Atomically dispersed nickel–nitrogen–sulfur species anchored on porous carbon nanosheets for environment friendly water oxidation

Electrocatalyst synthesis and characterizations

The artificial process of the S|NiNx−PC/EG nanosheets is illustrated in Supplementary Fig. 1. A ternary supramolecular composite (TSC) was first obtained by the cooperative meeting of dicyandiamide, thiophene, and NiCl2 on the floor of EG foil underneath hydrothermal situations. Fourier remodel infrared spectra and liquid-state 1H nuclear magnetic resonance spectroscopy verified the profitable coordination of thiophene with dicyandiamide/Ni2+ (Supplementary Figs. 2 and three), ensuing within the formation of TSC, which was additional supported by a colour distinction between the merchandise (Supplementary Fig. four). Subsequent pyrolysis at 900 °C underneath an Ar environment and acid leaching remedy transformed the TSC/EG precursor into S|NiNx−PC/EG (Supplementary Fig. 5). In the course of the pyrolysis course of, the continual decomposition of TSC was accompanied by releasing N- and S-containing gases28,29, which generated porous buildings.

X-ray diffraction (XRD) patterns and Raman spectra affirm the formation of graphitic carbon within the S|NiNx−PC/EG throughout pyrolysis (Fig. 1a and Supplementary Figs. 6 and seven)30. X-ray photoelectron spectroscopy (XPS) reveals that the S|NiNx−PC/EG is principally consisted of Ni, N, S, C, and O components (Supplementary Fig. eight). The high-resolution Ni 2p spectra of S|NiNx−PC/EG show the binding energies of the Ni 2p3/2 and Ni 2p1/2 peaks at 854.9 eV and 872.three eV with two satellite tv for pc peaks at 861.2 eV and 879.eight eV, respectively, that are attribute of Ni2+ and Ni3+ (Supplementary Fig. 9)31. The high-resolution N 1s spectrum is deconvoluted into 5 kinds of N species (Fig. 1b), which correspond to pyridinic N (397.eight eV), Ni–Nx (398.eight eV), pyrrolic N ( eV), graphitic N (401.5 eV), and oxidized N (403.9 eV)32. Clear shifts within the binding vitality of the Ni 2p and N 1s peaks of S|NiNx−PC/EG in comparison with these of NiNx−PC/EG are noticed (Fig. 1b and Supplementary Fig. 10), indicating that the S atoms doubtless coordinate with the Ni atoms by partial alternative of the N atoms to type Ni–Sx websites19, which thus optimize the native digital construction of S|NiNx−PC/EG. The high-resolution S 2p XPS spectrum of S|NiNx−PC/EG confirms the existence of Ni–S and C–S bonds (Fig. 1c)33,34. The bonds between C and N or S (C–N/C–S) are additionally supported by the height centered at 285.three eV within the C 1s spectrum (Supplementary Fig. eight). The Nitrogen adsorption−desorption isotherm shows a mesoporous characteristic of S|NiNx−PC/EG with a Brunauer−Emmett−Teller (BET) floor space of 235 m2 g−1, a pore-size distribution centered at 18 nm and a complete pore quantity of zero.41 cm3 g−1 (Fig. 1d). As well as, the S|NiNx−PC/EG is very hydrophilic with a small contact angle of 35.four°, which permits the electrolyte to entry the lively floor (Supplementary Fig. 11).

Fig. 1Fig. 1

Morphological and structural characterizations. a Raman spectrum, b, c Excessive-resolution N 1s and S 2p XPS spectra, d N2 adsorption isotherm and corresponding pore-size distributions (inset), e, f FESEM photos, g AFM picture, h, i TEM and HRTEM photos of S|NiNx−PC/EG. Inset in i: SAED sample of S|NiNx−PC/EG. Information for NiNx−PC/EG can also be proven

Area-emission scanning electron microscopy (FESEM) photos of S|NiNx−PC/EG present a two-dimensional (2D) sheet-like morphology with a lateral measurement of as much as a number of micrometers and the looks of some observable mesopores (Fig. 1e, f and Supplementary Fig. 12). Atomic drive microscopy (AFM) reveals that the thickness of the S|NiNx−PC/EG nanosheets is ~32 nm (Fig. 1g). Elemental mapping spectroscopy confirms that the S|NiNx−PC/EG consists of Ni, N, S, C, and O components (Supplementary Fig. 13). Additional transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) evaluation manifest the partially graphitized nature and extremely porous construction of those 2D S|NiNx−PC/EG nanosheets (Fig. 1h, i).

Electrocatalytic OER

The electrocatalytic exercise of S|NiNx−PC/EG towards OER was investigated in M KOH. For comparability, NiNx−PC/EG, Ni/S co-doped PC/EG (Ni-S−PC/EG), N/S co-doped PC/EG (N-S−PC/EG), and EG had been additionally ready underneath comparable situations. Amongst them, S|NiNx−PC/EG exhibited the very best OER efficiency with the smallest onset potential of 1.50 V and the best catalytic present density throughout the complete potential vary (Fig. 2a), which was superior to the onset potentials of NiNx−PC/EG (1.53 V), Ni-S−PC/EG (1.57 V), N-S−PC/EG (1.58 V), and EG (1.60 V), revealing the pivotal impact of the S|NiNx advanced doped within the carbon matrix for OER. Furthermore, the S|NiNx−PC/EG afforded present densities of 10 and 100 mA cm−2 at overpotentials of 1.51 and 1.56 V, respectively. The achieved overpotentials are the bottom amongst all heteroatom- and/or transition metal-doped carbon electrocatalysts for OER reported so far (Supplementary Desk 1), they usually even surpass the state-of-the-art industrial Ir/C catalyst (1.59 V at 10 mA cm−2). The mass exercise of S|NiNx−PC/EG was 941.eight mA mg−1 at 1.58 V, which is ~16.2 occasions greater than that of business Ir/C (58.1 mA mg−1). Assuming that every one the Ni websites had been electrochemically lively within the OER course of, the calculated turnover frequency (TOF) of S|NiNx−PC/EG reached 10.9 s−1 (Supplementary Fig. 14 and Supplementary Desk 2). We additional explored the affect of the pyrolysis temperature (700–1000 °C) and molar ratio of dicyandiamide:thiophene:Ni2+. The very best OER exercise was achieved with a pyrolysis temperature at 900 °C and molar ratio of 10:10:1 (Supplementary Figs. 15–18). The corresponding Tafel slope of S|NiNx−PC/EG was measured as 45 mV dec−1 (Fig. 2b), which is smaller than that of the Ir/C catalyst (88 mV dec−1), suggesting its favorable catalytic kinetics for OER. The electrochemical impedance spectra (EIS) revealed that S|NiNx−PC/EG possessed the smallest charge-transfer resistance amongst all 4 samples (Supplementary Fig. 19), additional justifying the promoted OER kinetics35.

Fig. 2Fig. 2

Electrocatalytic OER efficiency. a Polarization curves of EG, NiNx−PC/EG, Ni-S−PC/EG, N-S−PC/EG, S|NiNx−PC/EG, and Ir/C for OER. b The corresponding Tafel plots. c Multi-current electrochemical strategy of S|NiNx−PC/EG. d Polarization curves of S|NiNx−PC/EG earlier than and after 2000 cycles. Inset: Chronopotentiometry curves of S|NiNx−PC/EG underneath totally different present densities of 10 and 100 mA cm−2. All experiments had been carried out in M KOH

The multi-step chronopotentiometric curve confirmed that at first of 40 mA cm−2 (Fig. 2c), the potential instantly leveled off at 1.55 V and remained unchanged for the remaining 500 s. The opposite steps additionally confirmed comparable outcomes as much as 200 mA cm−2, implying that the excellent mass transport property and mechanical robustness of S|NiNx−PC/EG. The polarization curve of S|NiNx−PC/EG exhibited a negligible loss even after 2000 cycles, indicating its excessive electrochemical stability (Fig. second). Moreover, the sturdiness checks revealed that S|NiNx−PC/EG retained its catalytic exercise over 10 h at each 10 and 100 mA cm−2 (inset of Fig. second), which is superior to that of NiNx−PC/EG (Supplementary Figs. 20–22).

Understanding the lively websites

Management experiments exhibit that the S|NiNx−PC/EG ready with out acid leaching led to a lower in exercise (Fig. 3a), highlighting that the metallic nickel or nickel oxide nanoparticles fashioned throughout pyrolysis are inactive or may block the lively website for OER. Acid leaching eradicated the inactive Ni species and elevated the publicity of S|NiNx species, as confirmed by the HRTEM evaluation and N2 sorption research (Fig. 3a and Supplementary Fig. 23). These outcomes, coupled with the XPS evaluation, energy-dispersive X-ray spectrometer (EDX) mapping, and the affect of steel ions (Co2+, Fe3+, and Ni2+) on the OER exercise (Supplementary Fig. 24), determine the essential position of well-dispersed S|NiNx species as lively facilities in the direction of OER. To verify the presence of NiNx facilities, cyanide poisoning experiments of S|NiNx−PC/EG and NiNx−PC/EG had been performed. After remedy with potassium cyanide, each samples suffered from decreased exercise (Fig. 3b and Supplementary Fig. 25), which undoubtedly indicated that the NiNx lively websites with S-doping had been the origin of the OER exercise for S|NiNx−PC/EG36,37. Atomic-resolution high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) photos disclose that quite a few atomically dispersed brilliant spots marked with inexperienced cycles, akin to heavier Ni atoms, are distinguished within the porous carbon frameworks (Fig. 3c). The enlarged view of the chosen area and atomic electron vitality loss spectroscopy (EELS) of the brilliant dots (Fig. 3c–e) demonstrates that every Ni atom is coordinated by N and S components and additional hybridized within the carbon matrix. This commentary correlates properly with the DFT–simulated STEM and scanning tunneling microscopy (STM) photos, exhibiting that the Ni–N–S heart is embedded within the carbon lattice, forming secure bonds with neighboring carbon atoms (Fig. 3f, g). The HAADF-STEM photos and corresponding EDX mapping of S|NiNx−PC/EG additional exhibit that the Ni, N, and S atoms are homogenously distributed all through the entire pattern (Supplementary Fig. 26). The Ni content material of S|NiNx−PC/EG is zero.2 wt.%, as measured by inductively coupled plasma–optical emission spectrometry.

Fig. threeFig. 3

Understanding the construction of lively websites. a Comparability of the OER exercise of S|NiNx−PC/EG and S|NiNx−PC/EG earlier than etching. Insets are TEM photos exhibiting that the Ni nanoparticles had been eliminated by acid etching remedy. b Polarization curves of S|NiNx−PC/EG with and with out 10 mM KCN, indicating that CN− ions strongly poison the S|NiNx−PC/EG. Insets: illustrations of S|NiNx facilities blocked by the CN− ions. c HAADF-STEM picture of S|NiNx−PC/EG and corresponding electron vitality loss spectroscopy atomic spectra of Ni, N, and S components from the brilliant dots, as proven by the inexperienced circle arrow in c. d, e Atomic-resolution HAADF-STEM photos of S|NiNx−PC/EG. f, g Simulated HRTEM and STM photos for e. h Ni Okay-edge XANES spectrum and that i Ni Okay-edge k3-weighted EXAFS spectrum of S|NiNx−PC/EG; knowledge for the Ni foil, NiO, Ni porphyrin, and S|NiNx−PC/EG earlier than etching are additionally proven. The insets are the magnified photos. j Schematic structural mannequin for S|NiNx−PC. The metal blue, blue, yellow, grey, and purple spheres signify Ni, N, S, C, and O atoms, respectively

To additional probe the chemical state and native coordination construction of the Ni atoms in S|NiNx−PC/EG, X-ray absorption near-edge construction (XANES) and prolonged X-ray absorption high quality construction (EXAFS) spectroscopy measurements had been carried out (Fig. 3h, i and Supplementary Figs. 27 and 28). As proven in Fig. 3h, a definite shoulder peak at 8338 eV within the inset XANES determine (with arrow) presents a attribute peak that includes the Ni–N4 sq. planar D4h symmetry within the reference Ni porphyrin38. The S|NiNx−PC/EG exhibits a smaller change within the shoulder peak than that of Ni porphyrin, indicating distorted Ni-[N/S]four bonding atmosphere deviated from the best sq. planar geometry, which might be supported by the corresponding radial distribution operate (RDF) of the Fourier-transformed (FT) EXAFS spectra (Fig. 3i). In comparison with the RDF of the reference Ni porphyrin, which exhibits a symmetric FT peak (Ni–N4) within the inset RDF determine, S|NiNx−PC/EG has a well-separated FT peak that includes shorter and longer bond distances of 1.85(three)Å and a couple of.33(1)&Aring, which correspond to Ni–N and Ni–S, respectively. With the EXAFS becoming course of, the coordination numbers of Ni–N and Ni–S in S|NiNx−PC/EG are calculated to be 2.eight(2) and zero.eight(three), in comparison with these of the Ni–N4 in Ni porphyrin (Fig. 3j). The substitution of bigger sulfur for one nitrogen within the D4h native construction leads to the shortening of the Ni–N bond distance and a slight native distortion from the best sq. planar symmetry39.

Theoretical investigation

To rationalize the four-electron response mechanism and excessive electrocatalytic exercise of S|NiNx−PC/EG for OER, the correlative theoretical calculations had been carried out by means of DFT. A considerable amount of attainable N–S, Ni–S, Ni–N4, Ni–N3S, Ni–N2S2, and Ni–NS3 fashions had been judiciously constructed (partial fashions might be present in Supplementary Figs. 29–34). Within the calculations, the N–S and Ni–S fashions are the N/S and Ni/S co-doped graphene buildings, respectively; the Ni–N4, Ni–N3S, Ni–N2S2, and Ni–NS3 fashions are the Ni–N4-, Ni–N3S-, Ni–N2S2-, and Ni–NS3-doped graphene buildings. From the formation vitality calculations, the Ni–N4 and Ni–N3S fashions have the bottom formation vitality values and are probably the most secure buildings amongst these fashions. Contemplating the correction of zero level vitality, the formation energies for Ni–N, Ni–S, and N–S fashions are greater than that of Ni–N3S mannequin, which signifies that the Ni–N3S mannequin might be extra thermally stabilized than Ni–N, Ni–S, and N–S fashions. Every worth of the overpotential η for the catalytically lively websites on all of the fashions is calculated to additional consider the catalytic actions of various electrocatalysts. For the N–S mannequin, the S and C atoms neighboring the N atom, that are typical electron donors40, possess high-potential obstacles for the rate-limiting step in OER. In the meantime, from our calculations, the OER pathways on S within the Ni–N3S fashions and a few C atoms, that are neighboring the N atoms within the Ni–N, Ni–N4 and Ni–N3S fashions, additionally present excessive free-energy values within the third steps (OOH* era) due to edge results41. These potential obstacles stop them from exhibiting higher catalytic actions than the Ni atoms of Ni–N4 and Ni–N3S in these fashions. Consequently, the Ni atoms within the Ni-doped fashions (such because the Ni–N, Ni–N4, and Ni–N3S fashions) are confirmed to be probably the most lively catalytic websites for OER42.

To additional examine the catalytic mechanism, the inhabitants distributions of the associated doped graphene supplies are offered in Fig. 4a–h. By discussing the overpotential profiles, the construction with the best catalytic efficiency is recognized because the Ni–N3S-doped graphene construction, as seen in Fig. 4g, h. In contrast with the Ni–N4 mannequin, the S atom within the Ni–N3S mannequin is the electron donor43 and might cut back the electron donation of the Ni atom to its neighboring N atoms, thus tuning the hybridization states between Ni and the neighboring N atoms, which improves the native digital construction of the catalytic website and boosts the OER catalytic exercise (Fig. 4i–ok). Determine 4i exhibits a typical volcano plot for numerous lively websites on totally different fashions in alkaline environments. The values of the calculated OER overpotential are zero.346, zero.461, zero.478, and zero.516 V for the Ni–N3S, Ni–N4, Ni–S, and N–S fashions, respectively. The Ni–N3S mannequin has the bottom overpotential worth. The outcomes point out that the potential barrier of the third step clearly decreases, and the Ni–N3S-doped graphene exhibits the best catalytic efficiency amongst all fashions. Owing to the hybridization states with the neighboring C and Ni atoms, the existence of the S atom renders a excessive positive-charge density and optimizes the density-of-states distributions, which may improve the electron switch means within the Ni–N3S mannequin (Fig. 5a)44.

Fig. fourFig. 4

Theoretical calculations. Inhabitants distributions for the DFT-calculated consultant fashions: a N−S co-doped armchair nanoribbon, b N−S co-doped zigzag nanoribbon, c Ni−S co-doped armchair nanoribbon, d Ni−S co-doped zigzag nanoribbon, e Ni−N4-doped armchair nanoribbon, f Ni−N4-doped zigzag nanoribbon, g Ni−N3S-doped armchair nanoribbon, h Ni−N3S-doped zigzag nanoribbon. i OER volcano plot of the overpotential η vs. the distinction between the adsorption free vitality of O* and OH* for the N−S, Ni−S, Ni−N4, and Ni−N3S fashions. j Adsorption free vitality of OH* vs. the distinction between the adsorption free vitality of O* and OH* for the N−S, Ni−S, Ni−N4, and Ni−N3S fashions. ok Schematic free-energy profile for the OER pathway on the Ni−N3S mannequin in alkaline media

Fig. 5Fig. 5

PEC-OER efficiency. a DFT-calculated projected density-of-states for the Ni−N4, Ni−N3S, and Fe2O3 fashions. b Variation within the photocurrent density vs. utilized voltage for Fe2O3-NA and S|NiNx−PC/EG/Fe2O3-NA underneath darkish and AM 1.5G irradiation. c IPCE spectra of Fe2O3-NA and S|NiNx−PC/EG/Fe2O3-NA underneath AM 1.5G irradiation. d, e Cost-transfer efficiencies and cost transport efficiencies of Fe2O3-NA and S|NiNx−PC/EG/Fe2O3-NA. f Transient photocurrent responses of Fe2O3-NA and S|NiNx−PC/EG/Fe2O3-NA underneath AM 1.5G irradiation at 1.23 V. All experiments had been carried out in M NaOH

The elementary response steps in the direction of the OER course of over the Ni–N3S mannequin in alkaline environments are demonstrated in Fig. 4k. Although the fourth step (OOH* to O2 manufacturing) is spontaneous, the OER steps have apparent potential obstacles from the primary to 3rd steps when the electrode potential U is zero V. When U will increase to zero.965 V (zero.346 V in overpotential), the free vitality of elementary response steps lower to under zero, which signifies that the entire OER course of can happen spontaneously over this approximate potential. In the meantime, the catalytic actions of Ni–N2S2 and Ni–NS3 fashions are studied. Each of them have greater overpotential values than the Ni–N3S mannequin. The excessive potential obstacles present within the transition from O* to OOH* can decelerate and even block the O2 evolution. The boundary results of Ni–N4 and Ni–N3S buildings on the armchaired graphene nanoribbon are additionally investigated. From the calculations, the very best catalytic efficiency for Ni–N4 buildings is on the middle of the armchaired graphene nanoribbon, and the very best catalytic efficiency for Ni–N3S buildings is on the sting of the armchaired graphene nanoribbon. Thus, the cost redistribution, change within the group adsorption energy, and potential obstacles induced by the dopant play essential roles within the catalytic actions.

PEC water oxidation

Based mostly on our DFT calculations (Fig. 5a), the conduction band of Fe2O3 is near the Fermi degree, and the valence band is barely decrease than the work operate of Ni–N3S-doped graphene when Ni–N3S-doped graphene is built-in into Fe2O3-NA. This band alignment can promote simpler switch of photogenerated cost carriers between Fe2O3 and Ni–N3S-doped graphene, which facilitates the PEC-OER course of (Supplementary Fig. 35)45,46. Thus, we additional studied S|NiNx−PC/EG as a co-catalyst with Fe2O3-NA photoanode (S|NiNx−PC/EG/Fe2O3-NA) for photo voltaic water oxidation in alkaline answer (AM 1.5G, 100 mA cm−2, Supplementary Fig. 36). In Fig. 5b, the S|NiNx−PC/EG/Fe2O3-NA delivered a excessive photocurrent density of 1.58 mA cm−2 at 1.23 V, which is 2.59 occasions bigger than the Fe2O3-NA (zero.61 mA cm−2), and better than these reported for different Fe2O3-based inorganic photoanodes (Supplementary Desk three). Additionally, a outstanding cathodic shift within the onset potential from zero.eight V for Fe2O3-NA to zero.7 V for S|NiNx−PC/EG/Fe2O3-NA was noticed, revealing that the S|NiNx−PC/EG certainly promoted PEC-OER. The utmost photoconversion effectivity of S|NiNx−PC/EG/Fe2O3-NA achieved zero.24% at zero.92 V (Supplementary Fig. 37), tripling that of Fe2O3-NA (zero.07% at zero.96 V). The incident photon-to-current conversion effectivity (IPCE) measurement exhibits that the S|NiNx−PC/EG/Fe2O3-NA possessed a most IPCE worth of 30.9% at 300 nm at 1.23 V (Fig. 5c), which is about 2.68 occasions greater than that of Fe2O3-NA (11.5%).

To grasp the impact of S|NiNx−PC/EG on the promotion of photogenerated cost separation, the cost transport (ηtransport) and charge-transfer efficiencies (ηtransfer = JH2O/JH2O2) of the S|NiNx−PC/EG/Fe2O3-NA had been decoupled and quantified through the use of H2O2 as a gap scavenger (Supplementary Fig. 38)47,48,49,50,51,52. As proven in Fig. 5d, the addition of S|NiNx−PC/EG considerably elevated the ηtransfer of Fe2O3-NA all through the complete potential vary. Particularly, at 1.23 V, the S|NiNx−PC/EG/Fe2O3-NA delivered a a lot greater ηtransfer of 78.2% than the Fe2O3-NA (43.6%), illustrating that the S|NiNx−PC/EG can successfully weaken floor cost recombination and enhance charge-transfer from Fe2O3-NA to electrolyte, thus facilitating water oxidation kinetics53,54,55,56. The outcomes might be additional supported by EIS research (Supplementary Fig. 39), wherein S|NiNx−PC/EG/Fe2O3-NA exhibited a a lot decrease charge-transfer resistance than Fe2O3-NA each in darkish and underneath irradiation, suggesting that simpler interfacial charge-transfer occurred on the S|NiNx−PC/EG/Fe2O3-NA interface57. Furthermore, the S|NiNx−PC/EG/Fe2O3-NA exhibited the next cost transport effectivity (ηtransport) of 22.eight% at 1.23 V (Fig. 5e), as compared with the Fe2O3-NA (ηtransport = 16.three%), which is probably ascribed to the fashioned heterojunction between Fe2O3-NA and S|NiNx−PC/EG that may facilitate the cost transport in bulk Fe2O3-NA49,58. These outcomes counsel that the introduction of S|NiNx−PC/EG serving as co-catalyst not solely enhance bulk cost transport, however it additionally cut back floor cost recombination, thus rising the general effectivity of PEC water oxidation (Supplementary Fig. 40). No vital change in present density of S|NiNx−PC/EG/Fe2O3-NA was noticed inside 10,000 s of irradiation (Fig. 5f), indicating wonderful stability.

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