Supporting Information

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1 Supporting Information Stacked Porous Iron-Doped Nickel Coalt Phosphide Nanoparticle: An Efficient and Stale Water Splitting Electrocatalyst Chaiti Ray, a Su Chan Lee, a Bingjun Jin, Aniruddha Kundu, a Jong Hyeok Park, Seong Chan Jun a * a School of Mechanical Engineering, Yonsei University, 5 Yonsei-ro, Seodaemun-gu, Seoul, , Repulic of Korea Department of Chemical and Biomolecular Engineering, Yonsei University, 5 Yonsei-ro, Seodaemun-gu, Seoul, , Repulic of Korea *Corresponding Author: Seong Chan Jun scj@yonsei.ac.kr; Fax: ; Tel: The supporting information contains: Numer of pages: 15 Numer of figures: 12 Numer of tales: 3 S1

2 S1. Experimental Section Chemicals Nickel(II) nitrate hexahydrate (Ni(NO 3 ) 2 6H 2 O), coalt(ii) nitrate hexahydrate (Co(NO 3 ) 2 6H 2 O), iron(iii) nitrate nonahydrate (Fe(NO 3 ) 3 9H 2 O), urea, sodium hypophosphite monohydrate (NaH 2 PO 2 H 2 O), Pt/C (Pt 2% wt.), ruthenium oxide (RuO 2 ), potassium hydroxide (KOH), and 5 wt% Nafion were analytical grade (AR) and used without further purification. Doule distilled (DI) water was used throughout the study. Measurement of electrochemical H 2 and O 2 evolution Electrochemical H 2 and O 2 evolution was measured using an electrochemical analyzer (CHI66D) in a 1 ml la-faricated Pyrex reactor. A 1. M KOH aqueous solution was used as the electrolyte, and NiCoFe x and Pt wire were used as the anode and the counter electrode, respectively, at amient temperature and atmospheric pressure in a two-electrode system with an applied ias voltage of 1.6 V (vs. RHE) using a chronoamperometric technique. In a typical experiment, the sample was immersed in the KOH electrolyte, which was uled with nitrogen for 3 min to ensure almost complete removal of dissolved oxygen. The amount of generated gas was collected y taking 5 µl of the mixed gas from the headspace of the Pyrex reactor using a syringe and analyzing it using gas chromatography (Agilent technologies 789A GC system, USA) with a thermal conductivity detector and a 5Å-molecular-sieve column with Ar as the carrier gas. Preparation of standard electrodes To compare the HER, OER, and overall water splitting activities of our samples with those of standard materials, Pt-C and RuO 2 were coated on CC at a mass loading of 2 mg cm -2. Specifically, mg of Pt/C or 2.55 mg of RuO 2 was dispersed in 1 ml ethanol, and the otained suspension was coated onto CC (1 cm 2 ) efore drying under amient conditions. S2

3 Then, 5 µl 1.% Nafion solution prepared y dilution in ethanol was drop casted on the coated CC. Ni 2+ 2p 3/2 Ni 3+ Sat. a Ni 2p 1/2 Ni 2p Co 3+ 2p 3/2 Co 2+ Sat. 2p 1/2 Co 2p p 3/2 c Fe 2p d O 1s Fe 3+ 2p 1/2 Sat. Ni-O-H Fe-O-H Co-O-H H 2 O e f Figure S1. (a) Ni 2p, () Co 2p, (c) Fe 2p, and (d) O 1s XPS spectra of NiCoFe x OH/CC. (e, f) FESEM images of NiCoFe x OH/CC in low and high magnification, respectively. S3

4 Intensity Ni 5 P 4 [JCPDS No ] Intensity CoP [JCPDS No ] a c Intensity NiCoP [JCPDS No ] θ / degree θ / degree d e f θ / degree Figure S2. (a,, c) XRD patterns and (d, e, f) FESEM images correspond to NiFeP, CoFeP, and NiCo, respectively. Figure S2a and show the XRD pattern of NiFeP and CoFe. The diffraction peaks of NiFeP have well agreement with XRD pattern of Ni 5 P 4 (JCPDS no ). Again, in case of CoFeP the diffraction peaks arise for CoP (JCPDS no ). Note that no additional peaks of other Fe-phase can e oserved, suggesting that Fe is incorporated into the nickel and coalt phosphide lattices. The diffraction peaks appear to slightly shift to larger angles with Fe-doping, also indicate sustitutional incorporation of Fe-ions in nickel and coalt phosphide lattices. However, the XRD peaks of nickel coalt phosphide (Figure S2c) are completely matched with NiCoP (JCPDS no ). The FESEM image of NiFe in Figure S2d shows decoration aggregated tiny nanoparticles over caron cloth. In case of CoFe (Figure S2e), ultra-thin nanosheets aggregated on the caron cloth fiers. In contrast, 1D NiCoP nanowires completely covered the caron cloth as shown in Figure S2f. However, in case of NiCoFe x exhiited welldefined 2D nanoplates architecture and presence of synergistic effect among all three transition metals leads to improved electron conductivity. S4

5 -1 a Pt-C/CC NiCoFe x -2 NiCo CoFe NiFe RuO 2 /CC NiCoFe x NiCo CoFe NiFe Figure S3. (a, ) ir-uncorrected polarization curves of various as-synthesized electrodes for HER and OER in 1. M KOH electrolyte, respectively a c d J a -J c /2 (ma cm -2 ) NiCoFe x NiCo CoFe NiFe e mf cm mf cm mf cm mf cm Scan rate (mv s -1 ) Figure S4. (a-d) Cyclic voltammograms at different scan rates in the region of V vs. RHE for NiFeP, CoFeP, NiCoP and NiCoFe x. (e) The half differences in current density (J a J c ) at.85 V vs. RHE plotted against scan rate fitted to a linear regression. S5

6 The enhanced water splitting activity of NiCoFe x compared to those of NiCoP, CoFeP, and NiFe could e attriuted to its high electrochemically active surface area (ECSA). First, the doule-layer capacitances (C dl ) of the corresponding electrocatalysts were calculated y performing cyclic voltammetry experiments in the potential range of V vs. RHE (Figure S4a-d). The plot of half capacitive current density, (J a -J c )/2, against scan rate gives a straight line with a slope equal to C dl (Figure S4e). The current measured in a narrow potential window in the non-faradic range originates from charging the doule layer and the capacitance developed shows a linear relationship with active surface area. C dl is converted into ECSA using the specific capacitance value of a standard 1 cm 2 flat surface, which is normally etween 2 and 6 µf cm -2. The ECSA value for NiCoFe x (815.5) is ~2.3, 1.7, and 1.5 times larger than those of NiFeP, CoFeP, and NiCo, presumaly due to its more porous structure, as revealed y TEM (Figure 2d). Current density/ma cm -2 ECSA a NiCoFe x NiCo CoFe NiFe Current density/ma cm -2 ECSA NiCoFe x NiCo CoFe NiFe Figure S5. Polarization curves from normalized to the electrochemical active surface area (ECSA) for (a) HER, () OER. S6

7 a c Co 2p As-prepared Post-HER d Ni 2p As-prepared Post-HER Post-OER Post-OER e Fe 2p As-prepared Post-HER Post-OER f P 2p As-prepared Post-HER Post-OER Figure S6. TEM images of NiCoFe x after (a) HER and () OER. (c) Ni 2p, (d) Co 2p, (e) Fe 2p, and (f) P 2p XPS spectra of NiCoFe x efore and after HER and OER. S7

8 Co-O Intensity Ni-O Post OER As-prepared Raman shift/ cm -1 Figure S7: Raman spectra of NiCoFe x P efore and after OER process. To gain insight into the OER mechanism and etter understand the OER process of the NiCoFe x, we carefully performed Raman spectroscopy to characterize the NiCoFe x P efore and after OER, as shown in Figure S7. The Raman spectrum of the as-prepared sample has no significant peak. However, two prominent peaks were oserved at around 55 and 679 cm -1 correspond to the nickel and coalt oxyhydroxides on the catalyst surface, respectively. S1, S2 This result reveals the possile phase transformation on catalyst surface during OER process. HER Mechanism: Ni-Fe ased Hydrogenase enzyme is well known due to its catalytic activity towards HER. 3 Liu et al. reported DFT calculation indicated O-atom sites with highly negative charged were the most stale proton attracting sites. With an electron transfer, the atomic H favored ond formation to Ni-Fe ridge sites (hydride acceptor). Then, the second atomic hydrogen also transferred on pre-occupied hydride acceptor site and lead to the formation of dihydrogen. They y DFT calculation showed that the dihydrogen onded with Fe is not stale and shifts to the Ni site spontaneously. To interact with dihydrogen, the metal sites have to transfer electrons to hydrogen. Fe with a igger positive charge did not ind dihydrogen as strongly as S8

9 Ni. Overall, they showed that the Ni site of the hydrogenase played an essential role in the HER. They also experimentally oserved that Ni acted as the active site for the reaction with hydrogen. The similarity etween as-synthesized NiCoFe x and [Ni-Fe] hydrogenase is the presence of Ni, Fe ions. The heterostructured Ni-ased composites are well known catalysts for HER alkaline medium. Again, the presence of heteroatoms (like, Co, Fe etc.) may alter the surface adsorption/desorption energy on neary Ni atoms and provide mostly active catalytic site in intermediate species. The multicomponent heterostructures can maximize the direct interfacial contact etween metal atoms which significantly enhances the electron transfer and shorten the diffusion distances, while the synergy among these components can result in enhanced activities. Therefore in case of As-synthesized NiCoFe x P, Ni active site at surface are involved in the water dissociation while the Co and Fe sites will involve in the generation and release of H a -2 Alkaline Electrolyte Neutral Electrolyte Acidic Electrolyte Alkaline Electrolyte Neutral Electrolyte Acidic Electrolyte c Alkaline Electrolyte Neutral Electrolyte Acidic Electrolyte Potential / V d Alkaline Electrolyte Acidic Electrolyte Neutral Electrolyte Time / h S9

10 Figure S8. ir-corrected polarization curve of NiCoFe x for (a) HER and () OER in differnt ph electrolyte at 2 mv s -1 scan rate. (c) Polarization curves for NiCoFe x under differnt ph electrolyte in a two-electrode system without ir-correction. (d) Chronoamperometric curves for water electrolysis y NiCoFe x in different electrolyte at constant 1.51 V potential. Figure S8 displays comparative electrochemical activity of NiCoFe x towards HER, OER and 2-electrode water splitting in acidic (.5 M H 2 SO 4, ph 1.), neutral (1. M PBS, ph 7.) and alkaline (1. M KOH, 14.) medium. Figure S8a indicates NiCoFe x exhiit excellent HER performances with overpotential of ~48, 12, and 39 mv to reach 1 ma cm -2 current density in acidic, neutral, and alkaline electrolyte, respectively. Again, the OER performance of the NiCoFe x electrode was evaluated in H 2 SO 4, PBS, and KOH medium (Figures S8). The overpotential required to achieve the current density of 5 ma cm -2 was ~33, 355 and 275 mv in acidic, neutral, and alkaline electrolytes, respectively. Interestingly, NiCoFe x electrode exhiited good full water splitting activity (Figure S8c) with cell voltage 1.57 and V in acidic and neutral electrolyte similar to that in asic medium (1.51 V) at 1 ma cm -2 current density. Moreover, the electrode showed ~9 % retention of the original current density (1 ma cm -2 ) after a 5 h chronoamperometry test at 1.51 V in all different ph electrolyte(figure S8d). S1

11 -1 a NiCoFe x CoFe -2 NiFe Ni Co NiCoFe x CoFe NiFe Ni Co c NiCoFe x CoFe NiFe Ni Co Potential / V Figure S9. Polarization curves for (a) HER, () OER and (c) 2-electrode system of various assynthesized electrocatalysts. Figure S9a displays that for HER to achieve the current density 1 ma cm -2 CoP and Ni electrodes need 141 and 167 mv overpotentials, respectively. However, after Fe-doping in CoP and Ni the HER overpotentials decrease to 91 and 19 mv, respectively. Again, in case of Fe-doped NiCo the overpotential dramatically reduced to 39 mv. Thus, Fedoping into the catalyst plays a crucial role towards modulating electronic property and enhanced electrical conductivity. These factors lead to enhanced electrocatalytic activity of Fe-doped catalysts compared un-doped catalyst. Again, the Figure S9 and c clearly reveal the higher electrocatalytic performance of Fe-doped CoP and Ni compared CoP and Ni towards OER and 2-electrode full water splitting. S11

12 Intensity / a.u. a NiCoFe 1. NiCoFe.5 NiCoFe.2 NiCoFe.1 NiCoFe θ / degree c d e f Figure S1. (a) XRD patterns of NiCoFe x with different compositions. FESEM images of () NiCoFe. P, (c) NiCoFe.1 P, (d) NiCoFe.2 P, (e) NiCoFe.5 P and (f) NiCoFe 1. in high magnification. S12

13 d e NiCoFe.1 f NiCoFe.1 NiCoFe NiCoFe.2 NiCoFe.5 NiCoFe.5 NiCoFe 1. 6 NiCoFe.1 : 67.5% Retention NiCoFe NiCoFe.2 : 77.8% Retention 3 NiCoFe.5 : 9.4% Retention NiCoFe a c NiCoFe 6.1 : 55.3% Retention NiCoFe.2 NiCoFe.2 : 66.% Retention -9 NiCoFe.5 NiCoFe.1 NiCoFe NiCoFe.5 : 93.5% Retention 1. NiCoFe.2-6 NiCoFe 1. : 72.1% Retention NiCoFe.5-6 NiCoFe Z" / ohm Z" / ohm Z' / ohm Z' / ohm Time / h NiCoFe 1. : 85.8% Retention Time / h Figure S11. Electrocatalytic performances of NiCoFe x electrodes with different compositions in alkaline medium. (a) LSV curves, () Nyquist plot at -.2 V (vs. RHE), and (c) chronoamperometric analysis for the HER. (d) Polarization curves of catalysts with different compositions for OER. (e and f) Corresponding Nyquist plot at 1.56 V (vs. RHE) and chronoamperometric plots for staility investigation, respectively. J a -J c /2 (ma cm -2 ) NiCoFe.1 P x /CC NiCoFe.2 P x /CC NiCoFe.5 P x /CC NiCoFe 1. P x /CC mf cm mf cm mf cm mf cm Scan rate (mv s -1 ) Figure S12. The half differences in current density (J a J c ) at.85 V vs. RHE plotted against scan rate fitted to a linear regression. S13

14 Tale S1. ICP-OES analysis results for the as-prepared NiCoFe x P. Catalysts Co At.% Ni At.% Fe At.% P At.% NiCoFe.1 P NiCoFe.2 P NiCoFe.5 P NiCoFe 1. P Tale S2. Summary of electrochemical parameters for the HER and OER. Materials η (mv) Tafel slope (mv dec -1 ) ECSA Mass loading (mg cm -2 ) R ct (ohm) HER OER HER OER HER OER (η 1 ) (η 5 ) NiFeP CoFeP NiCoP NiCoFe x P Pt-C RuO Tale S3. Summary of electrochemical parameters for the HER and OER of different composites. Materials η (mv) ECSA Mass loading (mg cm -2 ) R ct (ohm) HER OER HER OER (η 1 ) (η 5 ) NiCoFe.1 P NiCoFe.2 P NiCoFe.5 P NiCoFe 1. P S14

15 REFERENCES 1. Ai, L.; Niu, Z.; Jiang, J. Mechanistic Insight into Oxygen Evolution Electrocatalysis of Surface Phosphate Modified Coalt Phosphide Nanorod Bundles and Their Superior Performance for Overall Water Splitting. Electrochim. Acta 217, 242, Jiang, N.; You, B.; Sheng, M., Sun, Y. Bifunctionality and Mechanism of Electrodeposited Nickel Phosphorous Films for Efficient Overall Water Splitting. ChemCatChem 216, 8, Liu, P.; Rodriguez, J. A. Catalysts for Hydrogen Evolution from the [NiFe] Hydrogenase to the Ni 2 P (1) Surface: The Importance of Ensemle Effect. J. Am. Chem. Soc. 25, 127, S15