Iron phosphide/N, P-doped carbon nanosheets as highly efficient electrocatalysts for oxygen reduction reaction over the whole pH range

A High-Rate and Ultrastable Ammonium Ion-Air Battery Enabled by the Synergy of ORR and NH4 + Storage

Ammonium ion batteries (AIBs) offer cost-effectiveness, nontoxicity, and eco-friendly attributes in energy storage technology. However, the constrained capacity and poor stability of conventional cathode materials have impeded their widespread adoption. Herein, a synergistic approach is introduced to overcome these challenges, by enhancing the air cathode with NH4 + and simultaneously leveraging atmospheric oxygen as a reservoir for NH4 + storage. Notably, NH4 + significantly enhances the oxygen reduction reaction (ORR) performance in neutral environments. Through in situ Raman spectroscopy and quantum density functional theory calculations, it is elucidated how NH4 + can act as a proton donor, replacing H2O in neutral media and reducing energy barriers in the protonation of *O2 - and *O, thereby accelerating ORR kinetics. The resulting ammonium ion-air battery, comprising an air cathode and a polymer (PNP) anode, showcases impressive metrics: high energy density of 78 Wh kg-1 and power density of 9369 W kg-1 at 1 A g-1, an initial capacity of 94.3 mAh g-1 and exceptional cycling stability (70.4% capacity retention after 12 500 cycles) at 10 A g-1. This pioneering research highlights the synergistic relationship between ORR and NH4 + storage and opens up new avenues for the design and advancement of innovative, sustainable, and environment-friendly AIBs.

Roughing Nitrogen-Doped Carbon Nanosheets for Loading of Monatomic Fe and Electroreduction of CO2 to CO

The catalyst is the pivotal component in CO2 electroreduction systems for converting CO2 into valuable products. Carbon-based single-atom materials (CSAMs) have emerged as promising catalyst candidates due to their low cost and high atomic utilization efficiency. The rational design of the morphology and microstructure of such materials is desirable but poses a challenge. Here, we employed different Mg(OH)2 templates to guide the fabrication of two kinds of amorphous nitrogen-doped carbon nanosheet-supported Fe single atoms (FeSNC) with rough and flat surface structures. In comparison to flat FeSNC with saturated FeN4 sites, the rough FeSNC (R-FeSNC) exhibited unsaturated FeN4-x sites and contracted Fe-N bond length. The featured structure endowed R-FeSNC with superior capacity of catalyzing CO2 reduction reaction, achieving an exceptional CO selectivity with Faradaic efficiency of 93% at a potential of -0.66 V vs. RHE. This study offers valuable insights into the design of CSAMs and provides a perspective for gaining a deeper understanding of their activity origins.

Research Progress on Porous Carbon-Based Non-Precious Metal Electrocatalysts

The development of efficient, stable, and economic electrocatalysts are key to the large-scale application of electrochemical energy conversion. Porous carbon-based non-precious metal electrocatalysts are considered to be the most promising materials to replace Pt-based catalysts, which are limited in large-scale applications due to high costs. Because of its high specific surface area and easily regulated structure, a porous carbon matrix is conducive to the dispersion of active sites and mass transfer, showing great potential in electrocatalysis. This review will focus on porous carbon-based non-precious metal electrocatalysts and summarize their new progress, focusing on the synthesis and design of porous carbon matrix, metal-free carbon-based catalysts, non-previous metal monatomic carbon-based catalyst, and non-precious metal nanoparticle carbon-based catalysts. In addition, current challenges and future trends will be discussed for better development of porous carbon-based non-precious metal electrocatalysts.

Transition Metal Non-Oxides as Electrocatalysts: Advantages and Challenges

The identification of hydrogen as green fuel in the near future has stirred global realization toward a sustainable outlook and thus boosted extensive research in the field of water electrolysis focusing on the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). A huge class of compounds consisting of transition metal-based nitrides, carbides, chalcogenides, phosphides, and borides, which can be collectively termed transition metal non-oxides (TMNOs), has emerged recently as an efficient class of electrocatalysts in terms of performance and longevity when compared to transition metal oxides (TMOs). Moreover, the superiority of TMNOs over TMOs to effectively catalyze not only OERs but also HERs and ORRs renders bifunctionality and even trifunctionality in some cases and therefore can replace conventional noble metal electrocatalysts. In this review, the crystal structure and phases of different classes of nanostructured TMNOs are extensively discussed, focusing on recent advances in design strategies by various regulatory synthetic routes, and hence diversified properties of TMNOs are identified to serve as next-generation bi/trifunctional electrocatalysts. The challenges and future perspectives of materials in the field of energy conversion and storage aiding toward a better hydrogen economy are also discussed in this review.

Interfacial Electron Transfer Strategy to Improve the Hydrogen Evolution Catalysis of CrP Heterostructure

Though several Pt-free hydrogen evolution reaction (HER) catalysts have been reported, their employment for industry is challenging. Here, a facile pyrolysis method to obtain phase-pure CrP nanoparticles supported on N, P dual-doped carbon (CrP/NPC) is reported to be tuned toward industrial HER. Interestingly, CrP/NPC exhibits excellent HER activity that requires an overpotential of 34 mV to attain a current density of 10 mA cm^-2 , which is only 1 mV positive to commercial Pt/C and a potential of 55 mV to achieve a current density of 200 mA cm^-2 which is better than Pt/C. In addition, the long-term durability of CrP/NPC is far superior to Pt/C due to the strong interaction between CrP and C support, restricting any agglomeration or leaching. Density functional theory (DFT) calculations suggest that electronic modulation at the interface (CrP/NPC) optimizes the hydrogen adsorption energy. The Cr-Cr bridge site with required density of states near the Fermi level is found to be the active site. Overall, this report provides a practical scheme to synthesize rarely investigated CrP based materials along with a computational mechanistic guideline for electrocatalysis that can be utilized to explore other phosphides for various applications.

Heteroatom-Doped Metal-Free Carbon Nanomaterials as Potential Electrocatalysts

In recent years, heteroatom-incorporated specially structured metal-free carbon nanomaterials have drawn huge attention among researchers. In comparison to the undoped carbon nanomaterials, heteroatoms such as nitrogen-, sulphur-, boron-, phosphorous-, etc., incorporated nanomaterials have become well-accepted as potential electrocatalysts in water splitting, supercapacitors and dye-sensitized solar cells. This review puts special emphasis on the most popular synthetic strategies of heteroatom-doped and co-doped metal-free carbon nanomaterials, viz., chemical vapor deposition, pyrolysis, solvothermal process, etc., utilized in last two decades. These specially structured nanomaterials' extensive applications as potential electrocatalysts are taken into consideration in this article. Their comparative enhancement of electrocatalytic performance with incorporation of heteroatoms has also been discussed.

Engineering atomic Fe–N–C with adjacent FeP nanoparticles in N,P-doped carbon for synergetic oxygen reduction and zinc–air battery

FeP-900 doped with transition metals (FeP and Fe–N–C) and heteroatoms (N,P) was prepared via pyrolysis of a conjugated microporous polymer constituted by Fe–phthalocyanine and cyclotriphosphazene, in which FeP was wrapped in N,P-rich carbon matrix.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO₂ reduction, and N₂ reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

Phosphorus modification of cobalt-iron nanoparticles embedded in a nitrogen-doped carbon network for oxygen reduction reaction

For the electrochemical reduction of oxygen the development of heteroatom-doped carbon-based transition metal catalysts has become a recognized strategy to replace traditional noble metal catalysts. In this work a catalyst consisting of CoFe nanoparticles encapsulated in N-doped carbon-based materials (NC) supported by carbon nanotubes (CNTs), i.e. Fe3Co1@NC/CNTs, was modified via treatment with a phosphate salt to synthesize a P-Fe3Co1@NC/CNTs catalyst. The P-Fe3Co1@NC/CNTs exhibits with 5.29 mA cm-2 an enhanced current density which is comparable to a Pt/C catalyst. In addition, a stability and methanol resistance better than the Pt/C catalyst were observed which is ascribed to the carbon encapsulation and the synergies between the two transition metals. Finally, the reaction mechanism of P-doping was studied and discussed. These results provide possible directions for carbon-based catalysts and doping with heteroatoms for the improvement of catalytic activity. Moreover, the zinc-air battery assembled with P-Fe3Co1@NC/CNTs as the air-cathode exhibited a high-power density of 73 mW cm-2, which is comparable to that of Pt/C (71 mW cm-2) and a specific capacity of 763 mA h g-1. The prepared catalyst could potentially serve to take the place of precious metal catalysts in rechargeable Zn-air batteries.

Ferrocene-crosslinked polypyrrole hydrogel derived Fe–N-doped hierarchical porous carbon as an efficient electrocatalyst for pH universal ORR and Zn–air batteries

A Fe–N-doped catalyst (PF-800), prepared via pyrolysis of ferrocene-crosslinked polypyrrole hydrogel, shows impressive activity for ORR in various pH. Further Zn–air battery using PF-800 present high energy density and excellent long-term stability.

Porous Organic Polymer-Derived Fe2P@N,P-Codoped Porous Carbon as Efficient Electrocatalysts for pH Universal ORR

A new porous organic polymer (CP-CMP) was designed and synthesized via the direct polymerization of pyrrole and hexakis(4-formyl-phenoxy)cyclotriphosphazene, skipping the tedious synthetic procedure of porphyrin-monomers containing special groups. This special porous organic polymer (POP) serves as an "all in one" precursor for C, N, P, and Fe. Direct carbonization of this special POP afforded Fe2P@N,P-codoped porous carbons with hierarchical pore structure and high graphitization. Finally, the optimal catalyst (CP-CMP-900) prepared by carbonization of CP-CMP at 900 °C exhibited high efficiency for oxygen electroreduction. Typically, CP-CMP-900 presented an oxygen reduction reaction half-wave potential (E 1/2) of 0.85, 0.73, and 0.65 V, respectively, in alkaline, neutral, and acidic media, close to those of commercial Pt/C in the same electrolyte (0.843, 0.71, and 0.74 V). Furthermore, it also displayed excellent methanol immunity and long-time stability in various electrolytes better than commercial Pt/C (20%).

N,P Co-Coordinated Manganese Atoms in Mesoporous Carbon for Electrochemical Oxygen Reduction

The increasing interest in fuel cell technology encourages the development of efficient and low-cost electrocatalysts to replace the Pt based materials for catalyzing the cathodic oxygen reduction reaction (ORR). In the present work, a nitrogen and phosphorus co-coordinated manganese atom embedded mesoporous carbon composite (MnNPC-900) is successfully prepared via a polymerization of o-phenylenediamine followed by calcination at 900 °C. The MnNPC-900 composite shows a high ORR activity in alkaline media, offering an onset potential of 0.97 V, and a half-wave potential of 0.84 V (both vs reversible hydrogen electrode) with a loading of 0.4 mg cm^-2 . This performance not only exceeds its phosphorus-free counterpart (MnNC-900), but also is comparable to the Pt/C catalyst under identical measuring conditions. The significantly enhanced ORR performance of MnNPC-900 can be ascribed to: i) the introduction of phosphorus assists the generation of mesopores during the pyrolysis and endows the MnNPC-900 composite with large surface area and pore volume, thus facilitating the mass transfer process and increases the number of exposed active sites. ii) The formation of N,P co-coordinated atomic-scale Mn sites (MnN_x P_y ), which modifies the electronic configuration of the Mn atoms and thereby boosts the ORR catalytic activity.

An Interpenetrating Porous Organic Polymer as a Precursor for FeP/Fe2 P-Embedded Porous Carbon toward a pH-Universal ORR Catalyst

Interpenetrating porous organic polymers (PNFc-POP) inspired by the structure of DNA were synthesized through a two-stage polymerization method under catalyst-free conditions. A ferrocene-rich hyper-crosslinked polymer (Fc-melamine) was interwoven with cyclotriphosphazene-based conjugated porous polymer (PN-CMP) to obtain an interconnected polymer network (PNFc-POP). The sequential interpenetrating polymer network contained a diverse range of heteroatoms (P, N, O and Fe) and exhibited a large BET surface area. Simple pyrolysis of the dual polymer interweaved skeletons at 900 °C afforded nanocrystalline FeP/Fe₂ P-embedded N and P codoped porous carbon composites. The optimal catalyst obtained by the pyrolysis of PNFc-POP at 900 °C (PNFc-900) exhibited hierarchical porosity and large BET surface areas. It also exhibited excellent oxygen reduction reaction catalytic activities over the entire pH range. The onset potential (E_onset =1.01 V) and half-wave potential (E_1/2 =0.86 V) of PNFc-900 exceeded those of commercial Pt/C (E_onset =0.99 V and E_1/2 =0.84 V) in alkaline conditions. The obtained catalysts with a four-electron transfer pathway for the reduction of oxygen also displayed excellent long-term stability and methanol tolerance.

Electrochemical impacts of sheet-like hafnium phosphide and hafnium disulfide catalysts bonded with reduced graphene oxide sheets for bifunctional oxygen reactions in alkaline electrolytes

Non-noble metal-based catalysts with efficient catalytic activities for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are critical for energy conversion devices, including fuel cells and metal-air batteries. In this work, novel hafnium phosphide-reduced graphene oxide nanosheets (HfP-rGO NS) and hafnium disulfide-reduced graphene oxide nanosheets (HfS2-rGO NS) were synthesized and investigated as bifunctional electrocatalysts for OER and ORR. The prepared HfP-rGO NS and HfS2-rGO NS catalysts showed nanosheet structures, where the HfP or HfS2 nanosheet was closely packed with rGO. A unique methodology was adopted to lodge the non-metal oxide catalytic sheets (i.e., HfP and HfS2) over the rGO sheets, which positioned the oxide layer on the catalytic sheet surface for instant oxygen evolution. Low intensity X-ray diffraction patterns and Raman spectra confirmed the sheet-like structure of HfP-rGO NS and HfS2-rGO NS. Scanning electron microscope mapping images revealed that all elements (i.e., Hf, P, C and O for HfP-rGO NS and Hf, S, C and O for HfS2-rGO NS) were equally distributed in the synthesized heteroatomic nanosheets. Moreover, both the HfP-rGO NS and HfS2-rGO NS demonstrated excellent durability for both ORR and OER. This outperforms the most state-of-the-art non-precious-metal-based bifunctional catalysts, which is attributed to the synergistic effect of rGO and Hf-based catalysts. The different ORR and OER reaction potentials in HfP-rGO NS and HfS2-rGO NS likely result from the influence of HfP and HfS2.

Interface self-assembly preparation of multi-element doped carbon nanobowls with high electrocatalysis activity for oxygen reduction reaction

Developing an efficient, stable and low cost oxygen reduction reaction electrocatalyst is desirable for fuel cells and metal-air batteries. Here, we have successfully prepared multi-element doped carbon nanobowls by simply mixing the porous carbon nanobowls and sulfur doped graphitic carbon nitride quantum dots in FeCl₃ solution and subsequent high temperature treatment processes. Compared with the commercial Pt/C electrocatalyst, the multi-element doped carbon nanobowls display a comparable half-wave potential of 0.82 V, much larger limiting diffusion current density (0.4-0.8 V), better methanol-tolerance and higher long-term stability for the oxygen reduction reaction in alkaline media. The robust three-dimensional porous structure of carbon nanobowls and multiple active centers derived from Fe, N, S and O co-doping are responsible for the excellent performance. This work suggests that such multi-element doped carbon nanobowls can be a promising alternative for Pt-based catalysts in fuel cells.

Deflagration synthesis of nitrogen/fluorine co-doped hollow carbon nanoparticles with excellent oxygen reduction performance

NaN₃/C₅F₅N deflagration is developed to synthesize hollow carbon nanospheres with high N/F doping level, graphitization degree and specific surface area in one step, which exhibit excellent ORR performance.

Efficient hierarchically synthesized Fe2P nanoparticles embedded in an N,P-doped mesoporous carbon catalyst for the oxygen reduction reaction

Novel Fe₂P nanoparticles encapsulated in an N,P-doped three-dimensional porous carbon catalyst were synthesized and displayed excellent ORR performance.