Supporting Information

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1 Supporting Information Highly Active and Stable Catalysts of Phytic Acid-Derivative Transition Metal Phosphides for Full Water Splitting Gong Zhang, a, e Guichang Wang, d Yang Liu, c Huijuan Liu, *a, e Jiuhui Qu, b, e and Jinghong Li *c a State Key Laboratory of Environmental Aquatic Chemistry, and b Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing , China c Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Beijing Key Laboratory for Microanalytical Methods and Instrumentation, Tsinghua University, Beijing , China d Center of Theory and Computational Chemistry, College of Chemistry University, Nankai University, Tianjin , China e University of Chinese Academy of Sciences, Beijing , China Corresponding Author *hjliu@rcees.ac.cn *jhli@mail.tsinghua.edu.cn S1

2 Supporting Materials Contains Following Sections: Notes Figure S1 Figure S2 Figure S3 Figure S4 Figure S5 Figure S6 Figure S7 Figure S8 Figure S9 Figure S10 Figure S11 Figure S12 Figure S13. Figure S14 Figure S15 Figure S16 Figure S17 Figure S18 Figure S19 Figure S20 Figure S21 Table S1 Table S2 Table S3 Experimental Section Molecular structure of Phytic acid SEM images of the TMPs catalysts Full XPS spectrum of the MoP and CoP HER performance of MoP catalysts Two-time constant models for fitting EIS response Estimation of C dl of MoP via plotting current density variation Estimation the C dl of MoP@RGO in 0.5 M H 2 SO 4 and 1 M KOH Cyclic voltammogrames for the RGO ranges in 1M KOH Raman and FTIR spectra of RGO before and after reaction Mole ratio of Fe and Co in CoP precursor XRD patterns for the FeP species The amount of O 2 catalyzed by CoP@RGO in OER process XRD patterns of the T-MoP and MoP@RGO Fitted pairs of EXAFS for TMP samples in real space (R-space) High resolution XPS scan of Mo 3d Atomic and band structures of MoP and CoP High resolution XPS scan for P 2p before and after reaction Models of P-terminated CoP(001) and O-doped CoP(001) Estimation of C dl for the two species of CoP Contact angle measurements Raman spectra of the product by pyrolyzing PA Fitted values of EIS data Curve fitting results for EXAFS Calculated adsorption energy of single H atom on MoP S2

3 Supplementary Notes 1. Experimental Section 1.1 Preparation of MoP: Graphene oxide (GO) sheets was synthesized from graphite powder (Sigma-Aldrich) based on Hummer s method but some modifications. 1 The Phytic acid (PA) (98% Aldrich) was wisely diluted with the same volume of ethanol to obtain the solution A. 0.5 mmol Ammonium molybdate tetrahydrate (Aldrich) was carefully dissolved in 5 ml GO solution (1 mg ml -1 ) (deionized water as comparison) in 2 min to obtain a transparent mixture, and denoted as solution B. Afterward, 2.5 ml B solution was dropwisely transferred to 5 ml A solution under vigorous stirring. After reaction 60 min, mixture was first dried in an electric-oven at 60 C about 6 h. Subsequently, as-formed gel was calcined in H 2 with a temperature ramp from 30 to 650 C at a heating rate of 5 C min -1, and hold at 650 C for 2 h, followed by cooling to room temperature. For comparison, (NH 4 ) 2 HPO 4 as conventional P source was applied for preparation of traditional MoP. 2 The step was described as following: a certain amount of citric acid as a chelating agent was added to a solution of (NH 4 ) 6 Mo 7 O 24 4H 2 O and (NH 4 ) 2 HPO 4 with molar ratio of Mo : P : CA=1 : 1 : 2. The transparent solution was kept at 90 C overnight and dried at 120 C. The dried sample was then calcined at 500 C for 10 h in a muffle furnace to obtain the precursors. The samples were subsequently reduced in H 2 /Ar (10 wt%). Finally, the obtained product was denoted as T-MoP. 1.2 Fabrication of CoP: 3.5 mmol cobalt acetate (C 4 H 6 O 4 Co 4H 2 O) was dissolved in 5 ml GO solution (1 mg ml -1 ) to obtain the transparent purple mixture (solution C). S3

4 The mixture obtained by the introduction of 2.5 ml solution C into solution A was the precursor. Product denoted as CoP@RGO was finally obtained by calcining precursor in the H 2 condition under 650 o C with the same temperature-programmed process. Meanwhile, a common synthetic strategy (inorganic phosphorus (IP) as P precursor) to preparation of CoP-IP was performed as comparison. The process was comprised of the Co-containing precursor synthesis followed by phosphidation treatment using NaH 2 PO Preparation of MoP/CoP loaded carbon cloth (CC): CC was washed repeatedly with aqueous solutions of HCl (19 wt.%) and HNO 3 (10 wt.%), followed by washing with pure water repeatly. The pre-oxidized CC (5 cm 5 cm) was first immersed into A+B or A+C solutions for minutes. Subsequently, PA crosslinked metal complexes -loaded CC was first dried at 60 o C in an electric-oven, followed by the temperature -programmed reduction process in H 2 at 650 o C for 2h. After cooling down to room temperature, TMP loaded CC electrodes with loading amount around 1.0 mg cm 2 were finally obtained. 1.4 Characterizations The crystal structure of sample was characterized by powder X-ray diffraction (XRD) (PANalytical Inc.) using Cu Kα irradiation operating at 40 KV and 40 ma with a fixed slit. Morphology of sample was observed by the Zeiss Field Emission Scanning Electron Microscopy (FESEM). TEM images were measured using a high-resolution JEM-ARM200F TEM/STEM for investigating the information on lattice and fringe. X-ray photoelectron spectroscopy (XPS) analyses were performed with a PHI5000 S4

5 Versa Probe system (Physical Electronics, MN), and binding energy was calibrated against reference of C1s peak at ev. Mo and Co K-edge X-ray absorption fine structure spectroscopy (XAFS) was carried out at BL 14W1 beamline at Shanghai Synchrotron Radiation Facility (SSRF) China. 1.5 Electrochemical Measurements Electrochemical measurements were performed at room temperature using a rotating disk working electrode made of glass carbon (GC) (0.196 cm 2 ) connected to a Gamry potentiostat. GC electrode was first polished to a mirror finish and thoroughly cleaned before use. A Pt wire and Ag/AgCl (3.5 M) were applied as the counter and reference electrodes, respectively. Before use, Ag/AgCl (3.5 M) electrode was first calibrated in 0.5 M H 2 SO 4 and 1 M KOH saturated with purity hydrogen ( %) using Pt plate as the working electrode. 4 Potentials were scanned from V to V or V to V vs Ag/AgCl at scan rate of 1 mv s -1, and the Cyclic voltammogrames (CVs) were recorded as following. For each individual, the average of two potentials at which the current cross zero was taken as thermodynamic potential for the hydrogen electrode reactions. Potentials vs RHE through RHE calibration were calibrated as following: S5

6 a) 0.5 M H 2 SO 4 So in 0.5 M H 2 SO 4 E RHE = E Ag/AgCl V b) 1.0 M KOH So in 1.0 M KOH E RHE = E Ag/AgCl V The preparation of working electrodes containing catalysts can be depicted as follows. 5 mg catalyst powder was dispersed in 1 ml of 3 : 1 v/v DIW/isopropanol mixed with 0.04 ml of Nafion solution (5 wt%), then a homogeneous ink was obtained after ultrasonicating mixture 30 min. Next, different volumes of dispersion was transferred onto GC disk. The catalyst film was then dried at room temperature. Before electrochemical measurement of HER, electrolyte (0.5M H 2 SO 4 ) was degassed by bubbling pure hydrogen for 30 min to ensure H 2 O/H 2 equilibrium at 0V vs RHE at S6

7 a rotation rate of 1,600 r.p.m. The polarization curves were obtained by sweeping the potential from -1.0 to 0 V vs Ag/AgCl at room temperature and 1,600 r.p.m. For OER performance, electrochemical characterization was carried out in 1M KOH electrolyte. The OER polarization curves were obtained by sweeping the potential from 0 to 0.8 V vs Ag/AgCl at room temperature and 1,600 r.p.m., with a sweep rate of 20 mv s -1. The polarization curves were replotted as overpotential (η) vs log current (log j) to get Tafel plots for assessing HER and OER kinetics of investigated catalysts. By fitting linear portion of Tafel plots to following Tafel equation, Tafel slope can be obtained. η=blog (j) +a (1) Electrochemical impedance spectroscopy (EIS) analysis was performed over a frequency range from 0.1 Hz to 1 MHz. Catalysts loaded CC were cut into 1 cm 1 cm pieces before applying as electrodes to collect chronoamperometry data. 1.6 Computational model and details Gibbs free energy calculations: This study uses Vienna ab initio simulation package (VASP) to perform the periodic, 5 the self-consistent density functional theory (DFT) calculation of HER or OER activity. Generalized gradient approximation (GGA) in form of Perdew-Burke-Ernzerhof (PBE) was used to calculate electronic structures. 6 The inner cores were described by project-augment wave (PAW) scheme. 7 Electronic wave functions were expanded in a plane wave basis which the kinetic cut-off energy was 400 ev. The structures were relaxed until residual force on each atom is less than 0.01 ev/å. Brillouin zone sampling was carried out using (5 5 1) Monkhorst-Pack grids. 8 S7

8 Stability of hydrogen was measured by differential hydrogen chemisorption energy E H, which can be calculated as follows: E H = E(MoP+nH) E(MoP+(n-1)H) 1/2 E(H 2 ) (2) where E(MoP+nH) is the total DFT energy for MoP system with n hydrogen atoms adsorbed on MoP, E(MoP+(n-1)H) is the total DFT energy for (n-1) hydrogen atoms adsorbed on MoP and E(H 2 ) is DFT energy for a hydrogen molecule in gas phase. The Gibbs free energy of hydrogen adsorption was calculated based on the following equations: G 0 = E + E T S (3) H H where E H is differential hydrogen chemisorption energy from the DFT calculations, E ZPE is the difference in zero point energy between the adsorbed state and the gas phase and S H is entropy difference between the adsorbed state and gas phase. 1 S H = SH S H (4) 2 2 ZPE H where SH 2 is the entropy of H 2 in the gas phase at standard conditions. The DFT optimized lattice parameters of MoP (a = b = Å and c = Å) are close to experimental data. The MoP (001) facet was chosen in the present study and a p (2 2) (001) MoP surfaces were constructed by cleaving optimized bulk MoP in a six-atom-layer-thick slab contains three-layers Mo and three-layers P. Moreover, since the P-terminated MoP (001) was considered as active site for HER reaction based on the theoretical study, 9 so we mainly study the catalytic properties on P-terminated MoP(001). Moreover, in order to study the oxygen effect on the HER reaction, the O-doped P-terminated MoP(001) was also taken into account. S8

9 The reaction mechanism of OER can be expressed as: H 2 O+* OH* + H + + e - (5) OH* O* +H + +e - (6) O* + H 2 O OOH* +H + + e - (7) OOH* O 2 + H + + e - (8) The Gibbs free energy for the OOH* formation is: ΔG 3 = ΔG OOH ΔG O (9) here ΔG O =E O/slab E slab (E H2O E H2) and ΔG OOH =E OOH/slab E slab (2E H2O 3/2E H2), where E A/slab is the total energy of slab with adsorbate A, E slab is the total energy of the bare slab.e H2O and E H2 are the total energy of H 2 O and H 2 in gas phase. Thus, Gibbs free energy of equation 8 will be transferred to ΔG 3 = ΔG OOH ΔG O = E OOH/slab E O/slab (E H2O-1/2E H2). DOS calculations: Oxygen-incorporated systems, together with pristine system were handled at the level of density functional theory (DFT) as implemented in the VASP. Supercells consisting of unit cell for O-incorporated and pristine TMPs were selected. GGA was used to account for exchange and correlation in the PBE form. In our calculations, single-particle equations were solved by using the PAW method with a plane-wave cutoff of 500 ev. The Brillouin zone was sampled with a k-mesh of Monkhorst-Park scheme. All atomic coordinates were fully optimized until the residual Hellmann-Feynman forces are smaller than 0.01 ev/å. S9

10 Supplementary Figures Figure S1. Molecular structure of Phytic acid (PA). S10

11 Figure S2. SEM images of the MoP, T-MoP and CoP. MoP particles with diameter 20 nm in the presence of layered RGO, which was evidently observed by means of scanning electron microscopy (SEM). Meanwhile, bulk MoP aggregate was observed without the introduction of GO into precursor. Crack surface was noticed in the T-MoP aggregates. Without GO, CoP aggregate was also observed after calcining. S11

12 Figure S3. Full XPS spectrum of the MoP and CoP. S12

13 Figure S4. (a) HER performance and (b) Tafel plots of different electrocatalysts in 0.5 M H 2 SO 4 solution. Tafel slopes for HER were determined from triplicate parallel measurements. S13

14 Figure S5. Two-time constant models for fitting EIS response of hydrogen evolution reaction on MoP electrodes, where R s is series resistance, R ct denotes charge transfer resistance, R p relates to porosity of electrode surface, and the double layer capacitance is represented by the elements CPE1 and CPE2. S14

15 Figure S6. Cyclic voltammogrames of (a) T-MoP, (b) MoP, and (c) with various scan rates in 0.5 M H 2 SO 4. (d) Estimation of C dl via plotting current density variation (Δj=(j a -j c ), at 150 mv vs RHE, the data of which derived from CV plots. S15

16 Figure S7. Estimation of C dl via plotting current density variation (Δj= j a -j c ) at 150 mv. Data were from cyclic voltammogrames (inset) of MoP@RGO with various scan rates in 0.5 M H 2 SO 4 and 1 M KOH. S16

17 Figure S8. Cyclic voltammogrames of RGO with scan rate of 20 mv s -1 in 1M KOH. Two types of RGO were prepared via reducing GO sheets by H 2 in the temperature of 400 o C and 650 o C, which were denoted as RGO-400 and RGO-650. Meanwhile, the same procedure was repeated to the PA-GO compound (matrix of our TMPs) under 650 o C. Obvious current densities can be observed in RGO-400 and RGO-600 before water oxidation, indicating oxidization of RGO more favorable than water oxidation. However, the PA-GO mixture calcined under 650 o C showed no current before water oxidation. S17

18 Figure S9. (a) Raman and (b) FTIR spectra of the RGO-400 and RGO-650 before and after linear sweep voltammetry (LSV) range from 0.6 V to 1.23 V vs RHE 10 times in 1.0 M KOH. S18

19 Figure S10. Mole ratio of Fe, Co, and Ni in CoP precursor (diluted multiples of 20000), with concentration of metal impurities in PA solution inset. The biological compound PA (myo-inositol 1,2,3,4,5,6-hexakisphosphate) was directly extracted from plants, and thus, the Fe ions, with the concentrations of 5.6 mg L -1, can be detected in PA solution using inductively coupled plasma mass spectrometry (ICP-Ms). Due to strong chelation by PA, it was extremely difficult to break chelation on premise that maintains the PA molecular structure. Meanwhile, the mole ratio of Fe and Co was measured in the CoP precursors. Ratio of Co and Fe was confirmed around 300, and the amount of Fe in this condition was far from enough to complete doping process. S19

20 Figure S11. XRD patterns of FeP species after different calcined process. As another prerequisite, the formation of Fe-P is necessary for the doping or substitution. We prepared FeP ranges using same fabrication method except calcined temperature. An amorphous structure in XRD pattern was observed when the calcined temperature was set at 650 o C. The structures belong to FeP can be obtained after rising the calcined temperature beyond 850 o C, and we thereby confirmed that the formation of the Fe-P bond was impossible in 650 o C. S20

21 Figure S12. Theoretical calculated and experimentally measured amount of O 2 versus time for CoP@RGO at 1.60 V vs RHE. Figure S13. XRD patterns of the T-MoP and MoP@RGO. Diffraction peaks of samples could be indexed to hexagonal MoP. Comparing with T-MoP, characteristic diffraction peaks slightly shifted to lower 2θ degrees in MoP@RGO, which is mainly due to lattice expansions by the O incorporation S21

22 Figure S14. (a) The Mo-O, Mo-P and Mo-Mo pairs of EXAFS for MoP samples were fitted in real space (R-space). (b) Co-O and Co-P pairs of EXAFS for CoP samples were fitted in real space (R-space) Figure S15. High resolution XPS scan for Mo 3d of MoP and T-MoP. High shifted peaks for Mo 3d in PA-derivative MoP corresponded to the high valence states, which were resulted from the oxygen incorporation. S22

23 Figure S16. (a-b) Atomic and (c-d) band structures of MoP and CoP, respectively. S23

24 Figure S17. High resolution XPS scan for P 2p of CoP@RGO before and after CV cycle treatment. The XPS for P2p was performed for investigating the transformation of the P during the OER process. At a high potential in KOH, Co-P on surface was readily to be oxidized to form the nano-layerd CoO x films, resulting in the substitution of O for surface P elements, which was confirmed by the observed phosphate groups after different CV treatments. S24

25 Figure S18. Models of (a) P-terminated CoP and (b) O-doped CoP. Azure, purple, and red indicate Co, P, and O atoms, respectively. Average bond length of Co-P is Å in the pure P-terminated CoP, and it is increased to an average of Å after introduction of oxygen atom into CoP. S25

26 Figure S19. Cyclic voltammogrames of CoP with various scan rates in 1M KOH, and the estimation of C dl via plotting current density variation (Δj=j a -j c ) at 925 mv vs RHE inset. C dl value of CoP@RGO was 7.18 mf cm -2, 1.5 times higher than value of 3.93 mf cm -2 presented by the CoP-IP. A higher ECSA in CoP@RGO indicated that a rapid charge transfer between the active sites and electrolyte was achieved. S26

27 Figure S20. Contact angle measurements of drops of water, PA solution, ethanol -water mixture and PA-ethanol mixture onto carbon cloth (CC) surface. In contrast to water, PA, ethanol-water mixture, PA-ethanol mixture (solvent for TMPs precursor) can evenly distributed on CC surface, providing a great potential to form the flexible TMPs/CC electrode. S27

28 Figure S21. Raman spectra of the as-pyrolysis PA (Top) and the RGO sheets (from hydrothermal method). Peaks appeared in 1338 and 1592 cm -1 corresponded to D and G bands, the intensity of which reflected its disorder degree. The low I D /I G value in the as-pyrolysis PA indicated the presence of high degree graphitized carbon, which benefited for the electron transfer from the electrode to catalysts. S28

29 Supplementary Tables Table S1. Fitted values of the EIS data Sample Overpotential (mv vs. RHE) R ct (Ω) R p (Ω) Cd1 (mf/cm 2 ) Cd2 (F) MoP MoP@RGO The EIS fitting model consists of a series of resistances: one at high frequency region is related to the surface porosity (Cd1-R p ); the other one at low frequency depends on overpotential (Cd2-R ct ), reflecting the charge transfer process during electrochemical performance. R ct denoting the charge transfer resistance, Rp relating to the porosity of electrode surface, and Cd corresponding to double layer capacitance, can be observed. S29

30 Table S2. Curve fitting results for EXAFS. Name Shell CN R(Å) σ 2 (10-3 Å 2 ) R factor (%) Mo-O MoP@RGO MoP T-MoP Mo-P Mo-Mo Mo-O Mo-P Mo-Mo Mo-P Mo-Mo CoP@RGO Co-P CoP-IP Co-P XANES was normalized with edge height. Data were analyzed using Athena/Artemis from IFeffit software package. EXAFS oscillation χ(k) was extracted by using spline smoothing with a Cook-Sayers criterion, and the k 2 -weighted χ(k) was Fourier -transformed into R space in k ranges of 2-10 Å for Mo (Co) K edge with a Hanning function window. In curve-fitting step, the backscattering amplitude and phase shift were calculated using the FEFF8.4 code. For standard samples, coordination numbers (CN), bond lengths (R), edge corrections (ΔE), and Debye-Waller (DW) factors were all set to be adjustable. S30

31 Table S3. Calculated adsorption energy of single H atom on MoP(001) (unit: ev). Site Mo-terminated P-terminated O-doped P-terminated Top Bridge Moves to fcc site Moves to top site Moves to top site fcc hcp Note: Fcc: without P atom on the underneath. Hcp: with one P atom on underneath. Supplementary Reference (1) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228. (2) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X. Adv. Mater. 2014, 26, (3) Yang, H. C.; Zhang, Y. J.; Hu, F.; Wang, Q. B. Nano Lett. 2015, 15, (4) Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Zhou, J. G.; Wang, J.; Regier, T.; Dai, H. J. Nat. Mater. 2011, 10, 780. (5) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, (6) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, (7) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, (8) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, (9) Xiao, P.; Sk, M. A.; Thia, L.; Ge, X. M.; Lim, R. J.; Wang, J. Y.; Lim, K. H.; Wang, X. Energy Environ. Sci. 2014, 7, S31