Extraction of non-noble metal columbite-tantalite as a highly efficient electrocatalyst for water splitting

The development of robust and inexpensive electrocatalysts that are capable of catalyzing the overall water splitting reaction is highly essential for large scale production of hydrogen. Herein, we report the successful liquid-liquid extraction and hydrothermal synthesis of a highly stable columbite-tantalite electrocatalysts (Fe 0.79 Mn 0.21 Nb 0.16 Ta 0.84 O 6 ) with remarkable HER and OER performance in alkaline media. The extracted Fe 0.79 Mn 0.21 Nb 0.16 Ta 0.84 O 6 electrocatalyst shows a low overpotential of 190.2 and 284.8 mV at 10/mA cm -2 in current density in situ for HER and OER, respectively. The electrocatalyst also exhibited low Tafel slopes of 56.36 mV/dec for HER and 112.85 mV/dec for OER, verifying their rapid catalytic kinetics. The electrolyzer maintained the cell voltage of 1.63 V and potential-time stability close to that of Pt/C & RuO 2 /C. The intrinsic mechanism for the exceptional HER and OER performance was further unravelled through first-principles density functional theory (DFT) calculations, predicting very low Gibbs free energy of hydrogen adsorption (ΔG H* ≈ 0.09 eV) and low overpotential (η =0.47 eV at the Mn sites) for OER on the Fe 0.75 Mn 0.25 Ta 1.875 Nb 0.125 O 6 catalyst. Our results demonstrate that columbite-tantalite electrocatalysts offer great promise for efficient overall water splitting. and superior electrocatalyst and 284.8 in current density in situ for and Complementary DFT+ calculations confirm that the incorporation of and Nb 6 to form 0.75 1.875 efficient this demonstrates a real potential for the design electrocatalyst with high electrocatalytic activity and stability electrolysis.


Introduction
The rising concerns regarding the depletion of carbon-rich fossil fuels and the increase of environmental pollution necessitates the development of renewable and clean energy technologies.
Water splitting to obtain hydrogen and oxygen has been considered as one of the most promising approaches to store renewable electricity in the form of hydrogen fuel 1,2 . Photoelectrochemical water splitting consists of two half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Electrochemical hydrogen production from water electrolysis is however, impeded by the sluggish OER kinetics, which requires high overpotential 3 .
Noble metals (e.g., Pt, Rh, and Ir) and noble metal oxides (e.g., IrO2, RuO2) have receive significant attention as the state-of-the-art electrocatalysts owing to their highly active geometry, long term stability, high current density, and the ability to provide low overpotential to forward HER and OER processes 4 . Nevertheless, the high cost of these catalyst materials limits their widespread practical application 5 . The past decades have therefore, seen a lot of constructive efforts in the development of earth-abundant transition metal-based electrocatalysts for water splitting, such as metal oxides, 6,7 metal hydroxides, 7-9 metal chalcogenides, 10 and metal phosphides, 11,12 Nevertheless, the facile oxidation and corrosion of transition metal based catalysts limits their use as HER and OER electrocatalysts. 13,14 Therefore, the rational design and development of transition metal-based electrocatalysts with superior stability and remarkable overall water splitting activity is still in high demand to improve the overall efficiency of water splitting.
Multimetal oxide catalysts have been reported as attractive photo-electrocatalysts in water oxidation/reduction [15][16][17] . The use of multimetal instead of single-metal oxide catalysts is shown to result in improvements in catalyst stability and performance due to the synergistic effects from the different metal components that tailor/modify the intrinsic properties affecting the HER and OER activity. For example, the incorporation of Ni into Co3O4 to form NiCo2O4 nanosheet array is demonstrated to enhanced the OER activity, which was attributed to increase of the number of active sites [18][19][20] . Besides that the NiCo2O4 nanosheet produces a small cell voltage of 1.59 V to drive a current density of 10 mA cm -2 compared to for Co3O4 catalyst [21][22][23] . Therefore, it is crucial to engineer different types of metal-ligand coordination to create abundant active sites for the electrocatalysis based on the lattice structure [24][25][26][27][28]

Mineral extraction and structural analyses
The coltan (CT) ore was mined in the Democratic Republic of Congo, supplied by the Bisunzu  The crystalline structure of the raw (CT) and extracted (WOCT) were revealed by the X-ray diffraction (XRD) as shown in (Fig. 2a In the case of CT, the corresponding peaks were identified with reference codes: Ta2O5 (00-054  The X-ray photoelectron spectroscopy (XPS) was used to find more information about the electron valence states of the WOCT. All the chemical elements were verified in the XPS spectra of the WOCT (Fig. 2c). Fig. 3 valence in the Nb2O5, Ta4d at 258.08 to 221.08 eV represents Ta 5+ species in the Ta2O5, and O1s located at 540.08 to 525.08 eV is attributed to the Metal-OH and the oxygen vacancy. Furthermore, Ta in WOCT shows two different spectra: Ta4d located at 230.81 eV (Fig. 3a) and Ta4f from 40.08 eV to 20.08 eV (Fig. 3b) indicate that Ta atoms in WOCT exist in the form of Ta 4+ 38,39 . Raman spectroscopy was also used to compare the raw (CT) and extracted (WOCT) materials (Fig. 2d).
The CT peak at 618.    and RuO2/C as benchmark reference catalyst, and NF were compared through the polarization curves without IR compensation. Fig. 6 When tested for overall water-splitting (H2 and O2 bubble formation), WOCT showed excellent performance, reaching 1.63 V at 10 mA cm -2 , approaching the Pt/C and RuO2/C activity ( Fig. 6(g)).

Density Functional Theory (DFT)
To gain further insights into the electrochemical performance of (Fe, Mn)(Nb- Ta surface. The ΔGH* is a good descriptor of the electrocatalytic activity of materials toward HER [49][50][51] . A ΔGH* as close as possible to zero is preferred as it shows that free energy of adsorbed H is close to that of the reactant or product. Fig. 7  where * stands for the active sites on the surface and O*, HO*, and HOO* denote the adsorbed oxygenated species. The ΔGI, ΔGII, ΔGIII, and ΔGIV represents the reaction Gibbs free energies. The overpotential η is determined as η = max (ΔGI, ΔGII, ΔGIII, ΔGIV) − 1.23 eV.
The Gibbs free energy profile for the proposed 4e-mechanism of oxygen evolution reaction at the pure FeTa2O6 and doped Fe0.75Mn0.25Ta1.875Nb0.125O6 (100) surfaces is presented in Fig. 8 (b-d).
The optimized structures of the *OH, *O, and *OOH intermediates for the OER are shown in Fig.   9. The rate determining step (i.e. the largest Gibbs free energy difference) is predicted to be Step-II for all active sites on the pure FeTa2O6 and doped Fe0.75Mn0.25Ta1.875Nb0.125O6 (100) surfaces, as shown in Fig. 9. The Gibbs free energy difference for the Fe-site on the pure FeTa2O6 (100) surface is predicted at ΔGII =2.19 eV, which is larger than that of the Fe (ΔGII =1.77 eV) and Mn  Fig. 8 (e and f). The incorporation of Mn introduced states at the valence band edge whereas Nb introduces states at the conduction band edge. The band gap is also narrowed with Mn and Nb incorporation, suggesting improvement in the electric conductivity of the doped material. This is consistent with lower overpotential and therefore improved OER performance predicted for the doped Fe0.75Mn0.25Ta1.875Nb0.125O6 than the pure FeTa2O6.

Mineral extraction
The columbite-tantalite ore (coltan) was crushed and sieved (<100 μm) at the beginning and 10 g was weighed for the digestion process. For leaching of the columbite-tantalite ore samples, twostep binary acid system was employed using the mixture of hydrofluoric (HF) and sulfuric acids

Electrochemical measurement
The electrochemical characterization was analyzed by Potentiostat/Galvanostat (ZIVE SP2, WonATech Co. Ltd., Seoul, Korea) with the conventional three-electrode electrochemical cell composed of WOCT, WOCTs, Pt/C, and RuO2/C deposited on the nickel foam (1 cm × 1 cm) as a working electrode, graphite rod as a counter electrode, and the Hg/Hg as a reference electrode (where the potential was converted in RHE by the ERHE = EHg/HgO + 0.098 + 0.059 pH). The measurement of the HER and OER polarization curves was conducted at the scan rate of 5 mV s -1 and 1M KOH at room temperature. The stability was studied at the j = -20 and 10 mV cm -2 and the electrochemical impedance spectroscopy (EIS) was accomplished at an amplitude of 10 mV and 0.054 V potential in a frequency range from 10 5 to 10 -2 Hz. The cyclic voltammetry (CV) was performed at different scan rates (10 to 100 mV s -1 ) in the -10 mV to -100 mV vs. RHE region to calculate double-layer capacitance value (Cdl) by plotting the Δj (ja -jc) at 0.98 V vs. RHE. The working electrode was prepared by dissolving 0.5 mg of WOCT, WOCTs, Pt/C, and RuO2/C in 5 mL ethanol containing 20 l of Nafion ® 115 solution (5%) sonicated for 10 min. Then, the ink was drop-casted onto the surface of 3D nickel foam and dried at 60°C for 5 h.

Theoretical calculation
The density functional theory (DFT) calculations were performed within the VASP -Vienna ab-Initio Simulation Package [63][64][65] . The electron-ion interactions were described using the projector augmented wave (PAW) pseudopotentials method 66  Ta with Nb resulted in the formation of Fe0.75Mn0.25Ta1.875Nb0.125O6, with the Fe 2+ or Mn 2+ ions occupying the A-sites, and Nb 5+ or Ta 5+ the B-sites. The optimized structures and the corresponding unit cell parameters as given in Supplementary Fig. 8. The predicted most stable (100) surface ( Supplementary Fig. 9) was used to characterize the HER and OER processes. To avoid interactions between periodic slabs, a vacuum size of 15 Å was added in the z-direction. Nete