Preprint of "Synthesis of Hollow Spherical Phosphide Catalysts with Industrial-Scale Potential for Alkaline Hydrogen Evolution Reaction"

Supporting Information Synthesis of Hollow Spherical Phosphide Catalysts with Industrial-Scale Potential for Alkaline Hydrogen Evolution Reaction Magdalena Streckova a*, Alena Fedorockova b, Alexandra Guboova a, Gabriel Sučikb, Vladimir Girman a, Akbar Hussain c, d, Tomas Bystron e, Michael Vorochta f, Jozef Strečka g a Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01, Kosice, Slovak Republic, b Faculty of Materials, Metallurgy and Recycling, Technical University of Kosice, Letna 9, 042 00 Kosice, Slovakia. c Department of Chemistry, Quaid-i-Azam University, Islamabad, 45320, Pakistan d Department of Physical Chemistry, Faculty of Science, P.J. Šafárik University, Moyzesova 11, SK-04154 Košice, Slovak Republic e University of Chemistry and Technology Prague, Department of Inorganic Technology, Technicka 5, Prague 6, 166 28, Czech Republic f Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague 8, Czech Republic. g Department of Theoretical Physics and Astrophysics, Faculty of Science, P.J. Šafárik University, Park Angelinum 9, 040 01 Košice, Slovak Republic *E-mail: mstr[email protected] Contents 2. Experimental ........................................................................................................................................ 3 2.1 Chemical and Materials ................................................................................................................. 3 2.2 Structural methods and characterization ...................................................................................... 4 2.3 Electrochemical characterization .................................................................................................. 7 2. Experimental 2.1 Chemical and Materials The source compounds for the preparation of phosphides included: ammonium phosphate dibasic ((NH4)2HPO4 (Centralchem, 99%), iron (III) nitrate nonahydrate (Fe(NO3)3 9H2O (Centralchem, 98%), cobalt (II) nitrate hexahydrate Co(NO3)2 6H2O (Centralchem, 99%), nickel (II) nitrate hexahydrate Ni(NO3)2 6H2O (Centralchem, 98%), ammonium molybdate tetrahydrate (NH4)6Mo7O24 4H2O(Centralchem, 99%), citric acid C₆H₈O₇ (Centralchem, 99.5% ). For electrochemical performance analysis, potassium hydroxide (KOH, Centralchem, 85%) was used as the electrolyte and Platinum on carbon (PtC, Sigma-Aldrich, Mw = 195.08 g·mol-1) and Iridium oxide (IrO, Sigma-Aldrich, Mw =224.22 g·mol-1)were used as standard catalysts. Fig. S1 Drying device TEFIC Biotech; TFS-2L used for the preparation of phosphide precursors Fig. S2 MoFeP, MoFeNiP and MoFeCoP (A) powder precursors after spray drying (B) powders after calcination in a reducing atmosphere of H2 at 650 °C 2.2 Structural methods and characterization The scanning electron microscope (SEM, JEOL, JSM-7000F, Japan) and transmission electron microscope (TEM, HRTEM, SAED, JEOL, JEM-2100F, Japan) were used for morphology visualisation. The phase composition of the samples was realized by XRD analysis (PhilipsX' PertPro) using the X`Pert Pro (Philips) diffractometer equipped with Cu Kα radiation, operating at 40 kV and 50 mA. The patterns were recorded at the 2 theta range between 10 and 60°. For evaluation of the thermal degradation, Differential thermal analysis (DTA)/Thermogravimetric analysis (TG) using simultaneous thermal analyser NETZSCH STA449F3 Jupiter at a heating rate of 10 °C.min-1 up to 1200 °C in Al2O3 crucible under air/argon atmosphere. The samples were processed in a vacuum before analysis. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a custom-built spectrometer (SPECS Surface Nano Analysis GmbH, Germany) operating under ultra-high vacuum conditions (2×10-9 mbar). The system was equipped with a high-intensity, monochromated, micro-focused Al Kα X-ray source (SPECS µ-FOCUS 600) and a multichannel electron energy analyzer (PHOIBOS 150 NAP 1D-DLD). Samples were pressed into pellets and mounted on a stainless-steel holder using a stainless-steel strip. Lowresolution survey spectra were recorded with an acquisition step size of 1 eV (pass energy of 50 eV). Detailed XPS regions (Fe 2p, Mo 2p, Co 2p, Ni 2p, O 1s, C 1s, and P 2p) were acquired with an acquisition step size of 0.05 eV (pass energy of 20 eV). Deconvolution of A B detailed spectra was performed using KolXPD (1.8.0) software. A Shirley background and Voigt profiles were used for deconvolution. All 2p components were fitted with the ratio of p3/2 : p1/2 peak intensities fixed at 2 : 1. Analogously, intensities of 3d5/2 and 3d3/2 peaks in 3d peaks were fixed at 3 : 2. The binding energies (BE) were adjusted using the main adventitious sp3 carbon peak referenced at 284.9 eV. The charging correction was in all cases about 1.1 eV, suggesting reasonable electronic conductivity of the samples. Fig. S3 Survey X-ray photoelectron spectra of investigated samples Fig. S4 Detailed X-ray photoelectron C1s spectra of investigated samples with deconvolution. 2.3 Electrochemical characterization The electrochemical experiments were conducted using a Vionic potentiostat/galvanostat (Metrohm, Switzerland). The potential was calculated with respect to the reversible hydrogen electrode (RHE) according to Eq.(1). Evs RHE = Evs Ag/AgCl + EAg/AgCl + 0.0591 × pH (S1) Table S1 Values of overpotentials at current densities of -10, -20, and -50 mA.cm-2 (η-10, η-20, η50) and Tafel slopes (b) in 1 M KOH for all samples Sample η-10 [mV] η-20 [mV] η-50 [mV] b [mV dec-1] Pt RDE -100 -128 -178 51 MoFeCoP -258 -292 -357 82 MoFeP -337 -372 -440 92 MoFeNiP -421 -467 -554 89 GCE -718 -756 -851 112 Fig. S5 The circuits used for fitting EIS measurements in 1M KOH The double-layer capacitance of real electrode-electrolyte interfaces is often well described by a Constant Phase Element (CPE) instead of a simple capacitor. CPE has impedance ZCPE defined by the Eq. 2: ZCPE = (1/Y0)/(jω) (S2) where Y0 and α (α ≤ 1) are CPE constants and ω is angular velocity. When α = 1, then CPE behaves like a capacitor with capacitance C, i.e. Y0 = C . χ2 indicates an error in EIS fit The double layer capacitance (Cdl) values were determined using the cyclic voltammetry (CV) method at scan rates of 20, 50, 100, 200, and 400 mV/s. The electrochemically active surface area (ECSA) of the samples is typically estimated using a simple CV approach. The ECSA of a catalyst sample is calculated from the the double layer capacitance (Cdl) according to Eq S3: ECSA = Cdl Cs (S3) However, determining the exact surface area of the material is challenging due to the unknown capacitive behavior (Cs) of the specific catalysts. Nevertheless, relative surface areas can be reliably estimated, as the Cdl is expected to be linearly proportional to the effective active surface area. The Cdl is determined by plotting the charging current density around the open circuit potential (Δj), calculated according to Eq. S4 against the scan rate. Δ𝑗 = (𝑗a – 𝑗c)/2 (S4) ja and jc are oxidation and reduction current density at open circuit potential, respectively. Effective differential capacitance Ceff of a circuit consisting of parallelly connected resistor (with resistance Rp) and CPE (with impedance ZCPE) can be calculated using the formula: 𝐶eff = 𝑌 0(1 𝛼)𝑅p(1 𝛼−1) (S5) Fig. S6 CV for ECSA measured for fibrous samples in 1M KOH