Native compressive strain-induced anti-corrosion over remoted Ru-decorated Co3O4 for environment friendly acidic oxygen evolution

Chemical substances

Cobalt(II) nitratehexahydrate (Co(NO₃)₂·6H₂O, 99%, Thermo Scientific), 2-methylimidazole (2-MIM, 99%, Thermo Scientific), Cetyltrimethylammonium bromide (CTAB, 99%, Shanghai Maclin Biochemical Expertise Co., Ltd), Tannic acid (ACS reagent, Sigma-Aldrich), Ruthenium (III) chloride hydrate (ACS reagent, AIKE REAGENT), Perchloric acid (HClO₄, 70%, Sigma-Aldrich), Ethanol (C₂H₅OH, absolute, VWR Chemical substances BDH), 2-propanol (C₃H₈O, HPLC grade, VWR Chemical substances BDH).

Synthesis of ZIF-67 nanocubes (NCs)

ZIF-67 NCs had been ready based mostly on a way reported beforehand³⁰. Intimately, 292 mg of Co(NO₃)₂•6H₂O was dissolved in 10 mL of deionized water together with 4 mg of CTAB. Following this, the answer was rapidly added to 70 mL of an aqueous combination that included 4.54 g of 2-MIM and stirred for 40 minutes at ambient temperature. The ultimate product was collected by centrifugation and totally washed a number of occasions with ethanol.

Synthesis of Co-tannic acid advanced nanoboxes (Co-TA NBs)

Initially, 60 mg of ZIF-67 nanocubes (NCs) had been dispersed in 20 mL of ethanol earlier than being transferred right into a solvent combination comprising 300 mL of ethanol and DI water in a 1:1 quantity ratio containing 1 mg mL^-1 of tannic acid (TA). The ensuing combination was stirred at room temperature for 10 minutes. Subsequently, the product was remoted by way of centrifugation and subjected to a number of ethanol washes. Lastly, the product was dried in an oven at 60 °C.

Synthesis of Co₃O₄ NBs

The Co-TA nanoboxes had been subjected to annealing at 350 °C for 3 hours, with the temperature rising at a fee of 1 °C per minute in air, adopted by pure cooling to room temperature.

Synthesis of Ru-Co₃O₄ NBs

50 mg of Co-TA NBs was dispersed into 10 mL of ethanol. Subsequently, 4 mL DI water containing 10 mM RuCl₃•xH₂O was added dropwise to the aforementioned dispersion, which was then stirred at room temperature for 3 h. The resultant product was harvested by way of centrifugation, subsequently washed a number of occasions with ethanol, and eventually dried in an oven at 70 °C. The obtained powder was positioned in a tube furnace and heated at 350 °C for 3 h with a heating fee of 1 °C min⁻¹ in air.

Supplies characterizations

X-ray diffraction (XRD) patterns had been obtained inside the vary of 5° to 90° 2θ utilizing a Bruker D8 Advance X-ray diffractometer, working at a scan fee of three° per minute. The instrument was geared up with Cu Kα radiation (λ = 1.5406 Å) and a beryllium window. The constructions and morphologies of supplies had been characterised by TEM (Titan Themis Z). HRTEM and elemental mapping had been recorded by the Titan Cs Probe instrument. Microstructure and morphologies had been characterised by scanning electron microscopy (SEM, Quattro FEG, FEI Ltd, CZ). The samples had been wet-digested and analyzed utilizing an Agilent 5110 ICP-OES. XPS knowledge was obtained by ESCALAB 250 xi. The wide-angle X-ray scattering (WAXS) measurements had been performed at Beijing Synchrotron Radiation Facility (BSRF), 1W1A beamline, utilizing X-ray with a wavelength of 1.54 Å. Characterization outcomes of Ru-Co₃O₄ pattern proven in Figs. 1 and a couple of are based mostly on the pattern with a Ru content material of 4.49 wt%.

X-ray absorption wonderful construction experiments

XAFS spectra had been acquired in fluorescence mode on the 1W1B beamline of the BSRF, using a double crystal Silicon (111) monochromator to tune photon power with a mean power decision ΔE/E of 1-3 × 10⁻⁴ @ 9 keV. The electron storage ring operated at an power degree of two.5 GeV with a present vary of 200 mA. The depth of the beam was quantified utilizing an ionization chamber full of 100% N₂ for Co Okay-edge detection and 100% Ar for Ru Okay-edge detection. Lytel detector full of 100% Ar was used to gather fluorescence sign. Co and Ru foils’ spectra had been acquired within the transmission mode to function the reference for power calibration. XANES and EXAFS had been employed to research the valence states, coordination environments, and structural dysfunction inside the samples. The information evaluation was performed using the ATHENA and ARTEMIS modules (model 0.9.26) inside the IFEFFIT software program suite⁶². EXAFS capabilities underwent Fourier transformation into R area, using a k-weight of two within the vary of two.98–11.83 Å⁻¹ for Co edge and a couple of.78–10.79 Å⁻¹ for Ru edge with a hanning home windows (dk = 1.0 Å⁻¹). For the absorption edges of Co and Ru, the amplitude discount elements (S₀²) had been decided to be 0.7 and 0.72, respectively, by separate fittings of Co and Ru foil XAFS knowledge. Fittings had been constantly carried out in R area with a k-weight of two. For the EXAFS knowledge processing, this research focuses on the Co-O coordination shells of curiosity: Co_Td-O, Co_Oh-O, Co_Oh-Co_Oh, and Co_Td-Co_Oh, with distance ranges of 1.81–3.35 Å. Contemplating elements akin to signal-to-noise ratio, knowledge high quality, and the bodily traits of the system, we decided that the R-space vary (1.15–3.45 Å) is probably the most applicable for EXAFS becoming. The Ru Okay-edge XANES spectra had been simulated utilizing full a number of scattering (FMS) and the finite distinction methodology (FDM) by fixing the Schrödinger equation. Theoretical XANES calculations on the Ru Okay-edge had been carried out utilizing the FDMNES code, using the real-space FMS scheme and using the Muffin-tin approximation for the potential⁶³. To simulate the XANES options, the calculation is carried out together with all atoms inside 6.0 Å away from the central Ru atom and solely the transition electrical dipolar element is calculated within the calculation. The calculations utilized power steps of 0.5, 0.2, 0.5, 1.0, and a couple of.0 eV similar to the power ranges of [−200.0, −2.0], [−2.0, 3.0], [3.0, 10.0], [10.0, 20.0], and [20.0, 300.0], respectively. The calculation outcomes are convoluted by an arctangent operate to acquire the ultimate spectrum. Comfortable XAS knowledge for the O Okay-edge had been obtained utilizing the full electron yield (TEY) mode at beamline 4B7B of the BSRF. The storage ring operated at an power of two.5 GeV with a most electron present of 250 mA. The power step sizes employed throughout knowledge assortment had been 0.5 eV, 0.05 eV, 0.1 eV, 0.2 eV, and 0.3 eV for the power ranges of [−33.1, −19.1] eV, [−19.1, −14.6] eV, [−14.6, −2.5] eV, [−2.5, 1.3] eV, and [1.3, 28.0] eV, respectively.

Operando XAFS experiments

The in situ Ru and Co Okay-edge spectra of the catalyst had been obtained utilizing an in situ response cell full of 0.1 M HClO₄ electrolyte. The powdered pattern was ready as a homogeneous catalyst ink at a focus of 5 mg/mL and utilized to a 1 × 1 cm² carbon paper to attain a catalyst loading of 1 mg/cm². The carbon paper with the catalyst was then positioned into the in situ response cell and sealed with Kapton movie. To check the evolution of the lively web site throughout the electrochemical response, a spread of consultant potentials (1.03–1.63 V) was utilized to the electrode. Every spectrum was measured thrice consecutively utilizing quick-scanning XAFS methods to make sure dependable signal-to-noise XAFS knowledge. Through the knowledge assortment course of for the Co Okay-edge, the next power steps had been employed for his or her respective power ranges: 4.0 eV for [−200, −20] eV, 1.0 eV for [−20, −10] eV, 0.5 eV for [−10, 50] eV, 1.0 eV for [50, 150] eV, 2.0 eV for [150, 300] eV, 3.0 eV for [300, 600] eV, and 4.0 eV for [600, 740] eV. Equally, for the Ru Okay-edge, the power steps used had been 4.0, 1.5, 2.0, 2.0, 3.0, and 4.0 eV for the power ranges of [−240, −20], [−20, 50], [50, 200], [200, 300], [300, 600], and [600, 740], respectively.

Electrochemical measurements

The electrochemical efficiency of the samples for the OER was evaluated utilizing a regular three-electrode system in a 0.1 M HClO₄ electrolyte at ambient temperature. 0.1 M HClO₄ electrolyte answer (pH = 1) was ready by diluting 70% HClO₄ in deionized water at room temperature. The answer was stirred repeatedly for 30 minutes to make sure the formation of a homogeneous answer. The Hg/Hg₂SO₄ reference electrode utilized in electrochemical measurements requires calibration. Particularly, the calibration was carried out in a hydrogen-saturated, high-purity 0.1 M HClO₄ electrolyte, utilizing a Pt wire because the working electrode, Hg/Hg₂SO₄ because the reference electrode, and one other Pt wire because the counter electrode. Cyclic voltammograms (CVs) had been obtained at a scan fee of 1 mV s⁻¹, with the thermodynamic potential for the hydrogen electrode reactions decided from the purpose the place the present crossed zero (Supplementary Fig. 17a, b). To arrange the catalyst ink, 5.0 mg of the catalysts had been dispersed in a combined answer consisting of 0.50 mL deionized water, 0.48 mL isopropanol, and 0.02 mL of 5 wt% Nafion answer. The combination was then subjected to ultrasonication for a minimum of 30 minutes to make sure the formation of a homogeneous ink. The catalyst ink was utilized onto 0.5 × 0.5 cm² carbon paper to attain a catalyst loading of 0.7 mg cm⁻². A platinum sheet electrode clamp was used to repair carbon fiber papers loaded with catalysts. Measurements had been performed on a CHI 760E electrochemistry workstation. The setup included a carbon rod because the counter electrode and a Hg/Hg₂SO₄ electrode because the reference electrode, whereas carbon fiber papers impregnated with catalysts functioned because the working electrode. The Nernst equation (left({E}_{{{{rm{RHE}}}}}={E}_{{{{rm{Hg}}}}/{{{{rm{Hg}}}}}_{2}{{{{rm{SO}}}}}_{4}}+0.0591times {{{rm{pH}}}}+0.656right)) was utilized to regulate all potentials in opposition to a reversible hydrogen electrode (RHE). Notably, linear sweep voltammetry (LSV) at a scan fee of 5 mV s⁻¹ was employed to evaluate the catalysts electrochemical exercise in direction of the OER. The overpotential (η) for the OER at a present density of 10 mA cm⁻² was calculated utilizing the equation η = E_RHE – 1.23 V. Furthermore, the Tafel slope was calculated utilizing the Tafel equation, expressed as η = b log j + a. On this equation, η denotes the overpotential related to the OER, j signifies the present density, and b represents the Tafel slope. To evaluate the electrochemical traits, Electrochemical impedance spectroscopy (EIS) was performed at a selected potential, using a ten mV amplitude over a frequency spectrum starting from 0.1 Hz to 100 kHz. An iR compensation of 90% was utilized to all polarization curves until acknowledged in any other case. The uncompensated resistance (R) was decided by electrochemical impedance spectroscopy. The resistance values had been 11.85 Ω for Co₃O₄, 13.24 Ω for RuO₂, 11.95 Ω for Ru-Co₃O₄ (1.33 wt%), 12.03 Ω for Ru-Co₃O₄ (3.21 wt%), 11.89 Ω for Ru-Co₃O₄ (4.49 wt%), 11.94 Ω for Ru-Co₃O₄ (7.06 wt%), and 11.50 Ω for Ru-Co₃O₄ (8.67 wt%). The electrochemically lively floor space (ECSA) of the catalysts was estimated from the electrochemical double-layer capacitance (C_dl) of the catalytic floor utilizing the next equation:

the place the C_dl was obtained from the CVs that had been depending on the scan fee, recorded within the non-Faradaic area between 0.81 and 0.91 V vs. RHE in 0.1 M HClO₄, using scan charges of 10, 20, 30, and 50 mV s⁻¹. On this context, S denotes the precise floor space of the graceful steel electrode, which usually corresponds to the geometric space of the glassy carbon electrode, the place S = 0.28 cm². The particular capacitance (C_s) for a flat floor usually ranges between 20 and 60 μF cm⁻². On this research, a C_s worth of 40 μF cm⁻² was used to estimate the ECSA.

Molecular simulations

The traditional parameters used throughout ab initio molecular dynamics (AIMD) in CP2K/Quickstep^64,65 using the hybrid Gaussian airplane wave scheme are set as follows: The PBE purposeful⁶⁶ is employed to explain the exchange-correlation interactions, supplemented by a DFT-D⁶⁷ correction to account for dispersion results. The remaining core electrons had been described by Goedecker-Teter-Hutter (GTH) pseudo-potentials⁶⁸. The airplane wave cutoff was 400 Ry with a double-zeta DZVP foundation set. The time step was set to 0.5 fs maintained at 300 Okay by utilizing canonical ensemble (NVT) with Nosé-Hoover thermostat throughout AIMD. Nicely-tempered AIMD⁶⁹ simulations had been carried out to speed up the sampling of configuration area and discover the response panorama because the coordination quantity adjustments mixed with the PLUMED plugin⁷⁰. The bias time period is outlined as a sum of Gaussian hills with peak h and width ω:

Right here, the bias issue was set to 100. Gaussian hills of 0.3 width and three.3 kJ mol⁻¹ preliminary peak had been utilized in a two-dimensional set of collective variables, described by coordination numbers (CN). The chosen collective variables are based mostly on coordination numbers. To explain the coordination of cobalt oxide Co_j with a set of oxygens CN is outlined by:

r_ij – inter-atomic distance between atom i and atom j,

d₀ – the Co-O equilibrium distance,

r₀ – the acceptance distance of the switching operate, which, coupled with the n/m ratio, controls at which distance the O atom is not thought of as bonded to Co.

n, m – two integer exponents with n < m.

On this research, a ratio of 9/14 (n = 9, m = 14), a Co-O equilibrium distance of 1.5 Å and a r₀ of 0.9 Å has been chosen. Two collective variables are set as follows: The collective variable CN_W represents the coordination variety of probed Co atoms with water oxygens and excessive chloric acid oxygens. CN_L represents the coordination variety of probed Co atoms with the oxygen atoms belonging to the floor construction of Co₃O₄. The error related to this process is decrease than 2 kJ mol⁻¹. The AIMD configurations extracted from trajectories are offered in Supplementary Knowledge 1.

DFT computational particulars

All theoretical calculations had been carried out utilizing the Vienna ab initio software program package deal (VASP 5.4.4)⁷¹. DFT fashions used for theoretical calculations are constructed by contemplating the next elements: 1) The composition of metals within the mannequin is taken into account by measuring the precise composition of catalysts. 2) XRD, XANES, and EXAFS methods can present essential construction info of supplies, which could be very helpful for establishing DFT fashions. For instance, the WT k-R maps offered the native construction info, aiding within the identification of the one atom construction. A least-square EXAFS becoming was additional carried out to disclose quantitative structural parameters of Ru in Ru-Co₃O₄. Moreover, a simulated XANES spectrum that carefully matched experimental knowledge by way of power place and spectral depth for incorporates a, b, c, and d (Fig. 2e), supporting the conclusion that Ru species in Ru-Co₃O₄ predominantly occupy octahedral (Oh) websites coordinated with lattice oxygen, which agrees effectively with EXAFS becoming outcomes. This strongly signifies the construction of Ru-Co₃O₄ is within the type of Ru single atom. The projector augmented-wave methodology was utilized to deal with core electrons with default potentials whereas the Perdew-Burke-Ernzerhof (PBE) GGA exchange-correlation purposeful was employed with spin polarization⁶⁶. The self-consistent discipline (SCF) cycles had been set to converge at a criterion of 10⁻⁶eV per unit cell. The ENCUT parameter, which determines the dimensions of the premise set, was maintained at 450 eV. For slab calculations, the k-point grid was sampled utilizing a 3 × 3 × 1 Monkhorst-Pack grid⁷², whereas a 12 × 12 × 12 grid was employed for bulk calculations. Core electrons had been handled by the projector-augmented airplane wave (PAW) methodology. Geometry optimization convergence was achieved when forces reached 0.01 eV Å⁻¹.

Adsorption energies (∆E_adverts) are used to explain the energetic properties of connected molecules on the surfaces and are outlined as:

the place E_adverts/sub  is the full power of the optimizer construction with adsorbed species, E_sub is the full power of the optimized clear floor construction, E_mol is the full power of adsorbed gasoline species.

The differential Gibbs free energies of adsorption (∆G_adverts) had been obtained based mostly on DFT complete energies, with corrections utilized for entropic change (T∆S, the place T represents temperature and S denotes entropy) and for the distinction in vibrational power (∆E_vib) and zero-point power (∆E_ZPE), each derived from vibrational frequencies:

The optimized structural fashions are offered in Supplementary Knowledge 1.