Supplemental Information (SI): Cobalt-iron (oxy)hydroxide oxygen evolution electrocatalysts: The role of

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1 Supplemental Information (SI: Cobalt-iron (oxyhydroxide oxygen evolution electrocatalysts: The role of structure and composition on activity, stability, and mechanism Michaela S. Burke, Matthew G. Kast, Lena Trotochaud, Adam M. Smith and Shannon W. Boettcher Table S1. Supporting numerical data. S1: Insert a concise title for the table here Deposition Solution Fe content (0.1 M total metals content 0% 0% 20% 40% 60% 80% 90% 100% A As Deposited Deposition rate (ma cm / ex situ XPS Fe (% ±2% 0* 0* ** B Pre η = 350 mv Polarization (Initial Cyclic Voltammetry (CV 1M KOH Purified Standard Standard Standard Standard Standard Standard Standard Range Mass Deposited (μg cm ± 2 11 ± 2 8 ± 2 10 ± 3 10 ± 2 10 ± 3 12 ± 1 11 ± 5 Tafel Slope (mv dec ± ± 2 38 ± 1 34 ± 4 29 ± 3 30 ± 1 35 ± 2 45 ± 1 Fraction Co electrochemically available Oxidation wave (1 st CV, cycle 7 ± ± ± ± ± ± 0.11 Fraction Co electrochemically available Oxidation wave (2 nd CV, cycle 0.22 ± ± ± ± ± ± 3 Fraction Co electrochemically available Reduction wave (2 nd CV, cycle 0.23 ± ± ± 7 4 ± ± ± ± 0.12 After 1 min polarization at η = 350 mv TOF mass (s ± ± ± ± ± ± ± 1 16 ± 03 TOF, (s ± ± ± 1 5 ± ± ± ± 0.15 TOF, (s ± ± ± ± ± 1 16 ± 03 Post 120 min polarization at η = 350 mv TOF mass (s ± ± 06 9 ± ± ± 5 5 ± ± ± 004 TOF, (s ± ± ± ± ± 8 ± ± 0.1 S1

2 TOF, (s ± ± ± ± ± ± ± 004 Mass change (% 11 ± 6 7 ± 4 4 ± 3 5 ± 5 1 ± 6-18 ± ± 9-44 ± 12 Current change (% -16 ± ± 5-41 ± 9-62 ± ± ± ± ± 2 Tafel Slope (mv dec ± 1 39 ± 2 45 ± 4 41 ± 3 36 ± 4 36 ± 3 41 ± 2 49 ± 4 Fraction Co electrochemically available ex situ Fe content from XPS C 0.19 ± ± ± ± ± ± ± 6 (% ± 2% 0* ** in situ effective conductivity 1M KOH purified Standard Standard Standard Deposition rate (ma cm 2- Thickness (μm 0.12 ± ± ± ± 4 Potential (WE1 (V Conductivity (S cm E-05 All films were dipped in 18.2 MΩ cm water and air dried prior to XPS measurements. All kinetic data is based on CV taken pre- (initial CV, 2 nd cycle and post-polarization (final CV after 2 h polarization, 2 nd cycle. The error in Fe content (% as determined by XPS between multiple samples deposited from the same solution is ± 2%. This was calculated based on the average of 4 pairs of samples. Errors in TOF, Tafel slope, mass, and percent mass change are based on the standard deviation of three separate electrodes. Individual samples were prepared for subsections A-C using the deposition-solution Fe content indicated in the first row of the table. See SI Figure 1 for further explanation of sample treatments. *samples with no quantifiable Fe peak ** samples that have no Co and were not measured with Mg Kα XPS. S2

3 Deposition 18.2 MΩ cm 0.1 M Metal salt f i Cathodic Initial rinse deposition 18.2 MΩ cm f dep Final rinse As Deposited Electrochemical Testing 1 M KOH 1 M KOH 1 M KOH f 1min f 120CV CV Post- 2 h (η = 350 mv f 0CV CV Pre- 2 h (η = 350 mv Overpotential ((V-iR u CV Pre 2 h ( η = 350 mv (1 st cycle (OX f 120min Constant Potential b Potential (E(V-iR u vs. Hg/HgO Post 2 h at Ƞ = 350 mv Figure S1. (a Schematic of sample treatment steps (squares. Important data points recorded at each step for mass monitoring are shaded in grey. Solution composition for each experimental step is shown at the top of each square (e.g MΩ cm H 2 O and the step name (e.g. initial rinse is at the bottom of each square. f indicates recorded frequency on the QCM, f 0CV is recorded after the 2 nd CV cycle. The crystals are transferred to a new QCM holder prior to electrochemical testing to prevent contamination between the deposition and electrochemicaltesting solutions (see description in the experimental section of the main text. The multi-step Deposition and Electrochemical Testing subsections indicate all the sample treatments prior to the As Deposited and ƞ = Post 350 mv sample states as denoted in Table S1. Mass calculations are given below, where A is area (1.38 cm 2. First (b and second (c CV cycle of a Co 0.46 Fe 4 (OH 2 film; the shaded area indicates an example integration based on the oxidative (OX, black or reductive (RED, blue peak. The decision to integrate over the entire voltage range measured (V vs Hg/HgO to OER onset is due to the lack of a single, well-defined redox wave for many of the films (RED (OX a Overpotential ((V-iR u CV Pre 2 h ( η = 350 mv (2 nd cycle c Potential (E(V-iR u vs. Hg/HgO S3

4 Calculations: A Hz cm 64.5 μg mass A Hz cm 64.5 μg Current A at 350 mv ƞ 96,485 C TOF s mol 4 mass MW Co Fe OH TOF, s Current A at 350 mv ƞ 96,485 C mol 4 mass 1x MW Co Fe OH TOF, s Current Aat 350 mv ƞ 96,485 C mol 4 mass x MW Co Fe OH Current Aat 350 mv ƞ 96,485 C TOF s mol 4 Integrated peak C 96,485 C mol S4

5 Current (ma st cycle 2 nd cycle Potential (E(V vs. Hg/HgO a Mass Lost (g cm -2 Current (ma st cycle 2 nd cycle 0% Fe 33% Fe Potential (E (V vs. Hg/HgO b Mass Lost (g cm -2 Figure S2. Mass change during first and second CV cycles directly after deposition (pre 2 h at η = 350 mv for (a Co(OOH in purified 1 M KOH and (b Co 0.67 Fe 0.33 (OOH in standard 1 M KOH. Grey arrows indicate axis of importance for designated data. Current (ma Overpotential (V Standard 0.2 Current (ma Purified a b c Potential (E(V vs. Hg/HgO Potential (E(V vs. Hg/HgO 5 0 Mass (g cm Time (min 0.3 Mass change ( g cm Figure S3. Control measurements on bare QCM crystals. (a Redox features of Au in standard (cyan and purified (black 1 M KOH. This highlights the effect of Fe on the OER catalysis ability of Au. Also evident is the small Au redox wave visible for the cleaned Au/Ti substrates that is not clearly visible after deposition of Co 1-x Fe x (OH 2. (b Typical change in mass loss (peach during CV (black in purified 1 M KOH (c Current response and mass change during polarization at η =350 mv on initially clean Au QC in standard base. S5

6 Current density (ma cm -2 Potential (V vs. Hg/HgO Potential (V vs Hg/HgO a 0% Fe Potential (E (V vs. Hg/HgO c 0.10 WE1 WE e Step 1 0% Fe % Fe Time (s Step 2 WE1 WE Time (s 54 Step Step 4 Current (ma Current (ma 1.2 b d l w WE 1 WE 2 WE1 WE f Step 1 a b Time (s Step 2 WE1 WE Time (s d Δ Step 3 Step 4 Figure S4. Details of conductivity measurements. (a CVs for films measured for effective conductivity with WE1 and WE2 shorted together. (b Schematic of electrodes and conductivity calculation. σ eff is the effective conductivity as shown in Figure 6 in the main text, I cond is the through-film conductivity current (calculated below, w is the IDA gap spacing (2 µm, N is the number of electrodes (130, d is the film thickness (See SI Table 1, and is the voltage offset between WE1 and WE2. (c Example of raw data for Co 1-x Fe x (OOH x < 1 demonstrating potential stepping with 10 mv offset between WE1 and WE2. (d Current response of WE1 and WE2 from potential stepping as shown in (c. (e Example of raw potential data for FeOOH demonstrating how alternating 10 mv steps in the relative potentials of WE1 and WE2 were S6

7 used to extract the conductivity component of the current. (f Current response of WE1 and WE2 from potential step as shown in (e. Calculations: I cond via steady state at a series of polarizations with 10 mv offset between WE1 and WE2 (Co 1- xfe x (OOH x < 1 (Figure S4, d: 2 I cond via alternating 10 mv steps (Co 1-x Fe x (OOH x = 1(Figure S4, f: Step 1 (point a: Step 2 (point b: S7

8 d-spacing (Å a d-spacing (Å % Fe 54% Fe c (003 -Co(OH 2 (Liu et al. x10 (001 (006 x10 x5 (101 (100 (012 (015 (011 (002 (0015 (110 (113 (1013 (116 ( (Degrees d-spacing (Å (018 (0012 (1010 (0111 (202 -Co(OH 2 (Liu et al. (012 (110 (003 ( % Fe b (103 (003 (200 (101 Fougerite Fe(OH 2 (OH 0.25 (H 2 O ICSD (006 (201 (012 (301 (210 (111 (015 (018 -FeOOH ICSD (212 (511 (003 -Co(OH 2 (Liu et al. (110 (020 (200 (200 (101 (021 (130 (201 (400 (130 (211 (111 (301 (301 (210 (111 (150 (002 (141 -FeOOH ICSD 1545 (151 -FeOOH ICSD (600 (212 (251 (511 (132 (541 -FeOOH ICSD x10 (001 (006 x10 x5 (101 (012 (015 (100 (011 (002 (018 (0012 (1010 (0111 (0015 (110 (113 (1013 (116 (0114 (202 -Co(OH 2 (Liu et al. (012 (110 (003 (111 ( (Degrees (Degrees Figure S5. Grazing Incidence X-ray diffraction of Co 1-x Fe x (OOH thick x = 0 (a, x = 1 (b and x = 4 (c films. hkl patterns are visually depicted beneath the raw data and ICSD numbers are in the upper right hand corner of corresponding cells. Lines depicted with a multiplier were reduced by that factor. hkl patterns for α and β have been adapted from Lui et al. 1 S8

9 Figure S6. SEM images of as-deposited Co-Fe (oxyhydroxide thin films. Compositions as determined by XPS are given in the lower-right corners of each image. Scale bars for rows A and C indicate 100 nm and 200 nm in the insets. Scale bars for row B and D indicate 1 μm. All images are of samples deposited on Au/Ti QCM crystals. Average film thicknesses for each composition are given in Table S1*. S9

10 Normalized Intensity Normalized Intensity O1s Co 2p3 0% Fe % Fe Fe 2p Normalized Intensity Binding Energy (ev Fe auger Binding Energy (ev Figure S7. XPS characterization. Changes in (a O 1s, (b Co 2p3, and (c Fe 2p Al Kα XPS spectra with 2 h polarization at η = 350 mv. Fe content as determined by XPS is shown in the upper- (pre-polarization %Fe and lower-right/left (post-polarization % Fe of each graph Fe auger Co auger 29 Co auger Binding Energy (ev Fe auger Pre- Post Pre- Post- Pre- Post- S10

11 Figure S8. SEM images of Co-Fe (oxyhydroxide thin films post- 2 h polarization at η = 350 mv in 1 M KOH. Compositions as determined by XPS are given in the lower-right corners of each image. Scale bars for rows A and C indicate 100 nm and 200 nm in the insets. Scale bars for rows B and D indicate 1 μm. All images are of samples deposited on Au/Ti QCM crystals. Average film thicknesses for each composition are given in Table S1*. S11

12 Overpotential ((V-iR u Overpotential ((V-iR u Overpotential ((V-iR u CV (pre- 1st cycle CV (pre-2 nd cycle CV (post- 2 nd cycle 0% Fe Potential (E(V-iR u vs. Hg/HgO Overpotential ((V-iR u CV (pre- 1st cycle CV (pre- 2 nd cycle CV (pre- 1st cycle CV (pre- 2 nd cycle CV (pre- 1st cycle CV (pre- 2 nd cycle CV (post 2 nd cycle 0% Fe CV (post 2 nd cycle 14% Fe Potential (E(V-iR u vs. Hg/HgO Potential (E(V-iR u vs. Hg/HgO Overpotential ((V-iR Overpotential ((V-iR u u CV (pre- 1st cycle CV (pre- 2 nd cycle CV (pre- 1st cycle CV (pre- 2 nd cycle CV (post- 2 nd cycle 33% Fe CV (post- 2 nd cycle 54% Fe CV (post- 2 nd cycle 79% Fe Potential (E(V-iR u vs. Hg/HgO Potential (E(V-iR u vs. Hg/HgO Potential (E(V-iR u vs. Hg/HgO Overpotential ((V-iR u Overpotential ((V-iR u CV (pre- 2 nd cycle CV (pre- 1st cycle CV (pre- 1st cycle CV (pre- 2 nd cycle CV (post- 2 nd cycle 92% Fe Potential (E(V-iR u vs. Hg/HgO CV (post- 2 nd cycle 100% Fe Potential (E(V-iR u vs. Hg/HgO Figure S9. Changes in CV pre- (CV 1 st and 2 nd cycle and post- 2 h at η = 350 mv (2 nd cycle. The as-deposited compositions are listed in the lower right hand corner. S12

13 Overpotential (V CV (pre- 1st cycle CV (pre- 2 nd cycle CV (post 2 h in 1 M KOH 1 st cycle 14% Fe Potential (V vs Hg/HgO a Potential (V vs Hg/HgO Figure S10. (a 1 st (black and 2 nd (orange cycle pre- and 1 st (grey cycle of x = 0.14 in 1 M KOH post 2 h with no applied potential (not O 2 saturated (b representative cycles of sequential anodic (1 st cycle black, 2 nd cycle green, cathodic (grey, and anodic (1 st cycle dark pink, 2 nd cycle light pink cycling of x = 0.33 showing reversibility of the initial large anodic peak with Co 1-x Fe x (OOH compositions stable at reducing potentials Cathodic 1 st cycle (pre- cathodic 1 st cycle (post cathodic Anodic 2 nd cycle (pre- cathodic 2 nd cycle (post cathodic 33% Fe b 4 Equation 1: Model TOF (s η= 350 mv,, Equation 2: Equation 3:,, Fe content, x (% Figure S11. (a Model data for the expected measured TOF based on situations where all the metals are considered active sites (TOF mass, Equation 1, grey, only Co are considered active sites (TOF,, Equation 2, red, and only Fe are considered active sites (TOF,, Equation 3, black. The model assumes a constant Co (TOF and Fe (TOF activity. TOF was calculated based on the average of TOF,, at x = 0 during steady-state polarization at η = 350 mv after 1 min and 120 min (~06 s -1. TOF was calculated based on the average of TOF,, at as-deposited x = 0.14, 0.33, 4, and 0.79 during steady-state polarization at η = 350 mv after 1 min and 120 min (~0.8 s -1. This data is overlayed on the experimental data in the manuscript. S13

14 Overpotential (V-iR u Pre- 2 h at η = 350 mv a 0 0 (S Log(Current Density, A cm -2 Log(Current Density, A cm -2 Figure S12. Tafel plots for a representative Co 1-x Fe x (OOH (x = 0 1 sample (a pre- and (b post- 2 h at η = 350 mv vs. Hg/HgO. Tafel slopes and error bars (Table S1 were calculated from averages and standard deviations, respectively, of three different samples. Due to the hysteresis observed due to capacitive and surface-faradaic transient current measured during voltammetry at 10 mv s -1, the forward and reverse sweeps were averaged prior to plotting on the semi-log plots and fitting to extract the Tafel slope. The averaged voltammetry data is shown as the light colored lines. Each line is labeled for the Fe content (x in the film at that point in the measurement Overpotential, (V-iR u0.40 Post- 2 h at η = 350 mv b 0 8(S S14

15 Figure S13. (a Fe 2p XPS spectra (Mg-XPS source and SEM images (right, c, d, and e of Co(OOH pre- (red, initial and post- 2 h polarization at η =350 mv (final in purified KOH (P, pink and standard KOH (S, grey. TOF int (s a 0.1 b 0.2 c Current density (ma cm mv η (0% mv η (54% 0 % Fe Integration (mc Potential (E(V - ir vs Hg/HgO Current density (ma cm Potential (E(V - ir vs Hg/HgO Figure S14. Loading dependence for x = 0 and x = 4 on apparent TOF with corresponding CVs on Au. The increase in loading is indicated by the arrow (light to dark. All samples analyzed in this paper were roughly the same loading as the samples highlighted in the grey box in (a. Previously, an electron withdrawing effect from Au has been shown to increase the activity of thin films of CoOOH and NiOOH. 2 Our preliminary thickness-dependence study S15

16 indicates that the samples under study here are in a loading regime where there is much less substrate effect than for the first few monolayers as observed by Yeo et al. Given the high conductivity of Co(FeOOH we do not expect the film conductivity to affect the measured activities in the thickness/loading range studied. REFERENCES (1 Liu, Z.; Ma, R.; Osada, M.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2005, 127, (2 Yeo, B. S.; Bell, A. T. J. Am. Chem. Soc. 2011, 133, S16