Supporting Information for Highly active catalyst derived from a 3D foam of Fe(PO 3 ) 2 /Ni 2 P for extremely efficient water oxidation Haiqing Zhou a,1, Fang Yu a,1, Jingying Sun a, Ran He a, Shuo Chen a,2, Ching-Wu Chu a,b,2, and Zhifeng Ren a,2 a Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204 b Lawrence Berkeley National Laboratory, Berkeley, California 94720 1 These two authors contributed equally to this work. 2 To whom correspondence may be addressed. Email: schen34@uh.edu, cwchu@uh.edu, or zren@uh.edu. 1
Fig. S1. Typical SEM images of the starting Ni foam. Fig. S2. Characterization of as-prepared Ni2P on Ni foam. (A-C) SEM images. (D) X-ray diffraction (XRD) pattern. 2
Fig. S3. The morphologies of as-prepared Fe(PO3)2 catalysts on three-dimensional Ni2P/Ni foam. Fig. S4. Characterization of the as-prepared Fe(PO3)2 catalyst by X-ray diffraction pattern. All XRD peaks of Fe(PO3)2/Ni2P/Ni foam can be indexed to Ni foam and Ni2P, which means the Ni2P was formed on the surface of the Ni foam together with Fe(PO3)2. Fig. S5. Electrocatalytic water oxidation performance. (A) Polarization curves recorded on different electrodes with a three-electrode configuration in 0.1 M KOH electrolyte. (B) Comparison of the overpotentials at which different electrodes reach a current density of 10 ma/cm2. The error bars represent the range of the overpotential values from three independent measurements. (C) Chronoamperometric measurements of the OER at 10 ma/cm2 on the Fe(PO3)2 electrode in 0.1 M KOH. 3
Fig. S6. Typical SEM morphologies of the Fe(PO3)2 electrode after 10,000-cycle OER testing. Fig. S7. Raman spectra of Ni2P catalysts before (black) and after (red) OER testing of 10,000 cycles. There are two prominent Raman peak frequencies appearing at 526 and 1050 cm-1 after OER testing, which are very similar to those of nickel oxide, suggesting the transformation of Ni2P to nickel oxide at the surface during electrocatalytic water oxidation. This observation is in good agreement with previous results on nickel phosphide (1). 4
Fig. S8. Elaboration of the XPS analysis of the Ni 2p3/2 binding energies in the original and post-oer Fe(PO3)2 electrode. For the original samples, a Ni 2p3/2 peak with a binding energy at 853.4 ev is detected, which can be ascribed to the formation of Ni2P compound, agreeing well with the XRD pattern shown in Fig. S4. While after OER testing, only one Ni 2p3/2 peak located at 855.7 ev is found, probably originating from the formation of nickel oxide/hydroxide at the surface, which is transformed from the Ni2P support during OER process. Fig. S9. Typical cyclic voltammetry curves at different scan rates. (A) Ni foam, (B) Ni2P, and (C) Fe(PO3)2 with scan rates ranging from 10 mv/s to 100 mv/s at intervals of 10 mv/s. The scanning potential range is from 1.025 V to 1.125 V vs RHE. 5
Table S1. Comparison of the catalytic OER performance of earth-abundant electrocatalysts in 0.1 M KOH electrolyte from the relevant literature. Materials Support η@10 ma/cm 2 η@100 ma/cm 2 j @ 300 mv Reference Fe(PO 3 ) 2 Ni foam 218 mv 301 mv 97 ma/cm 2 This work Ni-Co hydroxide ITO 460 mv NA negligible 2 NiFe LDH/CNTs Carbon paper 308 mv NA 11.5 ma/cm 2* 3 Perovskite CaCu 3 Fe 4 O 12 Glassy carbon 382 mv * 433 mv * < 0.5 ma/cm 2 4 Ultra-thin CoSe 2 nanosheets CoSe 2 nanobelts/ N-doped graphene Glassy carbon 320 mv > 520 mv * 7.5 ma/cm 2* 5 Ni foam 366 mv NA < 1 ma/cm 2* 6 Co 3 O 4 /C nanowires Cu foil 290 mv 490 mv * 12 ma/cm 2* 7 NiFe hydroxides Ni foam 240 mv >330 mv * 45 ma/cm 2* 8 *The value is calculated from the curves shown in the literature. 6
Table S2. Comparison of the catalytic OER performance of earth-abundant electrocatalysts in 1 M alkaline electrolyte from the relevant literature. Materials Support Tafel slope η@10 ma/cm 2 η@500 ma/cm 2 j @ 300 mv Electrolyte Reference Fe(PO 3 ) 2 Ni foam 51.9 mv/dec 177 mv 265 mv 1705 ma/cm 2 1 M KOH This work Ni x Fe 1-x Se 2 -DO Ni foam 28 mv/dec 195 mv NA NA 1 M KOH 9 NiFe LDH/r-GO Ni foam 39 mv/dec 195 mv NA NA 1 M KOH 10 Gelled FeCoW Au foam 37 mv/dec 190 mv NA NA 1 M KOH 11 NiFe LDH/CNTs FeOOH/Co/ FeOOH Carbon paper 31 mv/dec 247mV NA NA 1 M KOH 3 Ni foam 32 mv/dec NA NA 100 ma/cm 2* 1 M NaOH 12 CoNi(OH) x Cu foil 77 mv/dec 280 mv 425 mv * 35 ma/cm 2* 1 M KOH 13 h-nis x Ni foam 96 mv/dec 180 mv ~ 320 mv * 440 ma/cm 2* 1 M KOH 14 NiFe hydroxides Ni foam 28 mv/dec 215 mv NA 400 ma/cm 2* 1 M KOH 8 NiSe Ni foam 64 mv/dec 251 mv * NA 40 ma/cm 2* 1 M KOH 15 Co 4 N nanowire arrays Ni 2 P nanoparticles Carbon cloth 44 mv/dec 257 mv NA 30 ma/cm 2* 1 M KOH 16 Glassy carbon 59 mv/dec 290 mv NA 17 ma/cm 2* 1 M KOH 1 *The value is calculated from the curves shown in the literature. Table S3. XPS analysis of the atomic ratios of the original Fe(PO 3 ) 2 electrode. Elements Fe (%) Ni (%) P (%) O (%) Original 10.6 0.5 21.3 67.6 7
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