Synthesis and characterization of MESA nanocubes and HESA NCsA typical synthesis process for PBA-based cyanogel with HESA and cubic hollow structure consists of two steps: (1) synthesis of cyanogel nanocubes, and (2) subsequent spatially selective etching with ammonia solution, as illustrated schematically in Fig. 1b. First, five d- and p-block metal cations: Co(II), Cu(II), Zn(II), Cd(II), and In(III) from metal chlorides or nitrates, were selected as metal sources and mixed with K3[Fe(CN)6] to fabricate MESA nanocubes by a simple room temperature ligand exchange/substitution method. The ligand exchange reactions between metal ions and Fe-cyano molecules resulted in the generation of new covalent bonds and linear chains of M–N≡C–Fe–C≡N–M (M = Co, Cu, Zn, Cd, In) as constitutional unit. After 3D polymerization, the lattice was composed of single metal atoms coordinated with six C or N atoms, in which medium-entropy single atoms with a fcc lattice were anchored and stabilized in the PBA lattice. In the synthesis process, the molar ratio between K3[Fe(CN)6] and all other metal salts was fixed at 1:2 to ensure the formation of cyanogel with a cubic shape. In the second step, the interior compositions could be selectively etched by breaking the bonds between M and –N≡C–Fe(III) and thus generating lattice vacancies of Mn+ (VM) and [Fe(III)(CN)6]3− enclosed by VM in a controlled manner34,35, resulting in the formation of cyanogel NCs with HESA and extraction of K+ ions. In addition, a redox reaction may also be involved in the etching process for the reduction of Fe(III) to Fe(II)34,35.The morphology of the MESA nanocubes were characterized by scanning electron microscopy (SEM) and bright-field transmission electron microscopy (BF-TEM) imaging, as shown in Supplementary Fig. 1a and Fig. 2a. The as-synthesized MESA particles exhibited a well-defined cubic shape with a slightly concave surface and particle sizes from 50 to 150 nm. The high-resolution TEM (HRTEM) garnered on the corner of a MESA nanocube shows a profile around 90° (Supplementary Fig. 2d), indicating the dominance of {100} facet exposed on the surface of the nanocube with a cubic crystal structure. It is noteworthy that the zoomed gel nanocubes always went through slight morphology and crystallinity changes under intensive electron beam due to their limited beam stability. Powder X-ray diffraction (PXRD) was performed to investigate the crystal structure and lattice parameters of MESA nanocubes. The PXRD pattern exhibits a set of characteristic diffraction peaks of PBA lattice without excessive peaks for other phases, demonstrating a single-phase PBA structure of the product (Fig. 2k)15. The slight shift of diffraction peaks to higher degree relative to FeCo PBA ( JCPDS: #46-0907) implies that the introduction of multi-metals would result in a certain degree of lattice expansion23. In addition, the composition of MESA nanocubes were mensurated by inductively-coupled plasma mass spectrometry (ICP-MS) and energy dispersive X-ray (EDX) spectrometry, as shown in Supplementary Table 1 and Supplementary Fig. 2e, demonstrating a Fe:Co:Cu:Zn:Cd:In atomic ratio of 39:6:9:10:26:10 and thus a mixed configurational entropy (\(\Delta {S}_{{{{{\rm{mix}}}}}}^{{{{{\rm{conf}}}}}}\)) of 1.46R26. The and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and corresponding element mapping analysis of an individual nanocube demonstrate the homogeneous distribution of C, N, and Fe throughout the nanocube (Fig. 2b, i), in line with the presence of –[M–N≡C–Fe–C≡N]– constitutional unit composing the fcc framework of the lattice. Remarkably, heterogeneous and localized distribution of Co, Cu, Cd, and In were observed. Specifically, the Cd and In are mainly confined in the interior region of the nanocube, while Co and Cu are preferentially located in the exterior and surface region of the nanocube (Supplementary Fig. 3). The inhomogeneous elemental distributions were also confirmed by EDX line scans (Fig. 2d), which is crucial for subsequent spatially selective etching with changes over morphology, composition, and configurational entropy of the final product. According to previous studies, the insertion of K+ ions leads to the coexistence of Fe(III) and Fe(II) in the lattice, resulting in differential electron donating ability of the N atom in the –N≡C–Fe and thus distinct bond strength between M and –N≡C–Fe19,20,27. To confirm the localization of Fe(III) and Fe(II) lattice points, we conducted Fe-L-edge electron energy-loss spectroscopy (EELS) across the nanocube. As shown in Fig. 2c, the intensive EELS profiles apparently reveal a core@shell distribution of Fe(III)@Fe(II), indicating different reactivity and etching resistivity across the nanocube. In addition to the K+ ions induced charge, the presence of Fe(II) may also be ascribed to involvement of In(III) which can disrupt the charge balance of the system. The crystalline characteristics of the nanocubes were also investigated by selected-area electron diffraction (SAED) pattern collected using cryo-TEM at 80 kV to increase their beam stability and thus maintain their crystallinity. The hexagonal symmetry pattern was observed with clear diffraction spots of (220) and (02-2) lattice planes of PBA (Fig. 2a), which can be ascribed to [111] zone axis diffraction of fcc framework.Fig. 2: Synthesis and characterizations of MESA nanocubes and HESA NCs.a BF-TEM image and SAED pattern of MESA nanocubes. Scale bar is 2 1/nm. b HAADF-STEM of an individual MESA nanocube. c Fe-L-edge EELS throughout the MESA nanocube. d EDS line scans of an individual MESA nanocube. e BF-TEM image and SAED pattern of HESA NCs. Scale bar is 2 1/nm. f HAADF-STEM of an individual HESA NC. g zoom-in HAADF-STEM image and localized Fe-L-edge EELS on the corner and wall of the HESA NC, respectively. h EDS line scans of an individual HESA NC. EDX elemental maps of an individual MESA nanocube (i) and HESA NC (j). Scale bars are 50 nm for all images. k PXRD patterns of MESA nanocubes (black line) and HESA NCs (red line). Red dashed lines: FeCo PBA (JCPDS: #46-0907).The as-obtained MESA nanocubes serve as a precursor to HESA NCs via a spatially selective etching process. Supplementary Fig. 1b and Fig. 2e show the SEM and BF-TEM images of the HESA NCs, revealing a cubic shape as well as high hollowness and enhanced surface flatness of the particles with a size range from 50 to 150 nm. The rectangular profile observed using HRTEM at the corner of the NC demonstrate a well-defined cubic hollow structure with the preservation of {100} facet on the surface and a wall thickness around 12 nm (Supplementary Fig. 2i). PXRD pattern of HESA NCs shows diminished crystallinity with reduced number of characteristic diffraction peaks, indicating the increased symmetry of the lattice (Fig. 2k). The diffraction peaks shifted to higher degree relative to those of MESA nanocubes, suggesting a lattice contraction after etching due to the extraction of K+ ions from the nanocubes (Supplementary Table 1)24,25. The composition of HESA NCs was also confirmed ICP-MS and EDX spectrometry, (Supplementary Table 1 and Supplementary Fig. 2j), indicating a Fe:Co:Cu:Zn:Cd:In atomic ratio of 29:17:15:10:18:11 and thus a \(\Delta {S}_{{{{{\rm{mix}}}}}}^{{{{{\rm{conf}}}}}}\) of 1.59R. The increase of compositional equality and thus \(\Delta {S}_{{{{{\rm{mix}}}}}}^{{{{{\rm{conf}}}}}}\) from 1.46R to 1.59R resulted in the change from MESAL (∆S < 1.5R) to HESAL (∆S > 1.5R) with an increased thermodynamic stability (Supplementary Fig. 4). The presence and homogeneous distribution of all elements were also verified by HAADF-STEM image and corresponding element mapping and line scan analysis of an individual HESA NC (Fig. 2h, j). Fe-L-EELS profile collected on the wall of the NC indicates the dominance of Fe(II) lattice points, as shown in Fig. 2g. Figure 2e shows the SAED pattern of the HESA NCs along [111] zone axis, demonstrating a lower crystallinity with a similar hexagonal symmetry pattern including diffraction spots of (220) and (02-2) lattice planes of PBA but additional diffraction spots and slight amorphous characteristics.The local coordination environment and chemical state of different metal elements before and after the etching process were studied by X-ray photoelectron spectroscopy (XPS) and transmission X-ray absorption fine structure (XAFS). The core-level Fe 2p spectra validate that there are corresponding divalent and trivalent species of Fe in both MESA nanocubes and HESA NCs (Fig. 3a)38. Specifically, the fitted spectra of the Fe 2p region exhibit two sets of peak doublets at binding energies of 708.5 (Fe 2p3/2)/721.6 (Fe 2p1/2) eV and 710.0 (Fe 2p3/2)/723.5 (Fe 2p1/2) eV for Fe2+ and Fe3+, respectively. Notably, the decreased peak area ratio of Fe3+/Fe2+ after etching indicates the reduced amount of Fe3+, which is in agreement with Fe-L-EELS analysis. X-ray absorption near edge structure (XANES) spectra at the Fe K-edge for MESA nanocubes (before etching) and HESA NCs (after etching) are shown in Fig. 3b, with Fe foil, FeO and Fe2O3 as references. The curve tendencies of gel samples demonstrate that these materials feature different near-edge absorptions from those of Fe foil, FeO and Fe2O3. The XANES profiles before and after etching are nearly identical, indicating that the etching process barely changed the coordination environment adopted by Fe moieties in gel samples. Remarkably, the absorption edge position of Fe has apparently shifted to lower energy level after etching, which implies that the etching process resulted in the decrease of oxidation state caused by the preferential removal of Fe(III) from the lattice, in line with the Fe-L-EELS result41. XANES spectra of other doped metals indicate similar valence states originating from their metal precursors (Supplementary Fig. 5). The Fe, Co, Cu, Zn, and In K-edge k2-weighted Fourier transformed (FT) extended X-ray absorption fine structure (EXAFS) spectra for gel samples before and after etching demonstrates evident Fe–C, Co–N, Cu–N, Zn–N, Cd–N, and In–N (Fig. 3c and Supplementary Fig. 6) scattering path, further signifying that Fe, Co, Cu, Zn, Cd, and In are successfully introduced and atomically dispersed into the PBA framework38,41. The Fe, Co, Cu, Zn, Cd, and In K-edge XANES and EXAFS spectra indicate that both MESA nanocubes and HESA NCs feature similar near-edge absorptions, which indicates isolated metal atoms states coordinated with C or N in both MESA nanocubes and HESA NCs.Fig. 3: Structural analysis and formation mechanistic study of HESA NCs.a XPS spectra of Fe 2p of gel samples before and after the etching process. b Fe K-edge XANES spectra of Fe foil, FeO, Fe2O3, and gel samples before and after the etching process. c FT k2-weighted χ(k) function of EXAFS spectra for the Fe K-edge. d Atomic percentage of metal elements and corresponding ∆S after etching MESA nanocubes by ammonia with concentration of 0 (E0), 2 (E1), 5 (E2), 10 (E3), 15 (E4), and 20 (E5) mmol. ATR-FTIR localized spectra (e) and PXRD patterns (f) of the etched products. g Schematic illustration of the lattice evolution during the etching process with localized dissolution of [Fe(III)(CN)6]3− and increasing ∆S via the ligand substitution mechanism.The XPS was also employed to unravel the surface chemical compositions and valence state of different elements of cyanogels before and after etching. (Supplementary Fig. 7). The Co 2p spectra verify the coexistence of divalent and trivalent species of Co, while the Cu 2p spectra demonstrate the presence of monovalent and divalent Cu species in both MESA nanocubes and HESA NCs40,42. In addition, the Zn 2p, Cd 3d, and In 3d spectra validate that the Zn, Cd, In cations mainly exist in the form of Zn2+, Cd2+, and In+, respectively38,40,43. Specifically, the fitted Co 2p spectra show two sets of peak doublets at binding energies of 782.1 (Co 2p3/2)/797.4 (Co 2p1/2) eV and 784.2 (Co 2p3/2)/799.4 (Co 2p1/2) eV for Co3+ and Co2+, respectively, while no substantial changes of peak area ratio of Co3+/Co2+ were observed after etching (Supplementary Fig. 7b). The fitted spectra of the Cu 2p3/2 region were found to display peaks at 931.6 and 934.7 eV that can be assigned to Cu+ and Cu2+, respectively, together with their shake-up satellites (Supplementary Fig. 7c)38,40,42. The increase of Cu+ after etching signifies an internal redox reaction from Cu2+ to Cu+ occurred during the etching process. A major Zn 2p peak doublet was detected at 1022.0 eV (Zn 2p3/2) and 1045.1 eV (Zn 2p1/2), revealing the existence of Zn2+ before and after etching (Supplementary Fig. 7d). A single Cd 3d peak doublet at 404.8 eV (Cd 3d5/2) and 411.5 eV (Cd 3d3/2) (Supplementary Fig. 7e). These binding energies are typical of Cd in its preferred valence state of 2+40. The In 3d eak located at 445.5 eV (In 2p5/2)/453.1 (In 2p3/2) eV and 446.8 eV (In 2p5/2)/454.4 (In 2p3/2) eV can be assigned to In+ and In3+, respectively, verifying the dominance of In+ in both gel samples and suggesting an internal redox reaction from In3+ to In+ in the formation of MESA nanocubes (Supplementary Fig. 7f). The influence of etching on the surface valence band of the cyanogels was also studied, revealing the shift of the valence band away from the Fermi level (EF) after the etching process (Supplementary Fig. 8). The chemical structure change of the cyanogels caused by etching was also investigated by Fourier transform infrared (FTIR) spectroscopy analysis. As shown in Supplementary Fig. 9a, the sharp peak at 1608 cm−1 and the broadband at ~3400 cm−1 in both samples originate from the O–H bending vibration and H–O–H stretching vibration, respectively, of water molecules existing in the samples44. The typical C≡N group stretching vibration was detected from the adsorption bands in the range of 2000–2200 cm−1 45. Specifically, the v (C≡N) peak at 2155 cm−1 (I) for Fe(III)–C≡N–M exists in the MESA nanocubes but disappears in the HESA NCs, whereas the v (C≡N) peak at 2100 cm−1 (II) for Fe(II)–C≡N–M is present in both samples (Supplementary Fig. 9b)45. In addition, an additional v (N-H) peak at 1414 cm−1 of NH4+ was detected after etching34. These FTIR analysis can be summarized into three major changes of chemical structure during etching: (1) the water molecules adsorbed in the lattice of the cyanogels were partially removed by ammonia solution; (2) the linear chains of M–N≡C–Fe(III)–C≡N–M were substantially eliminated, which is attributed to the preferential bond breaking between M and –N≡C–Fe(III) in the internal region of the nanocubes; (3) occurrence of a redox reaction during the ammonia treatment for the reduction of Fe(III) to Fe(II) and generation of NH4+ 34,35.To further evaluate the versatility of the synthetic strategy of HESA NCs shown in Fig. 1b, we designed more control experiments and found that the hollowness of NCs can be governed by simply altering the feed ratio of metal precursors in the reaction mixture (see Supplementary Table 2 for details), resulting in NCs with wall-thickness of 40, 35, 12, and 6 nm after etching of gel nanocubes with different elemental compositions (Supplementary Fig. 10). In addition, it was found that variation of feed ratio of metal precursors (see Supplementary Table 3 for details) could also lead to gel nanocubes with lower surface flatness, which could serve as a precursor to NCs with enlarged cavities on the walls (Supplementary Fig. 11). The tunable hollowness and openness of HESA NCs with well-controlled parameters validate the versatility of the synthetic strategy to produce gel nanostructures with controlled parameters. Furthermore, to demonstrate the potential synthetic scalability, we attempted our synthesis up to 100 times just by proportionally increasing the volumes of reaction solutions. Noticeably, the stirring rate was increased from 400 rpm to 600 rpm to promote mass transfer and thus uniform dispersal of the metal precursors to enable homogeneous nucleation via the ligand exchange cyano reactions. Supplementary Fig. 12 shows SEM and BF-TEM images of the HESA NCs synthesized using the scale-up protocols in which the NCs feature a well-defined cubic shape and hollowness with a size in the range of 50–150 nm, consistent with that of the product from a standard synthesis. According to the ICP-MS analysis and EDX spectra shown in Supplementary Fig. 13 and Table 3, the HESA NCs delivered a Fe:Co:Cu:Zn:Cd:In atomic ratio of 27:16:17:9:19:12 close to the standard product. These results confirm the scalability of the synthetic strategy for HESA NCs.Mechanistic understanding of the spatially selective etchingTo fundamentally understand the formation mechanism of the HESA NCs via spatially selective etching, systematic characterizations of intermediates at different reaction stages were performed. The functional ions derived from ammonia solution for etching were firstly figured out by replacing ammonia solution with the same amount of KOH or NH4Cl as etchants, respectively, while other experimental parameters were kept the same. As shown in Supplementary Fig. 14, the barely changed products after etched by KOH eliminates the possibility of OH− ions as the etchant, while the obvious hollowness of NCs etched by NH4Cl demonstrates the key role played by NH3 or NH4+ ions during the etching process. To trace and quantify the changes of MESA nanocubes in terms of morphology, composition, chemical structure, and lattice parameters during etching, we designed a series of ex-situ experiments to simulate the time-dependent in-situ characterizations. Since the etching-induced changes occurred instantly upon contacted with ammonia, typically within a few seconds, it was almost impossible to conduct ex-situ time-dependent experiments to collect corresponding intermediates. As such, we attempted to collect the “pseudo time-dependent products” by using ammonia solutions with a range of reduced concentrations (Supplementary Table 4). Using this method, we have successfully captured intermediate samples (denoted as E0 to E5) during the fast etching process. Supplementary Fig. 15a–f shows the structure and morphology of the intermediate products collected at different reaction stages. As shown in Supplementary Fig. 15b, initial reaction with ammonia resulted in the generation of a yolk@shell structure likely due to the contact of outmost lattice points of Mn+ coordinated with –N≡C–Fe(III) in the core region with ammonia. As the reaction continues, the internal region of the nanocube became visibly void, generating a double shell structure (Supplementary Fig. 15c). This special structure might be caused by the redox reaction between terminal [Fe(III)(CN)6] and ammonia, resulting in newly generated M–N≡C–Fe(II) with a higher etching resistibility inside the nanocube34,35. As the etching proceeded, the inner shell was completely removed while the outer shell with a larger thickness and high internal surface roughness was observed (Supplementary Fig. 15d), which was considered as the involvement of dissolution and recrystallization processes of Fe(II)/Fe(III) lattice points33. Finally, a hollow nanobox with flat internal and external surfaces was obtained with increased extent of etching (Supplementary Fig. 15e). Further etching of the nanobox increased the porosity of the walls, producing a nanocage with observable cavities on the walls (Supplementary Fig. 15f). This result indicates the existence of M–N≡C–Fe(III) on the walls of the nanobox which could be etched away after sufficient etching time or under a robust etching condition. The summarized schematic illustration of the spatially selective etching process of intermediates are shown in Supplementary Fig. 15g.The compositional evolution during etching was evaluated by ICP-MS and EDX spectra of intermediate samples. As shown in Supplementary Fig. 16, different variation tendencies of elements can be distinguished when normalized to the amount of Fe in the samples. For example, increased amount of Cd is exhibited from E0 to E3 followed by a decrease from E3 to E5, while a continuous increase of Co is shown from E0 to E5. The quantified change of atomic percentage of each element obtained by ICP-MS analysis is shown in Fig. 3d, demonstrating the substantial decrease in Fe and Cd from 39% and 26% to 29% and 18%, respectively. Meanwhile, an apparent increase in Co from 6% to 17% was also observed. This difference can be attributed to the heterogeneous distribution of Fe, Co, and Cd in the nanocubes, in which Fe(III) and Cd are confined in the inner core while Co is mainly located in the outer shell. As such, the spatially selective etching process preferentially eliminated a greater number of elements that distributed in the interior region of the nanocubes with a higher atomic percentage, resulting in nanocages with improved compositional equality and thus elevated \(\Delta {S}_{{{{{\rm{mix}}}}}}^{{{{{\rm{conf}}}}}}\). In addition to the compositional evolution, the chemical structure info was further studied by FTIR (Supplementary Fig. 17). As shown in Fig. 3e, the peak at 2155 cm−1 disappeared from E0 to E1, indicating the fast chains scission of M–N≡C–Fe(III)–C≡N–M and transformation of lattice points from Fe(III) to Fe(II) via redox reaction upon etching. The emergence of v (N-H) peak for NH4+ at 1414 cm−1 from E0 to E1 (Supplementary Fig. 17) further verify the occurrence of the redox reaction. Noticeably, there is a shift of the stretching v (C≡N) band at 2100 cm−1 to a lower wavenumber of 2089 cm−1. During the etching process, simultaneous dissolution and recrystallization reactions inside the nanocubes were anticipated to prompt dynamic changes of compositions on the inner surface of the shell, resulting in the change of metal ions linked to the C≡N bonds. The stretching v (C≡N) band gradually shifted back to the original location from E1 to E5, suggesting the discontinued variation of metal ions linked to the C≡N bonds and the termination of the etching process.To further investigate the crystal structure evolution throughout the etching process, PXRD patterns of intermediate samples were collected (Fig. 3f). From E0 to E3, the diffraction peaks of (111) and (200) were shifted to higher degree, indicating lattice contraction and internal lattice strain due to the extraction of K+ ions39,40. In addition, the (220) peak split into two peaks, together with new peaks present around 20° and 30°, indicating reduced lattice symmetry in the initial stage of etching. In the subsequent etching process as represented from E3 to E5, the peak merging of (220) and the gradual disappearance of characteristic peaks demonstrate the increased symmetry of the lattice39,40,45. On the basis of the above observations, we propose an etching mechanism based on a ligand substitution reaction and a scheme of lattice evolution pathway towards the HESA NCs. As illustrated in Supplementary Fig. 4, the presence of NH3 could act as coordination ligands with M6/n[Fe(III)(CN)6]2 for the generation of [M(NH3)6]n+ and evacuation of gel nanocubes34,35. The atomic level illustration of the etching mechanism is shown in Fig. 3g, in which the generation of lattice vacancies can be divided into three major steps: (1) rapid dissolution of Mn+ derived from M–N≡C–Fe(III) via ligand exchange; (2) release of K+ ions from the lattice; (3) generation of [Fe(III)(CN)6]3− vacancies in the center of the lattice and occurrence of redox reaction producing Fe(II).Catalytic performance and mechanism insights into NO3RR over Fe-HESA NCsIt is recently recognized that the high-entropy-based electrocatalysts may be advantageous in multistep reactions requiring multiple adsorption sides due to the large combination of elements and other entropy-induced effects benefiting the electron transfer process46. As such, we chose NO3RR as a model reaction and evaluated performance of HESA NCs by loading them onto carbon black (Vulcan XC-72R) (Supplementary Fig. 18). As a comparison, Fe-Co gel NCs with a size of ~100 nm were synthesized using the same method except for the involvement of only Co precursor for ligand exchange (Supplementary Figs. 19–21). Since both gel catalysts are composed of Fe-based PBA frameworks, we rename them as Fe-HESA NCs and Fe-Co NCs, respectively, to emphasize their similarity and difference hereinafter. The polarization curves for NO3RR were acquired in a gas-tight H-cell at room temperature in a 0.5 M Na2SO4 solution containing 100 mM NaNO3 (Supplementary Fig. 22). As shown in Fig. 4a, the Fe-HESA NCs exhibited an onset potential of −0.08 V versus reversible hydrogen electrode (RHE), much more positive than that of Fe-Co NCs (−0.23 V) as a widely accepted descriptor of promoted reaction kinetics. In the absence of NO3−, the barely observed current density of polarization curve reveals a substantially diminished hydrogen evolution reaction (HER) activity of Fe-HESA NCs. In contrast, Fe-HESA NCs delivered a remarkable enhancement in current density (90.4 mA cm−2 at −0.8 V) in the presence of NO3−, while the increase of current density on Fe-Co NCs was very limited (16.8 mA cm−2 at −0.8 V). The mass activity (MA) obtained by the partial current density of NH3 normalized by the mass of Fe on Fe-HESA NCs was also higher than that on Fe-Co NCs over a range of potentials (Fig. 4b). Additionally, the corresponding turnover frequency (TOF) for NO3−-to-NH3 conversion per Fe site directly indicates at least 5 times enhancement of rate on individual Fe sites of Fe-HESA NCs relative to that of Fe-Co NCs, further distinguishing their intrinsic activities towards NO3RR.Fig. 4: Electrocatalytic NO3RR performance.a Linear scan voltammetry curves of Fe-HESA NCs and Fe-Co NCs normalized to the geometric area. b MA and TOF for NH3 production at various potentials. c FE of NH3 over Fe-HESA NCs (blank pattern) and Fe-Co NCs (slash pattern) at different potentials. d YR of NH3 over Fe-HESA NCs and Fe-Co NCs at different potentials. All potentials are not iR corrected. e 1H NMR spectrum of the products generated during the electrocatalytic NO3RR over Fe-HESA NCs in 0.1 M Na15NO3 or 0.1 M Na14NO3 at −0.6 VRHE. f Quantification of NH3 via UV-Vis and NMR measurements at −0.6 V vs. RHE. g Long-term chronoamperometry test for 150 h and the cycling test (inset) at −0.6 VRHE. Error bars denote the standard deviations calculated from three independent measurements.Chronoamperometry measurements of the catalysts were performed at diverse potentials for subsequential qualification of products (Supplementary Fig. 23). The indophenol blue spectrophotometric method and the Griess test were used to quantify the produced NH3 and NO2−, respectively (Supplementary Fig. 24). Faradaic efficiency (FE) and yield rate (YR) of NH3 over the catalysts are shown in Fig. 4c, d, respectively. The Fe-HESA NCs showed high selectivity towards NH3 from the NO3RR with a FE of 93.4% and a YR of 81.4 mg h−1 mg−1 at −0.6 V versus RHE. As a main byproduct of the NO3RR on Fe-HESA NCs, the FE of NO2− was over 25% at −0.3 V versus RHE while gradually reduced with increased potentials (Supplementary Fig. 25). As a comparison, Fe-Co NCs displayed FE and YR of NH3 lower than those of Fe-HESA NCs over different potentials, indicating that the enhanced electrocatalytic performance of NO3RR is potentially originated from the HESA. Meanwhile, no H2 can be detected at considered potentials on both Fe-HESA and Fe-Co NCs (Supplementary Fig. 26), indicating the complete inhibition of HER over the catalysts. Control experiments were carried out at −0.6 V versus RHE in 0.5 M Na2SO4 solution without NaNO3. As shown in Supplementary Figs. 27a, b and 28a, b, NH3 was barely detected in the electrolyte, excluding the N contaminants from other sources including the catalyst, electrolyte, and environment. 15N isotope labeling experiments were conducted to further verify that the produced NH3 was resulting from the feeding NO3− electrolyte (Supplementary Figs. 27c, d and 28c, d). After electrolysis at −0.6 V versus RHE, triple coupling and doublet peaks corresponding to 14NH4+ and 15NH4+ were detected in the 1H NMR spectra of the electrolytes containing 14NO3− and 15NO3−, respectively (Fig. 4e), proving that the produced NH3 was derived from the NO3RR. The FE and YR were very close to those determined by colorimetric methods (Fig. 4f), while the decreased current density, FE, and YR imply a potentially negative effect of 15N on NH3 production.The outstanding NO3RR performance of Fe-HESA NCs underline the consequence of HESA. To clearly demonstrate this, we compared the NO3RR performance of Fe-HESA NCs, Fe-Co NCs, and other state-of-art Fe-based catalysts. As summarized in Supplementary Table 5, the Fe-HESA NCs delivered a superiority relative to other catalysts. This demonstrates the key role of HESA in boosting NO3RR performance, endowing Fe-HESA NCs as one of the best reported neutral NO3RR electrocatalysts (Supplementary Table 6). The stability of NO3RR performance was also evaluated on Fe-HESA NCs by conducting successive 20 electrolysis cycles at a fixed potential (Supplementary Figs. 29 and 30). As shown in Fig. 4g, the FE and YR of NH3 exhibited slight fluctuations around 90.8% and 84.1 mg h−1 mg−1, respectively, over the consecutive electrolysis cycles. After the stability test, the catalysts were detached from the glassy carbon electrode by sonication and characterized by BF-TEM, XRD, and XPS. As shown in Supplementary Fig. 31, both the cubic and hollow morphologies were preserved after the long-term stability test. Besides, the crystal structure of Fe-HESA NCs was also well-preserved (Supplementary Fig. 32). Moreover, XPS analysis demonstrates the absence of Fe3+ while no apparent alteration was observed in the core levels spectra of other elements after the stability test, suggesting that the Fe sites were electrochemically reduced during NO3RR as active sites while no chemical states changed for other metals after the stability test (Supplementary Fig. 33). The long-term stability of the catalyst was investigated by a chronoamperometry test for 150 h at −0.6 V versus RHE. As demonstrated in Fig. 4g, the current density went through a slight decrease in the first 22 h, after which it became quite stable over days, demonstrating the reasonable current stability realized by the catalyst.To investigate the reaction process and mechanism of NO3RR on the Fe-HESA NCs, Operando synchrotron-radiation Fourier transform infrared spectroscopy (SR-FTIR) was conducted (Supplementary Fig. 34). As shown in Fig. 5a, b, infrared signals in the range of 4000–2800 cm−1 and 2200–1000 cm−1 under applied potential between −0.1 and −0.6 V versus RHE were collected and analyzed. In the SR-FTIR operation results of 3450–3150 cm−1 (Fig. 5a, b), the IR intensity corresponding to v (N-H) of NHx species increased as the voltage decreased from −0.1 to −0.6 V, indicating an increase in NH3 yield47. Within the range of 2200–1000 cm−1, peaks observed at 1229 and 1118 cm−1 were ascribed to the NO2− and *NH2OH intermediates, respectively48,49. It is apparent that both the production rates of NO2− and *NH2OH have nearly linear correlations with the applied potential decreased from −0.1 to −0.5 V versus RHE (Fig. 5b). In addition, the Co-Had and Had signals were also observed at 2110 and 2060 cm−1, respectively, starting from −0.2 V versus RHE50. As shown in Fig. 5c, the fast increase of Had signals indicates the enhanced water dissociation at higher voltage, which is beneficial to the hydrogenation process of NO3RR for the generation of NH3.Fig. 5: Mechanistic investigation of the NO3RR on HESA NCs.Operando SR-FTIR signals in the range of 4000–2800 cm−1 (a) and 2200–1000 cm−1 (b). c IR peak intensity versus the applied potential for the NO3RR process. d Conceptional illustration of the effects of configurational entropy and lattice symmetry on electronic property. The electron localization increases with increasing entropy and decreasing symmetry.Intriguingly, a previously developed survey of the symmetry evolution in carbon catalysts indicates a negative correlation between electrocatalytic activity and basal-plane symmetry41. This trend extends from the highest symmetry D6h observed in pristine graphene layers to D2h of nitrogen-doped single atom materials and Cs of high-entropy single atom materials. This phenomenon implies a positive correlation between catalyst activity and symmetry reduction, which helps to explain the origin of the catalytic activity of the HESA from the perspective of lattice symmetry and electron localization. As summarized in Fig. 5d, the electron localization is supposed to generate at carbon and nitrogen atoms due to the coordination bond of Fe-C5 and M–N5, respectively, and increase with symmetry reduction of lattice41. Compared with unitary and binary SA with a lattice symmetry of D3h, HESA displays a symmetry minimization in the system, and the spatial distribution of electrons is more localized and chaotic for delivering favorable electron transfer properties during electrocatalysis of NO3RR. And the maximized ∆S of HESA is anticipated to feature the highest entropic stabilization induced by incorporation of a high number of elements. This thermodynamically benefits the HESA phase and thus leads to additional enhancement of electrochemical stability of Fe-HESA NCs under the long-term stability test of NO3RR.
