SUPPORTING INFORMATION

Transcription

1 SUPPORTING INFORMATION An Investigation of Thin-Film Ni-Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen Mary W. Louie and Alexis T. Bell S 1. GOLD ELECTRODES S 1.1 Roughening Procedure Gold electrodes used as SERS substrates were electrochemically roughened 1 in.1 M KCl by cycling the potential between -8 and 1.22 V vs. Ag/AgCl (4 M KCl filling solution) at 5 mv s -1, with a 5 and 1 s dwell at -8 and 1.22 V, respectively. After this potential cycle was carried out 25 times, the potential was held at -.3 V for 5 s. S 1.2 Electrochemical Characteristics A representative cyclic voltammogram for a bare gold substrate is shown in Figure S1 for reference. The gold electrode shows redox transition at ~.3 V vs. Hg/HgO (1 M KOH) in.1 M KOH. The oxidation process is comprised of a peak at.35 V followed by broad feature visible between and V. The reduction process is better defined, with a peak at 3 V. The roughness factors (measured surface area divided by geometric area) for roughened gold substrates were determined both by electrochemical capacitance measurements and by integration of the gold oxide reduction peak. Capacitance measurements were similar to those carried out for Ni-Fe oxide/hydroxide films (1-1 mv s -1 in.1 M KOH and a specific capacitance of 6 µf cm -2 ), except that a potential window just above the gold redox peak, V vs. Hg/HgO, was selected. For surface area determination using the area of the gold oxide reduction peak (Figure S1), a value of 482 µc cm -2 was used. 2 Both methods yielded roughness factors of approximately 3. Apparent Current Density [ma cm -2 ] polished gold substrate 1 mv s Potential vs. Hg/HgO (1 M KOH) [V] Figure S1. Cyclic voltammogram (1 mv s -1 ) for bare gold substrate in.1 M KOH showing a redox couple at ~.3 V vs. Hg/HgO. S 2. ELECTRODEPOSITED NICKEL-IRON FILMS S 2.1 Electrodeposition of Ni-Fe The ph of the deposition solutions ranged between 5 (for.1 M FeSO 4 ) and 6 (for.1 M NiSO 4 ), depending on the relative concentrations of Ni and Fe. Deposition potential curves (Figure S2a) indicate that steady-state deposition conditions are reached by 2 3 s. The potential recorded at steady-state exceeds the equilibrium reduction potentials (at ph 5 6) for nickel or iron plating, sulfate ion reduction, and hydrogen evolution, likely due S1

2 to overpotentials of the deposition process(es). (Note that the oxygen reduction reaction is not considered as a possible reaction involved in the deposition process since oxygen gas is minimized in the solutions by N 2 sparging.) Therefore, any of these reactions are possible, and identification of the deposited phases cannot be made solely using the value of the deposition potential. Typically electrodeposited films are visually uniform and range from teal (for pure Ni films) to pink (for NiFe films with 5% Fe) to amber (for pure Fe films). S 2.2 Bulk Composition and Thickness Figure S2b is a plot of the composition of the Ni-Fe films, as determined by elemental analysis, as a function of the bath composition for both as-deposited (filled markers) and electrochemically-tested films (open markers). The measured film composition matches that of the deposition bath for, 5 and 1% Fe contents; preferential deposition of Fe and of Ni were obtained from baths with lower and higher Fe concentrations, respectively. That the compositions of as-deposited and electrochemically-tested films are in agreement implies minimal composition changes in the catalyst films as a result of the electrochemical testing conditions used in this work. Taking the deposition process to involve two electrons per metal atom deposited (applicable in the case of either electroplating or precipitation of metal hydroxide phases due to electro-generation of OH - by evolution of hydrogen 3,4 ), the Faradaic efficiencies for deposition of Ni and Fe (when deposited individually) were calculated to be 95 ± 9% and 81 ± 13%, respectively. The overall metal deposition efficiency for co-deposited films is 97 ± 14%. Using the measured quantities of Ni and Fe, and taking the density of the films to be a weighted average of those for Ni(OH) 2 and Fe(OH) 2 (4.1 and 3.4 g cm -3, respectively), we estimate the thickness of the films to be 7 nm when deposited on a smooth Au substrate. For films deposited on electrochemically-roughened Au substrates, the film thickness is estimated to be 2 nm, due to the increased surface area of the substrate. We note that by elemental analysis, we could not detect the presence of Fe impurities that could be incorporated into pure Ni films from KOH electrolyte. 5,6 Given the quantities of Ni and Fe in our films and the resulting Ni and Fe concentrations used for composition analysis, we estimate a lower detection limit of ~ 3% Fe. Potential vs. Ag/AgCl (4 M KCl) [V] (a) > 1 h N 2 sparge RPM - 5 µa cm -2.1 M NiSO M FeSO 4.1 M NiSO 4.1 M FeSO Time [s] at. % Fe (bath) Figure S2. (a) Electrodeposition potential profiles for three select electrolyte compositions for an applied current density of -5 µa cm -2, and (b) metal composition and estimated thickness of Ni-Fe films electrodeposited atop Au substrates, as determined by elemental analysis. The film thickness shown here was computed using the geometric area of the Au substrate. Filled markers correspond to elemental analysis performed for as-deposited films and open markers to films after OER characterization in KOH. at. % Fe (film) (b) Film Thickness [nm M(OH) 2 ] S2

3 S 2.3 Surface Area Representative plots for the determination of Ni-Fe surface area by electrochemical capacitance measurements are shown in Figure S3. The roughness factors for Ni-Fe films ranged from 2 to 6; such a variation in the surface area of the films is much smaller than changes in the OER activity when the film composition was varied (Figure S5). It should be noted that the surface area determined by electrochemical capacitance measurements was found to depend on ph; specifically, the measured surface area increases with ph, likely due to the greater number of OH - ions present when charging the film-electrolyte interface. As such, all roughness factors were determined using data collected under the same conditions (.1 M KOH). In addition, an inherent assumption when using electrochemical capacitance methods is that the nature of charging in the catalyst films is not dependent on film composition. That is, we neglect any changes in the capacitance due to movement of ions within the Ni-Fe oxide/hydroxide films. These observations imply that the measured surface area is not an absolute value and is only useful for comparison of specific current densities across this Ni-Fe catalyst system. Current [µa]. - Ni-Fe (5% Fe).1 (a) M KOH 1 mv s V [µa] (b) 22 µf Potential vs. Hg/HgO(1M) [V] Scan Rate [mv s -1 ] Figure S3. Electrochemical capacitance measurements: (a) typical voltammograms measured in.1 M KOH at RPM, shown here for a ~ 7 nm Ni-Fe film (5% Fe) on a gold electrode 5 mm in diameter, and (b) average capacitive current taken from center of potential window, at.13 V vs. Hg/HgO. In this example, the extracted slope is 22 µf. S 2.4 Ni(OH) 2 -NiOOH Redox Behavior Figure S4a are plots of the peak potentials and peak areas for the oxidation and reduction of Ni(OH) 2 /NiOOH. The potentials of the oxidation peak (visible for films with lower Fe contents) and the reduction peak trend in the same way, indicating that the shift in the reduction potential (discussed in the main text) is representative of a shift in the redox potential. Similarly, the integrated areas of the oxidation and reduction peaks are equal. Figure S4b are plots of the reduction peak potential and number of redox electrons transferred (or, equivalently, fraction of redox-accessible Ni, assuming a 1-electron redox process) as a function of film composition. The reduction potential is found to agree between thinner films (~ 2 nm) deposited on roughened gold substrates and thicker films (~ 7 nm) deposited on polished gold substrates. In the case of the fraction of redox-accessible Ni, the trend with Fe content is the same for the two types of films, but fraction of Ni that is accessible differs. This behavior can be clarified in Figure S4c which is the integrated peak area for a Ni-Fe film (~ 5% Fe) plotted as a function of the film thickness. We find that as the amount of deposited material is increased, the redox area scales S3

4 accordingly for thinner films but deviates noticeable starting at 3 nm. Furthermore, by examining the peak areas for the films both before and after polarization under OER conditions (335 mv, 3 m,.1 M KOH), we find that thinner films are almost completely oxidized at the start of electrochemical measurements while thicker films oxidized further at OER potentials. Dotted lines in the plot mark the estimated film thicknesses of the two films characterized in this work. Figure S4c suggests that thicker films have a smaller fraction of NiOOH formed under OER conditions, that is, only the outer layer of the film undergoes redox. Thin films (atop roughened gold electrodes), other the other hand, are almost entirely redox-accessible, as indicated by the corresponding dashed line in Figure S4c and by the passage of more than 1 electron in the redox process for pure Ni (Figure S4b). We assume there that variation of the Fe content does not change the accessibility of Ni atoms in the film. Peak Potential [V] Peak Area [mc] (a) (b) 1.2 films on (c) roughened Au Reduction Oxidation Catalyst Composition [at. % Fe] Peak Potential vs. Hg/HgO [V] No. of Redox Electrons substrate type roughened gold polished gold Catalyst Composition [at. % Fe] Figure S4. The Ni(OH) 2 /NiOOH redox transition in.1 M KOH for Ni-Fe films (a) comparison of the peak potentials and peak areas for oxidation and reduction as a function of composition, (b) reduction peak potential and number of redox electrons as a function of composition, with linear fits to each dataset, and (c) a plot of the peak area of Ni-Fe films (5% Fe) on polished Au as a function of film thickness, where dashed lines indicate thickness of films deposited on polished and roughened Au substrates. Peak Area [mc] 1..8 films on polished Au after polarization before polarization Film Thickness [nm] S 2.5 Activity Comparison to Bulk References For reference, we show in Figure S5 the specific current density for the OER for the Ni-Fe film along with that for aged Ni films, fresh and aged Ni metal electrodes, a Fe metal electrode, and an Au metal electrode which is used as the underlying substrate. The OER activity for as-deposited Ni films (6 ±.8 ma cm -2 ) agrees well with that for a fresh (un-cycled) Ni metal electrode (5 ma cm -2 ). The activity for Fe films (.17 ±.7 ma cm -2 ) is roughly three times lower than that for a bulk Fe metal electrode (.54 ma cm -2 ). Aged Ni films (4.2 ma cm -2 ) are over an order of magnitude more active than as-deposited films, consistent with the general consensus in literature that aged Ni (or β-niooh) is Specific Current 3 mv [ma cm -2 ] 1 aged Ni electrode E-3 aged Ni film fresh Ni electrode Au electrode/substrate Ni-Fe films Fe electrode Catalyst Composition [at % Fe] Figure S5. Specific current density at an overpotential of 3 mv for the OER in.1 M KOH for comparison of Ni-Fe films to bulk references. S4

5 preferred for the OER over γ-niooh. 7-1 Aged Ni films are slightly lower in activity compared to an aged Ni electrode (7.7 ma cm -2 ). Based on the low OER currents of ~.3 ma cm -2 for a bare Au electrode, we can conclude that the OER activities of the Ni-Fe films have no direct contributions from the underlying Au substrate. S 2.6 Effect of Rotation Rate The effect of rotation rate on the measured OER characteristics was examined. Figure S6 shows a representative plot of the effect of rotation rate on the OER current density for two select films. These results indicate that mass transport effects, if uncorrected, would under-represent the activity of catalysts such that higher activity catalysts (e.g., films with 5% Fe) would be more impacted than low-activity catalysts (e.g., film with 1% Fe). It should be noted that the effect of rotation rate on the OER current density results in a change in the OER current by less than two-fold when the rotation rate is increased from to 24 RPM; this is change in current is negligible compared to the effect of composition, which results in a 1-fold increase when the Fe content is changed from to 4% Fe (Figure S5). Specific Current 3 mv [ma cm -2 ] % Fe % Fe Rotation Rate [rpm] Figure S6. Effect of rotation rate on the OER current density for Ni and Ni-Fe (5% Fe) in.1 M KOH. Data obtained from voltammograms swept at 1 mv s -1. S 2.7 Stability of Ni-Fe Catalysts Figure S7 shows the stability of a typical high-activity Ni-Fe film (~ 5% Fe) in.1 M KOH (lowest concentrations used in this work). In Figure S7a, we show the electrode potential at a constant applied (apparent) current density of 1 ma cm -2. The noise and the drift in the recorded potential are attributed to bubbles which accumulate at the electrode. Removal of these bubbles (by lifting the RDE out of the electrolyte solution) results a large jumps in the potential readings which are followed by a return to the original potential values. This potential changes from ~ 635 to ~ 65 mv over the course of 2 h; these correspond to overpotentials of 27 and 285 mv, respectively. Voltammograms measured before and after this 2-h polarization (Figure S7b) are almost identical, indicating that neither the catalyst composition (as indicated by the redox feature) nor the activity (as indicated by the OER currents) has changed. S5

6 (a) Apparent Current Density [ma] (b) before 2 h at 1 ma cm -2 after 2 h at 1 ma cm Potential vs. Hg/HgO (1 M KOH) [V] Figure S7. Stability of a Ni-Fe film (~ 5% Fe) in.1 M KOH: (a) electrode potential (vs. Hg/HgO) for an apparent current density of 1 ma cm -2 applied for 2 h, (b) cyclic voltammograms (1 mv s -1 ) measured before and after the stability test in (a). Measurements were carried out at 16 RPM. The equilibrium potential for OER at these conditions is 365 mv vs. Hg/HgO (1 M KOH). We also demonstrate how the presence of Fe stabilizes the Ni-Fe films (Figure S8). Freshlydeposited Ni films display an increase in the OER currents with cycling in the KOH electrolyte; this is attributable to the well-known phenomenon of aging from an initially disordered γ-niooh to the more ordered β-niooh. On the other hand, the addition of even a small amount of Fe (~ 1% Fe) causes the OER currents to stabilize. Apparent Current Density [ma cm -2 ] % Fe, cycle 2 1% Fe, cycle 5 % Fe, cycle 2 % Fe, cycle 5 with ~1% Fe Ni film Potential vs. Hg/HgO (1 M KOH) [V] Figure S8. Cyclic voltammograms in 4.6 M KOH collected at 1 mv s -1 and 24 RPM. Shown are cycles 2 and 5 for a freshly deposited Ni film and a films with ~1% Fe. S6

7 S 3. KINETIC STUDIES S 3.1 Effect of Aging on Ni The effect of aging on the current-potential characteristics of Ni film immersed in 4.6 M KOH is shown in Figure S9. Repeated cycling of the film, in this case, up to 5 cycles results in an positive shift of the redox potential for the Ni(OH) 2 /NiOOH transformation (Figure S9a). A corresponding increase in the OER current is observed, as also indicated in Figure S5. Deliberate aging of this film in 1 M KOH for 3 days followed by subsequent cycling in KOH results is stabilization of the voltammograms to that shown in Figure S9a. The Tafel slope was observed to decrease with cycle number (Figure S9b), from 55 mv dec -1 for Cycle 2 to 41 mv dec -1 for Cycle 5. The latter is the same as that obtained for aged Ni films, 42 ± 2 mv dec -1. The reduction peak potential for aged Ni films is 6 V vs. Hg/HgO. Specific Current Density [ma cm -2 ] (a) 1 mv s, 24 RPM, 4.6 M KOH aged..1.3 Cycle 5 Cycle Overpotential [V] Overpotential [V] Figure S9. Effect of aging on the current-potential characteristics of Ni films in 4.6 M KOH: (a) Cyclic voltammograms for a Ni film during the 2 nd and 5 th cycles, compared to films aged for three days in 1 M KOH, and (b) corresponding Tafel slopes Specific Current Density [ma cm -2 ] mv dec -1 (b) aged Cycle 5 6 mv dec -1 Cycle 2 S 3.2 Determination of Kinetic Parameters Representative current-voltage curves for the Ni-Fe system, with specific current density plotted on a logarithmic scale, are shown in Figure S1. Tafel slopes of 4 and 6 mv dec -1 are shown for reference. The Tafel slopes for mixed Ni-Fe films and aged Ni films (possibly with the exception of 91% Fe) are close to 4 mv dec -1. Fresh, un-aged Ni films and pure Fe films exhibit higher Tafel slopes of 55 ± 6 and 54 ± 2 mv dec -1, respectively. The influence of O 2 and OH - concentration on the OER current is shown in Figure S11. Figure S11a shows two current-voltage plots measured for a catalyst film with Specific Current Density [ma cm -2 ] (4 mv/dec) -1 % Fe 1% Fe Overpotential [V] Figure S1. Current density (log scale) plotted against OER overpotential for Ni-Fe catalysts in.1 M KOH. Tafel slopes of 4 and 6 mv dec -1 shown for reference. Curves acquired at 1 mv s -1 and 24 RPM aged Ni (6 mv/dec) S7

8 37% Fe, one collected after the electrolyte was sparged for 1 hour with N 2 gas and the other with O 2 gas. The two plots can be overlaid almost perfectly, indicating that the OER current does not depend on the amount of O 2 in the bulk of the electrolyte. We note that this observation does not necessarily imply a reaction order in O 2 concentration of zero, since the generation of oxygen bubbles implies O 2 -saturation of the electrolyte solution immediately adjacent to the catalyst. Therefore, unless the reaction rate can be kept low (e.g., at low applied potentials) and/or the rotation rate kept high to eliminate concentration gradients in the electrolyte, obtaining the true dependence of the reaction rate on O 2 concentration is a challenge. The reaction order in the OH - concentration (for a film with 41% Fe, in this example) was determined for KOH concentrations between.1 and 4.6 M (ph 13 and 14.7, respectively). The dashed curves in Figure S11b correspond to measurements carried out at the end of the experiment to verify the stability of the catalyst as the ph was varied. The oxygen evolution current was found to increase with ph while the Tafel slope remained constant, consistent with an OER reaction pathway which is not changing for the concentration range used in this reaction order study. The reaction order for OH - was determined by plotting the potential, determined at a constant current density of 5 ma cm -2 (in this example), against the activity of KOH (Figure S11c). 11 The absolute value of the slope was divided by the Tafel slope to obtain the reaction order, i.e., ( j) Δφ ( C ) b ln ( C ) ln 1 m = = ln i Δφ, C i k i j, Ck i where j is the OER current density, C i is the concentration of species i, ϕ is the interfacial potential across the electrode-electrolyte interface, and b is corresponding Tafel slope for the system. This method (of using constant current density rather than constant potential) ensures consistency between the extracted reaction order and the Tafel slopes reported in this work, since determining the reaction order at a constant current density, rather than constant interfacial potential, eliminates the effects of mass transport limitations which has a larger contribution for catalyst with higher OER activities. Using this method, the reaction order for the data shown in Figure S11(b-c) is 1.3. The reaction orders for catalysts with different film compositions were are all close to unity; the value averaged across all samples is 1.1 ±.1. Spec. Current Density [ma cm -2 ] O 2 -sparged N 2 -sparged Spec. Current Density [ma cm -2 ] i,f i,f (a) (b) (c) Potential vs. Hg/HgO(1 M) [V] Potential vs. Hg/HgO (1 M) [V] log(a KOH ) Figure S11. Effect of (a) gas environment and (b) OH - concentration on the current-potential curves for OER, measured at 1 mv s -1 and 24 RPM. To determine the reaction order in OH -, the potential extracted from (b) at a current density of 5 ma cm -2 was plotted against the activity of KOH, shown in (c). The reaction order was obtained by dividing by the Tafel slope. Compositions are 41% Fe for (a) and 37% for (b, c). Potential vs. Hg/HgO(1 M) [V] mv/dec S8

9 S 3.3 Discussion of Kinetic Parameters The Tafel slopes of 55 ± 6 and 42 ± 2 mv dec -1, for as-deposited and aged Ni catalysts, respectively, fall within the values of mv dec -1 found in literature. 5,6,9,1,12-18 The wide range of Tafel slopes reported by previous groups is attributable to different preparation methods which likely result in the existence of different phases of Ni oxides/hydroxides. The relatively large standard deviation in the Tafel slope for as-deposited Ni films (55 ± 6 mv dec -1 ) is likely a consequence of a the instability of the films, that is, as-deposited films change more rapidly immediately upon immersion in the electrolyte, making reproducibility across samples challenging. On the other hand, the Tafel slope of 42 ± 2 mv dec -1 for aged films and 41 ± 1 mv dec -1 for cycled films is highly reproducible. The Tafel slope of 42 mv dec -1 and the corresponding reaction order in OH - of 1 for aged Ni agree well with values reported by Lyons and Brandon 1 who is one of two groups which examined both kinetic parameters. The same group found the kinetic parameters for Fe to depend on pretreatment of the Fe electrode, that is, minimally-cycled, un-aged electrodes yielded a Tafel slope and reaction order for OH - close to 4 mv dec -1 and 1, respectively, 19 while multi-cycled, aged electrodes yielded values close to 6 mv dec -1 and 1.5, respectively. 2 In a report by Doyle and Lyons, 21 a Tafel slope of 4, 6 or 12 mv dec -1 and reaction order in OH - of.5, 1. or 1.5 were reported, depending the concentration of the electrolyte in which the Fe oxide/hydroxide films were formed. The Tafel slope for the Fe films measured in this work is 54 ± 2 mv dec -1 and the reaction order is close to unity; therefore, it appears that electrodeposited films contain phases of Fe oxide/hydroxide which may be similar to those formed over Fe metal in 1 M KOH. 21 Tafel slopes reported for Ni-Fe catalysts in literature range between 25 and 6 mv dec -1, 5,12-15,22 and even within a given study, range between 46 and 61 mv dec -1 as a result of preparation conditions. 12 The Tafel slopes reported by Miller and Rocheleau (for films with 1-5% Fe) 22 and Landon et al. (for films with 5-2% Fe) 15 are in agreement with our value of ~ 4 mv dec -1. No systematic study of the reaction order has been determined for the Ni-Fe series, although Li et al. presents electrochemical data for various OH - concentrations. 14 From this data, we extracted a reaction order for OH - close to unity for a Ni-Fe catalyst with ~ 1% Fe. S9

10 S 4. X-RAY PHOTOELECTRON SPECTROSCOPY S 4.1 Ni films and Fe films: Before and After OER The XPS spectra for pure Ni films, both as-deposited and after testing under OER conditions, are shown in Figure S12. The Ni 2p spectra show negligible change after OER testing and is consistent with either a Ni(OH) 2 or NiOOH phase, both of which have Ni 2p 3/2 binding energies close to ev. 23,24 While there is minimal shape and binding energy changes observed in the Ni 2p spectra, the O 1s spectra for as-deposited films show a single peak at 531. ev attributable to a hydrated phase (Ni-O-H) and the appearance of a Ni-O species (529. ev) after OER polarization. These spectra are consistent with an initial, as-deposited Ni(OH) 2 phase at the film surface which then transforms to NiOOH during OER. The XPS spectra for pure Fe films, both as-deposited and after testing under OER conditions, are shown in Figure S13. The Fe 2p spectra show the reduction in the amount of metallic Fe (77, 72 ev) after OER. 25 Due to similarities in the binding energies and spectral shapes of the higher oxides of iron, 25 it not possible to uniquely identify the iron phases present; the iron oxide phases which are consistent with the observed spectra include Fe 3 O 4, Fe 2 O 3 and FeOOH. The O 1s spectra show an increase in the amount of oxide phases (Fe-O at ev) relative to hydrated phases (Fe-O-H at ev) after OER polarization, suggesting a greater iron oxide contribution after OER. Thus, after OER polarization, the Ni film surface is comprised primarily of Ni(OH) 2 with a minor contribution from NiOOH while the Fe film surface is comprised of Fe 3 O 4 and Fe 2 O 3, with a minor contribution from FeOOH. (a) Normalized Intensity as deposited after OER Binding Energy [ev] Binding Energy [ev] Figure S12. X-ray photoelectron spectra for Ni films, as-deposited (grey) and after OER polarization (black), measured at electron takeoff angle. Shown are Ni 2p (a) and O 1s (b) regions. (a) Normalized Intensity as deposited after OER * 71.8 Binding Energy [ev] Binding Energy [ev] Figure S13. X-ray photoelectron spectra for Fe films, as-deposited (grey) and after OER polarization (black), measured at electron takeoff angle. Shown are Fe 2p (a) and O 1s (b) regions. Asterisks indicate contribution from metallic Fe. * (b) (b) S 4.2 Effect of Composition The Ni 2p and Fe 2p spectra do not change significantly (in both binding energy and spectral shape) with film composition for as-deposited films (not shown) and post-oer films (Figure S14). Figure S14 shows that the Ni 2p spectra are virtually identical for films across the entire composition range, indicating the presence of Ni(OH) 2 /NiOOH at the surfaces of all the Ni-containing films. In the case of the Fe 2p spectra, some metallic Fe S1

11 (77 and 72 ev) is evident for films with 9 and 1% Fe. It should also be noted that the Fe 2p spectral background has contribution from a Ni LMM Auger peak 26 which is most apparent for the film with % Fe. The variation in the O 1s spectra with composition is consistent with the presence of oxygen species associated with Ni and with Fe. That is, the spectra appear to be comprised of a mixture of the Fe and Ni oxides/hydroxides identified in Section S 4.1. These results are in contrast to recently reported XPS spectra for annealed Ni-Fe catalysts which indicate the presence of NiO. 6,15 Recent work by Trotochaud et al. indicate that initial the NiO phase slowly transforms to Ni(OH)/NiOOH under potential cycling in alkaline solutions. 6 Ni 2p Fe 2p % Fe O 1s Intensity [arb. units] % Fe % Fe % Fe 1% Fe 1% Fe Binding Energy [ev] Binding Energy [ev] Binding Energy [ev] Figure S14. X-ray photoelectron spectra for Ni-Fe films after OER polarization at 335 mv overpotential in.1 M KOH. S 4.3 Effect of Electron Takeoff Angle For pure Ni films after OER polarization, the Ni 2p spectra show negligible changes with electron-takeoff angle (Figure S15a), while the O 1s spectra indicate a higher Ni-O-H contribution relative to Ni-O at higher take-off angles, i.e., closer to the film surface (Figure S15b). In the case of pure Fe films after OER polarization (Figure S15c), the Fe spectra indicate a decreasing contribution from metallic Fe for higher take-off angles, consistent with more iron oxide/hydroxide phases at the surface of the films. The O 1s spectra for these Fe films show a similar trend as Ni films, that is, increasing Fe-O-H species relative to Fe-O species near the film surface (Figure S15d). The surface compositions of the films were determined by integration of the Ni 2p and Fe 2p peak areas. To obtain accurate values for the surface composition, the relative areas of the Ni 2p and Ni Auger peaks were used to proportionally correct (based on the surface composition) for the contribution of the Ni Auger peak area to the Fe 2p peak area. Figure S16a-b shows how the surface compositions of the films compare with their bulk compositions both before and after OER polarization, for two select take-off angles of º and 75º (deepest and most shallow sampling depth, respectively). Figure S16c shows how the surface compositions vary with take-off angle; dotted lines indicate the bulk compositions of the films as determined by elemental analysis. Based on this results, we can conclude that the iron content of the films do not vary significantly between the bulk and the surface of the films. S11

12 The absence of a composition difference between the bulk and the surface of Ni-Fe films is consistent with the XPS results of Landon et al. for annealed films 15 but in contrast to the Auger measurements of Kamnev at al. for coprecipitated Ni-Fe which showed enrichment of either Ni or Fe, 27,28 depending on the composition. Ni 2p (a) O 1s (b) Fe 2p (c) O 1s (d) Intensity [arb. units] Intensity [arb. units] Binding Energy [ev] Binding Energy [ev] Binding Energy [ev] Binding Energy [ev] Figure S15. X-ray photoelectron spectra for Ni films (a, b) and Fe films (c, d) after OER polarization at 335 mv overpotential in.1 M KOH, as a function of electron takeoff angle (defined with respect to sample normal). Surface Composition [% Fe] Electron Take-off Angle (deeper) 75 (shallow) (a) (b) (c) Surface Composition [% Fe] Electron Take-off Angle (deeper) 75 (shallow) Surface Composition [% Fe] post OER as-deposited Bulk Composition [% Fe ] Bulk Composition [% Fe ] Figure S16. Surface composition plotted against bulk composition for (a) films after OER polarization, and (b) as-deposited films. (c) Variation of surface composition with electron take-off angle. Dashed lines in (a) and (b) correspond to identical surface and bulk compositions and those in (c) correspond to the bulk compositions. Electron Take-off Angle [ ] S12

13 S 5. RAMAN SPECTROSCOPY S 5.1 Gold Electrodes Raman spectra for a roughened gold substrate in.1 M KOH is shown in Figure S17. Observed bands at 422 and cm -1 are consistent with reported bands in literature. 29,3 Gold immersed in KOH exhibits bands at 56 and 6 cm -1 which are attributable to Au-O vibrations in Au(OH) 3 and Au 2 O 3, respectively. Bands in the range of cm -1 have been attributed to adsorbed hydroxide ions. 29 No O-H vibrations (except for broad bands at cm -1 for water) are visible at high wavenumbers. V.1.1 V Intensity [arb. unit] V Raman Shift [cm -1 ] Raman Shift [cm -1 ] Figure S17. In situ Raman spectra of roughened gold substrate in.1 M KOH as a function of potential (vs. Hg/HgO), for low and high wavenumbers, during an oxidation scan at 1 mv s -1. The equilibrium potential for the OER is.365 V. S 5.2 Identification of Raman Bands in Iron Films The assignment of the Raman bands observed in the electrodeposited Fe films (~ 57, 64-7 cm -1 ) is difficult owing to the many possible oxide and hydroxide phases of iron. In order to gather more information about these bands, we monitored these bands under a reducing potential sweep, from V to potentials as low as -1.2 V vs. Hg/HgO. As seen in the recorded spectra (Figure S18a), the primary band at 57 cm -1 shifts to lower wavenumbers at more reducing potentials, from 567 cm -1 at V to 545 cm -1 at - V. At this potential, the band decreases in intensity and then disappears by a potential of -.7 V vs. Hg/HgO. The band at 77 cm -1 also decreases in intensity with reducing potential and disappears by approximately -.84 V. The band at 667 cm -1 is difficult to discern at V but sharpens starting at a reducing potential of -.7 V. At even higher reducing potentials, the only band which remains is that at 667 cm -1 ; this band disappears completely at a potential of V. Additional bands at ~ 3 cm -1 and ~ 48 cm -1 can be observed at potentials between -.5 and -.84 V, visible likely due to the decreased contribution of the 545 cm -1 band. The corresponding voltammogram (Figure S18b) shows three reduction waves, with the first (denoted as I) attributed to the reduction of oxygen gas in the electrolyte. The second reduction wave (II) corresponds well with the disappearance of bands at 567 and 77 cm -1, and the reduction wave at (III) corresponds well with the disappearance S13

14 of the band at 667 cm -1. This observation suggests that the iron phase(s) with bands at 567 and 77 cm -1 have a higher oxidation state than those with a band at 667 cm -1. This is consistent with the assignment of the 667 cm -1 band to Fe 3 O 4 which is known to exhibit its strongest band at ~ 67 cm The bands at , 48, and ~ 3 cm -1 and the potential range in which they are present are consistent with α-feooh; the band at cm -1 is also consistent with Fe 3 O 4 which is reported to have a band at 55 cm The broad feature at can be assigned to two known Fe(III) phases, including γ-fe 2 O 3 (65-74 cm -1 ) and γ-feooh (66 cm -1 ). 31 However, only γ-fe 2 O 3 exhibits its strongest Raman band in this range, whereas the strongest Raman band for γ-feooh is at 252 cm Therefore, these bands likely arise from γ-fe 2 O 3. Note that there is also some evidence of α-fe 2 O 3 which exhibits strong bands at ~3 and 412 cm -1. (This is not surprising since α-fe 2 O 3 and α-feooh have rather similar structures, as is the case with γ-fe 2 O 3 and γ-feooh. 32 ) These small α-fe 2 O 3 contributions are more apparent in Figure S18a when the primary band at cm -1 is reduced in intensity. It is important to note that the low intensities of these bands make reliable assignment difficult; for example, the band at 412 cm -1 is more apparent in other Fe films (e.g., Figure 6, Main Text). Finally, it should be noted that the spectra measured in this work are similar to those observed by Schroeder and Devine 3 who studied corrosion layers formed over Fe metal in buffered ph 8 electrolyte. In their work, they conclude the presence of Fe 3 O 4 and γ-fe 2 O 3. Here, we cannot exclude contribution from α-feooh. Intensity [arb. unit] (a) V Current [ma] Raman Shift [cm -1 ] Potential vs. Hg/HgO (1 M) [V] Figure S18. Electrochemical reduction of Fe films electrodeposited on roughened Au during a cathodic sweep from to -1.2 V vs. Hg/HgO (1 M) at a scan rate of 1 mv s -1 : (a) in situ Raman spectra as a function of potential, and (b) the corresponding voltammogram. The potential windows at which observed Raman bands are stable are noted in (b). (b) III II cm cm cm cm cm -1 I S14

15 S 5.3 Nickel Films and the 495 cm -1 Band We found, by systematically examining the effect of both laser illumination and the applied potential, that prolonged illumination of pure Ni films by the laser result in irreversible changes in the Ni(OH) 2 structure. This effect is summarized in Figure S19 which shows Raman spectra collected in.1 M KOH under a cyclic potential sweep. As a consequence of continuous illumination of the analysis volume by the laser, we see changes in the relative intensities of the Raman bands at 449 and 495 cm -1. Specifically, we see that when the intensity of the 449 cm -1 grows relative to the 495 cm -1 band, the 3579 cm -1 band grows relative to the bands at 3597 and 3666 cm -1. Thus, we show that the 495 cm -1 which is assigned to a defective Ni(OH) is, indeed, counter-correlated with the ordered β-ni(oh) 2 phase (which exhibits bands at ~ 45 and 358 cm -1.) This provides additional evidence that this 495 cm -1 band is associated with a disordered or defective nickel phase. Furthermore, we find that during both oxidation and reduction, the band is increased in intensity as Ni(OH) 2 is transforming to NiOOH. This seems to suggest that that the number of Ni-O species associated with defects is increased during the Ni(OH) 2 -NiOOH transformation. It should be noted that a surface-enhanced Raman study of Ni(OH) 2 /NiOOH films by Desilvestro et al. 4 reports a weak band near 5 cm -1 which appears -.3 V prior to appearance of the pair of NiOOH bands, similar to what is observed in our measurements; they attribute this band to a change in the structure of the Ni(OH) 2 film. Intensity [arb. unit] V Raman Shift [cm -1 ] Raman Shift [cm -1 ] Figure S19. In situ Raman spectra of Ni films in.1 M KOH for an oxidation sweep followed by a reduction sweep (1 mv s -1 ), showing the correlation between the bands at 449 and 358 cm -1. Potentials are relative to Hg/HgO. V S15

16 S 5.4 Ni-Fe Films S The 495 cm -1 Band Figure S2 shows potential-dependent Raman spectra for films with Fe contents of 8, 19 and 32%. We show that the ~ 495 cm -1 is visible during the Ni(OH) 2 -NiOOH transformation in all cases. In the case of pure Ni films (Figure S19), the 495 cm -1 band is not visible once the NiOOH phase is obtained. As the Fe content is increased, the 495 cm -1 band diminishes in intensity for the Ni(OH) 2 phase (e.g., at V) but is stabilized at high potentials when NiOOH is formed. This implies that although the film becomes more disordered with Fe content, the Ni-O band associated with the presence of defects becomes stabilized at high potentials. This latter stabilization effect is more pronounced for films with 3-8% Fe. (See Figure 8a in Main Text) Intensity [arb. unit] V V V (a) (b) (c) Intensity [arb. unit] Raman Shift [cm -1 ] Raman Shift [cm -1 ] Raman Shift [cm -1 ] Figure S2. In situ Raman spectra collected in.1 M KOH as a function of potential (vs. Hg/HgO) on oxidation and reduction for Ni-Fe films with (a) 8% Fe (b) 19% Fe, and (c) 32% Fe. The equilibrium potential for the OER is.365 V vs. Hg/HgO. 554 V Intensity [arb. unit] V V S Raman Characteristics in O-H Regime Figure S21 shows the Raman spectra for Ni-Fe films in.1 M KOH at a potential of V vs. Hg/HgO (1 M KOH) which corresponds to an OER overpotential of 235 mv. The broad bands present at cm -1 are wellknown characteristics of water 34,41 which, given the focus of the laser beam at the surface of electrode, is likely in the vicinity of the electrode surface. A spectrum for the O-H stretches of water (also provided in Figure S21) was collected with the microscope objective immersed in KOH but withdrawn from the electrode surface so as to remove contributions from the electrode. S High-Activity Ni-Fe Film The Raman spectra for highly active Ni-Fe films (51% Fe) in.1 M KOH are shown in Figure S22. At low potentials, a broad band at 552 cm -1 and a smaller band near 67 cm -1 are visible. At higher potentials, the two bands S16

17 characteristic of a NiOOH phase can be seen (476, 548 cm -1 ). At high wavenumbers, bands attributable to water (3212 and 345 cm -1 ) are visible. There is a shoulder visible at 364 cm -1 which is likely due to disordered β-ni(oh) 2 or α-ni(oh) 2. Intensity [arb. unit] % Fe electrolyte (objective withdrawn) Raman Shift [cm -1 ] Figure S21. In situ Raman spectra (at high wavenumbers) for Ni-Fe films at V vs. Hg/HgO in.1 M KOH. Spectrum for the electrolyte (gray) shown for reference Intensity [arb. unit] V V Raman Shift [cm -1 ] Raman Shift [cm -1 ] Figure S22. In situ Raman spectra of a Ni-Fe film (51% Fe) as a function of potential (vs. Hg/HgO) during an oxidation sweep in.1 M KOH. The equilibrium potential for the OER is.365 V V 3212 V S Smooth Gold Substrates: Elimination of SERS Effects In order to reduce the contribution of Raman bands from gold and iron oxide/hydroxide phases to the reported changes in the relative intensities of the NiOOH bands (Figure 8, Main Text), we electrodeposited Ni-Fe films (, 25, 5, 75% Fe) onto un-roughened gold substrates. Figure S23a shows a film with 75% Fe content at two potentials, 35 and 6 mv vs. Hg/HgO. At 35 mv, the bands for Ni, Fe and Au phases, all of which are expected at this potential, are significantly lower in intensity compared to that for the bands observed at 6 mv. By comparison to Figure 8 in the main text, it can be readily seen that elimination of the roughened Au substrates noticeable reduces the signal of the band at ~ 56 cm -1. Thus, the change in the relative intensities of the NiOOHtype bands, and specifically the growth of the band at ~ 555 cm -1, cannot solely be a consequence of the Raman bands for Fe phases (which would increase in relative intensity for increasing Fe content). Figure S23b shows Ni-Fe films, deposited on un-roughened Au substrates, as a function of composition. These spectra indicate clear change of the relative intensities and shapes of the two NiOOH bands. Furthermore, a band at 493 cm -1 becomes apparent, particularly for films with more Fe (5 and 75% Fe). S17

18 Intensity [arb. unit] 6 mv 35 mv * (a) * (b) % Fe Raman Shift [cm -1 ] Raman Shift [cm -1 ] Figure S23. In situ Raman spectra of Ni-Fe films atop smooth Au: (a) Ni-Fe film (75% Fe) at 35 and 6 mv vs. Hg/HgO, and (b) Ni-Fe films at 6 mv vs. Hg/HgO as a function of composition. Asterisks indicate bands from PFA film used to protect the objective from the electrolyte. S 5.5 Sampling Depth of In Situ Raman Spectroscopy The in situ Raman spectra acquired for Ni-Fe films (~ 2 nm) cannot be solely attributed to vibrations at the film electrolyte interface. In general, when only SERS effects are present, it is expected that the strongest signals originate from the portion of the film closest to the roughened gold substrate where the enhancement effect is the greatest. This is likely the case for Ni films acquired at low potentials (below the Ni(OH) 2 /NiOOH redox potential). That is, Ni(OH) 2 bands in the Raman spectra correspond to species residing near the Au Ni interface, with lesser contribution from the bulk of the film. However, at higher potentials, NiOOH is formed. Since NiOOH undergoes resonance with 633 nm excitation, the Ni-O vibrations have very high cross-section and therefore can generate Raman intensities comparable to those from SERS. Desilvestro et al. (Ref. 33) examined the effect of thickness on NiOOH films (ranging from < 1 to 15 mc cm -2 in thickness) deposited on SERS and non-sers gold substrates. On SERS substrates, the Raman signal reaches a maximum at 1 mc cm -2 and then decreases with increasing thickness, approaching, at 15 mc cm -2, the intensities attained for films on non-sers substrates (for which the signal is attributed to resonance effects). This means that for sufficiently thick Ni films, such as those in our work (~ 25 mc cm -2 ), the Raman signal originates primarily from the bulk of film. If the film thickness exceeds that for the penetration depth of the laser into the film (depending on the absorption characteristics of NiOOH), then the Raman signal will be that from the outer portion of the film, equal to the penetration depth of the laser. In either case of surface- or resonance-enhancement, the Ni electrolyte interface is not dominating the observed Raman spectra. When Fe is incorporated into the film, the origin of the Raman spectra is more difficult to assess, since Fe phases do not appear to exhibit resonance effects under our experimental conditions. Based on the results shown in Figure S23a, we can expect that, at low potentials, the observed Ni and Fe oxide/hydroxide signals arise from SERS effects. At high potentials, when NiOOH is formed, NiOOH vibrations are more characteristic of the bulk, while Fe-O vibrations are likely from the portion of the film closer to the gold surface. The exact volume/depth of each S18

19 phase probed by Raman spectroscopy is difficult to assess as it depends on many factors, including the relative cross-sections of the vibrational modes and the extent to which the phases absorb incoming and scattered light. S 5.6 Powder References Raman spectra of commercial powders of various Ni and Fe oxides/hydroxides are shown in Figure S24. These spectra were collected using an Olympus objective with (1 mag., N. A. =.9) and an acquisition time of 1 s. The oxides shown are: NiFe 2 O 4 (Alfa Aesar 8853) γ-feooh (Alfa Aesar 17531) α-feooh (99%, Alfa Aesar 16267) α-fe 2 O 3 (99%, Alfa Aesar 44666) Fe 3 O 4 (98%, Alfa Aesar 44665) FeO (99.5%, Alfa Aesar 3513) NiO (99%, Alfa Aesar 12359) β-ni(oh) 2 (Alfa Aesar 12517) O-H bands are visible only for Ni(OH) 2. The spectra for oxides without structural protons exhibit no bands at high wavenumbers and therefore are not shown. Intensity [arb. unit] , γ-feooh α-feooh α-fe 2 O Fe 3 O FeO 348, NiO β-ni(oh) Raman Shift [cm -1 ] NiFe 2 O 4 β-ni(oh) 2 α-feooh γ-feooh Raman Shift [cm -1 ] Figure S24. Raman spectra of commercial powders of Ni and Fe oxides/hydroxides collected in air. S19

20 S 6. CATALYST ACTIVITY-PROPERTY CORRELATION We show in Figure S25 correlation plots based on the specific current density (rather than the turn-over frequency based on Ni sites, Figure 1 in Main Text). While there is scatter in plots based on the TOF and the specific current density, the TOF clearly corrects for the points at high Fe content (low I 475 /I 555 or high peak potential). This suggests that the existence of a maximum OER current density at 4% Fe and therefore the drop in the current density at higher Fe content is a consequence of the decreased number of Ni sites, despite its enhanced catalytic properties due to interaction with Fe. Therefore, a plot of the TOF based on Ni sites better represents the catalytic activity of Ni-Fe catalysts. Spec. Current 3 mv [ma cm -2 ] aged Ni film (a) (b) 3 mv [ma cm ] aged Ni film Raman Peak Height Ratio I 475 /I 555 Peak Potential vs. Hg/HgO(1 M) [V] Average Ni Oxidation State Figure S25. Specific current density at 3 mv overpotential plotted against the corresponding (a) ratio of the intensities of the NiOOH Raman bands at 475 and 555 cm -1 (I 475 /I 555 ), (b) the NiOOH reduction peak potential, and (c) the average oxidation state of Ni, as determined from integration of the reduction peaks. Filled markers correspond to as-deposited Ni-Fe films, and the unfilled marker corresponds to an aged Ni film aged Ni film S 7. CALCULATION OF THE TURNOVER FREQUENCY The upper and lower bounds for the TOF based on the number of Ni sites involved in catalyzing the OER is presented in Table S1. The TOF values presented in the main text are denoted as TOF min and computed assuming that all of the Ni atoms in the electrodeposited film, as quantified by elemental analysis, are accessible for catalyzing the OER. The upper limit for the Ni activity, TOF max, is estimated by assuming that Ni sites at the very surface of the Ni-Fe film participate in the catalytic cycle, using a value of Ni atoms per cm 2 area 9 and the surface fraction of Ni as determined by XPS. TOF min values, at 3 mv overpotential in.1 M KOH, range from.5-.8 s -1 for pure Ni films to.5 s -1 for films with 41% Fe. These values are notably lower than that reported by Trotochaud et al. 6,.17 s -1 at 3 mv, which was computed by assuming complete accessibility of a 2-nm Ni oxide film to the OER. However, they are comparable to those estimated (at 3 mv) from the work of Godwin and Lyons 42 (~.9 s -1 ) and Corrigan 5 (~.3 s -1 ). In the former, 42 the redox-active Ni was used to compute the TOF for Ni hydroxide films formed over a Ni metal electrode by potential cycling in KOH. In the latter 5, we took all the Ni atoms in the electrodeposited film to be OER-active. The low values of TOF reported here and in Ref. 5 and 42 suggest that the Ni atoms in electrochemically generated films, and even in the redox-active portions of the film, may not all be accessible. In the S2

21 work of Trotochaud, the films are notably thinner (and also shown to be redox-active after extended measurements in KOH) and therefore, most likely have a majority of the Ni atoms accessible to the OER. Table S1. Computed TOF values for the OER based on Ni as the active site. Ni-Fe on polished Au Ni-Fe on roughened Au TOF % Fe TOF max TOF max TOF min % Fe TOF TOF max TOF max min min TOF min aged Ni S21

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