Hydrogen spillover in complex oxide multifunctional sites improves acidic hydrogen evolution electrocatalysis

Improving the catalytic efficiency of platinum for the hydrogen evolution reaction is valuable for water splitting technologies. Hydrogen spillover has emerged as a new strategy in designing binary-component Pt/support electrocatalysts. However, such binary catalysts often suffer from a long reaction pathway, undesirable interfacial barrier, and complicated synthetic processes. Here we report a single-phase complex oxide La2Sr2PtO7+δ as a high-performance hydrogen evolution electrocatalyst in acidic media utilizing an atomic-scale hydrogen spillover effect between multifunctional catalytic sites. With insights from comprehensive experiments and theoretical calculations, the overall hydrogen evolution pathway proceeds along three steps: fast proton adsorption on O site, facile hydrogen migration from O site to Pt site via thermoneutral La-Pt bridge site serving as the mediator, and favorable H2 desorption on Pt site. Benefiting from this catalytic process, the resulting La2Sr2PtO7+δ exhibits a low overpotential of 13 mV at 10 mA cm−2, a small Tafel slope of 22 mV dec−1, an enhanced intrinsic activity, and a greater durability than commercial Pt black catalyst. While renewable H2 production offers a promising route for clean energy production, there is an urgent need to improve catalyst performances. Here, authors design a Pt-containing complex oxide that utilizes atomic-scale hydrogen spillover to promote H2 evolution electrocatalysis in acidic media.

A ccelerated exhaustion of fossil fuels accompanied with ever-increasing environmental issues has raised great concerns about the exploitation of renewable energy sources (e.g., solar and wind power) 1,2 . To overcome the intermittent nature of the renewable energy, an attractive prospect is to store them in the form of chemical bonds in certain molecular fuels 3,4 . Among various chemical fuels, hydrogen (H 2 ) has been pursued as the future sustainable energy alternatives to fossil fuels in view of high gravimetric energy density and carbon-free characteristics [5][6][7] . Therefore, finding a way to produce hydrogen efficiently is crucial for the future hydrogen economy. Electrochemical water splitting powered by renewable energy offers a cost-effective and promising approach for clean hydrogen production with high purity [8][9][10] . Water splitting in acidic solid polymer electrolytes is more efficient than alkaline electrolysis because it holds some notable superiority such as greater energy efficiency, higher current density, lower crossover of gases and more compact system design 11,12 . As the cathodic reaction in water splitting, the hydrogen evolution reaction (HER) is sluggish and requires an efficient electrocatalyst to expedite the rate. Currently, although a host of non-platinum candidate materials for HER were studied, the metallic platinum (Pt) is still considered as "the Holy Grail" of HER electrocatalyst in acidic media with a nearly-zero onset overpotential and fast kinetics owing to its favorable hydrogen binding energy 13,14 . However, the natural scarcity, high cost and poor durability limit its large-scale commercial applications of water electrolyzers 15,16 . As a consequence, it is necessary to improve the intrinsically catalytic ability and the utilization efficiency of Pt for HER.
Hitherto, numerous efforts have been made to design Pt-based HER catalysts in acid [17][18][19][20][21][22][23][24] . For example, the catalytic efficiency of Pt metal could be boosted by the size 17 , composition 18 , morphology 19 , and crystal phase-engineering strategies 20 . Besides, hybridizing Pt with another component is also an effective way to achieve high-performance HER electrocatalysts via hydrogen spillover phenomenon [21][22][23][24] . Recently, hydrogen spillover opens new opportunities for improving the HER activity of binary metal/support catalysts with hydrogen-enriched Pt nanocrystals and hydrogen-deficient components, such as WO 3-x 21 , SiO 2 22 , RuCeO x 23 , CoP 24 . As schematically illustrated in Fig. 1a, hydrogen spillover-based binary-component catalyst (HSBCC) involves three main steps: (1) the strong proton (H + ) adsorption on metals (e.g., Pt with ΔG H-metal < 0), (2) the interfacial H diffusion and spillover from metals to supports, and (3) efficient H 2 desorption on supports (ΔG H-support > 0) 24 . Nonetheless, considerable barriers need to be overcome for the hydrogen spillover process in HSBCC systems due to long reaction path and undesirable interfacial resistance within two components (e.g., Schottky barrier and unmatched lattice space) 2, [25][26][27] . Furthermore, HSBCCs always suffer from complicated synthesis processes, which are disadvantageous for costeffective and large-scale fabrication. In light of the short reaction path and interface-free feature in single-component catalysts 2,25,28,29 , the creation of atomic-level multiple catalytic sites for strong H + adsorption, thermo-neutral H adsorption and facile H 2 desorption simultaneously in hydrogen spillover-based single-component catalyst (HSSCC) system (as illustrated in Fig. 1b), is highly desirable to boost the acidic HER activity; yet still, such single-phase catalysts have not been reported so far.
Multi-metal oxides have attracted great interest in many catalytic applications by virtue of their structural and compositional flexibility [30][31][32] . The multiple elements and variable crystal structures could endow multi-metal oxides with some unique geometrical and electronic properties, consequently tailoring the binding behavior of reaction intermediates and promoting their electrocatalytic activities [30][31][32] . Inspired by aforementioned considerations, here we demonstrate a unique family of complex metal oxide prepared by a facile solid-phase reaction method, La 2 Sr 2 PtO 7+δ , as a highly active and durable HER electrocatalyst in acid media. This complex oxide crystallizes in a hexagonal structure with alternating layers of [La 2 PtO 6 ] containing isolated Pt IV O 6 octahedra and [Sr 2 O 1+δ ] slabs. The La 2 Sr 2 PtO 7+δ oxide displays a remarkable HER activity with a low overpotential of 13 mV at 10 mA cm −2 and a small Tafel slope of 22 mV dec −1 in 0.5 M H 2 SO 4 , superior to state-of-the-art HSBCCs and other Ptbased catalysts ever reported. In addition, the La 2 Sr 2 PtO 7+δ shows significant enhancement in the intrinsic activity and operational durability as compared with the commercial Pt black catalyst. Comprehensive experiments and electrochemical measurements were performed to support the possible occurrence of hydrogen spillover in La 2 Sr 2 PtO 7+δ . First-principles calculations suggest that the hydrogen adsorption at La-Pt bridge site in La 2 Sr 2 PtO 7+δ is nearly thermo-neutral, which could serve as the mediators for favorable hydrogen spillover and accordingly result in exceptionally high activity. Specifically, a unique synergistic mechanism of multi-function catalytic sites in La 2 Sr 2 PtO 7+δ involving hydrogen spillover for HER was proposed: the O site serves as the proton enrichment, the thermo-neutral La-Pt bridge  Fig. 1), suggesting the bulk nature of the as-synthesized La 2 Sr 2 PtO 7+δ oxide by solid-state reaction method. As shown in Fig. 2e, the high-angle annular dark-field scanning transmission electron microscopy and elemental mapping images demonstrate the homogeneous distribution of all elements in the as-prepared La 2 Sr 2 PtO 7+δ material. For a direct comparison in this work, commercial Pt black catalyst was also included and the pure phase structure was confirmed by XRD patterns ( Supplementary  Fig. 2). The Pt black catalyst has broad XRD peaks, implying nanocrystalline feature of the metallic Pt as evidenced by small nanoparticles (~5 nm) in TEM image ( Supplementary Fig. 3).
Electronic structure. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) were carried out to explore the surface chemical state and electronic structure of La 2 Sr 2 PtO 7+δ . Supplementary Fig. 4 presents the full XPS spectrum of La 2 Sr 2 PtO 7+δ , which demonstrates the existence of La, Sr, Pt, and O elements on the surface. As seen from the highresolution Pt 4f core level spectra in Fig. 3a, two peaks at 74.9 and 78.1 eV were observed, which could be ascribed to the Pt 4f 7/2 and Pt 4f 5/2 orbitals of oxidized Pt (IV) species 13,23,34 . Notably, in contrast to the Pt 4f spectra of commercial Pt black, no signals from metallic Pt at 71.4 and 74.7 eV were detected, indicating the absence of metallic Pt in the La 2 Sr 2 PtO 7+δ .To further confirm the oxidation state of Pt ions in La 2 Sr 2 PtO 7+δ , X-ray absorption nearedge structure (XANES) spectra were also collected along with standard Pt foil as a reference (Fig. 3b). XANES spectrum at the 5d L 3 -edge is highly sensitive to the valence state of 5d elements: an increase of the valence state of the 5d metal ion by one causes a shift of the L 3 spectra by more than one eV toward higher energies 35,36 . The intensity of white line peak in Pt L 3 -edge XANES spectra associates with the electronic transition from 2p 3/2 to unoccupied 5d states and discloses the oxidation state of Pt species 16,17 . The white-line intensity of the La 2 Sr 2 PtO 7+δ is much higher than that of the metallic Pt foil, suggesting Pt oxide species in La 2 Sr 2 PtO 7+δ 16,17 . Moreover, the white line of Pt-L 3 of La 2 Sr 2 PtO 7+δ locates at about 1.5 eV higher in energy than that of Pt foil indicating further higher oxidation state of the former. It is also well known that the extended X-ray absorption fine structure (EXAFS) is sensitive tool to uncover the local coordination of 5d elements 37,38 . Figure 3c shows the k 3 -weighted Fourier transform (FT) curves at R space of Pt L 3 -edge EXAFS spectra for La 2 Sr 2 PtO 7+δ in comparison with the Pt foil reference. The most intense peak at 1.64 Å for La 2 Sr 2 PtO 7+δ was detected, corresponding to Pt-O bond 17,39 . Also, the isolated PtO 6 octahedra in La 2 Sr 2 PtO 7+δ is evidenced by the absence of the Pt-Pt coordination at~2.52 Å in the first-shell region relative to Pt foil 17 Supplementary Fig. 6), which presents the superior HER activity (η 10 of 27 mV and Tafel slope of 19 mV dec −1 ) among the state-of-the-art HER electrocatalysts (Supplementary Table 2). Besides, the stoichiometric La 2 Sr 2 PtO 7+δ oxide shows higher HER activity than other non-stoichiometric complex oxides ( Supplementary Fig. 7), indicating that La 2 Sr 2 PtO 7+δ with stoichiometric element composition is the optimized catalyst for HER. To examine the kinetics and reaction mechanism, Tafel plots were drawn in Fig. 4b TOF was plotted vs. potential (Fig. 4f) based on the calculated numbers of surface active sites according to the previouslyreported methods (see Supplementary Fig. 10 and Supplementary Note. 1) 47,48 . Remarkably, the La 2 Sr 2 PtO 7+δ delivers an high TOF value of 596 s −1 at overpotential of 0.05 V, which is about two orders of magnitude higher than the commercial Pt black. Although La 2 Sr 2 PtO 7+δ possesses higher intrinsic activity than commercial Pt black, its mass activity is inferior to Pt black due to lower surface area ( Supplementary Fig. 11); future ongoing investigation could shed more light on improving the surface area of La 2 Sr 2 PtO 7+δ . As a control sample, a well-known Pt/WO 3 HSBCC was also prepared for comparison ( Supplementary  Fig. 12a) 21 . As seen from Supplementary Fig. 12b, the singlecomponent La 2 Sr 2 PtO 7+δ shows much higher intrinsic activity than binary-component Pt/WO 3 , confirming the superiority of the HSSCC system over the HSBCC system. In addition to the catalytic activity, we also take the long-term durability of the La 2 Sr 2 PtO 7+δ catalyst into consideration to assess its potential for practical application. For this purpose, the accelerated durability tests (ADT) by continuous cycling within HER potential window were conducted. As shown in Fig. 4g, after 1000-cycling, the La 2 Sr 2 PtO 7+δ exhibits only slight activity decline but Pt black suffers from obvious activity loss. For example, La 2 Sr 2 PtO 7+δ displays a decay of only 10% whereas a nearly 5.4-fold faster decay rate (54%) was observed for Pt black during the period of continuous 1000-cycle operation (Fig. 4h, i). The poor stability of Pt black catalyst may be associated with the dissolution of Pt surface atoms and the agglomeration of Pt particles [49][50][51] . In conjunction with a series of post-HER measurements including XRD, XAS, TEM and STEM-EDS ( Supplementary Fig. 13-16 Fig. 17). Besides, the SEM image showing in Supplementary Fig. 18 indicates the nearly unchanged morphology of La 2 Sr 2 PtO 7+δ after HER. It should be noted that short-time cycling (e.g., 10-cycling) does not lead to the activity loss and Pt formation of La 2 Sr 2 PtO 7+δ , as reflect by almost overlapped polarization curve with the initial one and the absence of metallic Pt peak in XRD pattern after 10cycling. Besides, the negligible fluctuation of overpotential (@-10 mA cm −2 ) was observed for La 2 Sr 2 PtO 7+δ during 50 h chronopotentiometry test (Fig. 4j), which further confirms the robust operation durability for HER. Overall, the high electrode/ intrinsic activity and electrochemical durability endow La 2 Sr 2 PtO 7+δ as a promising HER electrocatalyst candidate for future practical application in acidic water electrolysis.
Experimental evidences for hydrogen spillover. In order to provide the evidences of possible atomic-scale hydrogen spillover in La 2 Sr 2 PtO 7+δ for acidic HER electrocatalysis, comprehensive experiments and electrochemical measurements were performed. The hydrogen spillover could be initially confirmed by the color change in the mixture of the catalyst and WO 3 52 . As seen from Supplementary Fig. 19, The WO 3 after HER test exhibits an unchanged color. However, the mixture of La 2 Sr 2 PtO 7+δ and WO 3 after HER test generates a dark blue color, which is because that the spilled-over hydrogen migrates and readily reacts with WO 3 to form dark blue H x WO 3 52,53 . Besides, the hydrogen spillover effect could be also verified by Tafel slope and pH-dependent HER experiments. The Tafel slope of La 2 Sr 2 PtO 7+δ catalyst is only 22 mV dec −1 , which is evidently lower than the value (30 mV dec −1 ) via conventional Volmer-Heyrovsky/Tafel mechanism, implying a hydrogen spillover-involving mechanism as also reported before [54][55][56] . This reaction mechanism was further supported by the pH-dependent relation of HER (Supplementary Fig. 20 and Fig. 5a). The reaction order of 1.52 for the La 2 Sr 2 PtO 7+δ catalyst is close to the theoretical value of 2 and similar with the reaction orders of the previously HSBCCs 54-56 . In addition, referring to previous study, in situ analyzing the hydrogen adsorption and desorption kinetics on catalysts can support the occurrence of hydrogen spillover 57 . To examine the hydrogen desorption kinetics, the operando CV investigations were implemented and hydrogen desorption peaks during CV scanning in the double layer region were monitored 21,58 . For comparison, non-hydrogen-spillover metallic Pt black and wellknown Pt/WO 3 HSBCC samples were also studied. The CV curves of Pt black, Pt/WO 3 and La 2 Sr 2 PtO 7+δ catalysts show the hydrogen desorption peak shift depending on the scan rate ( Fig. 5b and Supplementary Fig. 21). Thus, it is rational to quantify their hydrogen desorption kinetics via plotting hydrogen desorption peak position vs. scan rate and comparing the fitted slopes. As shown in Fig. 5c, the slope follows the order of Pt black > Pt/WO 3 > La 2 Sr 2 PtO 7+δ . The significantly reduced slope for La 2 Sr 2 PtO 7+δ suggests its accelerated hydrogen desorption kinetics. It was reported that the hydrogen desorption kinetics for metal-support electrocatalysts could be effectively accelerated by hydrogen spillover effect 21,57 . Therefore, the fast kinetics of the hydrogen desorption for La 2 Sr 2 PtO 7+δ could be originated from the efficient hydrogen spillover. The operando electrochemical impedance spectroscopy (EIS) investigations were further carried out on Pt black, Pt/WO 3 and La 2 Sr 2 PtO 7+δ catalysts at different overpotentials to investigate the hydrogen adsorption kinetics. The recorded Nyquist plots were simulated by a double-parallel equivalent circuit model (Fig. 5d, Supplementary Fig. 22 and Table 4). Following a previous recognition, the second parallel component R 2 (R 2 represents the hydrogen adsorption resistance) can reflect the hydrogen adsorption behavior on catalyst surface 24,57 . In view of the potential-dependent R 2 for all catalysts, it is reasonable to quantify their hydrogen adsorption kinetics via plotting logR 2 vs. overpotential and calculating the EIS-derived Tafel slopes by virtue of the Ohm's law 57,59 . As displayed in Fig. 5e, the decreased slope for La 2 Sr 2 PtO 7+δ indicates an accelerated hydrogen adsorption kinetics, which may be associated with the promoted hydrogen spillover 57 . Furthermore, H/D kinetic isotope effects (KIEs) can reflect the hydrogen or proton transfer kinetic information of chemical reactions, and the presence of KIEs (KIEs > 1.5) is considered as evidence that proton or hydrogen transfer is involved to affect the reaction rate [60][61][62][63] (Fig. 5e). The KIEs value of La 2 Sr 2 PtO 7+δ suggests that the hydrogen or proton transfer involved in HER process is one possible step affecting the reaction rate.
Overall, the above experiments collectively verify the possible occurrence of hydrogen spillover in La 2 Sr 2 PtO 7+δ . Currently, it's very difficult to directly observe the electrocatalytic hydrogen spillover phenomenon, but we expect that in the future it could be resolved with the development of more advanced methodologies and technologies, such as transient imaging technology with super-high spatial resolution and nano-sized three-electrode electrochemical system.

DFT calculations.
To gain atomic-scale insight into the origin of the exceptional intrinsic activity of La 2 Sr 2 PtO 7+δ for HER in acid, density functional theory (DFT) calculations were carried out. Based on prior structural data and the analysis of HRTEM and SAED, (001) surface slab models with different terminations of La 2 Sr 2 PtO 7+δ and the optimized structures are shown in Fig. 6a and Supplementary Fig. 23. Generally, the acidic HER process involves a three-state diagram with an initial proton (H + ), an intermediate adsorbed H* and a final H 2 state, and the hydrogen adsorption Gibbs free energy (ΔG H* ) is taken as a commonlyaccepted descriptor for accessing the intrinsic activity of electrocatalysts toward acid HER 64,65 . According to the Sabatier principle, the thermo-neutral active sites with an optimal | ΔG H* | value close to zero can facilely promote the adsorption and desorption process during HER 25,66 . Figure 6b shows the free energy diagram of La 2 Sr 2 PtO 7+δ on all the possible sites at the terminations of Pt-O-La, La-O and Sr-O along with Pt (111) metal as a reference. It is found that the H adsorption on the sites of La-O and Sr-O terminations is all too strong, which is not beneficial HER. On the Pt-O-La termination, the ΔG H* values on O and Pt sites are −0.61 and 1.96 eV, respectively, indicating that the H adsorption is either too strong or too weak. Impressively, the calculated G H* for the unique La-Pt bridge site is 0.11 eV, an optimal value close to a thermo-neutral state even exceeding that (−0.16 eV) for the state-of-art Pt (111) surface, suggesting the hydrogen adsorption is neither too strong nor too weak. Thus, the Pt-O-La termination of La 2 Sr 2 PtO 7+δ with three different H adsorption sites is chosen for studying the HER process.
As reported before, hydrogen spillover has been an efficient strategy to boost the intrinsic HER activity of the binary-component electrocatalysts [21][22][23][24] Fig. 6 DFT calculations. a (001) surface slab models with different terminations of La 2 Sr 2 PtO 7+δ and the optimized structures. The yellow, red, gray, light and dark green balls represent H, O, Pt, La and Sr, respectively. b Gibbs free energy diagram for hydrogen adsorption at different catalytic sites on La 2 Sr 2 PtO 7+δ and Pt (111). c The kinetic energy barrier of hydrogen migration/spillover process for La 2 Sr 2 PtO 7+δ . (IS the initial state, TS the transition state, FS the final state). d The free energy profiles of Heyrovsky step in HER on La 2 Sr 2 PtO 7+δ (with and without hydrogen migration/spillover). e Schematic illustration of catalysis mechanism for acidic HER via atomic-scale hydrogen spillover on the La 2 Sr 2 PtO 7+δ oxide. NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-28843-2 ARTICLE the surface with ΔG H-positive 23,24 . When a single-component catalyst contains more than one kind of catalytic site, the hydrogen spillover effect between different sites may also happen and can affect the overall catalytic activity. With regard to single-component La 2 Sr 2 PtO 7+δ catalyst, the H + preferentially adsorbs at O site among three possible sites (i.e., O site, La-Pt bridge site and Pt site) due to the negative ΔG H* value of −0.61 eV, suggesting significant proton trapping at O site which functions as hydrogen-enriched "component". Conversely, the H exhibits weak adsorption on the Pt site with ΔG H* value of 1.96 eV, indicating Pt site with high oxidation state resembles the hydrogen-poor "component" and is advantageous for H 2 desorption. More importantly, the thermoneutral La-Pt bridge site can serve as the mediator for favorable hydrogen spillover, similar to the interface site in the conventional HSBCCs. In view of the steadily weakened H adsorption, a feasible channel for the hydrogen spillover from O site → La-Pt bridge site → Pt site may be formed on the La 2 Sr 2 PtO 7+δ catalyst. Noticeably, the interface-free feature and short reaction distance within the crystal lattice on single-phase La 2 Sr 2 PtO 7+δ catalyst is beneficial for minimizing the kinetic barrier for hydrogen spillover 52,67 , somewhat analogous to the reported metal alloy system with low hydrogen spillover barrier [68][69][70][71] . The calculation shows that the hydrogen spillover step has a low energy barrier value of 0.22 eV (Fig. 6c and Supplementary Fig. 24), indicating that the H migration from O site to Pt site is a kinetically favorable process. Then, we calculated the energy barriers of Heyrovsky step with and without hydrogen spillover. As seen from Fig. 6d, the energy barrier on Pt site after H migration is 0.17 eV, which is lower than that (0.26 eV) on O site without H migration, demonstrating the superiority of hydrogen spillover/migration of La 2 Sr 2 PtO 7+δ for HER catalysis. It should be also noted that the theoretical overpotential (0.22 V) of La 2 Sr 2 PtO 7+δ for the whole HER process involving the hydrogen spillover is lower than that (0.69 V) for the conventional HER mechanism on Pt surface 72 . Based on these DFT results, a possible HER catalytic mechanism on La 2 Sr 2 PtO 7+δ in acidic media via the synergy of multi-function catalytic sites was proposed, as schematically illustrated in Fig. 6e. Namely, the O site serves as the proton enrichment, the thermo-neutral La-Pt bridge site serves as the mediator for favorable hydrogen spillover/ migration from O site to Pt site, and Pt site functions the final H 2 desorption. In order to further confirm the active sites of La 2 Sr 2 PtO 7+δ for HER, poisoning tests was performed by adding thiocyanate (SCN − ) and tetramethylammonium cation (TMA + ) ions into acidic solutions ( Supplementary Fig. 25) 73,74 . The La 2 Sr 2 PtO 7+δ exhibits evident performance decay after adding SCNor TMA + into H 2 SO 4 solution, indicating that the Pt and O ions are both active sites of La 2 Sr 2 PtO 7+δ for HER. However, Pt/ WO 3 shows evident activity loss after adding SCNand has nearly no activity loss after adding TMA + , implying that the metallic Pt is the sole active site. Such result demonstrates the direct actives of Pt and O ions for HER in La 2 Sr 2 PtO 7+δ , which is initiated by the hydrogen spillover/migration and accordingly contributes to the high performance.

Discussion
In summary, we have successfully synthesized a single-phase complex oxide La 2 Sr 2 PtO 7+δ with exceptional HER performance in acid medium via a facile and scalable solid-state reaction method. The La 2 Sr 2 PtO 7+δ displays an excellent HER activity with an ultralow overpotential of 13 mV at a current density of 10 mA cm −2 and a small Tafel slope of 22 mV dec −1 in 0.5 M H 2 SO 4 solution, surpassing state-of-the-art Pt-based catalysts and HSBCC ever reported. Besides, significant intrinsic activity and durability enhancement were observed for La 2 Sr 2 PtO 7+δ relative to the commercial Pt black. By coupling DFT simulations and comprehensive experiments, the high HER catalytic activity of La 2 Sr 2 PtO 7+δ in acid possibly results from an unusual atomicscale hydrogen spillover effect between multiple catalytic sites, whereby O site captures proton, H facilely diffuses from O site to Pt site with thermoneutral La-Pt bridge site serving as the mediator, and eventually as-formed H 2 favorably releases on Pt site. Our proof-of-concept investigations not only provides the atomic-level insight into the hydrogen spillover within La 2 Sr 2 PtO 7+δ for acidic HER, but also open a new avenue for the design of advanced electrocatalysts via constructing multifunctional catalytic sites.

Methods
Catalyst synthesis. La 2 Sr 2 PtO 7+δ catalyst was synthesized via the traditional solid-phase reaction method. Firstly, stoichiometric amounts of La 2 O 3 , SrCO 3 , and Pt were weighed and mixed in ethanol and water under the rotation speed of 400 rpm for 1 h through the high-energy ball-milling (Planetary Mono Mill, Pulverisette 6, Fritsch). Then the homogeneously dispersed mixture was dried and finally calcined at 1100°C in air for 10 h to obtain the resultant catalyst powders. Pt/WO 3 complex catalyst was prepared via the high-energy ball-milling method. In detail, WO 3 and Pt were weighed at mass ratio of 75:25 and mixed in ethanol and water under the rotation speed of 400 rpm for 1 h through the high-energy ballmilling. The homogeneously dispersed mixture was finally dried to obtain the resultant catalyst powders.
Characterizations. XRD patterns were measured using a Rigaku Smartlab diffractometer operating at 40 kV with filtered Cu Kα radiation. The Rietveld refinements were revealed using DIFFRAC plus Topas 4.2 software. SEM images were recorded through a scanning electron microscope equipped with the scanning electron microanalyzer (Hitachi S-4800). The HRTEM images were obtained utilizing the electron microscope (FEI Tecnai G2 F20) operating at 200 kV. STEM image and elemental mapping images were obtained using Tecnai F20 SuperTwin operating at 200 kV. Nitrogen adsorption-desorption isotherms were recorded on BELSORP II. Pt4f spectra were acquired on XPS (Perkin Elmer PHI 1600 ECSA system). X-ray absorption near-edge structure (XANES) and the EXAFS spectra were determined at the BL 07A beamline of the National Synchrotron Radiation Research Center in Taiwan. All samples were pretreated via cutting pellets in an ultrahigh vacuum chamber to obtain a clean surface. Hydrogen temperatureprogrammed desorption (H 2 -TPD) experiments were performed on Chembet Pulsar (Quantachrome Instruments, USA). In total, 50 mg of sample was pretreated at 200°C for 3 h in hydrogen atmosphere and then was cleaned with argon gas flow at 50°C for 30 min to remove weakly adsorbed H 2 . TPD process was performed by heating the sample from 50 to 600°C at a ramp rate of 2.5°C min −1 under argon atmosphere.
Electrochemical measurements. HER measurements in acid media were conducted in a standard three-electrode electrochemical cell (Pine Research Instrumentation) in an RDE configuration using a CHI 760E electrochemistry workstation. Catalysts cast on RDE (5 mm in diameter), graphite rod, and Ag|AgCl (3.5 M KCl) were used as the working electrode, counter electrode, and reference electrode, respectively. Working electrodes for HER measurements were prepared by a controlled drop-casting method, in accordance with the previous works 2, 64 . The mass loading of oxide catalysts and Pt black on the RDE is~0.232 and 0.058 mg cm −2 , respectively. Linear sweep voltammetry was recorded at 5 mV s −1 at the rotation of 2400 rpm in Ar or H 2 -saturated 0.5 M H 2 SO 4 . Tafel slopes were determined by plotting the overpotential vs. the logarithm of current density (log | j | ). CV curves involving in the H upd adsorption/desorption peak were obtained in N 2 -saturated 0.5 M H 2 SO 4 solution with potential window between 0.05 V and 1.1 V vs. RHE and a sweep rate of 100 mV s −1 . The ECSA was derived from the H upd desorption peak (0.03-0.35 V) area normalized by the total mass of the Pt element. ADT of catalysts was conducted through continuous potential cycling ranged from 0 to −0.4 V vs. Ag|AgCl for 1000 cycles at a scan rate of 100 mV s −1 . The chronopotentiometry tests were performed at a constant cathodic current density of 10 mA cm −2 to explore the durability of the electrocatalysts. EIS measurements were carried out on the Bio-Logic SP-300 workstation at RT in the frequency range of 100 kHz-0.1 Hz at various HER overpotentials.
Computational methods. All first-principle calculations were performed using DFT implemented in the Vienna Ab initio Simulation Package. The projector augmented wave pseudo potentials with the Perdew-Burke-Enrnzerhof functional were used. The cutoff of kinetic energy was set as 500 eV. La 2 Sr 2 PtO 7 bulk with R-3 space group was built for lattice structure optimization. Monkhorst-Pack 7 × 7 × 2 k-point mesh was used during the structure relaxation. (001) surface slab models with different terminations were cleaved from the optimized bulk lattice to investigate H adsorption on surface. A 15Å-thick vacuum layer was added to the surface in the z direction. In total, 2 × 2 supercells were built to simulate HER process on (001) surface with La-O-Pt termination. Half layers on the bottom were fixed to mimic bulk arrangements, while other layers were fully relaxed to represent surface features. The first Brillouin zone of the slab model was samples with a 5 × 5 × 1 k-point grid. The first Brillouin zone of the slab model and supercell was sampled with a 5 × 5 × 1 and a 3 × 3 × 1 k-point grid, respectively. Transition states were determined using climbing image nudge elastic band method with 8 images. All calculations were performed in a spin-polarized fashion. The force and energy convergence criteria were set to 0.02 eV Å −1 and 10 −5 e Å, respectively. Zero-point energy and entropy corrections were introduced for Gibbs energy calculations.
Reporting summary. Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Data availability
The data that support the findings of this study are available in the Source Data. Source data are provided with this paper.