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1 Electronic Supplementary Information Scalable Two-Step Synthesis of Nickel-Iron Phosphide Electrodes for Stable and Efficient Electrocatalytic Hydrogen Evolution Wai Ling Kwong a, Cheng Choo Lee b, and Johannes Messinger a,c * a Department of Chemistry, Kemiskt Biologiskt Centrum (KBC), Umeå University, S Umeå, Sweden b Umeå Core Facility for Electron Microscopy, Umeå University, S Umeå, Sweden c Department of Chemistry-Ångström Laboratory, Uppsala University, S Uppsala, Sweden *Corresponding author; Tel.: ; Fax: ; Johannes.Messinger@umu.se S1

2 Figure S1. (a) EDS spectrum of 1Ni-4Fe-P. The signals of O, Cl, and Ti are due to metal phosphate, metal chloride precursor, and Ti substrate, respectively. Elemental mapping over (b) the scanned area of µm 2 showing homogeneously distributed (c) Ni, (d) Fe, and (e) P. S2

3 Figure S2. EDS spectrum of the electrolyte used for the activation of 1Ni-4Fe-P. The electrolyte was drop-casted onto Si substrate and dried on a hot plate (100ºC) before being examined by EDS. Figure S3. XRD measurements of 1Ni-4Fe-P (a) before and (b) after the activation. Reference patterns of (c) FeP also is shown. 1 S3

4 Figure S4. LSVs of the activated 1Ni-4Fe-P measured in 0.5 M H 2 SO 4 after storing the sample for 13 days in air. Figure S5. LSVs (ir u -corrected) of the samples with 70% Fe. S4

5 Figure S6. (a-c) EIS data presented as Nyquist plots at overpotentials before ir u - correction (η*) of mv. The symbols and solid lines are the experimental data and fitted data, respectively. (d) Equivalent one-time-constant circuit used to fit the EIS data of all samples except 5Fe-P. (e) Equivalent two-time-constant circuit used to fit the EIS data of 5Fe-P. R u is the uncompensated series resistance; R ct and Q are the charge transfer resistance and the constant phase element, respectively, at the sample/electrolyte interface; R 2 and Q 2 are the charge transfer resistance and the constant phase element, respectively, corresponding to the electron transport within the sample. Electrochemical impedance measurements were performed at η* = 60, 90, and 110 mv (before ir u -correction) and are presented as Nyquist plots in Figure S6a-c. Except for 5Fe-P, the rest of the samples showed a semicircle whose diameter represents the resistance for charge transfer (R ct ) at the surface of the samples for HER. For 5Fe-P, the measurement was done only at low overpotentials since the electrode became increasingly unstable at higher overpotential. 2 successive semicircles were observed in Figure S6a, where the high-frequency semicircle (in the low Z region) arose due to the poor electron transport within the sample while the low-frequency semicircle (in the high Z region) is ascribed to the HER process. 2,3 The parameters obtained by fitting the plots using the equivalent circuits shown in Figure S6d-e were tabulated in Table S2. The value of R ct has been shown to be influenced by the geometric surface area and therefore may not represent the intrinsic catalytic properties. 4 S5

6 Figure S7. XRD measurements of 5Fe-P prepared with (a) g, (b) g, and (c) g NaH 2 PO 2 H 2 O. Reference patterns of (d) FeP, 1 (e) γ-fe 2 O 3, 5 and (f) α- Fe 2 O 3 also are shown. 6 Figure S7 shows the XRD measurements of 5Fe-P prepared with different amounts of NaH 2 PO 2 H 2 O. Since the position, intensity, and full-widths at half-maxima of Ti (substrate) peaks were approximately equivalent for all samples, a direct comparison of the Fe 2 O 3 peak intensities among the samples is facilitated. Although Fe 2 O 3 was still present in 5Fe-P prepared with g NaH 2 PO 2 H 2 O, the amounts were much lower (indicated by the lower peak intensity, notably at 35.5º 2θ) compared to 5FeP prepared with g NaH 2 PO 2 H 2 O. S6

7 Figure S8. LSVs (ir u -corrected) of 5Fe-P prepared with g, g (3x), and g (6x) NaH 2 PO 2 H 2 O. Figure S9. LSVs (ir u -corrected) of the samples of comparable mass loadings (approximately 1.0 mg cm -2 ). S7

8 Figure S10. LSVs (ir u -corrected) of 1Ni-4Fe-P of mass loadings of 1.1 mg cm -2 and 2.1 mg cm -2. Figure S11. High-resolution XPS spectra of activated 5Ni-P and 3Ni-2Fe-P, and freshly prepared 5Fe-P in (a) Ni 2p, (b) Fe 2p, and (c) P 2p energy regions. The vertical dashed lines mark the phosphide peak positions of 3Ni-2Fe-P, while the vertical dotted lines mark the peak positions of elemental (a) Ni 2p 3/2, (b) Fe 2p 3/2, and (c) P 2p 3/2. S8

9 Figure S12. LSVs (ir u -corrected) of five individual 1Ni-4Fe-P samples measured in 0.5 M H 2 SO 4. These 1Ni-4Fe-P samples were synthesized in different batches. Inset shows the photographs of 1Ni-4Fe-P before (as-sprayed; left side) and after (right side) the phosphidation. Figure S13. Amount of H 2 gas evolved by 1Ni-4Fe-P as a function of electrolysis time. The electrolysis was performed at j = -10 ma cm -2 in 0.5 M H 2 SO 4. Solid lines represent the amount of H 2 calculated at an assumed 100% Faradaic efficiency, while the green squares show the amount of H 2 measured experimentally. S9

10 Table S1. Comparison of overall HER activity (normalized by geometric surface area) of various H 2 evolution catalysts (HECs) in H 2 SO 4. HEC(morphology) /substrate η@-10 ma cm -2 / mv Tafel slope / mv dec -1 Electrolyte Ref. FeP(NWA)/CP M H 2 SO 4 FeP(NWA)/Ti M H 2 SO 4 Ni 2 P-G(NSA)/NF M H 2 SO 4 FeP(NRA)/Ti M H 2 SO 4 FeP(NTA)/CP M H 2 SO 4 FeP(NWA)/Fe M H 2 SO 4 1Ni-4Fe-P(NP)/Ti M H 2 SO 4 This work α-ins(ns)/gce M H 2 SO 4 Ni 12 P 5 (NP)/Ti M H 2 SO 4 FeP-NCNT(NP)/GCE M H 2 SO 4 WP(NP)/Ti M H 2 SO 4 FeP(NR)/GCE M H 2 SO 4 Ni 5 P 4 -Ni 2 P(NSA)/NF M H 2 SO 4 Ni 2 P-CNT(NP)/GCE M H 2 SO 4 Ni 2 P(NP)/Ti M H 2 SO 4 MoP(NP)/GCE M H 2 SO 4 Ni 2 P(NP)/Ti M H 2 SO 4 Ni 2 P(NP)/GCE M H 2 SO 4 CoSe 2 (NP)/CP M H 2 SO 4 Ni 5 P 4 (NSA)/Ni M H 2 SO 4 G: graphene; α-ins: iron-nickel sulfide; NCNT: nitrogen-doped carbon nanotubes; CNT: carbon nanotubes; NWA: nanowire array; NSA: nanosheet array; NRA: nanorod array; NTA: nanotube array; NP: nanoparticles; NS: nanosheets; NR: nanorods; CP: carbon fiber paper; Ti: titanium foil; NF: nickel foam; Fe: iron foil; GCE: glassy carbon electrode; Ni: nickel foil S10

11 Table S2. Summary of fitting parameters for the EIS measurements in Figure S6. Sample η* = 60 mv η* = 90 mv η* = 110 mv* Average C dl (standard R ct / Ω Y 0 / S n C dl / mf cm -2 R ct / Ω Y 0 / S n C dl / mf cm -2 R ct / Ω Y 0 / S n C dl / mf cm -2 deviation) / mf cm -2 5Ni-P (0.7) Ni-1Fe-P (0.6) Ni-2Fe-P (1.3) Ni-3Fe-P (4.9) Ni-4Fe-P (8.0) Fe-P R ct : charge transfer resistance; Y 0 : admittance of the constant phase element; n: exponent of the constant phase element; C dl : double-layer capacitance normalized by geometric surface area; η*: overpotential before ir u -correction Calculation of the double-layer capacitance (C dl ) C dl was calculated using the following equation: 24 1 C dl Y n 1 n 0 R -1 ct The obtained C dl was then normalized by the geometric surface area of the sample. Calculation of the real surface area With reference to the double-layer capacitance of a flat surface (C fs ) of 1 cm 2 geometric surface area and by assuming that the real surface area of the flat surface equals its geometric surface area, the real surface area of the sample was calculated using the following equation: real surface area sample C dl C fs real surface area flat surface The value of C fs was assumed to be 40 µf cm -2 in the calculation Real surface area / cm 2 S11

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