Fe–N–C single-atom catalysts (SACs) exhibit high activity for oxygen reduction reaction (ORR). However, it remains controversial how the active center mediates catalysis, and the predicted potential ...deviates from experimental results, hindering development of ideal SACs. Here, using first-principles calculations, we present a microkinetic model for ORR on Fe–N–C SACs, disclosing a self-adjusting mechanism induced by its intrinsic intermediate. The modeling results show that the single-atom Fe site of the FeN4 center of Fe–N–C is covered with an intermediate OH* from 0.28 to 1.00 V. Remarkably, such OH* becomes part of the active moiety, Fe(OH)N4, and can optimize intermediate bindings on the Fe site, exhibiting a theoretical half-wave potential of ∼0.88 V. Partial current density analysis reveals the dominating associative path over the dissociative ones. In addition, ORR on Mn–N–C and Co–N–C SACs is unveiled. This work demonstrates the necessity of assessing the effect of intrinsic intermediates in single-atom catalysis and provides practical guidance for rational design of high-performance SACs.
The conventional thermal transformation of metal–organic frameworks (MOFs) for electrocatalysis requires high temperature, an inert atmosphere, and long duration that result in severe aggregation of ...metal particles and non‐uniform porous structures. Herein, a precise and inexpensive laser‐induced annealing (LIA) strategy, which eliminates particle aggregation and rapidly generates uniform structures with a high exposure of active sites, is introduced to carbonize MOFs on conductive substrates under ambient conditions within a few minutes. By systematically considering 8 substrates and 12 MOFs, a series of LIA‐MOF/substrate devices with controllable sizes and good flexibility are successfully obtained. These LIA‐MOF/substrate devices can directly serve as working electrodes. Remarkably, LIA‐MIL‐101(Fe) on nickel foam exhibits an ultralow overpotential of 225 mV at a current density of 50 mA cm−2 and excellent stability over 50 h for facilitating the oxygen evolution reaction, outperforming most recently reported transition‐metal‐based electrocatalysts and commercial RuO2. Physical characterizations and theoretical calculations evidence that the high activity of LIA‐MIL‐101(Fe) arises from the favorable adsorption of intermediates at its Ni‐doped Fe3O4 overlayer that is formed during the laser treatment. Moreover, the LIA‐MOF/substrate devices are assembled for overall water splitting. The proposed LIA strategy demonstrates a cost‐effective route for manufacturing scalable energy storage and conversion devices.
A laser‐induced annealing (LIA) strategy is applied to synthesize a series of LIA‐metal–organic frameworks (MOFs) on conductive substrates under ambient conditions within a few minutes. The obtained LIA‐MOF/substrate devices with controllable sizes and good flexibility exhibit excellent performance for electrochemical water splitting due to the formation of an active Ni‐doped Fe3O4 overlayer during the laser treatment.
Despite wide applications of bimetallic electrocatalysis in oxygen evolution reaction (OER) owing to their superior performance, the origin of the improved performance remains elusive. The underlying ...mechanism was explored by designing and synthesizing a series of stable metal–organic frameworks (MOFs: NNU‐21–24) based on trinuclear metal carboxylate clusters and tridentate carboxylate ligands. Among the examined stable MOFs, NNU‐23 exhibits the best OER performance; particularly, compared with monometallic MOFs, all the bimetallic MOFs display improved OER activity. DFT calculations and experimental results demonstrate that introduction of the second metal atom can improve the activity of the original atom. The proposed model of bimetallic electrocatalysts affecting their OER performance can facilitate design of efficient bimetallic catalysts for energy storage and conversion, and investigation of the related catalytic mechanisms.
An iron atom in an Fe3 cluster is replaced by a second metal to form Fe2M clusters, which can serve as nodes to bridge with organic ligands and construct stable bimetallic MOFs. The introduction of the second metal atom can improve the activity of the original atom and thus improve the oxygen evolution reaction performance of electrocatalysts.
Transition metal chalcogenides have emerged as unique electrocatalysts for the oxygen evolution reaction (OER) during which they usually undergo an oxidation transformation into active ...oxides/(oxy)hydroxides. However, the transformation is so rapid that a high exposure of as-transformed (oxy)hydroxides cannot be achieved, thereby hindering the OER efficiency of the electrocatalyst. Herein, we report a simple self-sacrificing strategy to increase this exposure. A trimetallic selenide heterostructure (FeCoMo-Se) consisting of FeSe2, CoSe2 and MoSe2 is first one-step synthesized on a carbon cloth substrate. The heterostructure possesses a thin nanosheet morphology due to the support of MoSe2 nanosheets as a structural template. Under OER conditions, FeSe2 and CoSe2 are then in situ converted to FeCo-oxyhydroxide while retaining the nanosheet morphology of the heterostructure. Interestingly, MoSe2 is self-sacrificially dissolved and hence leaves considerable space to increase the exposure of FeCo-oxyhydroxide to the electrolyte. Such an advantageous nanostructure endows the FeCoMo-Se-transformed electrocatalyst with excellent OER performance in an alkaline medium, which is much higher than the non-MoSe2-containing selenide FeCo-Se. Density functional calculations demonstrate the favorable intermediate bindings in FeCo-oxyhydroxide. This novel self-sacrificing strategy opens up new avenues in the development of high-performance OER electrocatalysts with respect to their in situ oxidation transformation.
We have synthesized a porous Mo‐based composite obtained from a polyoxometalate‐based metal–organic framework and graphene oxide (POMOFs/GO) using a simple one‐pot method. The MoO2@PC‐RGO hybrid ...material derived from the POMOFs/GO composite is prepared at a relatively low carbonization temperature, which presents a superior activity for the hydrogen‐evolution reaction (HER) in acidic media owing to the synergistic effects among highly dispersive MoO2 particles, phosphorus‐doped porous carbon, and RGO substrates. MoO2@PC‐RGO exhibits a very positive onset potential close to that of 20 % Pt/C, low Tafel slope of 41 mV dec−1, high exchange current density of 4.8×10−4 A cm−2, and remarkable long‐term cycle stability. It is one of the best high‐performance catalysts among the reported nonprecious metal catalysts for HER to date.
Nanocomposite catalyst: A novel Mo‐based catalyst for the hydrogen‐evolution reaction has been synthesized by directly carbonizing a composite obtained from polyoxometalate‐based metal–organic frameworks and graphene oxide at a relatively low temperature. The Mo‐based catalyst exhibits a positive onset potential, low Tafel slope, high exchange current density, and long‐term stability for the hydrogen‐evolution reaction in acidic media.
A laser-induced immobilization strategy is applied to prepare an amorphous iron-phosphate/Fe3O4 (L-FePO) composite on a nickel foam (NF) support. By laser-irradiating an iron hydrogen phosphate ...(FeHP) precursor, a melting and oxidation process leads to the generation of L-FePO with hierarchical pores and an amorphous structure. L-FePO shows exceptional electrocatalytic performance for the OER in an alkaline electrolyte, demonstrating an overpotential of 256 mV at 100 mA cm−2, a Tafel slope of 71 mV dec−1, and good stability over 100 h. The active Fe3O4, partially dissolved phosphate, and newly formed FeOOH species provide abundant active sites, contributing to the excellent OER performance.
The nitrogen (N), phosphorus (P) and sulphur (S) ternary-doped metal-free porous carbon materials have been successfully synthesized using MOFs as templates (denoted as NPS-C-MOF-5) for oxygen ...reduction reaction (ORR) for the first time. The influences of porous carbons from carbonizing different MOFs and carbonization temperature on ORR have been systematically investigated. Due to the synergistic effect of N, P and S ternary-doping, the NPS-C-MOF-5 catalyst shows a higher onset potential as a metal-free electrocatalyst for ORR among the currently reported metal-free electrocatalysts, very close to the commercial Pt-C catalyst. In particular, the kinetic limiting current density of NPS-C-MOF-5 catalyst at -0.6 V is up to approximate -11.6 mA cm(-2), which is 1.2 times higher than that of the commercial Pt-C catalyst. Furthermore, the outstanding methanol tolerance and excellent long-term stability of NPS-C-MOF-5 are superior to those of the commercial Pt-C catalyst for ORR in alkaline media.
Core–shell nanohybrids containing cheap inorganic nanocrystals and nanocarbon shells are promising electrocatalysts for water splitting or other renewable energy options. Despite that great progress ...has been achieved, biomimetic synthesis of metal phosphates@nanocarbon core–shell nanohybrids remains a challenge, and their use for electrocatalytic oxygen evolution reaction (OER) has not been explored. In this paper, novel nanohybrids composed of coralloid Co2P2O7 nanocrystal cores and thin porous nanocarbon shells are synthesized by combination of the structural merits of supramolecular polymer gels and a controllable thermal conversion technique, i.e., temperature programmable annealing of presynthesized supramolecular polymer gels that contain cobalt salt and phytic acid under a proper gas atmosphere. Electrocatalytic tests in alkaline solution show that such nanohybrids exhibit greatly enhanced electrocatalytic OER performance compared with that of Co2P2O7 nanostructure. At a current density of 10 mA cm–2, their overpotential is 0.397 V, which is much lower than that of Co2P2O7 nanostructures, amorphous Co-Pi nanomaterials, Co(PO3)2 nanosheets, Pt/C, and some reported OER catalysts, and close to that of commercial IrO2. Most importantly, both of their current density at the overpotential over 0.40 V and durability are superior to those of IrO2 catalyst. As revealed by a series of spectroscopic and electrochemical analyses, their enhanced electrocatalytic performance results from the presence of thin porous nanocarbon shells, which not only improve interfacial electron penetration or transfer dynamics but also vary the coordination environment and increase the number of active 5-coordinated Co2+ sites in Co2P2O7 cores.
Dispersing active materials on a working electrode without any binder is always desirable for electrocatalytic processes. Here we report a promising method to grow polyoxometalate-based MOFs (POMOFs) ...in situ on a copper foam (CF) substrate. Impressively, CF can serve as metal nodes for the direct growth of MOFs on its 3D skeleton tightly. After ammoniating the POMOFs/CF precursor, a nitride-based composite (MoN–Cu-NPC/CF) is achieved with such a unique structure with MoN and Cu nanoparticles embedded in N, P-doped amorphous carbon and decorated on CF uniformly. We find that MoN–Cu-NPC/CF has an improved HER activity over a wide pH range. Especially in an alkaline electrolyte, this composite shows a small onset overpotential of 60 mV, a Tafel slope of 152 mV dec −1 and an outstanding long-term stability (over 20 h), outperforming most of the nitride-based HER electrocatalysts to date. Density functional theory (DFT) calculations reveal that the synergy between MoN and Cu can modify the electronic structure of the active Cu sites, which significantly improves H* binding and water dissociation kinetics. This work provides a facile strategy for in situ growth of MOFs on a metal substrate without any binder and holds substantial promise for efficient HER applications.
Molybdenum disulfide (MoS2)-based bimetallic sulfides have drawn increasing research attention because of their unique structures and properties. Herein, a one-pot hydrothermal synthesis method is ...proposed to grow a series of bimetallic sulfides on carbon cloth (M-Mo-S/CC, M = Co, Ni, Fe) using Anderson-type polyoxometalates (POMs) as bimetallic sources for the first time. An ideal model of M-Mo-S/CC was used to study the growth process through the nucleation-doping competition mechanism. It is proved for the first time that M-Mo-S/CC possess certain compositions of bimetallic sulfides rather than metal doped MoS2 structures because the nucleation reaction is predominant in the nucleation-doping competition. Moreover, the nucleation rates of different metals can be compared to study the different morphologies of M-Mo-S/CC because Anderson-type POMs have fixed bimetal proportions and precise structures. Co-Mo-S and Ni-Mo-S show spherical heterostructures with CoS2 or NiS mainly inside and interconnected MoS2 nanosheets outside, while Fe-Mo-S exhibits uniform nanosheet morphology without stacking. As electrodes for alkaline water electrolysis, M-Mo-S/CC with different compositions and morphologies exhibit a variety of activities. Particularly, among the M-Mo-S/CC samples, Co-Mo-S/CC achieves the best performance for hydrogen evolution reaction, oxygen evolution reaction and overall water splitting. This study presents a facile strategy of using POMs as bimetallic precursors for studying the growth mechanism as well as the water electrolysis performances of MoS2-based bimetallic sulfides.
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