Electrospun noble metal-based nanofibers for water electrolysis

Abstract

The rational preparation of efficient and durable electrocatalysts is the key to advancing the development of water electrolysis technology. Noble metal-based materials, such as Pt, Ru, and Ir, have excellent catalytic performance and stability. However, their high cost and low abundance require researchers to explore effective strategies to improve their utilization efficiency. Electrospinning is a facile synthetic method to prepare one-dimensional nanofibers with the desired composition and structure, especially carbon-supported metal-based electrocatalysts with a large specific surface area and high conductivity, through post-processing strategies. This review introduces the recent progress in electrospinning to prepare noble metal-based catalysts for water electrolysis. Specifically, we summarize various strategies for incorporating noble metals into electrospinning nanofibers, as well as their electrocatalytic performance towards hydrogen evolution, oxygen evolution, and overall water splitting. Finally, we propose the opportunities and challenges faced by electrospinning technology in the creation of water electrolysis catalysts, as well as the prospects for future development.

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Boxin Xiao

Boxin Xiao received his MSc in 2021 from the Harbin University of Science and Technology. Then, he was employed as an Environmental Health and Safety Engineer at BYD Co., Ltd from 2021 to 2023. Now, he is a PhD candidate under the supervision of Prof. Xue Feng Lu at Fuzhou University. His current research interest is focused on the electrospinning synthesis of advanced functional materials towards hydrogen and oxygen electrocatalysis.

Jiaqing Liu

Jiaqing Liu graduated from Yanan University with a master's degree in 2022. Now, he is a PhD candidate under the supervision of Prof. Xue Feng Lu and Prof. Xinchen Wang at Fuzhou University. His current research interest focuses on the design and synthesis of nickel-based electrocatalysts for efficient small molecule oxidation.

Kunlong Liu

Kunlong Liu received his BS degree in Optical Information Science and Technology from Northwestern Polytechnical University in 2013 and his PhD degree in 2020 from Xiamen University under the guidance of Prof. Nanfeng Zheng and Gang Fu. During 2020–2022, he worked as a postdoctoral scholar at Xiamen University. He moved to Fuzhou University in 2022. His current research topic is focused on the surface and interface chemistry of photocatalytic overall water splitting for hydrogen production.

Shiqiang Feng

Shiqiang Feng is a Senior Engineer at the Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China. He received his BS degree in 2013 from Donghua University and his PhD degree in 2018 from Peking University. From 2022 to 2024, he served as a postdoctoral researcher at the Fujian Institute of Research on the Structure of Matter. Currently, his research is primarily focused on the catalytic properties of nanomaterials and electron microscopy characterization techniques.

Xue Feng Lu

Xue Feng LU is currently a full professor at the College of Chemistry, Fuzhou University, China. He obtained his PhD degree from Sun Yat-sen University in June 2017. After finishing his five-year postdoctoral training at Nanyang Technological University, Singapore, and the Hong Kong Polytechnic University, he joined Fuzhou University in July 2022. His research interests are focused on the design and synthesis of functional micro/nanomaterials for electrochemical energy storage and conversion. He has published over 80 peer-reviewed papers with more than 15 000 citations (h-index 54) and was selected as a Highly Cited Researcher by Clarivate in 2021–2024.

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1. Introduction

Water electrolysis for hydrogen production has been recognized as an attractive process for achieving carbon neutrality.¹ Although the theoretical conversion efficiency of water into hydrogen and oxygen is as high as 80%, the sluggish four-electron transfer kinetics in the process necessitate efficient electrocatalysts to overcome energy barriers.^2–4 Owing to pronounced kinetic advantages, Pt-, Ir-, and Ru-based noble metals remain the most effective electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).^5–8 However, the widespread deployment of water electrolysis is constrained by the prohibitive cost and scarcity of these noble metals.^9–13 Innovative catalyst design strategies aim to reduce noble metal consumption while enhancing intrinsic activity through intercomponent interactions, such as alloying noble metals with non-noble counterparts, engineering supports to anchor active sites, and constructing single-atom, dual-atom, or multi-atom catalysts.^14–18 Nevertheless, simplified, reproducible, and scalable synthesis methods for noble metal-based catalysts remain imperative to achieve these objectives.

The synthesis methods for noble metal-based electrocatalysts can be categorized as wet chemical synthesis, template-directed synthesis, electrochemical synthesis, vapor deposition synthesis, solid-state synthesis, electrospinning synthesis, etc.^19–22 Wet chemical synthesis can precisely control the morphology of catalysts in solution systems, but the surfactant residues and active site aggregation under high-temperature/pressure conditions compromise noble metal utilization.^19,23,24 Template-directed synthesis utilizes the spatial confinement effect of hard/soft templates to achieve controllable pores and morphologies in precious metal catalysts, especially in the construction of ordered mesoporous/macroporous structures.^20,25 However, this method typically requires multi-step operations and employs acid/alkali etching or high-temperature treatment for template removal, resulting in prolonged synthesis cycles, structural corrosion, and active site blockage.^26,27 In contrast, electrospinning demonstrates advantages in the fabrication of noble metal-based catalysts, especially one-dimensional (1D) nanomaterials.^28,29 For instance, their exceptional mechanical strength and flexibility render them particularly promising for integrated electrode applications, simplifying electrode fabrication processes.^30–32 Tailorable morphologies and porous architectures with large specific surface areas facilitate the exposure of active sites.^33,34 Furthermore, the interconnected nanofiber network architecture provides a high-speed path for charge transfer and prevents active components from agglomerating and falling off during electrocatalytic processes.^35–38

Electrospinning employs a high-voltage electric field to draw charged polymer solutions into nanofibers. Once the electrostatic force exceeds surface tension, a Taylor cone forms at the needle tip and a jet is ejected. The jet undergoes stretching, thinning, and solvent evaporation before being deposited as a continuous fiber network.^35,36 During electrospinning, fiber diameter and morphology depend on voltage, feed rate, and solvent. Higher voltages promote jet stretching, resulting in thinner fibers, while faster feed rates increase diameter due to insufficient stretching time. The volatility of the solvent controls the curing process; too rapid evaporation leads to beading, while too slow evaporation results in merging.³⁹

With the advancement of electrospinning technology, its substantial application potential in functional material fabrication has been demonstrated, accompanied by a proliferation of related review articles.^40–44 Against the backdrop of global hydrogen energy development and technological progress in electrospinning, increasing research efforts have focused on leveraging this technique for designing advanced water electrolysis catalysts, yielding numerous innovative and representative studies. Nevertheless, these studies remain isolated and lack systematic integration, and comprehensive reviews specifically focusing on this field remain scarce. This review systematically summarizes recent progress in electrospun noble metal-based nanofibers for water electrolysis (Fig. 1). Particular emphasis is placed on their catalytic performance in the HER, OER, and overall water splitting. Finally, we discuss current challenges and outline future opportunities for electrospinning technology in water electrolysis catalyst development.

Fig. 1 Schematic diagram of electrospinning to synthesize noble metal-based nanofibers for water electrolysis.

2. Electrospun noble metal-based nanofibers for the HER

Noble metal-based catalysts, especially Pt-, Ru-, and Ir-based catalysts, play a critical role in the HER due to their excellent electrocatalytic activity.² However, their high cost and limited reserves have prompted researchers to explore advanced structural designs for efficient utilization.⁴⁵ As an effective method for preparing nanofiber materials, electrospinning technology enables uniform dispersion of noble metal components and enhancement of charge transport properties, thereby significantly improving the atomic utilization and catalytic performance.⁴⁶

2.1 Pt-based catalysts for the HER

In the field of electrocatalysts for the HER in water electrolysis, Pt has become the benchmark catalytic material due to its optimized hydrogen adsorption free energy and excellent reaction kinetics. Its unique electronic structure enables efficient hydrogen evolution at low overpotentials in both acidic and alkaline electrolytes, with extremely low reaction energy barriers during the catalytic cycle, providing an important reference for the performance evaluation of other HER catalysts.^47–49 However, how to minimize the consumption of noble metals while maintaining the high intrinsic catalytic activity of Pt has become a core challenge in this field. In recent years, numerous studies have utilized electrospinning technology to engineer Pt nanostructures to the atomic scale, achieving a synergistic enhancement of catalytic efficiency and material utilization.^50–52

Single-atom catalysts (SACs) have emerged as a promising class of materials due to their well-defined coordination environments and unique electronic structures.^53–55 Recent studies have shown that structural design and optimization of supports via electrospinning technology, followed by anchoring of Pt SA onto the optimized support surfaces, can significantly enhance the charge/mass transport kinetics and interfacial stability of catalysts.^56,57 Zhang et al. reported a nitrogen-doped porous carbon nanofiber (pCNFs) as a support for engineering Pt SACs (Pt-SA/pCNFs) by electrospinning zeolitic imidazolate framework-8 (ZIF-8) nanoparticles into polyacrylonitrile (PAN) nanofibers, followed by carbonization, precursor impregnation, and pyrolysis (Fig. 2a).⁵⁸ During carbonization, PAN converts into an N-doped carbon framework, while ZIF-8 acts as a sacrificial template to generate micropores and promote nitrogen doping. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) unambiguously revealed the atomic dispersion of Pt (Fig. 2b). Pt-SA/pCNFs delivered outstanding HER performance, requiring an overpotential of only 21 mV to reach 10 mA cm⁻² (Fig. 2c). This work demonstrates a robust strategy to stabilize atomically dispersed Pt on N-doped porous carbon nanofibers, where the tailored Pt-N₂C₂ coordination effectively balances structural stability and hydrogen adsorption for enhanced HER performance. What's more, Du et al. dissolved platinum acetylacetonate, phosphomolybdic acid, and PAN in dimethylformamide, followed by one-step electrospinning to obtain composite nanofibers. These fibers were subsequently pre-oxidized in air and annealed under an argon atmosphere, ultimately yielding Pt/α-MoC_1−x-CNFs (Fig. 2d).⁵⁹ The high-resolution TEM (HRTEM) image reveals distinct lattice fringes of 0.24 nm, corresponding to the (111) plane of α-MoC_1−x (Fig. 2e). AC HAADF-STEM image further demonstrates atomic-scale bright spots uniformly anchored on the α-MoC_1−x surface without agglomeration, confirming the formation of isolated Pt SA sites (Fig. 2f). This strategy enables the in situ doping of Pt into metal carbides, precisely constructing atomic-level catalytic active sites. This demonstrates the electrospinning technology's advantage in regulating the component distribution and interfacial structure during the preparation of SACs.

Fig. 2 (a) Preparation of Pt-SA/pCNFs via electrospinning. (b) AC HAADF-STEM image of the Pt-SA/pCNFs. (c) Polarization curves of different catalysts. Reprinted with permission from ref. 58. Copyright 2023 Elsevier. (d) Schematic of the synthesis route. (e) HR-TEM and (f) HAADF-STEM images of α-MoC_1−xNP in the CNFs, highlighting isolated Pt atoms (red) and clusters (yellow). Reprinted with permission from ref. 59. Copyright 2020 Royal Society of Chemistry. (g) Illustration of spontaneous electron transfer forming MoS₂/Pt (0) redox reaction. (h) HAADF-STEM image of the Pt–MoS₂–Co@CHNF. (i) HER curves of different catalysts in 1 M KOH. (j) HER polarization curves of different catalysts. Stability test of the binder-free Pt–MoS₂–Co@CHNF through chronopotentiometry (inset of j) in seawater electrolyte. Reprinted with permission from ref. 61. Copyright 2024 Elsevier. (k) Simplified schematic of multi-metallic alloy nanoparticle formation. (l) TEM–EDS mapping of Pt–Ru–Ni nanoparticles. (m) Polarization curves of various catalysts in 0.5 M H₂SO₄. Reprinted with permission from ref. 64. Copyright 2022 Wiley-VCH GmbH.

Although SACs exhibit remarkable advantages of atomic-level utilization and customizable active sites, the high surface energy of Pt SA is prone to Ostwald ripening, causing active sites to spontaneously aggregate into irregular nanoclusters. This process disrupts the precise design of SACs’ active sites, thereby deteriorating the long-term stability and durability of catalysts. In contrast, nanocluster catalysts are synthesized via controlled methods to assemble a small number of atoms into ordered nanoclusters with specific geometric configurations and electronic properties. Such materials maintain high catalytic activity while effectively enhancing durability by virtue of the thermodynamic stability of cluster structures.⁶⁰ Han et al. reported the composite of cobalt-doped carbon hollow nanofibers (Co@CHNF) with Pt nanocluster-supported MoS₂ nanosheets.⁶¹ The HAADF image reveals a well-ordered honeycomb lattice characteristic of 2H-MoS₂ and shows Pt nanoclusters with an average diameter of approximately 1 nm (Fig. 2g and h). In seawater medium containing 1 M KOH, this catalyst exhibits a low overpotential of 301 mV at a current density of 500 mA cm⁻², providing a highly promising technical solution for the development of efficient electrolytic seawater hydrogen production catalysts. (Fig. 2i and j).

In addition to improving the Pt atom utilization through nanoscale size regulation, alloying modification is a core technical approach to breaking through the bottleneck of Pt's “high cost-low reserve”. The core advantage of electrospinning technology in synthesizing alloying catalysts lies in the atomic-level precision of alloy composition and interface regulation. Through the molecular-level mixing design of electrospinning precursor solutions, a highly uniform Pt–M alloy solid solution can be formed after calcination. Meanwhile, a second phase (such as oxides/carbides) can be in situ introduced during the electrospinning process to construct core–shell heterojunction interfaces.⁶² Jia et al. designed a nitrogen-doped multi-level porous nanofiber structure and used it as a carrier to anchor sub 5 nm octahedral Pt–Cu nanocrystals.⁶³ Thanks to the synergistic effect between N-doped porous carbon nanofibers and Pt–Cu nanocrystals, the catalyst exhibited excellent HER performance in acidic media, with an overpotential of only 13 mV at a current density of 10 mA cm⁻². Theoretical calculation approaches are used to guide the rational design of multimetallic Pt-based electrocatalysts. Kim et al. studied the optimal components and composition of multi-metal alloy catalysts using density functional theory (DFT) and machine learning (ML) techniques (Fig. 2k).⁶⁴ Based on theoretical guidance, a PtRuNi alloy catalyst with uniform element distribution and an average particle diameter of 9 nm was synthesized (Fig. 2l). Specifically, the best HER activity was shown when the ratio of Pt, Ru, and Ni was 0.65 : 0.3 : 0.05 (Fig. 2m). From the perspective of electronic structure, alloying Pt with transition metals shifts its d-band center relative to the Fermi level, thereby tuning the hydrogen adsorption free energy.⁴⁷ A moderate downward shift weakens the Pt–H bond and promotes H* desorption, whereas an upward shift strengthens adsorption. In Pt–M alloys such as Pt–Cu and PtRuNi, charge redistribution between Pt and the secondary metal fine-tunes the d-band position, achieving a balance between adsorption and desorption that accelerates HER kinetics.

2.2 Ru/Ir-based catalysts for the HER

Although the HER activity of Ru and Ir-based catalysts is generally lower than that of Pt-based catalysts, their unique advantages in cost and structural stability, as well as their huge potential for performance improvement, have made them the focus of continued and extensive research. Strategies such as secondary metal doping for electronic modulation, support engineering to strengthen nanoparticle anchoring, and heterostructure construction to induce interfacial electronic redistribution have been employed to enhance their performance. For example, Kim et al. fabricated 1D Ru–Ir nanofibers by electrospinning metal precursors with polymers, followed by oxidation and phosphidation to obtain Ir-doped Ru–P nanofibers (Fig. 3a).⁶⁵ Element mapping revealed the uniform distribution of Ru, P, and Ir elements (Fig. 3b). To further explore the influence of Ir incorporation on the electronic structure, X-ray photoelectron spectroscopy (XPS) was conducted. In the Ir 4f spectrum, distinct signals corresponding to Ir–P and Ir–O species were observed. With increasing Ir content, the intensity of the Ir 4f peaks gradually increased, while the presence of Ir–O was attributed to surface oxidation upon air exposure (Fig. 3c). The incorporation of Ir into ruthenium phosphide was found to effectively reduce the binding energy of hydrogen intermediates, thereby lowering the overpotential for hydrogen evolution. As a result, the catalyst exhibited an overpotential of 33 mV at 10 mA cm⁻² under acidic conditions (Fig. 3d).

Fig. 3 (a) Synthesis schematic of Ir–doped Ru–P NF catalysts. (b) HAADF-STEM and elemental mapping images of Ir–doped Ru–P catalyst. (c) XPS spectra of Ir 4f core-level binding energies. (d) LSV curves for the different catalysts. Reprinted with permission from ref. 65. Copyright 2023 Springer Nature. (e) Schematic illustration of the fabrication procedure of Ir-NCNFs. (f) HRTEM image of Ir-NCNFs-2. HER curves of different catalysts in (g) 1 M KOH and (h) 0.5 M H₂SO₄. Reprinted with permission from ref. 66. Copyright 2023 Elsevier. (i) Schematic diagram of catalyst synthesis. (j) HRTEM image of the Co–Ir-600. (k) Steady-state HER polarization curves of different catalysts. Reprinted with permission from ref. 67. Copyright 2022 Springer Nature. (l) Temperature-controlled synthesis route of different catalysts. (m) HRTEM image of Ru–Ru₂P@CNFs. LSV curves of the electrocatalysts for the HER in (n) 0.5 M H₂SO₄ and (o) 1 M KOH. Reprinted with permission from ref. 68. Copyright 2022 Springer Nature.

Beyond compositional tuning via doping, modifying the support to optimize the anchoring of noble metal nanoparticles also serves as an effective approach to enhance catalytic performance. Lu et al. proposed a feasible chelating adsorption-engaged strategy by introducing dedoped polyaniline with abundant amino groups to immobilize ultrafine Ir nanoparticles, resulting in the formation of Ir-NCNF carbon nanofibers (Fig. 3e).⁶⁶ The HRTEM image of Ir-NCNFs reveals particle diameters ranging from 2 to 5 nm, with a lattice spacing of 0.222 nm corresponding to the (111) plane of the Ir phase (Fig. 3f). The optimized Ir-NCNFs-2 catalyst exhibits outstanding HER activity, with overpotentials of only 23 mV in 1.0 M KOH and 8 mV in 0.5 M H₂SO₄ at 10 mA cm⁻²; this performance is attributed to the optimal Ir loading that maximizes active site exposure while avoiding agglomeration-induced activity loss (Fig. 3g and h). Similarly, Lu et al. fabricated Co–Ir nanofibrous catalysts via a multistep process involving electrospinning, calcination, in situ H₂ reduction, and galvanic replacement.⁶⁷ A homogeneous solution of Co(Ac)₂·4H₂O, PAN, and polyvinyl pyrrolidone (PVP) was electrospinning into precursor fibers, which were subsequently converted into metallic Co nanofibers, followed by galvanic replacement with Ir to yield the final catalyst (Fig. 3i). HRTEM analysis reveals lattice fringes of 0.207 nm and 0.216 nm, corresponding to the (111) planes of Co and Co–Ir alloy, respectively, confirming alloy formation with partial metallic Co observed at the interface (Fig. 3j). As shown in Fig. 3k, the catalyst exhibits excellent alkaline HER activity, requiring an overpotential of 48 mV at a current density of 10 mA cm⁻².

In addition to optimizing anchoring strategies, interfacial engineering has attracted considerable attention due to its potential to enhance the intrinsic activity of catalysts. Constructing heterostructures with well-defined phase boundaries enables interfacial charge redistribution and electronic structure modulation, thereby synergistically boosting catalytic performance. Wang et al. synthesized Ru-PA@PAN hybrid fibers by electrospinning a DMF solution of RuCl₃·3H₂O, phytic acid (PA), and PAN.⁶⁸ After preoxidation at 270 °C, the fibers were pyrolyzed under Ar at 750–850 °C to yield Ru₂P@CNFs, Ru–Ru₂P@CNFs, and Ru@CNFs (Fig. 3l). HRTEM image of Ru–Ru₂P@CNFs reveals a well-defined heterointerface between the (101) plane of Ru and the (112) plane of Ru₂P, with lattice spacings of 0.21 and 0.24 nm, respectively (Fig. 3m). This phase boundary facilitates interfacial charge redistribution and electronic coupling, enhancing the intrinsic activity of the catalyst. Electrochemical measurements in both acidic and alkaline media demonstrate that Ru–Ru₂P@CNFs exhibit outstanding HER performance. In 0.5 M H₂SO₄, it delivers an overpotential of just 11 mV at 10 mA cm⁻² (Fig. 3n), surpassing even commercial 20% Pt/C and other catalysts. In 1 M KOH, Ru–Ru₂P@CNFs again show the best activity, requiring only 14 mV to reach 10 mA cm⁻² (Fig. 3o).

In addition to Pt-based, ruthenium-based, and iridium-based catalysts, Lu et al. used electrospinning and wet chemical methods to prepare catalysts in which rhodium (Rh) nanoparticles were anchored on cobalt/nitrogen-doped carbon nanofibers, which can be used for efficient HER catalysts under both acidic and alkaline conditions.⁶⁹ The combination of Rh nanoparticles and Co-NCNFs provides abundant electrochemically active sites, while Co-NCNFs have excellent conductivity, significantly enhancing HER performance and accelerating reaction kinetics in both alkaline and acidic electrolytes.

By controlling the structure and composition, electrospun noble metal-based nanofibers have achieved excellent hydrogen evolution activity. These include Pt single atoms, Pt clusters coupled with transition metals, and high-entropy alloys (HEAs). Ru- and Ir-based catalysts have further enhanced their activity by inducing interfacial charge redistribution through secondary metal doping, support engineering, and heterostructure construction. Future research is needed to improve the catalyst stability in seawater and high current conditions, prevent single-atom aggregation, and enhance the reproducibility of electrospun structures. Combining in situ characterization techniques with theoretical calculations will help to more accurately understand the dynamics of active sites and guide the design of durable and industrially applicable hydrogen evolution catalysts.

3. Electrospun noble metal-based nanofibers for the OER

Compared to the HER, the four-electron transfer OER exhibits intricate reaction mechanisms involving multiple electron transfers and the formation of intermediate species, resulting in sluggish kinetics. To overcome these kinetic limitations, numerous electrocatalysts have been developed to enhance OER efficiency. To date, noble metals such as Ir and Ru remain among the most effective catalysts for the OER. Subsequently, we will separately discuss the latest research progress in Ir- and Ru-based noble metal electrocatalysts for the OER fabricated via electrospinning.

3.1 Ir-based catalysts for the OER

Ir-based noble metals, especially Ir-based oxides electrocatalysts, have emerged as promising candidates for the acidic OER due to their exceptional corrosion resistance and long-term durability. However, the intrinsic activity and conductivity limitations of Ir-based oxide electrocatalysts still necessitate substantial additional energy input during large-scale acidic water electrolysis systems. Consequently, advanced catalytic modification strategies such as heterojunctions, loadings, and solid solutions have been employed to enhance acidic OER efficiency in recent years.

The heterojunctions enable the introduction of functional components into the host material to achieve synergistic effects, demonstrating promising potential for enhancing the intrinsic activity. Particularly, enriching the heterojunction interface within amorphous structures can maximize this synergistic effect. Guided by these principles, Qu's group synthesized amorphous IrO_x/CeO₂ nanowires for acidic OER via a facile electrospinning/calcination approach.⁷⁰ Elemental mapping confirms the uniform distribution of Ir, Ce, and O in IrO_x/CeO₂, while lattice fringes in the HRTEM image verify the formation of heterojunctions with abundant interfaces (Fig. 4a and b). Combined experimental and theoretical investigations reveal that CeO₂ in IrO_x/CeO₂ functions as an electron buffer to modulate oxygen intermediate adsorption, reducing the activation energy barrier of IrO_x and significantly boosting OER activity (Fig. 4c).

Fig. 4 (a) The HAADF-STEM and elemental mapping, and (b) HRTEM images of IrO_x/CeO₂-0.6, insert: SAED pattern image. (c) LSV curves of various catalysts in 0.5 M H₂SO₄. Reprinted with permission from ref. 70. Copyright 2022 Elsevier. (d) HAADF-STEM and elemental mapping images of IrCoO_x@LLCF. (e) HRTEM image of IrCoOx@LLCF. (f) LSV curves of Ir-based catalysts in 0.1 M HClO₄. Reprinted with permission from ref. 71. Copyright 2023 Wiley-VCH GmbH. (g) Theoretically determined crystal structures. (h) OER overpotentials on surface Ir and Ti sites of A-Ti(Ir)O₂/P. (i) HRTEM image of SrTi(Ir)O₃. Inset: SAED pattern image. (j) Polarization curves towards OER of different catalysts in 0.1 M HClO₄. Reprinted with permission from ref. 72. Copyright 2020 Wiley-VCH GmbH.

Notably, the bulk phase of Ir-based oxides in heterojunctions remains challenging to fully utilize, rendering such configurations cost-ineffective. To achieve activity enhancement while reducing Ir consumption, strategies such as anchoring Ir-based oxides on supports or constructing solid solutions have been reported. For instance, Chong et al. synthesized La and Li co-doped Co₃O₄ (LLCF) nanorods as Ir supports via electrospinning technology.⁷¹ Intriguingly, subsequent annealing induced Co migration, leading to the formation of ultrafine IrCo nanoparticles. Elemental mapping confirmed the homogeneous distribution of Co, La, and Ir, while the HAADF-STEM and HRTEM images revealed uniformly dispersed IrCo ultrafine nanoparticles (∼1.1 nm in diameter) on the LLCF surface (Fig. 4d and e). Combined in situ characterization and theoretical simulations demonstrated that optimized local atomic structures, charge redistribution, and synergistic component interactions constituted key factors for activity enhancement (Fig. 4f).

In another example, Zou's group developed a SrTi(Ir)O₃ solid-solution model and theoretically analyzed the catalytic activity of Ti and Ir sites (Fig. 4g).⁷² Results revealed that the solid solution exhibited theoretically excellent catalytic activity compared to IrO, due to the synergistic effect of Ti–Ir (Fig. 4h). Guided by these predictions, porous hollow SrTi(Ir)O₃ solid solution nanotubes were synthesized via electrospinning and the Kirkendall effect. HRTEM image and corresponding (selected area electron diffraction) SAED patterns confirmed well-matched lattice fringes with theoretical crystallography, validating the accuracy of the perovskite phase model (Fig. 4i). As anticipated, the catalyst demonstrated an order-of-magnitude higher OER activity in acidic media than IrO₂ while containing 56% less Ir (Fig. 4j).

3.2 Ru-based catalysts for the OER

Ru-based electrocatalysts demonstrate advantages over Ir in the intrinsic catalytic activity due to the higher participation of d-orbital electrons in the OER. However, these catalysts are prone to forming high-valent Ru oxides during the OER, which triggers lattice destruction and Ru species dissolution. To address these limitations, researchers have employed electrospinning technology to synthesize Ru-based electrocatalysts with optimized electronic structure, aiming to enhance structural robustness and prolong operational lifespan. For example, Lu's group synthesized RuO₂/Ru embedded within carbon nanofibers (RuO₂/Ru-CNFs) through an electrospinning–carbonization–oxidation process (Fig. 5a and b).⁷³ The HRTEM image exhibits the concurrent presence of RuO₂ and Ru lattices in RuO₂/Ru-CNFs, demonstrating the formation of a heterojunction structure (Fig. 5c). Electrochemical performance tests revealed that the RuO₂/Ru-CNFs retained over 1000 cycling stability for the alkaline OER (Fig. 5d). Combined experimental and mechanistic investigations indicate that the carbon matrix not only enhances electrical conductivity but also effectively suppresses the aggregation of RuO₂/Ru ultrafine nanoparticles during OER operation.

Fig. 5 (a) SEM, (b) TEM, and (c) HRTEM images of RuO₂/Ru-CNFs-350. (d) Polarization curves before and after 500 and 1000 CV cycles, the insert is the i–t curve of RuO₂/Ru-CNFs-350. Reprinted with permission from ref. 73. Copyright 2022 Royal Society of Chemistry. (e)–(h) Low-magnification TEM images of synthesized Ru_xCr_1−xO_y_20. (i) Chronopotentiometry curve of Ru_xCr_1−xO_y_20. Reprinted with permission from ref. 75. Copyright 2023 Royal Society of Chemistry. (j) SEM image of LFRO–H–O, the inset is the HRTEM image. (k) AFM height image of an LFRO–H–O fiber before calcination. (l) Chronopotentiometry curve of LFRO–H–O. (m) H₂ TPR spectrum of LFRO, LFRO–H, and LFRO–H–O. Reprinted with permission from ref. 76. Copyright 2021 Elsevier.

Notably, carbon matrices suffer from severe corrosion and dissolution during OER operation, leading to membrane contamination and blockage in water electrolysis devices. To address the challenge, researchers have attempted to eliminate carbon matrices from nanofibrous materials. For instance, Kwon et al. synthesized Au-decorated RuO₂ through electrospinning and annealing, which was subsequently reduced into porous AuRu alloy nanofibers.⁷⁴ Mechanistic studies reveal that robust Ru frameworks and alloying strategies play crucial roles in enhancing stability. However, alloy inevitably undergoes oxidation due to strong acidic/alkaline electrolytes and oxidizing environments, leading to catalyst deactivation. In this regard, Ru-based reveals promising prospects due to the strong Ru–O chemical bond. Song et al. synthesized Ru–Cr nanofibers via electrospinning and subsequently converted them into Ru_xCr_1−xO_y oxide nanotubes through oxygen atmosphere annealing.⁷⁵ Oxygen concentration in the annealing atmosphere significantly modulates the morphology structure of Ru_xCr_1−xO_y nanotubes (Fig. 5e–h). Interestingly, among various nanofibers, the distinctive tube-in-tube architecture effectively enhances bubble desorption kinetics during the OER, which substantially improves structural stability of Ru-based oxides under prolonged operation (Fig. 5i).

Recent studies have demonstrated that support design provides unique perspectives for enhancing Ru-based electrocatalyst performance. As exemplified by Wu's group, hollow La_0.9Fe_0.92Ru_0.08O_3−δ (LFRO) ferrite nanofibers were synthesized via a modified electrospinning approach, followed by in situ precipitation and oxidation processes to anchor Ru/RuO₂ nanoparticles on LFRO surfaces (LFRO–H–O; Fig. 5j).⁷⁶ Experimental evidence reveals that the lower work function induced by abundant oxygen vacancies significantly accelerates charge transfer processes, thereby improving OER kinetics (Fig. 5k). The electronic interaction between Ru/RuO₂ species and LFRO support substantially reinforces the durability of LFRO–H–O (Fig. 5l). Consequently, the engineered LFRO–H–O exhibits exceptional catalytic performance in the alkaline OER (Fig. 5m).

Recent advances in noble metal-based nanofibers for the OER involve the development of Ir-based heterojunctions and solid-solution nanotubes, which enhance the adsorption of oxygen intermediates while reducing Ir consumption. Ru-based systems have also been improved through strategies, such as heterojunction design, alloying, and support interactions, leading to enhanced activity and stability across a broad pH range. Future research should focus on addressing the slow four-electron transfer kinetics and mitigating anodic corrosion. Emphasis should be placed on corrosion-resistant non-carbon supports and the use of in situ mechanistic studies to enable the design of durable, high-performance OER electrocatalysts.

4. Electrospun bifunctional noble metal-based nanofibers for water splitting

Although noble metal-based electrocatalysts exhibit excellent activity toward both HER and OER, their practical application in overall water splitting often requires coupling with additional functional catalysts, resulting in increased material complexity, difficulty in interface matching, and high system construction costs. In contrast, bifunctional electrocatalysts capable of simultaneously catalyzing both HER and OER offer significant advantages by simplifying device architecture, improving energy conversion efficiency, and enhancing overall stability. Therefore, we next summarize recent advances in electrospinning bifunctional noble metal-based catalysts for overall water splitting (OWS) applications.

Cheng et al. successfully synthesized a novel bifunctional electrocatalyst, Ru, Ni–CoP porous nanoparticles, using electrospinning and phosphating techniques in 2021.⁷⁷ The calculated H/OH co-adsorption energies for Ru (0.19 eV) and Ni (0.22 eV) are slightly lower than that of Co, indicating their role in promoting water dissociation. However, their highly negative Gibbs free energy values suggest overly strong H binding, which may impede H₂ formation and slow HER kinetics (Fig. 6a and b). After doping, the free energy change is significantly reduced, with Ru and Ni–CoP exhibiting a value of 1.65 eV lower than that of Ru–CoP and Ni–CoP, demonstrating that bimetallic doping enhances *OOH adsorption. This improved adsorption is attributed to the increased valence state of Co induced by bimetallic doping. Compared to single-metal doping, the dual-metal modification more effectively tunes the electronic structure of active sites, thereby improving water dissociation efficiency in the HER and OER, optimizing intermediate adsorption, and ultimately enhancing bifunctional catalytic activity. As shown in Fig. 6c, an alkaline electrolyzer utilizing Ru and Ni–CoP bifunctional porous nanofibers for overall water splitting achieved a low operating voltage of 1.757 V at a current density of 500 mA cm⁻², outperforming the Pt/C‖RuO₂ benchmark. This study provides a promising strategy for the development of high-performance electrocatalysts for electrochemical energy conversion.

Fig. 6 (a) Atomic structure of Ru, Ni–CoP (111). (b) Calculated OER Gibbs free energy diagram for different catalysts. (c) LSV curves of between electrolytic cell based on the Ru, Ni–CoP. Reprinted with permission from ref. 77. Copyright 2021 Elsevier. (d) XRD pattern of FeCoNiMnRu HEA. (e) Reaction energy profile for water dissociation of the FeCoNiMnRu HEA surface. (f) Gibbs free energy of the FeCoNiMnRu HEA surface. (g) Polarization curves for water splitting. Reprinted with permission from ref. 78. Copyright 2022 Springer Nature. (h) The established Ru, Fe–Ni₅P₄ structure model for DFT calculations. (i) The calculated adsorption free energy diagrams for the alkaline HER process. (j) LSV plots of different catalysts. Reprinted with permission from ref. 79. Copyright 2023 Elsevier. (k) Adsorption sites of H* on Mn-doped RuO₂. (l) Calculated free energies of H* adsorption. (m) The Gibbs free energy diagram for OER at the T_Mn site in the Mn-doped RuO₂ system. (n) Electrochemical tests of catalysts for OWS in 1 M KOH. Reprinted with permission from ref. 80. Copyright 2023 Wiley-VCH GmbH.

HEAs composed of multiple metal sites can simultaneously provide active centers for the HER and OER, enabling efficient bifunctional electrocatalytic performance. They can be readily synthesized via electrospinning followed by thermal treatment, which facilitates uniform dispersion of elements and precise control over nanostructure formation. In 2022, Zhu et al. designed a HEA system of FeCoNi_xRu (X = Cu, Cr, Mn), the electronegativity differences among the mixed elements induced charge redistribution, generating highly active Co and Ru sites with optimized energy barriers.⁷⁸ These active sites effectively stabilize OH* and H* intermediates, significantly enhancing water dissociation efficiency under alkaline conditions. XRD analysis confirmed a single-phase HEA structure in FeCoNiMnRu/CNFs after 1000 °C treatment for 3 hours, with diffraction peaks corresponding to FeNi alloy (Fig. 6d). Notably, the face-centered cubic (fcc) diffraction peaks of FeCoNiMnRu HEA exhibited a slight shift to lower angles due to lattice distortion caused by the incorporation of Ru, Mn, and Fe atoms, as well as the high entropy effect. As shown in Fig. 6e, the results indicate that the H–OH bond cleavage energy barrier at the Co site is the lowest, only 0.34 eV, suggesting that Co facilitates the adsorption and dissociation of H₂O, thereby accelerating the formation of H* intermediates. To further elucidate the role of different metal sites in hydrogen adsorption, the Gibbs free energy of adsorbed atomic hydrogen was calculated (Fig. 6f). The results show that the Ru site exhibits a ΔG_H* of only −0.07 eV, indicating that H* is preferentially stabilized at Ru sites. Specifically, an alkaline electrolyzer constructed using FeCoNiMnRu/CNFs as both the anode and cathode achieves a remarkably low operating voltage of 1.65 V at 100 mA cm⁻², significantly outperforming the Pt/C‖RuO₂ benchmark system (Fig. 6g).

In 2023, Cheng et al. synthesized a NiFeRu/C alloy catalyst via electrospinning, carbonization, and phosphating (Fig. 6h).⁷⁹ The energy barrier for the further dissociation of adsorbed H₂O was calculated to be 0.28 eV on the Ni surface and significantly reduced to 0.09 eV on the NiFeRu surface, highlighting the enhanced efficiency of the Volmer step on NiFeRu (Fig. 6i). Subsequently, an electrocatalytic cell was assembled using NiFeRu/C nanofibers as the cathode and Ru, FeNi₅P₄/C nanofibers as the anode. The system required only 1.569, 1.744, and 1.872 V to achieve current densities of 100, 500, and 1000 mA cm⁻², respectively, outperforming the Pt/C‖RuO₂ benchmark (Fig. 6j). Furthermore, it demonstrated excellent stability, maintaining performance for over 100 hours at 500 mA cm⁻². This synthesis strategy not only simplifies the preparation of different electrocatalysts but also provides a versatile approach for designing complementary catalysts to enhance overall water-splitting efficiency.

Lu et al. constructed uniformly Mn-doped RuO₂ nanofibers, where the surface electronic structure of RuO₂ was modulated via electrospinning combined with thermal treatment in 2023.⁸⁰ As shown in Fig. 6k, the crystal structure model after Mn doping indicates that Mn atoms partially replace Ru atoms at lattice centers, thereby altering the surrounding electronic environment and generating multiple potential active adsorption sites. Free energy comparisons of hydrogen adsorption at different sites revealed that the T_O1 site exhibits a ΔG_H* value closest to the value (0 eV), indicating the most favorable hydrogen adsorption capability (Fig. 6l). The Gibbs free energy diagram of the OER process at the T_Mn site shows a low overpotential of 0.38 V (Fig. 6m). Furthermore, a symmetric two electrode system was assembled using Mn_0.05Ru_0.95O₂ nanofibers as both the anode and cathode, requiring only 1.52 V to achieve a current density of 10 mA cm⁻², demonstrating excellent overall water splitting performance (Fig. 6n).

5. Summary and outlook

Electrospinning technology demonstrates prominent advantages in fabricating noble metal-based nanofibers for water splitting, including facile synthesis, compositional controllability, and scalability. By modulating precursor solution formulation and post-treatment strategies, this technique enables the synthesis of single-/multi-metallic electrocatalysts with tailored morphological diversity, encompassing porous, hollow, and core–shell architectures. Capitalizing on these merits, electrospun noble metal-based nanofibers exhibit outstanding catalytic performance for hydrogen/oxygen evolution and overall water splitting.

Notwithstanding remarkable advancements in electrospun noble metal-based electrocatalysts, this field presents coexisting opportunities and challenges (Fig. 7). Based on the aforementioned situation, we provide viewpoints on the challenges and prospects of noble metal-based electrocatalysts by electrospinning:

Fig. 7 The existing challenges in electrospun noble metal-based nanofibers for water electrolysis.

(1) Diversified catalyst structures. Electrocatalysts produced by electrospinning predominantly feature 1D nanowires with disordered porous structures, resulting in underutilized bulk structures. Multidimensional nanofibers present distinct advantages in this regard, notably through substantial increases in specific surface area and exposure of bulk active sites. In this context, the incorporation of cost-effective pore-forming agents/sacrificial templates into electrospinning solutions to generate 3D nanofibers with hierarchical porosity through post-treatment demonstrates promising application potential.

(2) Catalyst surface/interface engineering. The adsorption strength and configuration of water molecules at active sites critically govern reaction kinetics. Introducing hydrophilic/hydrophobic functional groups into electrocatalysts enables precise surface property modulation, thereby tailoring catalytic activity, reaction pathway, and durability. While electrospinning facilitates such surface modifications via accessible precursor solution composition adjustment, related investigations remain notably limited.

(3) Advanced post-processing protocols. Nanofibers synthesized by electrospinning that rely on prolonged high-temperature pyrolysis for polymeric template removal prove energy-/time-intensive, inducing noble metal agglomeration and sintering. To overcome these limitations, electrospinning requires integration with innovative post-treatment strategies, such as chemical bath deposition, electroplating, and electrophoretic deposition, to oxidize/reduce noble metal anions/cations within nanofibers, achieving controlled synthesis of uniformly dispersed surface-active sites.

(4) Integrated membrane electrode exploration. Most catalysts synthesized by electrospinning are evaluated for catalytic performance in powder form, which escalates manufacturing costs and hinders industrial scalability. Although nanofibers theoretically permit direct utilization as membrane electrodes in water electrolyzers, the objective remains unrealized. Strategic approaches, including 3D nanofiber membrane electrospinning technologies, precise nanofiber membrane thickness control, and direct electrospinning of conductive polymer/noble metal-based nanoparticles, could collectively advance the goal.

(5) Industrialization of electrospinning. The large-scale application of electrospinning remains constrained by limited productivity. To enhance production capacity, improvements in needle design, such as multi-needle and needleless electrospinning, have significantly increased spinning efficiency. In addition, innovations in collectors, such as rotating collectors, offer a viable strategy for continuous, large-area fiber membrane fabrication.⁴⁶ Nevertheless, these approaches still face challenges in maintaining fiber uniformity and stability. Therefore, combining process optimization with equipment upgrades is crucial for advancing the industrialization of electrospinning technology.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22409028).

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Footnote

† These authors contributed equally to this work.

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