Self-Bound Catalytic Layer of Porous Hollow Ru for Transport-Boosted Alkaline Water Electrolysis

Self-Bound Catalytic Layer of Porous Hollow Ru for Transport-Boosted Alkaline Water Electrolysis | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Self-Bound Catalytic Layer of Porous Hollow Ru for Transport-Boosted Alkaline Water Electrolysis Long Kuai, Suibao Shen, Qiao Qiao, Erjie Kan, Xianfu Li This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8719121/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Ru-based catalysts exhibit substantial potential for high-current-density alkaline water electrolysis. However, the ionomer incorporated into the final Ru catalytic layer (CL) used to disperse and bind catalyst particles during electrode fabrication introduces additional charge and mass transport resistances. This study demonstrates a self-bound Ru (Ru-SB) CL constructed from a superhydrophilic Ru precursor coordinated with polar N-vinylpyrrolidone (NVP). This design reduces the ionomer content required for stable ink dispersion to merely one-tenth of that in conventional systems; notably, the residual ionomer can be efficiently removed via a simple heat treatment. This process simultaneously yields a robust Ru-SB CL without compromising mechanical stability. When tested in an alkaline water electrolyzer (15% KOH, 60 °C), the Ru-SB CL delivers operating voltages of only 1.728 V and 1.916 V at current densities of 1.0 A cm⁻² and 2.0 A cm⁻², respectively — representing significant reductions of 81 mV and 107 mV relative to the conventional ionomer-bound Ru (Ru-IB) CLs. Furthermore, this study establishes a general and scalable aerosol-microdroplet method for screening more efficient superhydrophilic Ru-based precursors, enabling a further reduced operating voltage of 1.892 V at 2.0 A cm⁻². This work paves a way to build more efficient alkaline water electrolyzer for hydrogen production. Physical sciences/Materials science/Materials for energy and catalysis/Porous materials Physical sciences/Chemistry/Chemical synthesis/Synthetic chemistry methodology catalytic layer self-bound ionomer-free water electrolysis high-current density Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction High-current-density alkaline water electrolysis offers a promising pathway to enhance the unit capacity of hydrogen production. [1-3] However, operating at elevated current densities generally leads to increased cell voltages, which compromises energy conversion efficiency and raises energy consumption. [4-5] Reducing the overall cell voltage hinges on optimizing the catalytic layer (CL), which can be approached from three key aspects: (1) developing highly active and stable catalysts to minimize electrochemical polarization overpotential [6] ; (2) reducing charge transport resistance to mitigate ohmic losses [7] ; and (3) enhancing mass transport to facilitate the efficient diffusion of reactants and products [1, 8] . It is important to note that ohmic overpotential is highly sensitive to current density, while mass transfer overpotential becomes particularly dominant at high reaction rates. When the current density exceeds 1 A·cm⁻², even minor interfacial resistance can result in significant ohmic losses, and mass transfer limitations intensify sharply, becoming critically dependent on the diffusion performance of the CL. Therefore, an ideal CL for high-current-density alkaline water electrolysis should combine high intrinsic catalytic activity with a stable, low-resistance structure that supports efficient electron conduction and rapid mass transport. Recent significant advances in ruthenium (Ru)-based catalysts have positioned them as highly efficient alternatives for the hydrogen evolution reaction (HER) to addresses the issue of high overpotential associated with traditional Raney Ni catalysts. [9-12] Consequently, enhancing their operational efficiency under high-current-density conditions is crucial. Significant advances in Ru-based catalysts have been pursued through multiple structural design strategies, including the use of metallic Ru [13-16] , conductive supports for improved electron transport [17-19] , and engineered porous/hollow structures to enhance mass transport properties [20-22] . Despite these promising innovations at the material level, the efficiency of a Ru CL is still compromised by its fabrication process. Typically, as illustrated in Figure 1a , Ru-based CLs are prepared by dispersing catalyst particles with an ionomer to form an ink, which is then coated onto a substrate [23-26] . The ionomer serves a dual function: it disperses the catalyst powder for ink preparation and binds the catalyst particles to form the CL. However, this conventional process inevitably forms an ionomer film on the catalyst surface, which introduces additional resistance (Ri) beyond the inherent resistances of the electrode support (Rs) and the catalyst itself (Rc), thereby impeding electron transport, electrolyte wetting, ion/molecule migration, and gas evolution. [27] These factors severely limit the full utilization of the intrinsic activity of Ru catalysts in the high-current-density region. [28, 29] Trickily, the binding function provided by the ionomer is indispensable for maintaining the mechanical stability of CL. Simply reducing or removing the ionomer is not feasible, as it could lead to the detachment of CL under the intense gas flushing. [30-32] Consequently, developing low-ionomer or ionomer-free Ru-based CLs and integrating them with reported high-efficiency Ru catalyst architectures is crucial but challenge for constructing near-ideal CLs under high-current-density conditions. In this work, we eliminated ionomer-related resistances by developing a self-bound Ru (Ru-SB) CLs integrated with a porous hollow Ru architecture that facilitates local mass transport. Typically, this achievement is enabled by a rationally designed superhydrophilic Ru-based precursor coordinated with polar N-vinylpyrrolidone (NVP) molecules and structured into macroporous hollow (MacroPH) architectures. By utilizing the highly dispersible Ru-NVP precursor, the proposed strategy enables a substantial reduction in the ionomer/Ru ratio (I/Ru). The achieved I/Ru ratio of 0.03 is merely one-tenth of that required in the conventional route (0.4), as compared in Figure 1a and b . Figure 1c presents the TEM image and FT-IR spectra of the MacroPH Ru-NVP precursor particles via a carefully designed aerosol-microdroplet confinement method [33] (detailed in a subsequent section), containing numerous pores exceeding 50 nm in size and strongly polar NVP ligand. Thermogravimetric (TG) and heat flow analysis ( Figure S1 ) further confirm that there are 39% organic component in precursor, indicating a significant amount of NVP bound to Ru. Dynamic contact angle measurements ( Figure 1d ) verified the exceptional superhydrophilicity of the material, with nearly complete wetting achieved in 0.2 seconds. Figure 1e further demonstrates that as long as I/Ru ratio exceeds 0.03, the ink formulated with the MacroPH Ru-NVP precursor can remain stable for over one week, laying a solid foundation for large-scale industrial electrode production. For a fundamental comparison, the precursor was deposited onto an electrochemical-inert Ti mesh substrate to form a Ru-NVP pre-catalytic layer, which was subsequently converted into the final cathodic Ru-SB-c or anodic Ru-SB-a CL through thermal treatment. After screening via three-electrode HER and oxygen evolution reaction (OER) testing ( Figure S2 ), the thermal treatment conditions for converting the Ru-NVP precursor into both the Ru-SB-c and Ru-SB-a CLs were determined as calcination in a H₂/Ar atmosphere, followed by a subsequent calcination step in air. During heating, the Ru-NVP precursor underwent chemical transformation accompanied by the decomposition of the residual ionomer. This resulted in a robust, self-bound Ru-SB CL firmly adhered to the Ti substrate, which fully exposes the catalytic active sites and thereby reduces the transport resistance associated with Ru CL bound by ionomers (Ru-IB). Moreover, the SEM images of the finally formed HER CLs of Ru-IB/Ti ( Figure 1f ) and Ru-SB/Ti electrodes ( Figure 1g ) reveal markedly different morphologies. The ionomer-bound Ru-IB CL shows extensive coverage of its surface and interparticle gaps by the ionomer, whereas the self-bound Ru-SB CL exhibits an ionomer-free surface with larger interparticle voids. The unique physical structure of the self-bound CL by microspheres provides highly efficient channels for both water infiltration into the CL and bubble escape from within it. Furthermore, the water CAs on the final cathodic catalytic layers of Ru‑IB-c/Ti and Ru‑SB-c/Ti electrodes were further measured. Because the polar C=O group remains intact after thermal treatment (Figure 1c), the Ru‑SB CL ( Figure 1g ) possesses much superior wettability compared to the Ru‑IB layer ( Figure 1f ), which is also conducive to enhanced performance under high‑current‑density conditions. [34] The superior catalytic performance of the as-developed Ru-SB CL was clearly demonstrated by HER polarization curves measured in 10% KOH at 25 °C ( Figure 2a ). While the Ru-IB/Ti and Ru-SB/Ti electrodes showed nearly identical onset performance in the low-current-density region, the HER activity of the Ru-SB/Ti electrode became increasingly advantageous as the current density rises. Additionally, the conventional Ru-IB/Ti electrode requires approximately 20 cycles to reach a stable state, whereas the Ru-SB/Ti electrode essentially achieves stability starting from the 3rd cycle. Figure 2b presents the stabilized HER polarization curves and the corresponding Tafel fitting curves. Tafel analysis reveals a minimal difference in kinetic polarization overpotential (~50 mV) even at 1.0 A cm⁻². In contrast, during actual HER operation, the overpotential gap exceeds 200 mV at 0.5 A cm⁻². This indicates that the Ru-SB catalyst layer possesses a lower total transport resistance—encompassing both mass and charge transport—and that the benefit of this reduced resistance grows more pronounced with increasing operating current. [35] We also investigated the influence of I/Ru ratio on the HER performance of the as-prepared Ru-SB/Ti electrodes. As shown in Figure 2c , the electrode performance showed no significant difference within the I/Ru ratio range of 0–0.2, indicating that the HER performance can be effectively ensured as long as the ionomer is effectively removed. Finally, long-term HER cycling tests ( Figure 2d and S3 ) confirm the robustness and stability of the Ru-SB/Ti electrode fabricated with a low I/Ru ratio of 0.03, showing no performance degradation after 10,000 and 20,000 cycles. The advantage of the self-bound Ru-SB CL from superhydrophilic Ru-NVP precursor is also demonstrated in the anodic OER process ( Figure S4 ). This further corroborates that the self-bound CL approach paves the way for building advanced electrodes in high-current-density alkaline water electrolysis. To evaluate the practical performance of the self‑bound CLs in water electrolysis, a two‑electrode electrolyzer ( Figure 2e and S5 ) was constructed. The anode and cathode supports were Ni foam and Ti mesh, yielding the corresponding electrodes of Ru‑SB‑a/Ni and Ru‑SB‑c/Ti, each with a geometric active area of 2 × 2 cm², separated by a commercial membrane (thickness: 0.25 mm). The electrolysis was carried out in 15 wt% KOH solution at 60 °C. For comparison, an electrolyzer equipped with ionomer‑bound CLs (Ru‑IB‑a/Ni and Ru‑IB‑c/Ti) was also tested under the same conditions. Figure 2f shows that the cell voltage difference between the Ru‑SB‑a/Ni||Ru‑SB‑c/Ti pair (red plots) and the Ru‑IB‑a/Ni||Ru‑IB‑c/Ti pair (blue plots) remains minor at current densities up to 0.25 A/cm². However, as the current density increases, the cell voltage difference ( ΔE ) becomes more pronounced, reaching 81 mV at 1.0 A/cm² and further widening to 107 mV at 2.0 A/cm². These results confirm that the self‑bound design of CLs effectively reduces transport resistance at high current densities, leading to a lower cell voltage. Additionally, the performance of the Ru‑SB‑a/Ni||Pt/C‑IB‑c/Ti pair (grey plots) was evaluated, where a well-known HER catalysts of Pt/C ( Figure S6 ) was used to ionomer-bound Pt/C‑IB‑c CL instead of the Ru‑SB‑c CL. At current densities below 0.25 A/cm², this air exhibited performance comparable to that of both Ru‑IB‑a/Ni||Ru‑IB‑c/Ti and Ru‑SB‑a/Ni||Ru‑SB‑c/Ti references. However, its performance degraded more significantly than Ru‑IB‑a/Ni||Ru‑IB‑c/Ti at higher current densities. Figure 3a shows the processes of above-mentioned aerosol-microdroplet mediated synthetic methodology for constructing the superhydrophilic MacroPH Ru-based precursors. Briefly, an aqueous solution containing RuCl₃, NVP, and NaNO₃ is subjected to ultrasonic spraying, generating a multitude of microdroplets. These microdroplets are transported into a tube furnace, set at 700 °C, using a vacuum pump. At the outlet of the tube furnace, a powder filter collects the resulting MacroPH Ru-NVP precursor containing NaCl. Washing this collected powder yields precursor product. The key design here focuses on the reaction process of the microdroplets within the tube furnace, specifically tailored to achieve superhydrophilicity and a MacroPH structure. Firstly, as the solvent rapidly evaporates and the temperature rises, the NVP and NaNO₃ undergo a vigorous redox reaction. This reaction produces gases like COₓ and NOₓ, forming ultrafine bubbles. The aggregation of some of these bubbles creates cavities. Simultaneously, non-volatile inorganic components solidify, and the release of these bubbles leads to the formation of a MacroPH structure featuring an open window ( Figure 3b ) that is favorable for mass transfer [36] . The formation process of Ru-NVP is more complex. RuCl₃ and NaNO₃ participate in a molten-state reaction. Coupled with the reducing gases (CO, NO) generated from the reaction between NVP and NaNO₃, this process primarily yields metallic Ru along with NaCl ( Figure S7 ). This mechanism has been previously detailed in our studies on the Ir-glucose system. [37] More importantly, NVP was employed owing to the strong coordination capability of the carbonyl groups, enabling the formation of complexes with a fraction of RuCl₃, which subsequently deposit onto the above-mentioned Ru/NaCl, yielding the composite denoted as Ru-NVP/NaCl. Washing this intermediate yields the superhydrophilic MacroPH Ru-NVP microspheres. The distinct Ru, C, N, and O signals in the EDS mapping ( Figure 3b ) and characteristic signals of NVP in FT-IR spectra ( Figure 1c ) well agree with the composition of Ru-NVP. The HAADF-STEM image ( Figure 3c ) shows the MacroPH structure consistent with TEM image ( Figure 1c ). To validate the formation mechanism related to MacroPH structure, we conducted two control experiments: one without NVP and another without NaNO₃. Consequently, the product obtained in the absence of NVP exhibited a normal macroporous structure without hollow cavity ( Figure 3d and S8 ), while the microspheres formed without NaNO₃ appeared solid-like. ( Figure 3e and S9 ). Furthermore, the BJH pore size distribution analysis ( Figure 3f ) reveals a predominance of macropores in the derived MacroPH Ru sample. In contrast, the control sample without NVP possesses a greater proportion of small mesopores and significantly fewer macropores, while the sample lacking NaNO₃ exhibits minimal porosity. The electrochemical HER polarization curves ( Figure 3g ) clearly demonstrate the superior performance of the electrode fabricated from the MacroPH structural Ru-NVP precursor. Although the Ru-SB/Ti electrode exhibited an onset performance similar to those derived from NVP-free and NaNO₃-free precursors in the low-current-density region, its HER activity became increasingly superior at higher current densities. Moreover, the electrode obtained from the low-porosity NaNO₃-free precursor showed the weakest performance in the large-current-density region. The advantage of the MacroPH structural Ru-NVP precursor is also confirmed in the two‑electrode alkaline electrolysis ( Figure S10 ) in 15% KOH at 60 o C. The enhanced performance at high current densities can be attributed to the superior mass transfer properties of the MacroPH structure. [38] More importantly, the aerosol-microdroplet mediated synthesis procedure is applicable to a wide range of Ru-based composite precursors. As depicted in Figure 3h , a series of Ru-M-NVP (M=Co, Ni, etc. ) precursors—from binary Ru-Cu ( Figure S11 ) and Ru-Mn ( Figure S12 ), to ternary medium-entropy Ru-Co-Ni ( Figure S13 ), quaternary Ru-Fe-Co-Ni ( Figure S14 ), and even quinary high-entropy Ru-Fe-Co-Ni-Mn ( Figure S15 ) systems—have been successfully synthesized. Objectively speaking, not all particles possess macropores (e.g., Ru-Cu-NVP), but all exhibit a fundamental porous hollow structure. This versatility provides a methodological foundation for screening and developing CLs and electrodes with further improved intrinsic activity. Benefiting from this, we facilely identified a porous hollow FeNiRu₀.₂-NVP precursor ( Figure 4a and S16 ) and the corresponding anode of FeNiRu₀.₂‑SB‑a/Ni, which exhibits better performance at high current densities while using only 20% of the Ru loading compared to a pure Ru anode. As shown in Figure 4b , despite the reduced Ru content, it shows a slightly higher overpotential at low current densities compared to the Ru-SB‑a/Ni anode, but demonstrates superior performance at high current densities. Its performance is also relatively better than that of FeNi‑SB‑a/Ni anode. The two-electrode alkaline water electrolysis tests yielded consistent results. Figure 4c shows that when the current density exceeds 1 A/cm², the performance of the FeNiRu₀.₂‑SB‑a/Ni||Ru‑SB‑c/Ti electrode pair surpasses that of the Ru‑SB‑a/Ni||Ru‑SB‑c/Ti electrode pair. The voltages required at 2 A/cm² and 3 A/cm² are only 1.892 V and 2.048 V, respectively. This simple optimization not only enhances electrode performance but also reduces the consumption of the precious metal Ru. It is noteworthy that the presented alkaline water electrolysis system delivers high performance under significantly milder conditions (15% KOH, 60 °C) than the industrial standard (30% KOH, 80-90 °C). In summary, this study firstly demonstrate a self-bound Ru-SB CL for high-current-density alkaline water electrolysis, constructed from a superhydrophilic Ru precursor coordinated with polar N-vinylpyrrolidone. We find that the ionomer content required for stable ink dispersion to be reduced to just one-tenth of conventional levels, and the residual ionomer present in the precursor layer can be effectively removed via a straightforward heat treatment, which concurrently produced a robust, self-bound Ru-SB CL with no sacrifice in mechanical stability. The resulting CL structure fully exposes active sites and transport channels, thereby delivering catalytic activity that far surpasses that of conventional ionomer-bound Ru-IB CLs and allowing the electrolyzer to operate at substantially reduced voltages and milder working conditions. Demonstrated in the alkaline water electrolyzer at mild condition (15% KOH, 60 °C), the electrode with Ru-SB CL delivers current densities of 1.0 and 2.0 A cm⁻² at operating voltages of only 1.728 V and 1.916 V, respectively—representing significant reductions of 81 and 107 mV compared to a conventional Ru-IB CL. 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Wu, Regular mesoporous superparticles with a tailored opening window and tunable surface crisscrossed grooves. J. Am. Chem. Soc. 2025 , 147 , 26, 22460-22472. Liu, T. Huang, X. L. Yang, S. J. Liu, S. S. Wang, L. L. Xiang, G. M. Wang, L. Kuai, Microdrop-confined synthesis and regulation of porous hollow Ir-based catalysts for the mass transfer-enhanced electrolysis of pure water, Sci. Bull. 2024 , 69 , 1081-1090. M. Farber, N. M. Seraphim, K. Tamakuwala, A. Stein, M. Rücker, D. Eisenberg, Porous materials: The next frontier in energy technologies. Science 2025 , DOI: 10.1126/science.adn9391. Methods Preparation of superhydrophilic porous hollow Ru-NVP precursor The superhydrophilic porous hollow Ru-NVP precursor was synthesized using a microdroplet confinement approach. The experimental setup consisted of an ultrasonic atomizer, a tubular furnace, a sample collection unit, and a circulating water vacuum pump. A homogeneous solution was first prepared by dissolving 0.433 g of N-vinylpyrrolidone (NVP) and 0.761 g of NaNO₃ in 61 mL of deionized water under ultrasonic agitation. Subsequently, 11.2 mL of RuCl₃ solution (0.41 M) was added to the mixture and homogenized. The resulting solution was fed into the ultrasonic atomizer to generate fine microdroplets. Driven by the vacuum pump, the aerosol droplets were transported through the tubular furnace maintained at 700 °C, resulting in the formation of a brown powder collected in the sample receiver. The product was then washed three times with ultrapure water via centrifugation, followed by one washing step with ethanol. Finally, the powder was dried under vacuum to obtain the superhydrophilic porous hollow Ru-NVP precursor. Electrode fabrication with self-bound catalyst layer Ink preparation: A dried Ru-NVP precursor was weighed and transferred into an ink bottle. Isopropanol and Nafion solution were added to prepare a homogeneous ink with a concentration of 4 mg/mL of Ru-NVP. The mixture was ultrasonicated for 30 minutes in an ice-water bath to ensure uniform dispersion. Cathode fabrication: To make the self-bound cathodic (Ru-SB-c) catalyst layer, the Ru-NVP ink was sprayed onto Ti mesh substrates of two sizes: 1 × 1 cm² (for three-electrode electrochemical testing) with a Ru loading of approximately 0.6 mg Ru /cm², and 2 × 2 cm² (for two-electrode alkaline water electrolysis evaluation) with a Ru loading of about 1.2 mg Ru /cm². To obtained final cathode (Ru-SB-c/Ti), the Ru-NVP coated Ti mesh was then subjected to a two-step calcination process: first, it was heated at 360 °C (2 °C/min) under a 5% H₂/Ar atmosphere for 2 h, cooled naturally to room temperature, and subsequently calcined again at 300 °C (2 °C/min) in air for another 2 h, followed by cooling to room temperature, yielding the final Ru-SB-c/Ti cathode. Anode fabrication: The self-bound anodic (Ru-SB-a) catalyst layer and final anode (Ru-SB-a/Ni) were prepared following a similar procedure and the same ink, with the following modifications: the Ti mesh was replaced by Ni foam unless otherwise indicated, the reduction step in 5% H₂/Ar was conducted at 380 °C, and the subsequent calcination in air was performed at 400 °C. Three-electrode hydrogen/oxygen evolution reaction testing Three-electrode electrochemical performance tests for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were conducted on the Donghua Electrochemical Workstation (DH7003B) at 25 °C. The Pt sheet (1×1 cm²) served as the counter electrode, Hg/HgO electrode (GOE) as the reference electrode, and a home-made electrode (1×1 cm) as the working electrode. The electrolyte consisted of 10 wt% KOH (pH = 14.3). All potentials were expressed as reversible hydrogen electrode (RHE) potentials as the following equation: . Two-electrode alkaline water electrolysis testing The electrode with a spray-coated area of 2 × 2 cm², was aligned on both sides of a composite membrane with 0.25 mm in thickness (Jiangsu Weidao Energy Technology Co., Ltd.). The assembly was then thermally pressed using a hot press (YLJ-HP-9, Hefei Kejing Technology Materials Co., Ltd.) at 50 °C and 2 MPa for 1 minute. The Ni foams were used as current collector. The water electrolysis performance was subsequently evaluated on an electrolyzer test station manufactured by Anhui Contango New Energy Technology Co., Ltd. The tests were conducted under the following conditions: 15 wt% KOH as the electrolyte, a temperature of 60 °C, and electrolyte flow rates of 30 mL/min for the anode and 15 mL/min for the cathode. Additional Declarations There is NO Competing Interest. Supplementary Files Supportinginformation.docx Self-Bound Catalytic Layer of Porous Hollow Ru for Transport-Boosted Alkaline Water Electrolysis GA.png Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8719121","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":603680949,"identity":"6e2181ca-18e3-4d98-bd3c-3131dd41adab","order_by":0,"name":"Long Kuai","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5UlEQVRIie3RMQrCMBSA4SdCp4BrStEzRAp1kXqVlIJdijh2LAhOYlcFD1EQnB8E7KK7g0NF6NShN9AYcJNYN4f8kBBCPkIIgMn0h1E5sJRTD4ja6KStCJeTnf5C4EUYtiX2JrwhT67+vjgf7wTG/Ry7VakjDp0y5KcqPJxmkUtg6uZojZiODChnGCxF6GHsOQREkCOxqJ5EjSJuVr/I4ztxaKxu8RlVt+B3Yq/quXyL4PRSj4Y7FrpbYXlaQoto3zSJmPSy2CvrxO+vi0WlJe+CFMCSvyOX3TbnZRNQxGQymUwfegKPNkxml2iu2wAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-8633-0320","institution":"Anhui Polytechnic University","correspondingAuthor":true,"prefix":"","firstName":"Long","middleName":"","lastName":"Kuai","suffix":""},{"id":603680950,"identity":"04446d3e-dab6-4e1c-8793-3fcb1fd225e1","order_by":1,"name":"Suibao Shen","email":"","orcid":"","institution":"Anhui Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Suibao","middleName":"","lastName":"Shen","suffix":""},{"id":603680952,"identity":"370a6fce-ad29-456e-a8dc-dc79cb25819b","order_by":2,"name":"Qiao Qiao","email":"","orcid":"","institution":"Anhui Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Qiao","middleName":"","lastName":"Qiao","suffix":""},{"id":603680954,"identity":"a0a7e820-e4da-4a7d-b96b-eabc9e8192ed","order_by":3,"name":"Erjie Kan","email":"","orcid":"","institution":"Anhui Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Erjie","middleName":"","lastName":"Kan","suffix":""},{"id":603680956,"identity":"9984a2fb-f95a-42d1-b615-51ffc2c7eafe","order_by":4,"name":"Xianfu Li","email":"","orcid":"","institution":"Anhui Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Xianfu","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2026-01-28 10:19:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8719121/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8719121/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":104458743,"identity":"27630a5b-0e09-45d8-9f16-0fa66bc4017e","added_by":"auto","created_at":"2026-03-12 03:12:30","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":19480048,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDesign and characterization of self-bound Ru-based CLs.\u003c/strong\u003e(a) Schematic of the conventional approach to constructing a Ru CL with ionomer. (b) Proposed strategy for fabricating an ionomer-free Ru CL using a superhydrophilic Ru-NVP precursor. (c) TEM image, FT-IR and (d) Dynamic water CA measurement of the as-synthesized superhydrophilic MacroPH Ru-NVP precursor. (e) Photographs of Ru-NVP precursor inks with various I/Ru ratios (0, 0.03, 0.05, 0.1, and 0.2) after storage for 1 hour (1h), 1 day (1d), and 1 week (1w). SEM images of the CL in the (f) Ru-IB/Ti and (g) Ru-SB/Ti electrodes.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8719121/v1/7ceb3b07a66213bbb916e053.png"},{"id":104458739,"identity":"137477f3-8545-47e0-9803-5c30fb5c4642","added_by":"auto","created_at":"2026-03-12 03:12:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":4145072,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePerformance verification of self-bound CL.\u003c/strong\u003e HER activation curves for the (a) Ru-IB-c/Ti and (b) Ru-SB-c/Ti electrodes. (c) HER polarization curves of the Ru-SB/Ti electrode with CLs prepared using various I/Ru ratios (0, 0.03, 0.05, 0.1, and 0.2). (d) HER polarization curves of the Ru-SB/Ti electrode (I/Ru: 0.03) measured initially, after 10,000 and 20,000 cycles. (e) Schematic of a two‑electrode electrolyzer. (f) Alkaline water electrolysis performance equipped with Ru‑SB‑a/Ni||Ru‑SB‑c/Ti, Ru‑IB‑a/Ni||Ru‑IB‑c/Ti and Ru‑SB‑a/Ni||Pt/C‑IB‑c/Ti electrode pairs. All the penitential/voltage data are iR-correction free.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8719121/v1/90d3ffaa9d3be614a39778e7.png"},{"id":104458744,"identity":"3c1f1197-3da2-4d13-a10f-0be14b00f01a","added_by":"auto","created_at":"2026-03-12 03:12:30","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":22056638,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSynthesis, structural advantages, and generality of Ru-based MacroPH precursors.\u003c/strong\u003e (a) Schematic illustration of the aerosol-microdroplet mediated synthesis procedure for the MacroPH Ru-NVP precursor and the proposed formation mechanism of its mesoporous hollow architecture. (b) SEM image and corresponding EDS elemental mappings of MacroPH Ru-NVP precursor. (c-e) Comparative HAADF-STEM images of the MacroPH Ru-NVP precursor (c), alongside the Ru precursors synthesized in the absence of NVP (d) and without NaNO₃ (e). (f) BJH pore-size distribution profiles of the aforementioned precursors after thermal treatment in air. (g) HER polarization curves of the electrode derived from the aforementioned precursors. (h) Schematic demonstrating the preparation of various Ru-based composite MacroPH precursors via the versatile aerosol-microdroplet mediated synthesis route.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8719121/v1/c43471d41ce756e872e27698.png"},{"id":104458740,"identity":"e570f87e-c810-4e77-b663-9f95998bb6ba","added_by":"auto","created_at":"2026-03-12 03:12:30","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":6595373,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDevelopment of FeNiRu\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e-NVP precursors and water electrolysis application.\u003c/strong\u003e (a) HAADF-STEM-EDS mappings of porous hollow FeNiRu\u003csub\u003e0.2\u003c/sub\u003e-NVP precursor. (b) OER polarization curves of the anodes derived from the porous hollow FeNiRu\u003csub\u003e0.2\u003c/sub\u003e-NVP, FeNi-NVP and Ru-NVP precursors. (c) Alkaline water electrolysis performance equipped with Ru‑SB‑a/Ni||Ru‑SB‑c/Ti, FeNi‑SB‑a/Ni||Ru‑SB‑c/Ti, and FeNiRu\u003csub\u003e0.2\u003c/sub\u003e‑SB‑a/Ni||Ru‑SB‑c/Ti, electrode pairs.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8719121/v1/6f7898798b33fcf60f9282c6.png"},{"id":108006123,"identity":"7eabcb54-06c7-4975-b7b3-832c29b6f41e","added_by":"auto","created_at":"2026-04-28 12:53:44","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":48820657,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8719121/v1/fa0d99f0-6eac-4a32-bcb4-7326832a483c.pdf"},{"id":104458742,"identity":"9f853c4c-7fad-406c-801c-cbe918589180","added_by":"auto","created_at":"2026-03-12 03:12:30","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":6736484,"visible":true,"origin":"","legend":"Self-Bound Catalytic Layer of Porous Hollow Ru for Transport-Boosted Alkaline Water Electrolysis","description":"","filename":"Supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8719121/v1/e756e2248730f0a5390d4877.docx"},{"id":104458741,"identity":"1f048134-0936-4559-b9b4-047d4c06f6d4","added_by":"auto","created_at":"2026-03-12 03:12:30","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":6819809,"visible":true,"origin":"","legend":"","description":"","filename":"GA.png","url":"https://assets-eu.researchsquare.com/files/rs-8719121/v1/c6cdb8eaf697b4192b634728.png"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Self-Bound Catalytic Layer of Porous Hollow Ru for Transport-Boosted Alkaline Water Electrolysis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eHigh-current-density alkaline water electrolysis offers a promising pathway to enhance the unit capacity of hydrogen production.\u003csup\u003e[1-3]\u003c/sup\u003e However, operating at elevated current densities generally leads to increased cell voltages, which compromises energy conversion efficiency and raises energy consumption.\u003csup\u003e[4-5]\u003c/sup\u003e Reducing the overall cell voltage hinges on optimizing the catalytic layer (CL), which can be approached from three key aspects: (1) developing highly active and stable catalysts to minimize electrochemical polarization overpotential\u003csup\u003e[6]\u003c/sup\u003e; (2) reducing charge transport resistance to mitigate ohmic losses\u003csup\u003e[7]\u003c/sup\u003e; and (3) enhancing mass transport to facilitate the efficient diffusion of reactants and products\u003csup\u003e[1, 8]\u003c/sup\u003e. It is important to note that ohmic overpotential is highly sensitive to current density, while mass transfer overpotential becomes particularly dominant at high reaction rates. When the current density exceeds 1 A\u0026middot;cm⁻\u0026sup2;, even minor interfacial resistance can result in significant ohmic losses, and mass transfer limitations intensify sharply, becoming critically dependent on the diffusion performance of the CL. Therefore, an ideal CL for high-current-density alkaline water electrolysis should combine high intrinsic catalytic activity with a stable, low-resistance structure that supports efficient electron conduction and rapid mass transport.\u003c/p\u003e\n\u003cp\u003eRecent significant advances in ruthenium (Ru)-based catalysts have positioned them as highly efficient alternatives for the hydrogen evolution reaction (HER) to addresses the issue of high overpotential associated with traditional Raney Ni catalysts.\u003csup\u003e[9-12]\u003c/sup\u003e Consequently, enhancing their operational efficiency under high-current-density conditions is crucial. Significant advances in Ru-based catalysts have been pursued through multiple structural design strategies, including the use of metallic Ru\u003csup\u003e[13-16]\u003c/sup\u003e, conductive supports for improved electron transport\u003csup\u003e[17-19]\u003c/sup\u003e, and engineered porous/hollow structures to enhance mass transport properties\u003csup\u003e[20-22]\u003c/sup\u003e. Despite these promising innovations at the material level, the efficiency of a Ru CL is still compromised by its fabrication process. Typically, as illustrated in \u003cstrong\u003eFigure 1a\u003c/strong\u003e, Ru-based CLs are prepared by dispersing catalyst particles with an ionomer to form an ink, which is then coated onto a substrate\u003csup\u003e[23-26]\u003c/sup\u003e. The ionomer serves a dual function: it disperses the catalyst powder for ink preparation and binds the catalyst particles to form the CL. However, this conventional process inevitably forms an ionomer film on the catalyst surface, which introduces additional resistance (Ri) beyond the inherent resistances of the electrode support (Rs) and the catalyst itself (Rc), thereby impeding electron transport, electrolyte wetting, ion/molecule migration, and gas evolution.\u003csup\u003e[27]\u003c/sup\u003e These factors severely limit the full utilization of the intrinsic activity of Ru catalysts in the high-current-density region.\u003csup\u003e[28, 29]\u003c/sup\u003e Trickily, the binding function provided by the ionomer is indispensable for maintaining the mechanical stability of CL. Simply reducing or removing the ionomer is not feasible, as it could lead to the detachment of CL under the intense gas flushing.\u003csup\u003e[30-32]\u003c/sup\u003e Consequently, developing low-ionomer or ionomer-free Ru-based CLs and integrating them with reported high-efficiency Ru catalyst architectures is crucial but challenge for constructing near-ideal CLs under high-current-density conditions.\u003c/p\u003e\n\u003cp\u003eIn this work, we eliminated ionomer-related resistances by developing a self-bound Ru (Ru-SB) CLs integrated with a porous hollow Ru architecture that facilitates local mass transport. Typically, this achievement is enabled by a rationally designed superhydrophilic Ru-based precursor coordinated with polar N-vinylpyrrolidone (NVP) molecules and structured into macroporous hollow (MacroPH) architectures. By utilizing the highly dispersible Ru-NVP precursor, the proposed strategy enables a substantial reduction in the ionomer/Ru ratio (I/Ru). The achieved I/Ru ratio of 0.03 is merely one-tenth of that required in the conventional route (0.4), as compared in \u003cstrong\u003eFigure 1a\u003c/strong\u003e and \u003cstrong\u003eb\u003c/strong\u003e. \u003cstrong\u003eFigure 1c\u003c/strong\u003e presents the TEM image and FT-IR spectra of the MacroPH Ru-NVP precursor particles via a carefully designed aerosol-microdroplet confinement method\u003csup\u003e[33]\u003c/sup\u003e (detailed in a subsequent section), containing numerous pores exceeding 50 nm in size and strongly polar NVP ligand. Thermogravimetric (TG) and heat flow analysis (\u003cstrong\u003eFigure S1\u003c/strong\u003e) further confirm that there are 39% organic component in precursor, indicating a significant amount of NVP bound to Ru. Dynamic contact angle measurements (\u003cstrong\u003eFigure 1d\u003c/strong\u003e) verified the exceptional superhydrophilicity of the material, with nearly complete wetting achieved in 0.2 seconds. \u003cstrong\u003eFigure 1e\u003c/strong\u003e further demonstrates that as long as I/Ru ratio exceeds 0.03, the ink formulated with the MacroPH Ru-NVP precursor can remain stable for over one week, laying a solid foundation for large-scale industrial electrode production. For a fundamental comparison, the precursor was deposited onto an electrochemical-inert Ti mesh substrate to form a Ru-NVP pre-catalytic layer, which was subsequently converted into the final cathodic Ru-SB-c or anodic Ru-SB-a CL through thermal treatment. After screening via three-electrode HER and oxygen evolution reaction (OER) testing (\u003cstrong\u003eFigure S2\u003c/strong\u003e), the thermal treatment conditions for converting the Ru-NVP precursor into both the Ru-SB-c and Ru-SB-a CLs were determined as calcination in a H₂/Ar atmosphere, followed by a subsequent calcination step in air. During heating, the Ru-NVP precursor underwent chemical transformation accompanied by the decomposition of the residual ionomer. This resulted in a robust, self-bound Ru-SB CL firmly adhered to the Ti substrate, which fully exposes the catalytic active sites and thereby reduces the transport resistance associated with Ru CL bound by ionomers (Ru-IB). Moreover, the SEM images of the finally formed HER CLs of Ru-IB/Ti (\u003cstrong\u003eFigure 1f\u003c/strong\u003e) and Ru-SB/Ti electrodes (\u003cstrong\u003eFigure 1g\u003c/strong\u003e) reveal markedly different morphologies. The ionomer-bound Ru-IB CL shows extensive coverage of its surface and interparticle gaps by the ionomer, whereas the self-bound Ru-SB CL exhibits an ionomer-free surface with larger interparticle voids. The unique physical structure of the self-bound CL by microspheres provides highly efficient channels for both water infiltration into the CL and bubble escape from within it. Furthermore, the water CAs on the final cathodic catalytic layers of Ru‑IB-c/Ti and Ru‑SB-c/Ti electrodes were further measured. Because the polar C=O group remains intact after thermal treatment (Figure 1c), the Ru‑SB CL (\u003cstrong\u003eFigure 1g\u003c/strong\u003e) possesses much superior wettability compared to the Ru‑IB layer (\u003cstrong\u003eFigure 1f\u003c/strong\u003e), which is also conducive to enhanced performance under high‑current‑density conditions.\u003csup\u003e[34]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe superior catalytic performance of the as-developed Ru-SB CL was clearly demonstrated by HER polarization curves measured in 10% KOH at 25 \u0026deg;C (\u003cstrong\u003eFigure 2a\u003c/strong\u003e). While the Ru-IB/Ti and Ru-SB/Ti electrodes showed nearly identical onset performance in the low-current-density region, the HER activity of the Ru-SB/Ti electrode became increasingly advantageous as the current density rises. Additionally, the conventional Ru-IB/Ti electrode requires approximately 20 cycles to reach a stable state, whereas the Ru-SB/Ti electrode essentially achieves stability starting from the 3rd cycle. \u003cstrong\u003eFigure 2b\u003c/strong\u003e presents the stabilized HER polarization curves and the corresponding Tafel fitting curves. Tafel analysis reveals a minimal difference in kinetic polarization overpotential (~50 mV) even at 1.0 A cm⁻\u0026sup2;. In contrast, during actual HER operation, the overpotential gap exceeds 200 mV at 0.5 A cm⁻\u0026sup2;. This indicates that the Ru-SB catalyst layer possesses a lower total transport resistance\u0026mdash;encompassing both mass and charge transport\u0026mdash;and that the benefit of this reduced resistance grows more pronounced with increasing operating current.\u003csup\u003e[35]\u003c/sup\u003e We also investigated the influence of I/Ru ratio on the HER performance of the as-prepared Ru-SB/Ti electrodes. As shown in \u003cstrong\u003eFigure 2c\u003c/strong\u003e, the electrode performance showed no significant difference within the I/Ru ratio range of 0\u0026ndash;0.2, indicating that the HER performance can be effectively ensured as long as the ionomer is effectively removed. Finally, long-term HER cycling tests (\u003cstrong\u003eFigure 2d\u003c/strong\u003e and \u003cstrong\u003eS3\u003c/strong\u003e) confirm the robustness and stability of the Ru-SB/Ti electrode fabricated with a low I/Ru ratio of 0.03, showing no performance degradation after 10,000 and 20,000 cycles.\u0026nbsp;The advantage of the self-bound Ru-SB CL from superhydrophilic Ru-NVP precursor is also demonstrated in the anodic OER process (\u003cstrong\u003eFigure S4\u003c/strong\u003e). This further corroborates that the self-bound CL approach paves the way for building advanced electrodes in high-current-density alkaline water electrolysis.\u003c/p\u003e\n\u003cp\u003eTo evaluate the practical performance of the self‑bound CLs in water electrolysis, a two‑electrode electrolyzer (\u003cstrong\u003eFigure 2e and S5\u003c/strong\u003e) was constructed. The anode and cathode supports were Ni foam and Ti mesh, yielding the corresponding electrodes of Ru‑SB‑a/Ni and Ru‑SB‑c/Ti, each with a geometric active area of 2 \u0026times; 2 cm\u0026sup2;, separated by a commercial membrane (thickness: 0.25 mm). The electrolysis was carried out in 15 wt% KOH solution at 60 \u0026deg;C. For comparison, an electrolyzer equipped with ionomer‑bound CLs (Ru‑IB‑a/Ni and Ru‑IB‑c/Ti) was also tested under the same conditions. \u003cstrong\u003eFigure 2f\u003c/strong\u003e shows that the cell voltage difference between the Ru‑SB‑a/Ni||Ru‑SB‑c/Ti pair (red plots) and the Ru‑IB‑a/Ni||Ru‑IB‑c/Ti pair (blue plots) remains minor at current densities up to 0.25 A/cm\u0026sup2;. However, as the current density increases, the cell voltage difference (\u003cem\u003e\u0026Delta;E\u003c/em\u003e) becomes more pronounced, reaching 81 mV at 1.0 A/cm\u0026sup2; and further widening to 107 mV at 2.0 A/cm\u0026sup2;. These results confirm that the self‑bound design of CLs effectively reduces transport resistance at high current densities, leading to a lower cell voltage. Additionally, the performance of the Ru‑SB‑a/Ni||Pt/C‑IB‑c/Ti pair (grey plots) was evaluated, where a well-known HER catalysts of Pt/C (\u003cstrong\u003eFigure S6\u003c/strong\u003e) was used to ionomer-bound Pt/C‑IB‑c CL instead of the Ru‑SB‑c CL. At current densities below 0.25 A/cm\u0026sup2;, this air exhibited performance comparable to that of both Ru‑IB‑a/Ni||Ru‑IB‑c/Ti and Ru‑SB‑a/Ni||Ru‑SB‑c/Ti references. However, its performance degraded more significantly than Ru‑IB‑a/Ni||Ru‑IB‑c/Ti at higher current densities.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure 3a\u003c/strong\u003e shows the processes of above-mentioned aerosol-microdroplet mediated synthetic methodology for constructing the superhydrophilic MacroPH Ru-based precursors. Briefly, an aqueous solution containing RuCl₃, NVP, and NaNO₃ is subjected to ultrasonic spraying, generating a multitude of microdroplets. These microdroplets are transported into a tube furnace, set at 700 \u0026deg;C, using a vacuum pump. At the outlet of the tube furnace, a powder filter collects the resulting MacroPH Ru-NVP precursor containing NaCl. Washing this collected powder yields precursor product. The key design here focuses on the reaction process of the microdroplets within the tube furnace, specifically tailored to achieve superhydrophilicity and a MacroPH structure. Firstly, as the solvent rapidly evaporates and the temperature rises, the NVP and NaNO₃ undergo a vigorous redox reaction. This reaction produces gases like COₓ and NOₓ, forming ultrafine bubbles. The aggregation of some of these bubbles creates cavities. Simultaneously, non-volatile inorganic components solidify, and the release of these bubbles leads to the formation of a MacroPH structure featuring an open window (\u003cstrong\u003eFigure 3b\u003c/strong\u003e) that is favorable for mass transfer\u003csup\u003e[36]\u003c/sup\u003e. The formation process of Ru-NVP is more complex. RuCl₃ and NaNO₃ participate in a molten-state reaction. Coupled with the reducing gases (CO, NO) generated from the reaction between NVP and NaNO₃, this process primarily yields metallic Ru along with NaCl (\u003cstrong\u003eFigure S7\u003c/strong\u003e). This mechanism has been previously detailed in our studies on the Ir-glucose system.\u003csup\u003e[37]\u003c/sup\u003e More importantly, NVP was employed owing to the strong coordination capability of the carbonyl groups, enabling the formation of complexes with a fraction of RuCl₃, which subsequently deposit onto the above-mentioned Ru/NaCl, yielding the composite denoted as Ru-NVP/NaCl. Washing this intermediate yields the superhydrophilic MacroPH Ru-NVP microspheres. The distinct Ru, C, N, and O signals in the EDS mapping (\u003cstrong\u003eFigure 3b\u003c/strong\u003e) and characteristic signals of NVP in FT-IR spectra (\u003cstrong\u003eFigure 1c\u003c/strong\u003e) well agree with the composition of Ru-NVP. The HAADF-STEM image (\u003cstrong\u003eFigure 3c\u003c/strong\u003e) shows the MacroPH structure consistent with TEM image (\u003cstrong\u003eFigure 1c\u003c/strong\u003e). To validate the formation mechanism related to MacroPH structure, we conducted two control experiments: one without NVP and another without NaNO₃. Consequently, the product obtained in the absence of NVP exhibited a normal macroporous structure without hollow cavity (\u003cstrong\u003eFigure 3d and S8\u003c/strong\u003e), while the microspheres formed without NaNO₃ appeared solid-like. (\u003cstrong\u003eFigure 3e and S9\u003c/strong\u003e). Furthermore, the BJH pore size distribution analysis (\u003cstrong\u003eFigure 3f\u003c/strong\u003e) reveals a predominance of macropores in the derived MacroPH Ru sample. In contrast, the control sample without NVP possesses a greater proportion of small mesopores and significantly fewer macropores, while the sample lacking NaNO₃ exhibits minimal porosity.\u003c/p\u003e\n\u003cp\u003eThe electrochemical HER polarization curves (\u003cstrong\u003eFigure 3g\u003c/strong\u003e) clearly demonstrate the superior performance of the electrode fabricated from the MacroPH structural Ru-NVP precursor. Although the Ru-SB/Ti electrode exhibited an onset performance similar to those derived from NVP-free and NaNO₃-free precursors in the low-current-density region, its HER activity became increasingly superior at higher current densities. Moreover, the electrode obtained from the low-porosity NaNO₃-free precursor showed the weakest performance in the large-current-density region. The advantage of the MacroPH structural Ru-NVP precursor is also confirmed in the two‑electrode alkaline electrolysis (\u003cstrong\u003eFigure S10\u003c/strong\u003e) in 15% KOH at 60 \u003csup\u003eo\u003c/sup\u003eC. The enhanced performance at high current densities can be attributed to the superior mass transfer properties of the MacroPH structure.\u003csup\u003e[38]\u003c/sup\u003e More importantly, the aerosol-microdroplet mediated synthesis procedure is applicable to a wide range of Ru-based composite precursors. As depicted in \u003cstrong\u003eFigure 3h\u003c/strong\u003e, a series of Ru-M-NVP (M=Co, Ni, \u003cem\u003eetc.\u003c/em\u003e) precursors\u0026mdash;from binary Ru-Cu (\u003cstrong\u003eFigure S11\u003c/strong\u003e) and Ru-Mn (\u003cstrong\u003eFigure S12\u003c/strong\u003e), to ternary medium-entropy Ru-Co-Ni (\u003cstrong\u003eFigure S13\u003c/strong\u003e), quaternary Ru-Fe-Co-Ni (\u003cstrong\u003eFigure S14\u003c/strong\u003e), and even quinary high-entropy Ru-Fe-Co-Ni-Mn (\u003cstrong\u003eFigure S15\u003c/strong\u003e) systems\u0026mdash;have been successfully synthesized. Objectively speaking, not all particles possess macropores (e.g., Ru-Cu-NVP), but all exhibit a fundamental porous hollow structure. This versatility provides a methodological foundation for screening and developing CLs and electrodes with further improved intrinsic activity. Benefiting from this, we facilely identified a porous hollow FeNiRu₀.₂-NVP precursor (\u003cstrong\u003eFigure 4a\u003c/strong\u003e and \u003cstrong\u003eS16\u003c/strong\u003e) and the corresponding anode of FeNiRu₀.₂‑SB‑a/Ni, which exhibits better performance at high current densities while using only 20% of the Ru loading compared to a pure Ru anode. As shown in \u003cstrong\u003eFigure 4b\u003c/strong\u003e, despite the reduced Ru content, it shows a slightly higher overpotential at low current densities compared to the Ru-SB‑a/Ni anode, but demonstrates superior performance at high current densities. Its performance is also relatively better than that of FeNi‑SB‑a/Ni anode. The two-electrode alkaline water electrolysis tests yielded consistent results. \u003cstrong\u003eFigure 4c\u003c/strong\u003e shows that when the current density exceeds 1 A/cm\u0026sup2;, the performance of the FeNiRu₀.₂‑SB‑a/Ni||Ru‑SB‑c/Ti electrode pair surpasses that of the Ru‑SB‑a/Ni||Ru‑SB‑c/Ti electrode pair. The voltages required at 2 A/cm\u0026sup2; and 3 A/cm\u0026sup2; are only 1.892 V and 2.048 V, respectively. This simple optimization not only enhances electrode performance but also reduces the consumption of the precious metal Ru. It is noteworthy that the presented alkaline water electrolysis system delivers high performance under significantly milder conditions (15% KOH, 60 \u0026deg;C) than the industrial standard (30% KOH, 80-90 \u0026deg;C).\u003c/p\u003e\n\u003cp\u003eIn summary, this study firstly demonstrate a self-bound Ru-SB CL for high-current-density alkaline water electrolysis, constructed from a superhydrophilic Ru precursor coordinated with polar N-vinylpyrrolidone. We find that the ionomer content required for stable ink dispersion to be reduced to just one-tenth of conventional levels, and the residual ionomer present in the precursor layer can be effectively removed via a straightforward heat treatment, which concurrently produced a robust, self-bound Ru-SB CL with no sacrifice in mechanical stability. The resulting CL structure fully exposes active sites and transport channels, thereby delivering catalytic activity that far surpasses that of conventional ionomer-bound Ru-IB CLs and allowing the electrolyzer to operate at substantially reduced voltages and milder working conditions. Demonstrated in the alkaline water electrolyzer at mild condition (15% KOH, 60 \u0026deg;C), the electrode with Ru-SB CL delivers current densities of 1.0 and 2.0 A cm⁻\u0026sup2; at operating voltages of only 1.728 V and 1.916 V, respectively\u0026mdash;representing significant reductions of 81 and 107 mV compared to a conventional Ru-IB CL. In addition to the self-bound CL design, this work further establishes a general and scalable aerosol-microdroplet method for generally synthesizing and screening aforementioned superhydrophilic Ru-based precursors integrated with a porous hollow architecture, achieving a further lower operating voltage of 1.892 V at current density of 2.0 A cm⁻\u0026sup2; with a FeNiRu₀.₂-NVP precursor. Therefore, our work paves the way for future studies to overcome ionomer-induced transport barriers across diverse fields. By resolving these issues, low-cost alkaline water electrolysis can become a more competitive candidate for green hydrogen production, challenging the dominance of higher-cost AEM and PEM electrolyzer technologies.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eH. Li, G. X. Lin, L. Q. Wang, H. Lee, J. Du, T. Tang, G. H. Ding, R. Ren, W. L. Li, X. Cao, S. W. Ding, W. T. Ye, W. X. Yang, L. C. 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Alia, Role of the ionomer in supporting electrolyte-fed anion exchange membrane water electrolyzers. \u003cem\u003eACS Electrochem.\u003c/em\u003e \u003cstrong\u003e2025\u003c/strong\u003e, \u003cem\u003e1\u003c/em\u003e, 239-248.\u003c/li\u003e\n\u003cli\u003eQ. Li, B. Kim, Z. P. Li, R. Thapa, Y. F. Zhang, J.-M. Seo, R. N. Guan, F. Tang, J.-H. Baek, Y. H. Kim, J.-P. Jeon, N. Park, J.-B. Baek, Direct electroplating ruthenium precursor on the surface oxidized nickel foam for efficient and stable bifunctional alkaline water electrolysis. \u003cem\u003eAdv. Mater.\u003c/em\u003e \u003cstrong\u003e2024\u003c/strong\u003e, \u003cem\u003e36\u003c/em\u003e, 2403151.\u003c/li\u003e\n\u003cli\u003eQiao, X. Y. Li, S. B. Shen, X. H. Zhong, Q. Liu, L. Kuai, A composition-precise synthesis of PtCo alloy catalysts mediated by aerosol-microdroplet confinement. \u003cem\u003eSmall\u003c/em\u003e \u003cstrong\u003e2025\u003c/strong\u003e,\u003cem\u003e 21\u003c/em\u003e, e11541.\u003c/li\u003e\n\u003cli\u003eY. Yang, M. Driess, P. W. Menezes, Self-supported electrocatalysts for practical water electrolysis. \u003cem\u003eAdv. Funct. Mater.\u003c/em\u003e \u003cstrong\u003e2021\u003c/strong\u003e, \u003cem\u003e11\u003c/em\u003e, 2102074.\u003c/li\u003e\n\u003cli\u003eDong, C. Y. Zhang, Z. Y. Yue, F. R. Zhang, H. Zhao, Q. Q. Cheng, G. L. Wang, J. F. Xu, C. Chen, Z. Q. Zou, Z. L. Dou, H. Yang, Overall design of anode with gradient ordered structure with low iridium loading for proton exchange membrane water electrolysis. \u003cem\u003eNano Lett.\u003c/em\u003e \u003cstrong\u003e2022\u003c/strong\u003e, \u003cem\u003e22\u003c/em\u003e, 9434-9440.\u003c/li\u003e\n\u003cli\u003eK. Fan, J.Wang, Z. Y. Han, H. Li, L. X. Zhang, Y. J. Zhao, L. L. Duan, H. Shen, J. Li, L. P. Wang, W. H. Zhou, X. J. Gu, J. W. Zhang, D. L. Chao, Z. W. Zhao, D. Y. Zhao, L. M. Wu, Regular mesoporous superparticles with a tailored opening window and tunable surface crisscrossed grooves. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e2025\u003c/strong\u003e, \u003cem\u003e147\u003c/em\u003e, 26, 22460-22472.\u003c/li\u003e\n\u003cli\u003eLiu, T. Huang, X. L. Yang, S. J. Liu, S. S. Wang, L. L. Xiang, G. M. Wang, L. Kuai, Microdrop-confined synthesis and regulation of porous hollow Ir-based catalysts for the mass transfer-enhanced electrolysis of pure water, \u003cem\u003eSci. Bull.\u003c/em\u003e \u003cstrong\u003e2024\u003c/strong\u003e, \u003cem\u003e69\u003c/em\u003e, 1081-1090.\u003c/li\u003e\n\u003cli\u003eM. Farber, N. M. Seraphim, K. Tamakuwala, A. Stein, M. R\u0026uuml;cker, D. Eisenberg, Porous materials: The next frontier in energy technologies. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e2025\u003c/strong\u003e, DOI: 10.1126/science.adn9391.\u003c/li\u003e\n\u003c/ol\u003e\n"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003e\u003cem\u003ePreparation of superhydrophilic porous hollow Ru-NVP precursor\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe superhydrophilic porous hollow Ru-NVP precursor was synthesized using a microdroplet confinement approach. The experimental setup consisted of an ultrasonic atomizer, a tubular furnace, a sample collection unit, and a circulating water vacuum pump. A homogeneous solution was first prepared by dissolving 0.433 g of N-vinylpyrrolidone (NVP) and 0.761 g of NaNO₃ in 61 mL of deionized water under ultrasonic agitation. Subsequently, 11.2 mL of RuCl₃ solution (0.41 M) was added to the mixture and homogenized. The resulting solution was fed into the ultrasonic atomizer to generate fine microdroplets. Driven by the vacuum pump, the aerosol droplets were transported through the tubular furnace maintained at 700 \u0026deg;C, resulting in the formation of a brown powder collected in the sample receiver. The product was then washed three times with ultrapure water via centrifugation, followed by one washing step with ethanol. Finally, the powder was dried under vacuum to obtain the superhydrophilic porous hollow Ru-NVP precursor.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eElectrode fabrication with self-bound catalyst layer\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eInk preparation:\u0026nbsp;\u003c/em\u003eA dried Ru-NVP precursor was weighed and transferred into an ink bottle. Isopropanol and Nafion solution were added to prepare a homogeneous ink with a concentration of 4 mg/mL of Ru-NVP. The mixture was ultrasonicated for 30 minutes in an ice-water bath to ensure uniform dispersion.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eCathode fabrication:\u0026nbsp;\u003c/em\u003eTo make the self-bound cathodic (Ru-SB-c) catalyst layer, the Ru-NVP ink was sprayed onto Ti mesh substrates of two sizes: 1 \u0026times; 1 cm\u0026sup2; (for three-electrode electrochemical testing) with a Ru loading of approximately 0.6 mg\u003csub\u003eRu\u003c/sub\u003e/cm\u0026sup2;, and 2 \u0026times; 2 cm\u0026sup2; (for two-electrode alkaline water electrolysis evaluation) with a Ru loading of about 1.2 mg\u003csub\u003eRu\u003c/sub\u003e/cm\u0026sup2;. To obtained final cathode (Ru-SB-c/Ti), the Ru-NVP coated Ti mesh was then subjected to a two-step calcination process: first, it was heated at 360 \u0026deg;C (2 \u0026deg;C/min) under a 5% H₂/Ar atmosphere for 2 h, cooled naturally to room temperature, and subsequently calcined again at 300 \u0026deg;C (2 \u0026deg;C/min) in air for another 2 h, followed by cooling to room temperature, yielding the final Ru-SB-c/Ti cathode.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eAnode fabrication:\u0026nbsp;\u003c/em\u003eThe self-bound anodic (Ru-SB-a) catalyst layer and final anode (Ru-SB-a/Ni) were prepared following a similar procedure and the same ink, with the following modifications: the Ti mesh was replaced by Ni foam unless otherwise indicated, the reduction step in 5% H₂/Ar was conducted at 380 \u0026deg;C, and the subsequent calcination in air was performed at 400 \u0026deg;C.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eThree-electrode hydrogen/oxygen evolution reaction testing\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThree-electrode electrochemical performance tests for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were conducted on the Donghua Electrochemical Workstation (DH7003B) at 25 \u0026deg;C. The Pt sheet (1\u0026times;1 cm\u0026sup2;) served as the counter electrode, Hg/HgO electrode (GOE) as the reference electrode, and a home-made electrode (1\u0026times;1 cm) as the working electrode. The electrolyte consisted of 10 wt% KOH (pH = 14.3). All potentials were expressed as reversible hydrogen electrode (RHE) potentials as the following equation:\u0026nbsp; \u003cimg src=\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAS0AAAA6CAYAAADlRw95AAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAAFiUAABYlAUlSJPAAAAlwSURBVHhe7d0/TBPvHwfwd39xLDAcQ0ujgSKwGUrSJiaohGNBMDpIYhwKgxgTB5XGgQmNgyReEQZIgEkHy8BWWkz8E4hMGFrCYmhjcaHt4A3QW5ye78L1d/f0WloKtFc+r+QG7p4+vfs8D5977u45sDDGGAghxCT+x68ghJBqRkmLEGIqlLQIIaZCSYsQYiqUtAghpkJJixBiKpS0CCGmQkmLEGIqlLQIIaZCSYsQYiqUtAghpkJJixBiKpS0CCGmQkmLEFKyVCoFj8eDaDTKbzpzlLQIIUVTFAV9fX1YWFhAZ2cntra20NrailQqxRc9M5S0CCFFs1qt+Pr1K6xWKxYXF/Hjxw/8/v0bdrudL3pmKGkRQoqmjrQURcHo6Chu3Lhx7iMtC/3lUkJIqVKpFO7evYv5+Xm4XC5+85nKGWmlUim0trbCYrEUtaysrPBV1DSKz/HC4TA8Hg8sFsuJb9aWUodRmzQ2NuZ8RlEU+Hw+NDY2ZsssLS3pypRCHXXwba4deUSj0ez3aRe/389Xdy6MYlVoydd/7XY7Njc3zz1hAQBYHpFIhDmdTiYIAotEIvxmlslkmCiKLBgM8psuBIqPsWAwyACwQCDAGGMsEAgwQRBKikOpdUiSxADoFlEUWSaTyZZR20MURRaLxRg7qhcAkyRJU1vpIpEIEwRBt888dR/dbnf2+yvJzP03b9JSdzrfQbGjA19fX+dXXwgUn1zqL6/X682uU+PkdDpZMpnUlTdSah3F/nJJkmT4eUmSCrZhMQrtn0pNxNrjqiQz99+cy8NSuFwu3Lx5k19Njly0+Hz//h2yLGNoaCi7zmq1or+/H4lEAp8+fdKVN1JqHWtra3A4HBgcHNSt10qlUpibm0NLSwvq6up023p7ewEA09PTuvWkevtvWUmL1K5SJw8qioLV1VUIggCHw6Hb1tHRAQBYXV2Foii6bVql1qEoCqanp/Hx40d0dHTg/fv3hvWn02kcHBzwqwEAbW1t6OzsxMbGBlKpFBRFQTgcRn9/P/r6+pBOp+Hz+bL3eDweD+LxOF9NxZTaTrXgxEnr9evX5/qY02wuWnzi8Ti2t7fR0NAAm82m2+ZwOCAIAvb29pDJZHTbtEqtY21tDd++fQMAxGIxjI2Nobm5Oe/N4+O+HwCePn2KgYEBfP78GYeHh/D7/Xjy5AkYYwgEAkgkErh+/XrNJ4lq7r8nSlrxeBwbGxv8anKE4nM+BgcHwRhDPB7H1NQUBEGALMsYGRnRJRV1NJVIJLC1taWrI5PJYG9vL/vzhw8fEIlEIAgC6uvrMTExgba2NgDAgwcPMD4+DlmWa/pystr777FJS5ZldHV16R6Dtre3I5FI8EWrmt/vz3mcW2gp9pF0rcSnXPv7+5BlmV+tc3BwgHQ6za/OOmkdV69exYsXL/Dnzx94vV7IsoyXL19mLxWtViueP38OALqEpigKFhYWSmqrhw8fwul0Zi8ntRKJBJqamnL6ksViwZ07d3Rlq4UZ+++xSUsQBEQiERw9aQRjDLFYDE6nky9a1Xw+n+4Yjlt8Ph9fhaFaiU+51Mu3Qowu+7TKrcNqtWJ2dhaiKOZcCg4ODiIUCsHpdKKrqwuNjY2YmJhAfX09AKC7u7uoV1Hq6urQ0tJimDydTieSyWROX2KMIRgM6spWCzP232OTlpG2tjZ0d3fzq8kRs8XHaBTa1NSEnz9/5pyFLSWMQitBHVUZJZXbt29jc3MTjDH8/fsXfr8f29vbAKB7Wlmtzqudqr3/nihpAcDExERRZ6aLykzxMRqFJpNJuN3unLMwyzMKtdlsaGhoMEwW6mXfcaOZ06gDRY7YcDRbPRQKQRRF9PT08JsLKjTiOyun0U7Fqub+e+KkpYpGo7qnNflebRgeHjbcrq4fHh7O+Qy/lPNiptFZqtBy0rMUj4+PlvaYl5aW8ObNG90j+5WVFcN9yvdqSDnxKZfdbkd3dzdkWcb+/r5u2+7uLgDg2rVruvW806gDRwnu8uXL2RvoRlKpFO7fvw8AePfuHaxWK1/EkHrj3mjOVy2qyv7LzzZVFTNjNpPJsNHR0ZxZwPwsZXU2sHbWstfrzXl9QpKk7Ixhvg7GGFtcXMy7L+etnPiwo+PXvmrC/8zP4E4mk8zpdGZjdtbxSSaTzO12l1RfqbPZjZxGHWNjYwX3W9t2RjPp1X0w+j6jvlzM/p3VjPiTtBMzef/NO9JS58zkE4/Hce/ePfz79+/YYWRPTw9EUcyeLQspdCZ99OhRZV7QNFBOfFZWVhAKhXRn+NnZWdy6dQs4OhMtLCxgeXk5+1m73Y6ZmRm8ffs27xyhSsfH5XJhfHwcoVAI4XAYADA/P4/t7W3MzMzo4hCPx+HxeHImaxZbhzpi93g82XLqC9Fut9swDoqiYGlpCc3NzTg8PMSXL18KzqRPJBKQJCk7egiHwxgZGYHX69V9Tu0LiUQC6+vrmhr+T+37v379qorJqabuv3wWUzMi/wJqvoUfLTGDLGr0fle+kVa+TFxsFj5rpxEfSZJ0ZyUef5ZSac9WZx2fk57BmeYFZxR4QTgWizG32513ezF1aMsAYKOjo4bl1FgBYO3t7SwQCOSNPdOMtERRZFNTU9nvEARB90K0tl7tom07tS6+jFG/OIlS26kW+m9O0joNfGMaDYm9Xm9OgLRB4uvgk56Z8UNpbUcSRZG9evXKsFOoMdE2ei3Gp9K0SYtvA1L5/pv38vA0BINBBINB7O/vG74TJkmS7mmHJEl8EQSDQTDGMDk5yW8yNe08IrvdjuXlZbS3t2eH3Pw8IxjM3kYNx4dUt0r23zNNWjia1OdwODA/P89vKknR17smMDQ0lPNKic1mw5UrV2Cz2dDb24uDg4OcV07S6TQuXbqU/csEWrUUH1LdKt1/zzxpAcDk5CTm5ubyPjpV7ezsYGdnh1+dFY1Gy5p7Ui3UBxPPnj0zfMTrcrkwMDCg264+on/8+HHexq2V+FTa7u4uZFnG3t6eYftcdBXvv/z1Yrn4a1X1HpX6yJdf1Ptd+e5x8YvRI2qz4o+Zv/cncX+RUz32fDd3ay0+5y3fTWqjm9Gkcv2X/rEFIcRUzuXykBBCTgslLUKIqVDSIoSYCiUtQoipUNIihJgKJS1CiKlQ0iKEmAolLUKIqVDSIoSYCiUtQoipUNIihJgKJS1CiKlQ0iKEmAolLUKIqVDSIoSYCiUtQoipUNIihJgKJS1CiKn8B9HjYfOGmaR1AAAAAElFTkSuQmCC\" style=\"width: 265px;\"\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTwo-electrode alkaline water electrolysis testing\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe electrode with a spray-coated area of 2 \u0026times; 2 cm\u0026sup2;, was aligned on both sides of a composite membrane with 0.25 mm in thickness (Jiangsu Weidao Energy Technology Co., Ltd.). The assembly was then thermally pressed using a hot press (YLJ-HP-9, Hefei Kejing Technology Materials Co., Ltd.) at 50 \u0026deg;C and 2 MPa for 1 minute. The Ni foams were used as current collector. The water electrolysis performance was subsequently evaluated on an electrolyzer test station manufactured by Anhui Contango New Energy Technology Co., Ltd. The tests were conducted under the following conditions: 15 wt% KOH as the electrolyte, a temperature of 60 \u0026deg;C, and electrolyte flow rates of 30 mL/min for the anode and 15 mL/min for the cathode.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"catalytic layer, self-bound, ionomer-free, water electrolysis, high-current density","lastPublishedDoi":"10.21203/rs.3.rs-8719121/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8719121/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Ru-based catalysts exhibit substantial potential for high-current-density alkaline water electrolysis. However, the ionomer incorporated into the final Ru catalytic layer (CL) used to disperse and bind catalyst particles during electrode fabrication introduces additional charge and mass transport resistances. This study demonstrates a self-bound Ru (Ru-SB) CL constructed from a superhydrophilic Ru precursor coordinated with polar N-vinylpyrrolidone (NVP). This design reduces the ionomer content required for stable ink dispersion to merely one-tenth of that in conventional systems; notably, the residual ionomer can be efficiently removed via a simple heat treatment. This process simultaneously yields a robust Ru-SB CL without compromising mechanical stability. When tested in an alkaline water electrolyzer (15% KOH, 60 °C), the Ru-SB CL delivers operating voltages of only 1.728 V and 1.916 V at current densities of 1.0 A cm⁻² and 2.0 A cm⁻², respectively — representing significant reductions of 81 mV and 107 mV relative to the conventional ionomer-bound Ru (Ru-IB) CLs. Furthermore, this study establishes a general and scalable aerosol-microdroplet method for screening more efficient superhydrophilic Ru-based precursors, enabling a further reduced operating voltage of 1.892 V at 2.0 A cm⁻². This work paves a way to build more efficient alkaline water electrolyzer for hydrogen production.","manuscriptTitle":"Self-Bound Catalytic Layer of Porous Hollow Ru for Transport-Boosted Alkaline Water Electrolysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-12 03:12:25","doi":"10.21203/rs.3.rs-8719121/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"dbfbce06-a85b-4f51-b8dd-91f2bcb1b9fe","owner":[],"postedDate":"March 12th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":64234430,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Porous materials"},{"id":64234431,"name":"Physical sciences/Chemistry/Chemical synthesis/Synthetic chemistry methodology"}],"tags":[],"updatedAt":"2026-04-27T08:21:55+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-12 03:12:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8719121","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8719121","identity":"rs-8719121","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}