Supporting Information Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt- Phosphate Catalyst at Neutral ph Yogesh Surendranath, Matthew W. Kanan and Daniel G. Nocera* Department of Chemistry, 6-335, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307 nocera@mit.edu Index Experimental methods Fig. S1. Tafel plots comparing sequential runs Fig. S2. Tafel plots comparing independent runs Fig. S3. Tafel plots comparing incremented and decremented potential sweeps Fig. S4. Tafel plots comparing data collected on a RDE and a stationary electrode Fig. S5. ph dependence of electrode potential in the presence of 0.5 M KNO 3 Fig. S6. Dependence of Tafel behavior on Pi concentration Fig. S7. Koutecký-Levich plots of catalyst activity in 0.1 M NaClO 4, ph 8.0 Fig. S8. Tafel plots comparing activity in KPi electrolyte before and after data collection in 0.1 M NaClO 4, ph 8.0 Page S2-S8 S9 S10 S11 S12 S13 S14 S15 S16 S1
Experimental Methods Materials. Co(NO 3 ) 2 99.999% was used as received from Aldrich or Strem. KOH 85% (VWR International) and NaClO 4 (Fluka) were reagent grade and used as received. KH 2 PO 4 was used as received from Mallinckrodt. All electrolyte solutions were prepared with reagent grade water (Ricca Chemical, 18 MΩ-cm resistivity). 18 OH 2 (97% enriched) was used as received from Cambridge Isotope Laboratories. Indium-tin-oxide coated glass slides (ITO) were purchased from Aldrich (8-12 Ω/sq surface resistivity). Fluorine-tin-oxide coated glass (FTO; TEC-7) was purchased as pre-cut 1 cm 2.5 cm glass pieces from Hartford Glass. Unless otherwise stated, all experiments used FTO with 7 Ω/sq surface resistivity. Electrochemical Methods. All electrochemical experiments were conducted using a CH Instruments 730C or 760C potentiostat, a BASi Ag/AgCl reference electrode, and a high surface area Ni-foam or Pt-mesh counter electrode. Rotating disk electrode measurements were conducted using a Pine Instruments MSR rotator and a 5 mm diameter Pt-disk rotating electrode. All electrochemical experiments were performed using a three-electrode electrochemical cell with a porous glass frit separating the working and auxiliary compartments. Each compartment consisted of 50 ml of electrolyte solution. Unless otherwise stated, all experiments were performed at ambient temperature (21 ± 1 C) and electrode potentials were converted to the NHE scale using E(NHE) = E(Ag/AgCl) + 0.197 V. All overpotentials were computed using η = E(NHE) 0.81 V. Unless otherwise stated, the electrolyte was 0.1 M potassium phosphate, ph 7.0 (Pi electrolyte). Film Preparation. Catalyst films were prepared via controlled-potential electrolysis of Pi electrolyte solutions containing 0.5 mm Co 2+. Depositions were carried out using an FTO-coated glass piece as the working electrode. FTO-coated glass pieces were rinsed with acetone and water prior to use in all experiments and a ~0.5 cm wide strip of Scotch tape was affixed to the FTO coated side such that a 1 cm 2 area was exposed to solution. Unless otherwise stated, controlled potential electrolysis was carried out on quiescent solutions at 1.05 V with passage of 24 mc/cm 2. Cyclic Voltammetry. Catalyst films used for CV studies were prepared as described above, rinsed in fresh Pi electrolyte, and transferred, without drying, to Co-free Pi electrolyte solution. CV scans were initiated at the rest potential and five cycles were taken consecutively without pause in quiescent solution at a scan rate of 10 mv/sec. Potentiostatic Tafel Data Collection. Current-potential data were obtained by conducting controlled potential electrolyses in Pi electrolyte at a variety of applied potentials. Prior to film preparation, the solution resistance was measured in the electrolysis bath to be used for Tafel data collection using the IR test function. The electrolysis solution was exchanged for Co 2+ - containing Pi electrolyte and the film was prepared by controlled-potential electrolysis as described above. Following film preparation, the working electrode was rinsed in fresh Co-free S2
Pi electrolyte and transferred, without drying, to the same electrolysis bath from which the solution resistance was measured. The electrode was allowed to equilibrate with the electrolysis solution for 5 min while being held at the open circuit potential. The solution was stirred and steady-state currents were then measured at applied potentials that descended from 1.3 V to 1.09 V proceeding in 10-30 mv steps. For currents greater than 10 μa/cm 2, a steady state was reached at a particular potential in less than 400 sec. For currents lower than 10 μa/cm 2, longer electrolysis times (as great as 20 min) were utilized to ensure that a steady state had been achieved. Tafel data were collected twice in succession with a 5 min pause between runs during which the electrode was held at open-circuit. The solution resistance measured prior to the data collection was used to correct the Tafel plot for ohmic potential losses. Tafel data collected in succession using the same electrode exhibited good reproducibility (Figure S1). Good reproducibility was also observed for Tafel data collected using two independently prepared electrodes (Figure S2). Dependence of Tafel Data on Direction of Sweep. To examine whether Tafel data were sensitive to the direction of potential sweep, a set of Tafel data was collected using potential steps descending from 1.3 V to 1.09 V proceeding in 10-30 mv increments. The electrode was then held at open-circuit for 5 min and Tafel data were collected using ascending potential steps incremented from 1.09 V to 1.3 V proceeding in 10-30 mv increments. Tafel data are largely insensitive to direction of sweep over a roughly 2.5 decade range in current density from 10 6 to 10 3.5 A/cm 2 but exhibit a slight hysteresis at high current values (Figure S3). Dependence of Tafel Data on Film Thickness. To assess the dependence of the Tafel data on catalyst loading and film thickness, data were collected using the above procedure using catalyst films grown with passage of 6, 24 and 60 mc/cm 2 (Figure 5). For the 6 mc/cm 2 film, the deposition duration was approximately 800 sec. Thus, in all cases, non-faradaic charging currents contributed negligibly to the total charge. Tafel slopes of 61, 62 and 60 mv/decade were observed for the 6, 24 and 60 mc/cm 2 films, respectively. The thickness of the films was estimated by assuming that each cobalt is ligated by six oxygen atoms in an octahedral configuration. Assuming a Co O distance of ~1.9 Å (as indicated by EXAFS data), 1,2 the primary coordination sphere of each cobalt atom occupies a cube 3.8 Å in width. Taking note of the high phosphate content of these films, it is more reasonable to assume, as an upper limit, that each Co occupies a cube 5 Å in width, defining a volume of 125 Å 3. This estimate is in line with EXAFS data of thin films that points to an average structural unit consisting of 7 cobalt atoms arranged in a planar cobaltate lattice. This structural unit, taken together with several phosphates, defines a ~700 Å 3 slab. Thus, 125 Å 3 /Co serves as a reasonable upper limit. Using this estimate, a film monolayer requires a charge of 0.33 mc/cm 2 and is 0.5 1 2 Risch, M.; Khare, V.; Zaharieva, I.; Gerencser, L.; Chernev, P.; Dau, H. J. Am. Chem. Soc. 2009, 131, 6936. Kanan, M. W.; Yano, J; Surendranath, Y.; Dincă, M.; Nocera, D. G.; Yachandra, V. K. J. Am. Chem. Soc. 2010, 132, in press. S3
nm thick. As such, films prepared with passage of 6, 24 and 60 mc/cm 2 are estimated to be 9, 36 and 90 nm thick, respectively. Calculation of turnover frequency (TOF) of the Co-Pi is provided in the text. For Co 3 O 4 nanorods, 3 a turnover frequency for O 2 evolution of 1140 s 1 per cluster under photochemical conditions was reported. To normalize this turnover frequency per cobalt atom in each cluster, the reported average dimensions for each nanorod, taken together with the average number of rods per cluster and the density of the Co 3 O 4, was used to estimate that ~1.5 10 6 Co atoms (volume of a cylinder was used for a rod) comprised each nanocluster. Dividing the reported turnover frequency by this value yields a lower limit per cobalt of ~8 10 4 s 1. Rotating Disk Electrode Studies. To assess whether Tafel data were subject to mass transport limitations, data were collected using a rotating disk electrode. A Pt-disk rotating electrode, polished to a mirror shine with 5 μm alumina, was used as the substrate for catalyst deposition. Prior to film preparation, the solution resistance was measured in Co-free Pi electrolyte while the working electrode was rotated at the rate to be used for subsequent Tafel data collection. The electrolysis solution was exchanged for Co 2+ -containing Pi electrolyte and the film was prepared by controlled potential electrolysis as described above with a 0 RPM rotation rate. Following film preparation, the rotating disk electrode was rinsed in fresh Co-free Pi electrolyte and transferred, without drying, to the same electrolysis bath from which the solution resistance was measured. Rotation was initiated and Tafel data were collected as described above, but with the potential stepped from 1.3 V to 1.14 V. ir corrected Tafel data collected at 1000 rpm and 2000 RPM are compared with data collected on a stationary FTO electrode in a well stirred solution in Figure S4. Tafel plots collected at 1000 and 2000 RPM exhibit similar slopes (61 mv/decade) and current densities to those collected on a stationary FTO electrode (62 mv/decade) indicating that the observed currents are not mass-transport limited over this current/potential range. Phosphate Concentration Dependence. Tafel data were collected as described above at each Pi concentration using independently prepared electrodes. Electrode preparations were all carried out in 0.1 M Pi electrolyte as described above following measurement of solution resistance in the electrolysis bath to be used for Tafel data collection. Tafel data were collected using electrolysis solutions containing 0.03, 0.1, 0.3 and 1 M Pi electrolyte. The 0.03 M Pi solution also contained 0.07 M NaClO 4 to preserve solution conductivity. The ir corrected Tafel data for all four concentrations examined are shown in Figure S6. All Tafel plots exhibit similar Tafel slopes (60-62 mv/decade) and similar exchange current densities (3-4 10 11 A/cm 2 ). Interpolation of the Tafel data was used to derive current values at constant potential displayed in Figure 6. Unbuffered Electrolyte. For studies in unbuffered electrolyte, a Pt-disk rotating electrode, polished to a mirror shine with 5 μm alumina, was used as the substrate for catalyst deposition. 3 Jiao, F.; Frei, H. Angew. Chem. Int. Ed. 2009, 48, 1841-1844. S4
Prior to film preparation, the solution resistance was measured in 0.1 M NaClO 4 electrolyte, ph 8.0, while the working electrode was rotated at 2000 rpm. The electrolysis solution was exchanged for Co 2+ -containing Pi electrolyte, ph 7.0, and the film was prepared by controlled potential electrolysis as described above with a 0 rpm rotation rate. Following film preparation, the rotating disk electrode was rinsed in reagent grade water and transferred, without drying, to the same electrolysis bath from which the solution resistance was measured. Controlled potential electrolysis data were collected at applied potentials ranging from 1.24-1.09 V in 0.03 V increments. At each applied potential, data were collected while the electrode was rotated at 2500, 1600, 1111, 816 and 625 rpm in a stirred solution. The current was allowed to reach a steady state at each rotation rate for 200 sec. Throughout the experiment, the ph was continuously monitored with a semi-micro ph probe (VWR) positioned in the working compartment and small aliquots (1-3 μl) of 0.01 M NaOH were added periodically to maintain the ph of the working compartment at 8.00 ± 0.05. For each applied potential, the steady state current density data recorded over the last 50 sec at each rotation rate were averaged and used to produce the Koutecký-Levich plot (i 1 vs. ω 1/2 ) shown in Figure S7. The intercepts of linear fits to these plots were used to determine the activation controlled current density as a function of applied potential (Figure 5). The data shown in Figure 5 is the average of three independent experiments collected using independently prepared electrodes. Due to the low currents measured in all experiments (< 10 μa), ohmic potential losses were less than 1 mv in all cases and were ignored. In each experiment, following data collection in 0.1 M NaClO 4 electrolyte, the electrode was rinsed in reagent grade water and transferred, without drying, to 0.1 M Pi electrolyte, ph 8.0. Tafel data were collected as described above at a rotation rate of 2000 rpm with the potential stepped from 1.24 V to 1.08 V. Following Tafel data collection in Pi electrolyte, ph 8.0, the Ptdisk rotating electrode was polished and repositioned in the electrolysis cell with preservation of the relative position of the working and reference electrode. The solution resistance was measured and this value was used to generate ir corrected Tafel plots at ph 8.0. A representative example is shown in Figure 5 and is compared (Figure S8) to an ir corrected Tafel plot collected in an identical manner in Pi electrolyte, ph 8, using a freshly prepared catalyst film. ph Dependence. ph titrations were conducted using controlled potential and controlled current methods. For controlled current measurements, catalyst films were prepared as described above and transferred without drying to a two compartment electrochemical cell containing 50 ml of 0.1 M potassium phosphate electrolyte, ph 4.5, in both the working and auxiliary chambers. A chronopotentiometric experiment was initiated with an anodic current of 30 μa/cm 2. The potential was allowed to reach a steady state and small aliquots (2-20 μl) of 40 wt% KOH were added at 3 min intervals to the working and auxiliary chambers to raise the ph from 4.5 to 12.0 in ~0.2 ph unit steps. The ph was continuously monitored with a semi-micro ph probe (VWR) positioned in the working compartment. The potential stabilized at each new ph within 30 sec and the ph remained stable within 0.01 units over the course of each 3 min interval. Following S5
the titration, the solution resistance was measured over the entire ph range using a fresh FTO electrode placed in the same configuration with respect to the reference electrode as the catalystcoated electrode. These resistance values were used to correct the steady potentials obtained from the titration (Figure 3). To rule out diffuse double layer effects, data were collected following the same procedure with an electrolyte solution containing to 0.1 M potassium phosphate electrolyte, ph 4.5, with 0.5 M KNO 3 (Figure S5). For controlled potential measurements catalyst films were prepared using lower surface area FTO electrodes (0.2 cm 2 exposed to solution) to minimize ohmic potential losses. Catalyst films were prepared as described above and transferred without drying to a two compartment electrochemical cell containing 50 ml of 0.1 M potassium phosphate electrolyte, ph 5.8, in both the working and auxiliary chambers. Controlled potential electrolysis was initiated at 1.18 V (η = 0.37 V at ph 7.0). The potential was allowed to reach a steady state and small aliquots (2-20 μl) of 40 wt% KOH were added at 5 min intervals to the working and auxiliary chambers to raise the ph from 5.8 to 8.5 in ~0.2 ph unit steps. The ph was continuously monitored with a semi-micro ph probe (VWR) positioned in the working compartment. The potential stabilized at each new ph within 30 sec and the ph remained stable within 0.01 units over the course of each 3 min interval. Although the data displayed in Figure 2 cannot be corrected for ohmic potential losses, solution resistance measurements were conducted following the titration to ensure that a stable potential was maintained throughout the ph range of the titration. The solution resistance was measured over the entire ph range using a fresh FTO electrode placed in the same configuration with respect to the reference electrode as the catalyst-coated electrode. The measured solution resistance reached a maximum of 33 Ω at ph 8.5 resulting in a maximal ohmic potential loss of 4 mv. Mass Spectrometry. An Agilent Technologies 5975C Mass Selective Detector operating in electron impact ionization mode was used to collect mass spectrometric data. All experiments were performed in a custom built two-compartment gas-tight electrochemical cell with gas inlet and outlet ports, a glass frit junction, and pressure equalizing valve. In all cases, at the start of the experiment, the auxiliary compartment contained a Ni-foam auxiliary electrode whereas the working chamber contained no electrodes. Pi electrolyte (ph 7.0) consisting of natural isotope abundance (99.8% 16 OH 2 ; 0.2% 18 OH 2 ) was degassed by bubbling with ultra high purity He for 3-4 h with vigorous stirring and it was transferred to the working and auxiliary chambers of the electrochemical cell under He. The cell was connected to the He carrier gas and mass spectrometer and purged for at least 8 h. 18 O enriched catalyst films were prepared via electrodeposition at 0.9 V from 0.5 mm Co 2+ solutions of Pi electrolyte enriched with 87% 18 OH 2. 10-30 mc (1.5-6.8 mc/cm 2 ) were passed during the deposition. In all MS experiments, catalyst films were electrodeposited onto ITO coated glass slides (~1 cm 6 cm exposed to solution). Immediately following deposition, the catalyst film was transferred, without drying, to a Co-free Pi electrolyte solution enriched with 87% 18 OH 2 and a 100 sec, 1.3 V pulse was applied. The films were then held at open circuit S6
potential for 30 min to allow equilibration with the 18 OH 2 enriched solution. The above procedure was employed to minimize loss of the 18 O label through water oxidation, mediated by discharge of the film s capacity, during transfer to and purging of the gas-tight cell. After 30 min at open circuit, the catalyst films were rinsed with reagent grade water and further washed in triplicate by sequential immersion in 40 ml of fresh Pi electrolyte of natural isotope abundance for 1 min. Films were then transferred rapidly along with a reference electrode to the working compartment of the gas-tight electrochemical cell. The headspace was then rapidly purged by a dozen or more cycles of evacuation and backfill with He carrier gas until stable background signals were observed for all ions monitored. In all cases, the mass spectrometer was operated in selective ion mode with detection of 28 (N 2 ), 32 ( 16,16 O 2 ), 34 ( 18,16 O 2 ), 36 ( 18,18 O 2 ) and 35 (Cl 2 fragment) amu ions. Once a stable background was achieved, a mock burst injection was conducted. The working and auxiliary compartments were isolated from each other and the working compartment was isolated from the MS by closing upstream and downstream valves and diverting the flow of He carrier gas. After a duration of time similar to that to be utilized in subsequent burst injections, the working compartment was simultaneously opened to the MS and carrier gas supply allowing for rapid injection of headspace gases into the MS. Head space gases were then purged for a minimum of 30 min. In all cases, negligible amounts of 32 O 2, 34 O 2 and 36 O 2 were observed in the mock burst injection. Immediately following the mock burst, the electrochemical cell was again isolated from the MS. Controlled current or controlled potential electrolyses were then initiated with stirring and without IR compensation. Following electrolysis, head space gases were rapidly injected into the MS as described above. For electrolyses shorter than 1 h, the cell remained isolated for an additional 10-20 min following electrolysis to permit gas equilibration. The headspace was purged with real-time MS detection of analytes until the residual background values were observed (8-16 h). Subsequent burst injections were conducted in the same fashion. The raw count data for each burst injection was corrected for residual air as well as for the 0.2% 18 OH 2 found in unenriched water. The 28 amu signals were used to determine the residual air backgrounds. The 28/32 amu signal ratios were stable at 1.4-2.1 prior to initiation of each run and these ratios were used to obtain the background 32 ion signal at all points during each experiment. The background 34 amu signals were stable at 38-40 prior to each electrolysis and these values were used as the 34 ion background. In addition to this, the 34 ion signals were corrected for 34 O 2 arising from its natural abundance in air (0.4%) and 34 O 2 produced statistically through oxidation of residual 18 OH 2 (0.2%) found in natural abundance water. For this correction, 0.4% of the 32 amu singles, corrected for background but not for residual air, were subtracted from each raw 34 amu signal. The background 36 amu signals were stable at 37-38 prior to each electrolysis and these values were used as the 36 ion background. No additional corrections were applied to the 36 amu signals because 36 O 2 composes a negligible portion of air (0.0004%) and statistical solvent water oxidation (0.0004%). The 35 ion signals were monitored S7
to determine if any Cl 2 was produced during electrolysis via oxidation of adventitious Cl originating from the reference electrode. This signal remained at the baseline level throughout all experiments. Corrected counts for 32, 34 and 36 amu signals are shown for a representative burst injection in Figure 6. To permit more accurate quantitation of the relative amounts of 32 O 2 and 34 O 2 produced in a particular burst injection, the corrected 32 and 34 amu signals were integrated over a 6 to 8 h period following injection by summing each individual data point. The ratio of peak integrations for 32 O 2 and 34 O 2 are shown in Table 1 for all burst injections attempted. Noting that the catalyst films operate with quantitative Faradaic efficiency, the current passed in each electrolysis was used to determine the moles of O 2 produced and the 32 O 2 / 34 O 2 ratio was used to estimate the number of 18 O atoms extruded from the film as 34 O 2. Assuming a Co:O stoichiometry of 1:2 in the film, the charge passed in the deposition was used to estimate the number of 18 O atoms in the film and this value was used to calculate the percentage of the 18 O label extruded from the film in each run (Table 1). In all experiments, a small but significant amount of 36 O 2 was always observed. While the signal strength was too low to permit accurate integration, the 34 O 2 / 36 O 2 ratio was always approximately 10 to 1 in the min following initiation of the burst injection. S8
Figure S1. Tafel plots, V = (V appl ir), η = (V E ), of catalyst films operated in cobalt-free 0.1 M Pi electrolyte, ph 7.0. First data set ( ), which was collected on a freshly prepared electrode, is compared with a subsequent data set ( ) using the same electrode. Tafel slopes for both data sets are 62 mv/decade. S9
Figure S2. Representative Tafel plots, V = (V appl ir), η = (V E ), of two catalyst films prepared independently and operated in cobalt-free 0.1 M Pi electrolyte, ph 7.0. S10
Figure S3. Tafel plots, V = (V appl ir), η = (V E ), of catalyst films operated in cobalt-free 0.1 M Pi electrolyte, ph 7.0. Data collected with descending ( ) and ascending ( ) potential sweeps. S11
Figure S4. Tafel plots, V = (V appl ir), η = (V E ), of catalyst films deposited on Pt rotating disk electrodes and operated in cobalt-free 0.1 M Pi electrolyte, ph 7.0 at 1000 rpm ( ) and 2000 rpm ( ). Tafel plot ( ) of a catalyst film on a stationary FTO electrode in a stirred solution is shown for comparison. Tafel slopes are 61( ), 61( ), and 62( ) mv/decade. S12
Figure S5. ph dependence of steady-state electrode potential at constant current (i anodic = 30 μa/cm 2 ) for a catalyst film operated in 0.1 M Pi electrolyte ( ) and in 0.1 M Pi electrolyte with 0.5 M KNO 3 ( ). S13
Figure S6. Tafel plots, V = (V appl ir), η = (V E ), of catalyst films operated at ph 7.0 in cobalt-free solution consisting of: 0.03 M Pi and 0.07 M NaClO 4 ( ), 0.1 M Pi ( ), 0.3 M Pi ( ), and 1.0 M Pi ( ). S14
Figure S7. Koutecký-Levich plots of a catalyst film operated in cobalt-free 0.1 M NaClO 4, ph 8.0, at applied potentials of 1.24 ( ), 1.21 ( ), 1.18 ( ), 1.15 ( ), 1.12 ( ), and 1.09 ( ) V. S15
Figure S8. Tafel plots, V = (V appl ir), η = (V E ), of catalyst films deposited on Pt rotating disk electrodes and operated at 2000 rpm in cobalt-free 0.1 M Pi electrolyte, ph 8.0, immediately after deposition ( ) and following Tafel data collection in 0.1 M NaClO 4, ph 8.0, ( ). S16