Supporting Information (SI): Revised Oxygen Evolution Reaction Activity Trends for First- Row Transition Metal (Oxy)hydroxides in Alkaline Media

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1 Supporting Information (SI: Revised Oxygen Evolution Reaction Activity Trends for First- Row Transition Metal (Oxyhydroxides in Alkaline Media Michaela S. Burke, Shihui Zou, Lisa J. Enman, Jaclyn E. Kellon, Christian A. Gabor, Erica Pledger, and Shannon W. Boettcher* Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97, United States Experimental Methods Catalyst synthesis. All films were cathodically deposited from single or mixed-metal nitrate and or chloride solutions (. M total metal ion concentration, < depositions per session in a - electrode cell with a carbon cloth counter electrode (Fuel Cell Earth, untreated. Starting materials included Co(NO H O (Strem chemicals, % trace metal basis, FeCl H O (Sigma-Aldrich, > 9 %, and.5 M NaNO Mallinckrodt Chemicals, > 9 %, for Fe only materials, Ni(NO H O (Sigma Aldrich, %, and Mn(NO x H O (Sigma Aldrich, %. All Fe containing solutions were purged with N for ~ min prior to FeCl addition and between depositions (to prevent precipitation of Fe + oxyhydroxides from the deposition solution., Films were deposited on /Ti or /Ti quartz crystals (5 Mhz, Stanford Research Systems after an initial acid clean ( CV cycles in M H SO, two-electrode cell, coiled electrode counter and base rinse ( CV cycles in M KOH. Deposition rates vary based on material, see Table S for complete deposition parameters. Film masses were calculated based on the Sauebrey equation (, is the sensitivity factor,.5 Hz cm µg - in solution, is the experimental frequency change, and is the change in mass per. cm area in water pre- and post- deposition., For Co-containing films the quartz-crystal microbalance holder was exchanged for a cleaned one following the deposition step to prevent contamination of the analysis solution from Co adventitiously deposited on the electrode holder. In situ masses are based on mass changes measured during electrochemical analysis in M KOH solution. Electrochemical characterization. All electrochemical analysis was done in a - or -electrode glass-free cell using a BioLogic potentiostat (SP or SP, coiled -wire counter (regularly aqua regia cleaned in a plastic fritted compartment, and a Hg/HgO reference electrode (CH

2 Instruments. Ultra-high-purity O was bubbled ~ min prior to analysis and throughout characterization. A magnetic stir bar was positioned close to the working electrode surface and run during characterization to dislodge bubbles. Steady-state Tafel measurements were taken with either constant current or constant potential techniques. All films were analyzed via chronopotentiometry from low to high current densities ( min per step for the first two steps and min per step for all subsequent steps. The last - s of each step were averaged and used for analysis. The samples labeled Thick and Thin FeO x H y were measured with chronoamperometry ( min, corrected for ir u in real time with new samples for each applied potential (because FeO x H y slowly dissolves under anodic bias in base. The last s of each step were averaged and used for analysis. All measurements were corrected for uncompensated series resistance (R u. R u was calculated using the minimum impedance between khz and MHz where the phase angle was closest to zero. R u ranged from -7 Ω for /Ti crystals to 5 - Ω for /Ti crystals. The overpotential (η was calculated by η = E measured - E rev - ir u. E measured is the recorded potential vs Hg/HgO and E rev is the reversible potential (. V vs. Hg/HgO for the OER. Hg/HgO reference electrode was calibrated against a reversible hydrogen electrode (RHE at ph (.9 V vs. RHE. All Fe-free catalysts were measured in Fe-cleaned base unless otherwise mentioned. Specifically, Ni-based catalysts were tested in Ni(OH -cleaned base and Co-based catalysts were tested in Co(OH -cleaned base. First, high purity Ni or Co nitrate salts were precipitated with M KOH (Semiconductor Grade, Sigma Aldrich. The residual potassium nitrate was removed by washing with dilute KOH via several cycles of vigorous agitation and centrifugation. Semiconductor grade M KOH electrolyte was then added to clean M(OH precipitate and vigorously mixed for - min, left to sit overnight, and separated from the precipitate via centrifugation. In situ electrical conductivity measurements for MnO x H y were taken using a /Ti interdigitated array electrode (IDA (CH instruments, μm electrode width, μm gap, mm length, 5 pairs. Due to low effective conductivity at low potentials, care was taken to ensure the measured current was associated with electrical conductivity. First both working electrodes were poised at the same potential. Then the steady state current was measured with +/- mv offsets between WE and WE to determine the conductivity current component. This was done at several potentials relative to the reference electrode. See Burke et al (Ref. S5 for further calculation and experimental details. 5 Materials characterization. Scanning electron microscope (SEM images were taken using a Zeiss Ultra 55 SEM operating at 5 kev. Compositional information (Fe/Ni/Mn/Co ratio was determined by X-ray photoelectron spectroscopy (XPS with an ESCALAB 5 (ThermoScientific using a Mg Kα nonmonochromated flood source ( W, 75 ev pass energy. All samples were charge neutralized using an in-lens electron source and grounded to the stage with a conductive clip to minimize charging. The resulting spectra were analyzed with a Shirley background, calibrated using the carbon peak (.5 ev and peak fit using ThermoScientific Avantage.75 software.

3 Table S. Supporting numerical data. Sample MnO x H y Ni(MnO x H y NiO x H y Ni(FeO x H y FeO x H y FeO x H y CoO x H y Co(FeO x H y Deposition Solution Deposition rate (ma cm -. M.7 M Ni +. M Ni + Mn +. M Mn +.9 M Ni +. M Fe +. M Fe +.5 M NaNO. M Fe +.5 M NaNO. M Co +. M Co +. M Fe Electrolyte, M KOH Purified Purified Purified Standard Standard Standard Purified Standard Substrate Composition from ex situ XPS Mass deposited (µg cm - (after nd CV cycle MnO x H y N.M. NiO x H y Ni.7 Fe.9 O x H y FeO x H y FeO x H y CoO x H y Co.59 Fe. O x H y 9 ±. ±..7 ±.. ± ±..5 ±. ± ± Electrochemical data from -step galvanostatic analysis Tafel slope (mv dec - 7 ± ± ± 9. ±. ± 59 ± 5 ± y-intercept (V.9 ±.. ±..77 ±.5.9 ±..5 ±..55 ±..9 ±. TOF at η = 5 mv(s - TOF at η = 5 mv(s -. ±.. ±.. ±.. ±.. ±.. ±.. ±..5 ±.. ±.7 Mass change (% -5 ± -. ±.. ±.5. ±. -7 ± ± - ± Electrochemical data from potentiostat measurements (new FeO x H y film for each point TOF at η = 5 mv(s -. ±.. ±. TOF at η = 5 mv(s - 7. ±. 5 ± Tafel slope (mv dec -.7 ±. ± y-intercept (V. ±..5 ±. Substrate Composition from ex situ XPS Range mass deposited (µg cm - (post nd cycle MnO x H y Ni.9 Mn. O x H y NiO x H y Ni.5 Fe.5 O x H y FeO x H y FeO x H y CoO x H y Co.5 Fe. O x H y Electrochemical data from -step galvanostatic analysis ±.7. ± Tafel slope (mv dec ± 5 y-intercept (V TOF at η = 5 mv(s TOF at η = 5 mv(s Mass change (% ±.5 Electrochemical data from potentiostat measurements (new FeO x H y film for each point TOF at η = 5 mv(s -. ±.. ±. TOF at η = 5 mv(s -. ±.5. ±. Tafel slope(mv dec - 7. ±. ± y-intercept (V. ±..5 ±. All boxes with no applicable data are greyed. N.M = not measureable due to interference from ger peaks. The differences in electrodeposition solution composition and electro-

4 precipitated film composition is likely due to differences in the solubility of the different cations in the deposition solution. Overpotential (V-iR u A MnO x H y Mn(FeO x H y log current density (A cm - Mass Change (% Intensity B Fe p MnO x H y Mn(FeO x H y Binding Energy (ev Current density (ma cm C MnO x H y ( Mn(FeO x H y ( Mn(FeO x H y (... Overpotential (V-iR u MnO x H y Figure S: Fe contamination from semiconductor grade base in Mn (oxyhydroxide films. (a Tafel analysis of as deposited MnO x H y in standard semiconductor grade base (Mn(FeO x H y and Ni cleaned semiconductor grade base (MnO x H y, (b Fep spectra showing Fe impurities are present after Tafel analysis in semiconductor grade KOH, (c cyclic voltammetry before and after Tafel analysis in standard semiconductor grade base (Mn(FeO x H y and in Ni-cleaned base (MnO x H y. TOF : TOF FeO x H y (thick FeO x H y (thin CoO x H y η = 5 mv NiO x H y Increased activity on MnO x H y Co.59 Fe. O x H y Ni.5 Fe.5 O x H y Increased activity on Ni.9 Mn. O x H y Figure S: Ratio of TOF measured on relative to that measured on at η = 5 mv. Only the Fe sample shows substantial substrate enhancement effects. Some points overlap.

5 NiO x H y Ni.7 Fe.9 O x H y Overpotential (V - ir u NiO x H y Overpotential (V - ir u Ni.5 Fe.5 O x H y MnO x H y Ni.9 Mn. O x H y MnO x H y Overpotential (V - ir u Overpotential (V - ir u Ni.9 Mn. O x H y

6 CoO x H y Co.59 Fe. O x H y Overpotential (V - ir u CoO x H y Overpotential (V - ir u Co.5 Fe. O x H y - Figure S: Voltammetry of the catalysts on and. The second voltammetry cycle is shown prior to (initial, and after (final, the steady-state Tafel analysis. The inset depicts an expanded region showing the redox or pseudocapacitive response. Dotted line marks where η = mv. Anodization at η = 5 mv A substrate Thin initial Thick initial Thick final Thin final.. Anodization at η = 5 mv B - - Current Density Current Density substrate Thick initial Thick final Thin initial Thin final Figure S: Voltammetry of thick and thin FeO x H y on a and b prior to (initial and after ( min at η = 5 mv.

7 Figure S5: Scanning electron micrograph of thick and thin FeOxHy before and after polarization at η = 5 mv. Scale bars are um, nm, and nm for the left, middle and right columns, respectively. 7

8 NiO x H y log current density (A cm Mass Change (% CoO x H y log current density (A cm Mass Change (%.. Ni.7 Fe.9 O x H y - - Mass Change (%... Co.59 Fe. O x H y Mass Change (% log current density (A cm log current density (A cm MnO x H y log current density (A cm Mass Change (%.5... Ni.9 Mn. O x H y log current density (A cm - - Mass Change (% Figure S: Tafel Analyses of Ni, Co, Mn, NiFe, CoFe, NiMn (oxyhydroxide films on (performed in triplicate, some error bars are smaller than the symbol and on. The half-open symbols correspond to the mass measurements. The compositions listed are those measured on.

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11 Figure S7: SEM images of Mn, NiMn, Ni, NiFe, Co, CoFe (oxyhydroxide films on and after steady-state Tafel analysis. Scale bars are um, nm, and nm for the left, middle and right columns, respectively.

12 Fe p Fe p Intensity (A.U. MnO x H y (Al Kα Intensity (A.U. Ni.9 Mn. O x H y - Mn ger NiO x H y control Binding Energy Binding Energy Intensity (A.U. Fe p NiO x H y - NiO x H y - Intensity (A.U. Ni.7 Fe.9 O x H y - Ni.5 Fe.5 O x H y - Fe p Binding Energy Binding Energy Fe p Co.5 Fe. O x H y - Fe p Intensity (A.U. CoO x H y - CoO x H y - Intensity (A.U. Co.59 Fe. O x H y Binding Energy Binding Energy Figure S: Fe p XP spectra of both Fe-free films and those with Fe added. Interfering ger peaks are labeled. Measurements were taken with a Mg Kα source unless otherwise specified.

13 Supplementary References: ( Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution. J. Am. Chem. Soc.,, ( Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel-Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc.,, ( Sauerbrey, G. Z. Verwendung von Schwingquarzen Zur Wägung Dünner Schichten Und Zur Mikrowägung. Phys. A Hadron. Nucl. 959, 55,. ( Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. a; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev.,, 7. (5 Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt-Iron (oxyhydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 5, 7,.