Supporting Information Mixed-Node Metal-Organic Frameworks as Efficient Electrocatalysts for Oxygen Evolution Reaction Xiaohua Zhao, a Brian Pattengale, b Donghua Fan, c Zehua Zou, a Yongqing Zhao, a Jing Du, a Jier Huang b * and Cailing Xu a * a State Key Laboratory of Applied Organic Chemistry, Laboratory of Special Function Materials and Structure Design of the Ministry of Education, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China b Department of Chemistry, Marquette University, Milwaukee, Wisconsin, 53201 c School of Applied Physics and Materials, Wuyi University, Jiangmen, 529020, China Materials and Methods...S2-S5 Supporting figures and tables...s6-s17
Methods Materials Cobalt (II) nitrate hexahydrate (Co(NO)2 6H2O), Iron (II) chloride tetrahydrate (FeCl2 4H2O), N,N-dimethylformamide (DMF), ethanol and potassium hydroxide (KOH) were all of analytical grade without further purification. The 2, 5-dihydroxyterephthalic acid (H4DOBDC), Nafion and iridium (IV) dioxide (IrO2) were purchased from Xuanguang Company, Sigma-Aldrich and Alfa Aesar, respectively. Preparation of Co-Metal-Organic Frameworks-74 (Co-MOF-74) Typically, Co(NO)2 6H2O (0.107 g) and H4DOBDC (0.22 g) were dissolved in 7.5 ml water-dmf-ethanol mixture (1:1:1 (v/v/v)) under magnetic stirring at room temperature to form a homogeneous solution. The obtained homogeneous solution was transferred into the vial and sealed in a Teflon-lined stainless-steel autoclave to heat at 120 ºC for 24 h. The black powder was collected by washing/centrifugation with ethanol and DMF several times to remove organic residues and dried at 100 ºC for overnight. For CoxFe1-x-MOF-74 (x=0.8, 0.6, or 0.4), the same method were employed except for the different Co(NO)2 6H2O (0.086 g, 0.065g, or 0.054 g) and FeCl2 6H2O (0.02 g, 0.04 g, or 0.05 g. Fe-MOF-74 was obtained by adding 0.1 g FeCl2 6H2O and 0.22 g H4DOBDC into water-dmf-ethanol mixture. Ink Preparation In order to decrease the influence of the micrometer-size products on the dispersed ink, the catalysts were firstly grinded in an agate mortar for 60 min. Then, 5 mg of the
as-obtained Co-MO-74, Fe-MOF-74, CoxFe1-x-MOF-74 (x=0.8, 0.6, or 0.4), IrO2, and 20 µl 5 wt% Nafion solutions were dissolved in deionized water (1 ml) and ultrasonicated for 30 min to form an even suspension for the further electrochemical experiments. Characterization The electrochemistry measurements were performed by a Autolab PGSTST302N. Infrared spectra were recorded by a Bruker VERTEX 70v FT-IR spectrometer in the range of 400-4000 cm -1. Field emission scanning electron microscopy (FESEM) images and energy dispersive X-ray spectroscopy (EDS) were collected on a Hitachi S-4800 microscopy. The transmission electron microscopy (TEM) images were from FEI Tecnai G2F30 microscope. X-ray absorption spectroscopy was conducted at 12-BM beamline, Advanced Photon Source at Argonne National Laboratory. X-ray diffraction patterns (XRD) were obtained through a Rigaku D/M ax-2400 X-ray diffractometer in a 2θ range of 5-70. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) was performed on a PHI-5702 instrument. Binding energies (BE) were determined using the C 1s peak at 284.5 ev as a charge reference. A monochromatized He Іα irradiation from discharged lamp supplies photons with 21.22 ev for UPS. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were carried out to determine the concentration of Co and Fe. Electron spin resonance (ESR) experiments were performed on a JEOL, JES-FA 300 spectrometer (X-band at 8.969 GHz, power of 1 mw).
Electrochemical Measurement The OER performance of different catalysts was recorded at room temperature by using rotating disk electrode (RDE) with a rotation speed of 1600 rpm to remove the disturbance of dissovled O2. A standard three electrode setup was adopted: an Hg/HgO electrode as reference electrode, a Pt wire as auxiliary electrode and a RDE as working electrode (surface area = 0.0707 cm 2 ) in 1 M KOH solution. The surface of the polished RDE was firstly added 6 µl as-prepared inks and then dried under an infrared lamp for a few minutes. The reversible hydrogen electrode (RHE) were converted according to the following equation: ERHE = E 0 Hg/HgO + EHg/HgO + 0.059 ph, where the ph=13.6. Linear sweep voltammetry (LSV) scans were performed with a scan rate of 5 mv/s without ir-compensation. For the RRDE measurements, the as-synthesized Co0.6Fe0.4-MOF-74 ink was coated onto RRDE, consisting of a glassy carbon disk electrode and a Pt ring electrode. A scan rate of 10 mv s -1 and a rotation rate of 1500 rpm were applied for RRDE test. Firstly, in order to determine electron transfer number (N) for OER by detecting the HO2 - formation, the ring potential was constantly held at 1.50 V versus RHE in O2-saturated 1M KOH solution. The disk current was fixed at 200 µa and ring potential at 0.4 V versus RHE to determine to the Faradaic efficiency (ε) in N2-saturated 1 M KOH solution. The Faradaic efficiency (ε) was calculated by the equation as follows: ε = Ir/(IdNC), where Id is the disk current, Ir is the ring current, and NC is the current collection
efficiency (0.21 in this study), which was determined using the same configuration with an IrO2 thin-film electrode. The electron transfer number (N) can be calculated from the disk current (Id) and ring current (Ir) of RRDE N=4 Id/ (Id+Ir/NC) The values of TOF are calculated assuming that both Co and Fe ions in the catalysts are active and contribute to the catalytic reaction (the lowest TOF values were calculated) TOF= js/(4fn) Here, j (A/cm 2 ) is the measured current density at an overpotential of 350 mv; S is the surface area of RDE (0.0707 cm 2 ); the number of 4 means 4 electrons transfer in OER; F is Faraday constant (96485.3 C mol 1 ), and n is the Co (or Fe) ions molar number calculated from the mass loading of catalyst. Therefore, the TOF of Co and Fe ions are calculated. To measure the electrochemical double-layer (Cdl) from CV with different scan rates, a potential range in which no apparent Faradaic processes occur was assumed to be due to double-layer charging. The ECSA of catalyst is calculated from the double-layer capacitance according to the equation: ECSA = Cdl/Cs, where Cs is the specific capacitance of ideal planar for metal oxides under identical electrolyte conditions, usually taken to be 20-60 μf cm -2. In our work, Cs is estimated to be 40 μfcm 2, according to the most literatures 1,2.
Table S1. ICP-AES results of CoxFe1-x-MOF-74 samples (0<x 1 ) Feeding molar ratio (Co:Fe) Final product molar ratio (Co:Fe) Sample name 1 1 : 0 - Co-MOF-74 2 0.78 : 0.22 0.76 : 0.24 Co0.8Fe0.2-MOF-74 3 0.59 : 0.41 0.55 : 0.45 Co0.6Fe0.4-MOF-74 4 0.50 : 0.50 0.40: 0.60 Co0.4Fe0.6-MOF-74 5 0 : 1 - Fe-MOF-74
Figure S1. Fourier-transformed R-space spectra for Co K-edge (a) and Fe K-edge (b) with data shown in open points and solid FEFF best fit lines.
Figure S2. Full energy range EXAFS spectra at Co K-edge (a) and Fe K-edge (b); EXAFS fitting results in K-space for Co K edge (c) and Fe K edge (d), respectively.
Figure S3. FEFF input model composed of 6 metal atoms. The purple atom is the central atom that is either Co or Fe depending on the edge. The orange atom is either the same as the purple atom for single-metal MOF fits or different for dual-metal MOF fits. The light blue atoms are the same identity as the purple atom. The grey atoms are carbon and red atoms are oxygen.
Figure S4. TEM image of Co0.6Fe0.4-MOF-74.
Figure S5. (a) Steady-state polarization curves (scan rate: 5 mv/s), (b) Tafel plots of Co0.8Fe0.2-MOF-74, Co0.6Fe0.4-MOF, and Co0.4Fe0.6-MOF-74 for OER; (c) Nyquist plots at 1.51 V vs. RHE, (d) TOF of Co-MOF-74, Co0.8Fe0.2-MOF-74, Co0.6Fe0.4-MOF, Co0.4Fe0.6-MOF-74, and Fe-MOF-74.
Figure S6. CV curves between 1.35 and 1.45 V vs. RHE for (a) Co-MOF-74, (b) Co0.8Fe0.2-MOF-74, (c) Co0.6Fe0.4-MOF-74, (d) Co0.4Fe0.6-MOF-74 and (e) Fe-MOF-74. (f) difference of current density at 1.41 V vs. RHE as a function of scan rate for Co-MOF-74, Co0.8Fe0.2-MOF-74, Co0.6Fe0.4-MOF-74, Co0.4Fe0.6-MOF-74 and Fe-MOF-74.
Figure S7. (a) Steady-state polarization curves (scan rate: 5mV/s) and (b) Tafel plots of Co-MOF-74, Co0.8Fe0.2-MOF-74, Co0.6Fe0.4-MOF-74, Co0.4Fe0.6-MOF-74, Fe-MOF-74 for OER. The current density is normalized by ECAS obtained from non-faradic double-layer capacitance.
Figure S8. UPS of (a) Co-MOF-74, (b) Co0.6Fe0.4-MOF-74 and (c) Fe-MOF-74.
Figure S9. XPS survey spectra of (a) Co-MOF-74, (b) Co0.6Fe0.4-MOF-74, (c) Fe-MOF-74.
Fig. S10 (a) TEM image and (b) XRD patterns of Co0.6Fe0.4-MOF-74 catalysts after OER for 12h.
Table S2. Overpotential and Tafel slope of the reported MOFs and other Co-Fe based electrocatalysts for OER (η: overpotential at 10 ma cm -2 ) Catalyst Electrolyte η(mv) Tafel Slope (mv/dec) Reference CoFe-MOF-74 1 M KOH 280 56 This work Fe1Co1-ONS 1 M KOH 308 71.7 Adv. Mater. 2017, 29, 1606793. Fe-CoOOH/G 1 M KOH 330 37 Adv. Energy. Mater. 2017, 7, 1602148. CoFe-UMNs 1 M KOH 290 31 Nano energy 2018, 44, 345-352. Co2 Fe B 1 M KOH 298 62.6 ACS Appl. Mater. Interfaces 2017, 9, 40333-40343. CoFe2O4/PANI-M WCNT1:20 1 M KOH 314 30.69 J. Mater. Chem. A 2016, 4, 4472-4478. FeCo-Co4N/N-C 1 M KOH 280 40 Adv. Mater. 2017, 29, 1704091. E-CoFe LDHs 1 M KOH 300 41 NNU-23 0.1 M KOH 365 81.8 CoyFe10-yOx/NPC 1 M KOH 328 31.4 Chem. Commun. 2017, 53, 11778-11781. Angew. Chem. Int. Ed. 2018, 57, 9960-9964. J. Mater. Chem. A 2016, 4, 6505-6512. Co-Fe-N@MWC NT 1 M KOH 290 32 Electrochim. Acta 2017, 258, 51-60. Fe3-Co2 0.1 M KOH 283 43 J. Am. Chem. Soc. 2017, 139, 1778-1781.
Table S3. EXAFS fitting parameters of Co K Edge using the FEFF model. Additional shared parameters: Co K-edge fits: ΔEo = 2.35, So 2 = 1 for all paths. The σ 2 values are in units of x10-3 Å 2 and R in ± 0.02 Å. Path Co-MOF-74 Co0.6Fe0.4-MOF-74 N σ 2 R N σ 2 R Co-O 3 2.7 2.03 3 2.7 2.03 Co-O 1 2.7 2.08 1 2.7 2.08 Co-O 1 2.7 2.18 2 2.7 2.17 Co-C 2 1.0 2.91 2 1.5 2.92 Co-C 1 1.0 2.98 1 1.5 2.99 Co-Co 2 6.8 3.06 1.2 1.0 3.10 Co-Fe - - - 0.8 5.2 2.96 Co-O-C 4 1.0 3.04 4 6.6 3.13
Table S4. EXAFS fitting parameters of Fe K Edge using the FEFF model. Additional shared parameters: Fe K-edge fits: ΔEo = 5.89, So 2 = 1 for all paths. The σ 2 values are in units of x10-3 Å 2 and R in ± 0.02 Å. Path Fe-MOF-74 Co0.6Fe0.4-MOF-74 N σ 2 R N σ 2 R Fe-O 3 5.3 1.97 3 3.5 2.00 Fe-O 1 5.3 1.97 1 3.5 1.96 Fe-O 1 5.3 2.19 2 3.5 2.19 Fe-C 2 7.4 2.87 2 1.0 2.90 Fe-C 1 7.4 2.94 1 1.0 2.97 Fe-Fe 2 1.0 3.01 0.8 4.5 3.00 Fe-Co - - - 1.2 5.5 3.06 Fe-O-C 4 1.0 3.25 4 7.0 3.09 Reference (1) Liu, Z.-Q.; Cheng H.; Li, N.; Ma. T. Y.; Su, Y.-Z. ZnCo2O4 Quantum Dots Anchored on Nitrogen-Doped Carbon Nanotubes as Reversible Oxygen Reduction/ Evolution Electrocatalysts. Adv. Mater. 2016, 28, 3777-3784. (2) Liu, Y.; Liang, X.; Gu, L.; Zhang, Y., Li; G.-D.; Zou, X.; Chen, J.-S. Corrosion Engineering towards Efficient Oxygen Evolution Electrodes with Stable Catalytic Activity for Over 6000 Hours. Nat. Commun. 2018, 9, 2609.