results show that N and Ni co-doped MnO2nanotubes are a promising air electrode, which can provide some ideas for the research of aluminum-air batteries.

The effect of glass sealing stabilization on LSM–YSZ cathode poisoning and oxygen reduction reaction processes in solid oxide fuel cell

AbstractThe perovskite structure and electrochemical activity of (La0.80Sr0.20)0.95MnO3−x–(Y2O3)0.08(ZrO2)0.92 (LSM–YSZ) composite cathode can be significantly affected by volatile boron species originated from sealing glass–ceramics. Herein, we investigate the impact of doping the varying Er2O3 content (ranging from 0 to 4%) into an aluminoborosilicate glass to mitigate boron volatilization, its interaction with the LSM–YSZ cathode and consequent effects on its electrochemical performance. The results reveal that the volatility of boron species can be significantly suppressed by introducing Er oxide into the glass network by enhancing the conversion of [BO3] to [BO4] units, thereby increasing the binding energy of boron and strengthening of the glass structure. Moreover, the electrocatalytic performance for the O2 reduction reaction on the LSM–YSZ/8YSZ/LSM–YSZ symmetric cells is studied under open‐circuit conditions in the presence and absence of glass sealants. The analyses utilizing electrochemical impedance spectroscopy (EIS) and distribution of the relaxation times (DRT) analysis indicate that the electrocatalytic performance of LSM–YSZ cathodes can be enhanced in the presence of glass with 4% Er2O3 (GE4). This enhancement in the electrocatalytic activity of the LSM–YSZ electrode for the oxygen reduction reaction (ORR) is attributed to formation of the thin layer on the electrode surface, leading to a notable reduction in the polarization resistance (Rp) from 0.81 Ω cm2 in the glass absence to 0.45 Ω cm2 in the presence of GE4. However, DRT analysis demonstrates that the prolonged exposure of the cathode to GE4 causes a deterioration in cell performance primarily due to the blocking the cathode active sites, which impedes dominant cathode processes of oxygen dissociative adsorption/desorption and charge transfer and consequently reducing kinetics of the ORR.

Designing a series of RE based carbon composites (RE=Tb, Gd, Ho, Lu, Nd, Sm, Yb, Eu, and Ce)-especially Tb-N-Cs for oxygen reduction enhancement

Iron and nitrogen co-doped carbon materials (Fe–N–Cs) have enormous application prospects in cathodic oxygen reduction reaction (ORR) as efficient electrode materials. However, their stability is of great importance, but is still limited by severe Fenton reaction due to the preferential reaction between Fe2+ and H2O2. This paper intends to rationally design a series of rare earth (RE) metals based carbon matrix anchoring RE centers directly converted from RE oxides and small nitrogen-containing carbon molecules by employing the molten-salts-assisted pyrolysis. A class of prepared RE based carbon (RE-N-Cs) catalysts surprisingly exhibits ORR activities comparable to that of Pt/C. The Tb–N–Cs sample with the best catalytic activity among them was characterized by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) and X-Ray Diffraction (XRD). The characterization results show the presence of Tb single-sites in carbon substrate, with Eonset of 1.06 V and Jd of 6.37 mA cm−2, exceeding those of the commercial Pt/C (Eonset = 0.96 V vs. RHE; Jd = 5.64 mA cm−2).

Silicon-air batteries enabled by in-situ FeMn alloy-catalyzed nitrogen-doped carbon nanotube arrays as efficient air electrodes catalysts

Silicon-air batteries (SABs) have become promising candidates for energy conversion and storage devices due to their high theoretical energy density, cost-effectiveness, and inherent safety. However, the slow kinetics of the 4e− transfer in the oxygen reduction reaction (ORR) at the cathode during discharge, coupled with severe polarization, reduces the battery’s capacity and hinders the development of silicon-air batteries. The cathodes of currently developed SABs primarily rely on commercial Pt/C and MnO2, with limited research on low-cost, efficient, and stable air cathodes for SABs. To address this issue, we synthesized nitrogen-doped carbon nanotubes containing FeMn alloy particles (FeMn@NCNTs) as cathode ORR catalysts using a simple high-temperature pyrolysis method combined with chemical vapor deposition. In an alkaline medium, the catalyst’s half-wave potential (E1/2) reached 0.83 V. Moreover, the FeMn@NCNTs air cathode exhibited excellent compatibility with the silicon anode, and the constructed aqueous silicon-air battery demonstrated a high specific capacity (165 Ah kg−1) and power density (3.69 mW cm−2). Additionally, the quasi-solid-state SABs constructed with FeMn@NCNTs showed stable operation over a wide temperature range, providing a new solution for the development of low-cost, efficient silicon-air batteries suitable for a wide range of applications.

Engineering medium-entropy alloy nanoparticle nanotubes for efficient oxygen reduction

Fuel cells, as promising energy conversion devices, realize the direct conversion of hydrogen fuel chemical energy into electrical energy. The kinetics of cathodic oxygen reduction reaction (ORR) is slow and plays a decisive role in the output efficiency of fuel cells. Platinum catalysts have been commercialized, but their high cost, relatively low catalytic activity and poor cycling durability limit the development of fuel cells. The introduction of non-platinum components is an effective strategy to minimize the cost and maximize the activity. In this study, PtPdCu medium entropy alloy nanoparticle nanotubes (PtPdCu MEANPTs) that demonstrate high catalytic activity and long durability are presented. Combined with density functional theory calculations, the introduction of Pd and Cu modulates surface electronic structure and optimizes the oxygen adsorption energy, thus enhancing the catalytic activity. The 0D/1D hybrid structure exposes more surface sites and inhibits particle maturation. The atomic migration and rearrangement are emerged during the initial stage of potential cycling process, induing a palladium-rich outer layer and preventing the oxidation of platinum atoms. The best-performing Pt24Pd28Cu48 MEANPTs deliver the impressive ORR activity (1.42 A/mgPt at 0.9 V vs. RHE, which is 6.17 times that of commercial Pt/C) and durability (retaining 2.87 times the initial activity of benchmark catalyst after 30,000 potential cycles). Theoretical calculations have confirmed the regulation of alloying on the surface oxygen affinity and revealed the true active sites on the catalyst surface. This work explores the research direction of platinum-based multi-element catalysts with low platinum content and high catalytic activity.

Domestic wastewater treatment towards reuse by “self-supplied” microbial electrochemical system assisted UV/H2O2 process

Domestic wastewater is a potential source of water for non-potable reuse that may help address the global water, energy, and resource challenges. Herein, a “self-supplied” process through integrating microbial electrochemical system (MES) with UV/H2O2 was developed and investigated for wastewater treatment. H2O2 was “self-supplied” from MES while the MES catholyte was “self-supplied” from the final effluent of UV/H2O2. It was found that the MES accomplished > 80 % degradation of chemical oxygen demand (COD) through bioanode degradation, and produced 18 - 20 mg L−1 H2O2 via oxygen reduction reaction in the gas diffusion cathode. The MES effluent was further treated by the UV/H2O2 process, which achieved the complete removal of recalcitrant diclofenac and > 6 log inactivation of Escherichia coli. The enhanced treatment performance of UV/H2O2 was demonstrated via a comparison with the control experiments (UV or H2O2 treatment) and benefited from ·OH generation and sulfide removal. When treating the actual wastewater, the proposed system exhibited consistent treatment performance for the organic compounds and recalcitrant contaminants, and the quality of the treated water would meet the non-potable water reuse guidelines. The results of this study encourage the further exploration of emerging contaminant removal, system coordination, and use of renewable energy by the cooperation between MES and UV/H2O2.

Recent Advances in Confined Pt‐Based Electrocatalysts for Oxygen Reduction Reaction

AbstractDeveloping advanced electrocatalysts toward the cathodic oxygen reduction reaction is essential for the large‐scale application of fuel cells. Pt‐based catalysts have been known to exhibit optimal ORR performance compared with other metal‐based counterparts. However, their long‐term stability and toxic tolerance still need further improvement. Numerous optimization strategies have been employed in the past few years to enhance the ORR comprehensive performance of Pt‐based electrocatalysts, and tremendous progress has been achieved. Among various optimization strategies, the confinement strategy can not only inhibit the coalescence of catalysts during high‐temperature preparation but also mitigate the aggregation, detachment, and dissolution during the electrochemical process, which motivates this review. After addressing the fundamental ORR mechanism and the superiority of confined strategies, we categorize and review various Pt‐based catalysts confined by different materials, including organic compounds, inorganic oxides and carbon materials. Their synthesis routes are discoursed first, then their structure and performance optimization are highlighted. Subsequently, the principle of confinement strategy to enhance the catalyst ORR performance is also discussed. Finally, the challenges and perspectives on the materials selection, synthesis design, technology improvement, and practical application of confined Pt‐based catalysts are proposed.

N-doped carbon confined ternary Pt2NiCo intermetallics for efficient oxygen reduction reaction

Developing high performance electrocatalysts for the cathodic oxygen reduction reaction (ORR) is essential for the widespread application of fuel cells. Herein, a promising Pt2NiCo atomic ordered ternary intermetallic compound with N-doped carbon layer coating (o-Pt2NiCo@NC) has been synthesized via a facile method and applied in acidic ORR. The confinement effect provided by the carbon layer not only inhibits the agglomeration and sintering of intermetallic nanoparticles during high temperature process but also provides adequate protection for the nanoparticles, mitigating the aggregation, detachment and poisoning of nanoparticles during the electrochemical process. As a result, the o-Pt2NiCo@NC demonstrates a mass activity (MA) and specific activity (SA) of 0.65 A/mgPt and 1.41 mA/cmPt2 in 0.1 mol/L HClO4, respectively. In addition, after 30,000 potential cycles from 0.6 V to 1.0 V, the MA of o-Pt2NiCo@NC shows much lower decrease than the disordered Pt2NiCo alloy and Pt/C. Even cycling at high potential cycles of 1.5 V for 10,000 cycles, the MA still retains ∼70%, demonstrating superior long-term durability. Furthermore, the o-Pt2NiCo@NC also exhibits strong tolerance to CO, SOx, and POx molecules in toxicity tolerance tests. The strategy in this work provides a novel insight for the development of ORR catalysts with high catalytic activity, durability and toxicity tolerance.

Research Progress of Pt-Based Catalysts toward Cathodic Oxygen Reduction Reactions for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) have been considered by many countries and enterprises because of their cleanness and efficiency. However, due to their high cost and low platinum utilization rate, the commercialization process of PEMFC is severely limited. The cathode catalyst layer (CCL) plays an important role in manipulating the performance and lifespan of PEMFCs, which makes them one of the most significant research focuses in this community. In the CCL, the intrinsic activity and stability of the catalysts determine the performance and lifetime of the catalyst layer. In this paper, the composition and working principle of the PEMFC and cathode catalyst layer are briefly introduced, focusing on Pt-based catalysts for oxygen reduction reactions (ORRs). The research progress of Pt-based catalysts in the past five years is particularly reviewed, mainly concentrating on the development status of emerging Pt-based catalysts which are popular in the current research field, including novel concepts like phase regulation (intermetallic alloys and high-entropy alloys), interface engineering (coupled low-Pt/Pt-free catalysts), and single-atom catalysts. Finally, the future research and development directions of Pt-based ORR catalysts are summarized and prospected.

Electrodeposition of Ni/Cu Bimetallic Conductive Metal-Organic Frameworks Electrocatalysts with Boosted Oxygen Reduction Activity for Zinc-Air Batteries.

Zinc-air batteries employing non-Pt cathodes hold significant promise for advancing cathodic oxygen reduction reaction (ORR). However, poor intrinsic electrical conductivity and aggregation tendency hinder the application of metal-organic frameworks (MOFs) as active ORR cathodes. Conductive MOFs possess various atomically dispersed metal centers and well-aligned inherent topologies, eliminating the additional carbonization processes for achieving high conductivity. Here, a novel room-temperature electrochemical cathodic electrodeposition method is introduced for fabricating uniform and continuous layered 2D bimetallic conductive MOF films cathodes without polymeric binders, employing the organic ligand 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and varying the Ni/Cu ratio. The influence of metal centers on modulating the ORR performance is investigated by density functional theory (DFT), demonstrating the performance of bimetallic conductive MOFs can be effectively tuned by the unpaired 3d electrons and the Jahn-Teller effect in the doped Cu. The resulting bimetallic Ni2.1Cu0.9(HITP)2 exhibits superior ORR performance, boasting a high onset potential of 0.93V. Moreover, the assembled aqueous zinc-air battery demonstrates high specific capacity of 706.2mA h g-1, and exceptional long-term charge/discharge stability exceeding 1250 cycles.

Deciphering the enhanced oxygen reduction reaction activity of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ via constructing negative thermal expansion offset for high-performance solid oxide fuel cell

Solid oxide fuel cells (SOFCs) are one of the most efficient energy conversion devices. However, the sluggish oxygen reduction reaction (ORR) kinetics at low temperatures significantly challenge the performance and commercialization of SOFCs. Introducing negative expansion coefficient materials has been recognized as an effective approach to enhancing the ORR catalytic properties, but a clear understanding of this enhanced electrochemical performance is still lacking. In this work, the composite cathode of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) with different amounts of negative-thermal-expansion material Sm0.85Zn0.15MnO3 (SZM) is prepared, and in-depth analysis the effect of SZM on the catalytic activity of cathodic ORR is systematically investigated. Simultaneously, the mechanistic studies verify that the enhanced ORR activity might be attributed to the constructed compression strain during sintering, which significantly improves the adsorption, dissociation, and oxygen ion exchange process of the PBSCF cathode.

Correlating crystallinity regulation in spinel oxides and catalytic activity in electrocatalytic water oxidation

Recently, the electro-Fenton (EF) process by coupling cathodic two-electron oxygen reduction reaction (2e- ORR) with anodic oxygen evolution reaction (OER) has emerged as a potentially viable technology, while the sluggish OER is the main bottleneck. As promising OER catalysts, the amorphous and crystal phases of spinel oxides display high activities and electrical conductivity, respectively. Efforts to produce amorphous/crystal spinel oxides have progressed slowly, and how an amorphous/crystal structure benefits the catalytic performances remains elusive. Herein, Fe-doped NiCo2O4 (Fe-NiCo2O4) was used as a model catalyst to explore the effect mechanism of crystallinity controlled by annealing temperature on OER performance. The results display that the Fe-NiCo2O4 obtained at 300 ℃ shows amorphous/crystal structure, which facilitates the active site exposure and the charge transfer, leading to superior OER activity and catalytic stability. Based on the crystallinity regulation of OER catalysts, we propose an anodic optimized strategy to upgrade the EF-related processes by effectively providing O2 produced in the anode.

Oxygen-enriched Fe-N-C electrocatalyst for efficient oxygen reduction reaction

The cathode's oxygen reduction reaction (ORR) plays a crucial role in a variety of fuel cells. Developing affordable and efficient non-precious electrocatalysts for the ORR poses a major hurdle in the realm of electrochemical energy technologies. This work presents a transition metal-based catalyst, Fe-ONC, in which Fe is loaded in an oxygen-enriched nitrogen-doped carbon (NC) material. Oxygen enrichment of the catalyst was achieved by a pre-oxidation strategy. The XPS analysis reveals that Fe-ONC exhibits a higher abundance of oxidized species. The Fe of Fe-ONC exists in the form of Fe3O4 particles and Fe-Nx, respectively, which are uniformly distributed in the NC. The even spread of iron sites and the plentiful presence of oxygen-rich species are key factors in the superior ORR efficiency of Fe-ONC. Fe-ONC's half-wave potential (E1/2) in an alkaline environment (0.1 M KOH) stands at 0.891 V, markedly surpassing the standard commercial Pt/C value (0.840 V). Additionally, Fe-ONC demonstrated enhanced longevity and resistance to ethanol in comparison to commercially available Pt/C. Fe-ONC's outstanding efficacy highlights its potential as an alternative to precious metal catalysts. The document outlines a plan for creating ORR catalytic materials that are sustainable, economical, and efficient.

Insights into the Activity and Stability of Dual-Site Single Atom Catalysts for Oxygen Reduction Reaction Using a 2D Phthalocyanine-MOF

Hydrogen fuel cells have been considered as an effective strategy to shift fossil-fuel based economy to hydrogen economy. The performance of the fuel cells is often limited by the slow kinetics of the cathodic oxygen reduction reaction (ORR). Metal-organic frameworks (MOFs) based on 2D Phthalocyanine offer dual-site single atom catalyst (SAC) where M1 site typically amine coordinated node and M2 site is the phthalocyanine center (M1/M2 = Co, Cu, Ni). Here we conduct both computational studies and experimental measurements to elucidate the site selectivity and activity of two- and four-electron ORR process for pure (contains same element on both sites) and mixed (different M1 and M2) MOF system, and their overall stability. Using density functional theory (DFT), we find that regardless of site, Cobalt (Co) element shows better four-electron ORR activity than other elements in the mixed MOF system. Our integrated crystal orbital Hamilton population (ICOHP) calculations indicate that for pure MOF system, M1(d)-4N(2p) bonds are stronger than M2(d)-4N(2p), which raises d-band center of M2 sites closer to Fermi level, hence improves its activity towards four-electron ORR process. For mixed systems, M2 sites activity largely depends on the geometrical effects caused by M1 site elements. Our activity calculations reveal that Co (as M2) in the Ni-Co mixed MOF systems predicts lowest overpotential for ORR among all studied MOF system. Our dissolution mechanism and stability calculations show that Cu (as M1) causes stress/strain within the mixed MOF system denoting least stable among all studied during ORR process. In experiment, we found Co-based MOF, particularly Ni-Co demonstrates the highest kinetic activity and Cu is the least stable element, showing excellent agreement with computational studies. We hope this study will help in understanding the insight into the rational design of highly active and stable MOF for electrochemical processes. This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program to the SUNCAT Center for Interface Science and Catalysis.

Designing Switchable Transition Metal Nitrides/Carbides from High-Entropy PBA Derivative As Bifunctional Oxygen Electrocatalyst for Rechargeable Zinc-Air Batteries

In response to the growing energy and environmental challenges, considerable endeavors have been directed towards the development of sustainable and eco-friendly energy technologies as alternatives to fossil fuels. Moreover, rechargeable zinc-air batteries (RZABs) stand out due to their high energy density and environmental friendliness, although the sluggish cathode oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics restrict RZABs performance. Prussian blue analogues (PBAs), owing to their highly porous structure and tunable morphology, are often utilized as the precursors of the materials. Particularly, high-entropy PBAs (HEPBAs), formed by blending five or more metals in a random lattice, have achieved enhanced thermal and chemical stability via a robust structure with multiple transition metal ions. Meanwhile, transition metal carbides (TMCs) were investigated for ZAB applications due to their higher catalytic activity. Herein, we introduced CNT into HEPBA nanoparticles, followed by calcination processes, resulting in the synthesis of high-entropy metal carbides on CNT (HE-TMCs/CNT) which are effective for the bifunctional OER and ORR performance. HE-TMCs/CNT demonstrate superior activity towards the ORR (E1/2 = 0.77 V) in a 0.1 M KOH solution and OER (η = 330 mV @10 mA cm−2) in a 1 M KOH solution. The ZAB with HE-TMCs/CNT as the cathode demonstrated an open-circuit voltage of 1.39 V, providing a high energy density of 71 mW cm-2 under a current density of 102 mA cm-2. It exhibited the ability to be charged and discharged in cycles for up to 40 hours, revealing good stability under an applied current density of 5 mA cm-2. These results offer a straightforward pathway for the development of efficient bifunctional electrocatalysts for high-performance RZABs. Figure 1

Research on a Metal–Organic Framework (MOF)-Derived Carbon-Coated Metal Cathode for Strengthening Bioelectrochemical Salt Resistance and Norfloxacin Degradation

Microbial fuel cells (MFCs) can realize the conversion of chemical energy to electrical energy in high-salt wastewater, but the easily deactivated cathode seriously affects the performance of MFCs. To enhance the stability and sustainability of MFC in such circumstances, a bimetallic organic framework ZIF-8/ZIF-67 was utilized for the synthesis of a carbon cage-encapsulated metal catalysts in this study. Catalysts with different Co and Ce ratio (Co@C (without the Ce element), CoCe0.25@C, CoCe0.5@C, and CoCe1@C) were employed to modify the activated carbon cathodes of MFCs. The tests demonstrated that the MFCs with the CoCe0.5@C cathode catalyst obtained the highest maximum power density (188.93 mW/m2) and the smaller polarization curve slope, which boosted the electrochemical activity of microorganisms attached to the anode. The appropriate addition of the Ce element was conductive to the stability of the catalyst’s active center, which is beneficial for the stability of catalytic performance. Under the function of the CoCe0.5@C catalyst, the MFCs exhibited superior and stable norfloxacin (NOR) degradation efficiency. Even after three cycles, the NOR degradation rate remained at 68%, a negligible 5.6% lower than the initial stage. Furthermore, based on the analysis of microbial diversity, the abundance of electrogenic microorganisms on a bioanode is relatively high with CoCe0.5@C as the cathode catalyst. This may be because the better cathode oxygen reduction reaction (ORR) performance can strengthen the metabolic activity of anode microorganisms. The electrochemical performance and NOR degradation ability of MFC were enhanced in a high-salt environment. This paper provides an approach to address the challenge of the poor salt tolerance of cathode catalysts in MFC treatment, and presents a new perspective on resource utilization, low carbon emissions, and the sustainable treatment of high-salt wastewater.

Hydrogen Peroxide Spillover on Platinum-Iron Hybrid Electrocatalyst for Stable Oxygen Reduction.

Iron-nitrogen-carbon (Fe-N-C) catalysts, although the most active platinum-free option for the cathodic oxygen reduction reaction (ORR), suffer from poor durability due to the Fe leaching and consequent Fenton effect, limiting their practical application in low-temperature fuel cells. This work demonstrates an integrated catalyst of a platinum-iron (PtFe) alloy planted in an Fe-N-C matrix (PtFe/Fe-N-C) to address this challenge. This novel catalyst exhibits both high-efficiency activity and stability, as evidenced by its impressive half-wave potential (E1/2) of 0.93 V versus reversible hydrogen electrode (vs RHE) and minimal 7 mV decay even after 50,000 potential cycles. Remarkably, it exhibits a very low hydrogen peroxide (H2O2) yield (0.07%) at 0.6 V and maintains this performance with negligible change after 10,000 potential cycles. Fuel cells assembled with this cathode PtFe/Fe-N-C catalyst show exceptional durability, with only 8 mV voltage loss at 0.8 A cm-2 after 30,000 cycles and ignorable current degradation at a voltage of 0.6 V over 85 h. Comprehensive in situ experiments and theoretical calculations reveal that oxygen species spillover from Fe-N-C to PtFe alloy not only inhibits H2O2 production but also eliminates harmful oxygenated radicals. This work paves the way for the design of highly efficient and stable ORR catalysts and has significant implications for the development of next-generation fuel cells.