Ultrastable Fe-N-C Fuel Cell Electrocatalysts by Eliminating Non-Coordinating Nitrogen and Regulating Coordination Structures at High Temperatures

Modulating the electronic configuration of single-atom nanozymes using cobalt nanoclusters for enhanced mycotoxin degradation

Herein, Co- and Fe-based single-atom nanozymes (M/N-PC, M = Co or Fe) were successfully fabricated and their catalytic performances for patulin degradation were evaluated systematically. Co/N-PC, consisting of Co-N4 and nanoclusters sites, achieved a higher patulin degradation efficiency (99.4 %, within 60 min) than Fe/N-PC (only consisting of Fe-N5 sites). Synergistic interactions between Co-N4 and Co nanoclusters greatly enhanced electron density near the Fermi level in Co/N-PC, enabling its high catalytic performance. The degradation products of patulin exhibited negligible cytotoxicity. The M/N-PCs demonstrated good reusability, broad pH adaptability and high practical application potential for patulin degradation in apple juice. M/N-PC also exhibited high efficiency in degrading aflatoxin B1, deoxynivalenol and zearalenone (∼100 %, 10-40 min). This study provides in-depth insights into the relationship between metal active site structures in M/N-PCs and their catalytic properties for mycotoxin detoxification, offering guidance for the design of highly efficient single-atom nanozymes.

Asymmetric tacticity navigates the localized metal spin state for sustainable alkaline/sea water oxidation

Anodic oxygen evolution reaction (OER) that involves a spin-dependent singlet-to-triplet oxygen changeover largely restrains the water electrolysis efficiency for hydrogen production. However, the modulation of spin state is still challengeable for most OER catalysts, and there remains a debate on deciphering the active spin state in OER. Here, we pioneered an asymmetric Fe-incorporated NiPS3 tactic system to retune the metal localized spin for efficient OER electrocatalysis. It is unraveled that the synergistic effect of medium-spin FeIII site and P/S coordination can effectively boost OER activity and Cl resistance selectivity in alkaline/sea water. Resultantly, the Fe/NiPS3-based asymmetric electrodes exhibit low cell voltages of 1.50 volts/1.52 volts in alkaline/sea water at 10 milliamperes per square centimeter, together with a sustainable retention for 1000 hours. It also delivers the durable performance in anion exchange membrane water electrolyzers with a low operation voltage at 45°C. This research navigates the atomic localized spin state as the criterion in rationalizing efficient nonprecious alkaline/sea water oxidation electrocatalysts.

Breaking the Activity and Stability Trade-Off of Platinum-Free Catalysts for the Oxygen Reduction Reaction in Hydrogen Fuel Cells

Hydrogen fuel cells, which use hydrogen as fuel to generate electricity, hold great promises as future energy conversion devices for heavy-duty transport, due to their zero CO2 emissions, high energy conversion efficiency, and high power density. However, the adoption of hydrogen fuel cells has been slow due to their reliance on large amounts of costly and scarce platinum (Pt) for the oxygen reduction reaction. The replacement of Pt with Earth-abundant transition metals such as Fe, Co, Mn, and Sn with oxygen reduction reaction affinity has thus been a holy grail of electrocatalysis research. Pt-free catalysts must combine both high power density and high stability in hydrogen fuel cells to be considered viable alternatives to Pt. Despite promising progress on both fronts, a trade-off has emerged: Pt-free catalysts either achieve high power densities (≥1.5 W cm-2) but suffer from low stabilities (≥70% loss after 25 h) or more recently demonstrate improved stability (≤25% loss after 150 h), while delivering considerably lower power densities (<1 W cm-2) in hydrogen fuel cells. Herein, we summarize the recent progress in the synthesis of high power density M-N-C catalysts for hydrogen fuel cells and highlight the critical importance of uncovering the underlying mechanisms using operando methods. We then discuss the primary causes of catalyst degradation in hydrogen fuel cells and the most promising strategies to enhance the stability of the M-N-C catalysts. Finally, a roadmap is proposed to overcome the activity stability trade-off for Pt-free catalysts in hydrogen fuel cells.

When Graphitic Nitrogen Meets Pentagons: Selective Construction and Spectroscopic Evidence for Improved Four-Electron Oxygen Reduction Electrocatalysis

Nitrogen-doped carbon materials have emerged as promising metal-free electrocatalysts for oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. However, the structural inhomogeneity, particularly the coexistence of four nitrogen doping structures-pyridinic, graphitic, pyrrolic, and oxidized nitrogen-makes assessing their respective contributions challenging and controversial. The current understanding of the four nitrogen doping structures may be also oversimplified and even problematic. The development of a distinctive graphitic-N-doped carbon electrocatalyst is presented in which graphitic nitrogen coordinated with pentagon defects is selectively constructed. Contrary to the previously held belief that graphitic nitrogen has little effect on ORR electrocatalysis, the unique graphitic N configuration exhibited significantly enhanced four-electron ORR activity in both alkaline and acidic media. In situ electrochemical Raman spectroscopy combined with density functional theory calculations further revealed that graphitic nitrogen, when coordinated with pentagon defects, optimized the density of states near the Fermi level, leading to optimized binding energies with oxygen-containing intermediates. The results rationalize the long-standing controversy over the role of different nitrogen dopants in ORR electrocatalysis and suggest that there is considerable potential to precisely construct new nitrogen doping configurations to achieve superior electrocatalytic performance.

Heteroatom sulfur-doping in single-atom FeNC catalysts for durable oxygen reduction performance in zinc-air batteries

Heteroatom doping into the transition metal-based catalysts is an effective strategy to improve the oxygen reduction reaction (ORR) kinetics. Herein, we proposed a one-step, soft template assisted, and green method for the synthesis of Sulfur (S) doped single atom FeNC catalyst. XAFS demonstrated that the Fe active sites in the FeNSC were more likely to possess the Fe-N4 configuration. Density functional theory (DFT) calculations revealed the effect of S-doping into the single atom Fe-N4 symmetric structure, resulting in the delocalization of 3d electrons and asymmetric structure for the single atom FeNSC. The energy barrier of the rate-determining step decreased from 0.535 eV (for FeNC) to 0.474 eV for the FeNSC structure, indicating the possible good catalytic activity of the FeNSC catalyst. The following experiments demonstrated that the FeNSC catalyst showed an excellent ORR performance in both acidic medium with a half wave potential (E1/2) of 0.81 V vs. RHE and basic medium with an E1/2 value of 0.93 V vs. RHE. The high ORR performance is validated by assembling a homemade Zinc-air battery (ZAB) using the single atom FeNSC as a cathode, showing a high power density of 240 mW cm-2. The synthesized single-atom FeNSC catalysts outperformed the state-of-the-art 20 % Pt/C catalyst. The combination of physical characterization, experimental results, and DFT calculations unveiled exceptional improvements in the ORR activity through the incorporation of the S atom into the Fe-N4 matrix. Our findings offer a pathway towards sustainable energy solutions, driving innovation in the field of green energy technologies.

Tailoring the Catalytic Activity of Metal Catalysts by Laser Irradiation

In recent years, the rapid advancements in laser technology have garnered considerable interest as an efficient method for synthesizing electrocatalytic nanomaterials. This review delves into the progress made in laser-induced nanomaterials for electrocatalysis, providing a comprehensive overview of the synthesis strategies and catalytic mechanisms involved in defect engineering, morphology tuning, and heterostructure formation. The review highlights the various laser-induced synthesis techniques in producing nanomaterials with enhanced electrocatalytic properties. It discusses the underlying mechanisms through which laser irradiation can induce defects, modify morphology, and create heterostructures in nanomaterials, ultimately leading to improved catalytic performance. The comprehensive summary of these synthesis strategies and catalytic mechanisms provides valuable insights for researchers interested in utilizing laser technology for the fabrication of advanced electrocatalytic materials. Furthermore, this review identifies the existing challenges and outlines future directions within this booming research field. By addressing the current limitations and discussing potential avenues for exploration, the review provides important guidance for researchers looking to design and fabricate laser-induced nanomaterials with desirable properties for advanced electrocatalysis and beyond.

Concurrently Boosting Activity and Stability of Oxygen Reduction Reaction Catalysts via Judiciously Crafting Fe-Mn Dual Atoms for Fuel Cells

The ability to unlock the interplay between the activity and stability of oxygen reduction reaction (ORR) represents an important endeavor toward creating robust ORR catalysts for efficient fuel cells. Herein, we report an effective strategy to concurrent enhance the activity and stability of ORR catalysts via constructing atomically dispersed Fe-Mn dual-metal sites on N-doped carbon (denoted (FeMn-DA)-N-C) for both anion-exchange membrane fuel cells (AEMFC) and proton exchange membrane fuel cells (PEMFC). The (FeMn-DA)-N-C catalysts possess ample dual-metal atoms consisting of adjacent Fe-N4 and Mn-N4 sites on the carbon surface, yielded via a facile doping-adsorption-pyrolysis route. The introduction of Mn carries several advantageous attributes: increasing the number of active sites, effectively anchoring Fe due to effective electron transfer to Mn (revealed by X-ray absorption spectroscopy and density-functional theory (DFT), thus preventing the aggregation of Fe), and effectively circumventing the occurrence of Fenton reaction, thus reducing the consumption of Fe. The (FeMn-DA)-N-C catalysts showcase half-wave potentials of 0.92 and 0.82 V in 0.1 M KOH and 0.1 M HClO4, respectively, as well as outstanding stability. As manifested by DFT calculations, the introduction of Mn affects the electronic structure of Fe, down-shifts the d-band Fe active center, accelerates the desorption of OH groups, and creates higher limiting potentials. The AEMFC and PEMFC with (FeMn-DA)-N-C as the cathode catalyst display high power densities of 1060 and 746 mW cm-2, respectively, underscoring their promising potential for practical applications. Our study highlights the robustness of designing Fe-containing dual-atom ORR catalysts to promote both activity and stability for energy conversion and storage materials and devices.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Inhibiting Demetalation of Fe─N─C via Mn Sites for Efficient Oxygen Reduction Reaction in Zinc-Air Batteries

Demetalation caused by the electrochemical dissolution of metallic Fe atoms is a major challenge for the practical application of Fe─N─C catalysts. Herein, an efficient single metallic Mn active site is constructed to improve the strength of the Fe─N bond, inhibiting the demetalation effect of Fe─N─C. Mn acts as an electron donor inducing more delocalized electrons to reduce the oxidation state of Fe by increasing the electron density, thereby enhancing the Fe─N bond and inhibiting the electrochemical dissolution of Fe. The oxygen reduction reaction pathway for the dissociation of Fe─Mn dual sites can overcome the high energy barriers to direct O─O bond dissociation and modulate the electronic states of Fe─N4 sites. The resulting FeMn─N─C exhibits excellent ORR activity with a high half-wave potential of 0.92 V in alkaline electrolytes. FeMn─N─C as a cathode catalyst for Zn-air batteries has a cycle stability of 700 h at 25 °C and a long cycle stability of more than 210 h under extremely cold conditions at -40 °C. These findings contribute to the development of efficient and stable metal-nitrogen-carbon catalysts for various energy devices.

Improvement in ORR Durability of Fe Single-Atom Carbon Catalysts Hybridized with CeO2 Nanozyme

FeNC catalysts are considered one of the most promising alternatives to platinum group metals for the oxygen reduction reaction (ORR). Despite the extensive research on improving ORR activity, the undesirable durability of FeNC is still a critical issue for its practical application. Herein, inspired by the antioxidant mechanism of natural enzymes, CeO2 nanozymes featuring catalase-like and superoxide dismutase-like activities were coupled with FeNC to mitigate the attack of reactive oxygen species (ROS) for improving durability. Benefiting from the multienzyme-like activities of CeO2, ROS generated from FeNC is instantaneously eliminated to alleviate the corrosion of carbon and demetallization of metal sites. Consequently, FeNC/CeO2 exhibits better ORR durability with a decay of only 5 mV compared to FeNC (18 mV) in neutral electrolyte after 10k cycles. The FeNC/CeO2-based zinc-air battery also shows minimal voltage decay over 140 h in galvanostatic discharge-charge cycling tests, outperforming FeNC and commercial Pt/C.

PGM-free single atom catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells

The progress of proton exchange membrane fuel cells (PEMFCs) in the clean energy sector is notable for its efficiency and eco-friendliness, although challenges remain in terms of durability, cost and power density. The oxygen reduction reaction (ORR) is a key sluggish process and although current platinum-based catalysts are effective, their high cost and instability is a significant barrier. Single-atom catalysts (SACs) offer an economically viable alternative with comparable catalytic activity for ORR. The primary concern regarding SACs is their operational stability under PEMFCs conditions. In this article, we review current strategies for increasing the catalytic activity of SACs, including increasing active site density, optimizing metal center coordination through heteroatom doping, and engineering porous substrates. To enhance durability, we discuss methods to stabilize metal centers, mitigate the effects of the Fenton reaction, and improve graphitization of the carbon matrix. Future research should apply computational chemistry to predict catalyst properties, develop in situ characterization for real-time active site analysis, explore novel catalysts without the use of platinum-based catalysts to reduce dependence on rare and noble metal, and investigate the long-term stability of catalyst under operating conditions. The aim is to engineer SACs that meet and surpass the performance benchmarks of PEMFCs, contributing to a sustainable energy future.

TiN as Radical Scavenger in Fe─N─C Aerogel Oxygen Reduction Catalyst for Durable Fuel Cell

Fe─N─C is the most promising alternative to platinum-based catalysts to lower the cost of proton-exchange-membrane fuel cell (PEMFC). However, the deficient durability of Fe─N─C has hindered their application. Herein, a TiN-doped Fe─N─C (Fe─N─C/TiN) is elaborately synthesized via the sol-gel method for the oxygen-reduction reaction (ORR) in PEMFC. The interpenetrating network composed by Fe─N─C and TiN can simultaneously eliminate the free radical intermediates while maintaining the high ORR activity. As a result, the H2O2 yields of Fe─N─C/TiN are suppressed below 4%, ≈4 times lower than the Fe─N─C, and the half-wave potential only lost 15 mV after 30 kilo-cycle accelerated durability test (ADT). In a H2─O2 fuel cell assembled with Fe─N─C/TiN, it presents 980 mA cm-2 current density at 0.6 V, 880 mW cm-2 peak power density, and only 17 mV voltage loss at 0.80 A cm-2 after 10 kilo-cycle ADT. The experiment and calculation results prove that the TiN has a strong adsorption interaction for the free radical intermediates (such as *OH, *OOH, etc.), and the radicals are scavenged subsequently. The rational integration of Fe single-atom, TiN radical scavenger, and highly porous network adequately utilize the intrinsic advantages of composite structure, enabling a durable and active Pt-metal-free catalyst for PEMFC.

Atomically Dispersed Ce Sites Augmenting Activity and Durability of Fe-Based Oxygen Reduction Catalyst in PEMFC

In the cathode of proton exchange membrane fuel cells (PEMFCs), Fe and N co-doped carbon (Fe-N-C) materials with atomically dispersed active sites are one of the satisfactory candidates to replace Pt-based catalysts. However, Fe-N-C catalysts are vulnerable to attack from reactive oxygen species, resulting in inferior durability, and current strategies failing to balance the activity and stability. Here, this study reports Fe and Ce single atoms coupled catalysts anchored on ZIF-8-derived nitrogen-doped carbon (Fe/Ce-N-C) as an efficient ORR electrocatalyst for PEMFCs. In PEMFC tests, the maximum power density of Fe/Ce-N-C catalyst reached up to 0.82 W cm-2, which is 41% larger than that of Fe-N-C. More importantly, the activity of Fe/Ce-N-C catalyst only decreased by 21% after 30 000 cycles under H2/air condition. Density functional theory reveals that the strong coupling between the Fe and Ce sites result in the redistribution of electrons in the active sites, which optimizes the adsorption of OH* intermediates on the catalyst and increases the intrinsic activity. Additionally, the admirable radical scavenging ability of the Ce sites ensured that the catalysts gained long-term stability. Therefore, the addition of Ce single atoms provides a new strategy for improving the activity and durability of oxygen reduction catalysts.

Universal Sublimation Strategy to Stabilize Single-Metal Sites on Flexible Single-Wall Carbon-Nanotube Films with Strain-Enhanced Activities for Zinc-Air Batteries and Water Splitting

Single-metal-site catalysts have recently aroused extensive research in electrochemical energy fields such as zinc-air batteries and water splitting, but their preparation is still a huge challenge, especially in flexible catalyst films. Herein, we propose a sublimation strategy in which metal phthalocyanine molecules with defined isolated metal-N4 sites are gasified by sublimation and then deposited on flexible single-wall carbon nanotube (SWCNT) films by means of π-π coupling interactions. Specifically, iron phthalocyanine anchored on the SWCNT film prepared was directly used to boost the cathodic oxygen reduction reaction of the zinc-air battery, showing a high peak power density of 247 mW cm-2. Nickel phthalocyanine and cobalt phthalocyanine were, respectively, stabilized on SWCNT films as the anodic and cathodic electrocatalysts for water splitting, showing a low potential of 1.655 V at 10 mA cm-2. In situ Raman spectra and theoretical studies demonstrate that highly efficient activities originate from strain-induced metal phthalocyanine on SWCNTs. This work provides a universal preparation method for single-metal-site catalysts and innovative insights for electrocatalytic mechanisms.

Salt Effect Engineering Single Fe-N2P2-Cl Sites on Interlinked Porous Carbon Nanosheets for Superior Oxygen Reduction Reaction and Zn-Air Batteries

Developing efficient metal-nitrogen-carbon (M-N-C) single-atom catalysts for oxygen reduction reaction (ORR) is significant for the widespread implementation of Zn-air batteries, while the synergic design of the matrix microstructure and coordination environment of metal centers remains challenges. Herein, a novel salt effect-induced strategy is proposed to engineer N and P coordinated atomically dispersed Fe atoms with extra-axial Cl on interlinked porous carbon nanosheets, achieving a superior single-atom Fe catalyst (denoted as Fe-NP-Cl-C) for ORR and Zn-air batteries. The hierarchical porous nanosheet architecture can provide rapid mass/electron transfer channels and facilitate the exposure of active sites. Experiments and density functional theory (DFT) calculations reveal the distinctive Fe-N2P2-Cl active sites afford significantly reduced energy barriers and promoted reaction kinetics for ORR. Consequently, the Fe-NP-Cl-C catalyst exhibits distinguished ORR performance with a half-wave potential (E1/2) of 0.92 V and excellent stability. Remarkably, the assembled Zn-air battery based on Fe-NP-Cl-C delivers an extremely high peak power density of 260 mW cm-2 and a large specific capacity of 812 mA h g-1, outperforming the commercial Pt/C and most reported congeneric catalysts. This study offers a new perspective on structural optimization and coordination engineering of single-atom catalysts for efficient oxygen electrocatalysis and energy conversion devices.

Stable and High-performance Flow H2 -O2 Fuel Cells with Coupled Acidic Oxygen Reduction and Alkaline Hydrogen Oxidation Reactions

Conventional H2 -O2 fuel cells suffer from the low output voltage, insufficient durability, and high-cost catalysts (e.g., noble metals). Herein, this work reports a conceptually new coupled flow fuel cell (CF-FC) by coupling asymmetric electrolytes for acidic oxygen reduction reaction and alkaline hydrogen oxidation reaction. By introducing an electrochemical neutralization energy, the newly-developed CF-FCs possess a significantly increased theoretical open-circuit voltage. Specifically, a CF-FC based on a typical transition metal single-atom Fe-N-C cathode catalyst demonstrates a high electricity output up to 1.81 V and durability with an ultrahigh retention of 91% over 110 h, far superior to the conventional fuel cells (usually, < 1.0 V, < 50% retention over 20 h). The output performance can even be significantly enhanced easily by connecting multiple CF-FCs into the parallel, series, or combined parallel-series connections at a fractional cost of that for the conventional H2 -O2 fuel cells, showing great potential for large-scale practical applications. Thus, this study provides a platform to transform conventional fuel cell technology through the rational design and development of advanced energy conversion and storage devices by coupling different electrocatalytic reactions.

Integrating hydrogen utilization in CO2 electrolysis with reduced energy loss

Electrochemical carbon dioxide reduction reaction using sustainable energy is a promising approach of synthesizing chemicals and fuels, yet is highly energy intensive. The oxygen evolution reaction is particularly problematic, which is kinetically sluggish and causes anodic carbon loss. In this context, we couple CO2 electrolysis with hydrogen oxidation reaction in a single electrochemical cell. A Ni(OH)2/NiOOH mediator is used to fully suppress the anodic carbon loss and hydrogen oxidation catalyst poisoning by migrated reaction products. This cell is highly flexible in producing either gaseous (CO) or soluble (formate) products with high selectivity (up to 95.3%) and stability (>100 h) at voltages below 0.9 V (50 mA cm-2). Importantly, thanks to the "transferred" oxygen evolution reaction to a water electrolyzer with thermodynamically and kinetically favored reaction conditions, the total polarization loss and energy consumption of our H2-integrated CO2 reduction reaction, including those for hydrogen generation, are reduced up to 22% and 42%, respectively. This work demonstrates the opportunity of combining CO2 electrolysis with the hydrogen economy, paving the way to the possible integration of various emerging energy conversion and storage approaches for improved energy/cost effectiveness.

Research Progress on Atomically Dispersed Fe-N-C Catalysts for the Oxygen Reduction Reaction

The efficiency and performance of proton exchange membrane fuel cells (PEMFCs) are primarily influenced by ORR electrocatalysts. In recent years, atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have gained significant attention due to their high active center density, high atomic utilization, and high activity. These catalysts are now considered the preferred alternative to traditional noble metal electrocatalysts. The unique properties of M-N-C catalysts are anticipated to enhance the energy conversion efficiency and lower the manufacturing cost of the entire system, thereby facilitating the commercialization and widespread application of fuel cell technology. This article initially delves into the origin of performance and degradation mechanisms of Fe-N-C catalysts from both experimental and theoretical perspectives. Building on this foundation, the focus shifts to strategies aimed at enhancing the activity and durability of atomically dispersed Fe-N-C catalysts. These strategies encompass the use of bimetallic atoms, atomic clusters, heteroatoms (B, S, and P), and morphology regulation to optimize catalytic active sites. This article concludes by detailing the current challenges and future prospects of atomically dispersed Fe-N-C catalysts.

Flash Nitrogen-Doped Carbon Nanotubes for Energy Storage and Conversion

In recent years, nitrogen-doped carbons show great application potentials in the fields of electrochemical energy storage and conversion. Here, the ultrafast and green preparation of nitrogen-doped carbon nanotubes (N-CNTs) via an efficient flash Joule heating method is reported. The precursor of 1D core-shell structure of CNT@polyaniline is first synthesized using an in situ polymerization method and then rapidly conversed into N-CNTs at ≈1300 K within 1 s. Electrochemical tests reveal the desirable capacitive property and oxygen catalytic activity of the optimized N-CNT material. It delivers an improved area capacitance of 101.7 mF cm-2 at 5 mV s-1 in 1 m KOH electrolyte, and the assembled symmetrical supercapacitor shows an energy density of 1.03 µWh cm-2 and excellent cycle stability over 10 000 cycles. In addition, the flash N-CNTs exhibit impressive catalytic performance toward oxygen reduction reaction with a half-wave potential of 0.8 V in alkaline medium, comparable to the sample prepared by the conventional long-time pyrolysis method. The Zn-air battery presents superior charge-discharge ability and long-term durability relative to commercial Pt/C catalyst. These remarkable electrochemical performances validate the superiorities of the Joule heating method in preparing the heteroatom-doped carbon materials for wide applications.

Faraday cage-type self-powered immunosensor based on hybrid enzymatic biofuel cell

Self-powered immunosensors (SPIs) based on enzymatic biofuel cell (EBFC) have low sensitivity and poor stability due to the high impedance of the immune sandwich and the vulnerability of enzymes to environmental factors. Here, we applied the Faraday cage-type sensing mode on a hybrid biofuel cell (HBFC)-based SPI for the first time, which exhibited high sensitivity and stability. Cytokeratin 19 fragment (CYFRA 21-1) was used as a model analyte. Au nanoparticle-reduced graphene oxide (Au-rGO) composite was used as the supporting matrix for immunoprobe immobilized with detection antibody and glucose dehydrogenase (GDH), also the builder for Faraday cage structure on the bioanode in the presence of antigen. After the combination of immunoprobe, antigen, and the antibody on the bioanode, the Faraday cage was constructed in case the AuNP-rGO was applied as a conductive cage for electron transfer from GDH to the bioanode without passing through the poorly conductive protein. With the assistance of the Faraday cage structure, the impedance of the bioanode decreased significantly from 4000 to 300 Ω, representing a decline of over 90%. The sensitivity of the SPI, defined as the changes of open circuit voltage (OCV) per unit concentration of the CYFRA 21-1, was 68 mV [log (ng mL-1)]-1. In addition, Fe-N-C was used as an inorganic cathode material to replace enzyme for oxygen reduction reaction (ORR), which endowed the sensor with 4-week long-term stability. This work demonstrates a novel sensing platform with high sensitivity and stability, bringing the concept of hybrid biofuel cell-based self-powered sensor.

Single-Atom-Based Oxygen Reduction Reaction Catalysts for Proton Exchange Membrane Fuel Cells: Progress and Perspective

Single-atom catalysts (SACs) are regarded as promising non-noble-metal alternatives for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells due to their high atom utilization efficiency and excellent catalytic properties. However, the insufficient long-term stability issues of SACs under the working conditions seriously hinder their practical application. In this perspective, the recent progress of SACs with optimized ORR catalytic activity is first reviewed. Then, the possible degradation mechanisms of SACs in the ORR process and effective strategies for improving their ORR durability are summarized. Finally, some challenges and opportunities are proposed to develop stable single-atom-based ORR electrocatalysts in the future.

Efficient yolk-shelled Fe-N-C oxygen reduction electrocatalyst via N-rich molecular-guided pyrolysis

Fe-N-C catalysts with highly dispersed metal active centers were developed as promising non-precious metal materials for acidic oxygen reduction reaction (ORR) electrocatalysis. However, such kind of novel catalysts still suffer from major challenges in the manipulation of dispersion, utilization, and stability of the Fe-based metal centers. Herein, a N-rich molecular dual-guided pyrolysis strategy was proposed to develop an efficient yolk-shelled Fe-N-C ORR electrocatalyst. A unique yolk-shelled nanostructure with a relatively ordered shell and disordered yolk of a carbon skeleton was controllably constructed via this guided-pyrolysis route from the precursor of Fe-doped zeolitic imidazolate framework-8 (Fe-ZIF-8). Moreover, the atomic-level dispersion of Fe element in the carbon skeleton could be achieved via the dual guidance from phenanthroline and melamine molecules. The optimized Fe-N-C catalyst demonstrated a half-wave potential of 0.78 V vs. RHE in acid media, close to commercial 30% Pt/C, along with a small negative shift of 19 mV after an accelerated durability test. These enhanced electrocatalytic properties could be attributed to the preferred transformation of the Fe precursors to atomically dispersed Fe-Nx active configurations, as well as the enhanced three-phased interfacial reaction kinetics.

Catalyst Development for High-Temperature Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) Applications

A constant increase in global emission standard is causing fuel cell (FC) technology to gain importance. Over the last two decades, a great deal of research has been focused on developing more active catalysts to boost the performance of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC), as well as their durability. Due to material degradation at high-temperature conditions, catalyst design becomes challenging. Two main approaches are suggested: (i) alloying platinum (Pt) with low-cost transition metals to reduce Pt usage, and (ii) developing novel catalyst support that anchor metal particles more efficiently while inhibiting corrosion phenomena. In this comprehensive review, the most recent platinum group metal (PGM) and platinum group metal free (PGM-free) catalyst development is detailed, as well as the development of alternative carbon (C) supports for HT-PEMFCs.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.