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1 Using TiO2 as a Conductive Protective Layer for Photocathodic H2 Evolution. Brian Seger 1, Thomas Pedersen 2, Anders B. Laursen 1, Peter C.K. Vesborg 1, Ole Hansen 2, Ib Chorkendorff 1,* 1 Department of Physics, CINF, Technical University of Denmark, DK-2800, Kongens Lyngby, Denmark. 2 Department of Micro- and Nanotechnology, Technical University of Denmark, DK-2800, Kongens Lyngby, Denmark. Experimental Procedures Electrode Production Supporting Information Silicon with n + p junctions were produced in the same manner as in previous work 1. For the protective layer deposition, wafers with n + p silicon were dipped in buffered HF for 30 s, cleaned with Millipore water, dried and were then immediately placed in the sputter chamber. Either Ti or Pt was then sputtered onto the electrodes. For the case of the 100 nm TiO 2 /5 nm Ti/n + p Si electrodes, TiO 2 was reactively sputtered at room temperature directly after the Ti sputtering. All room temperature sputter depositions were done in a Lesker sputter system. Profilometer measurements were used to calibrate Ti, TiO 2, and Pt sputter thicknesses. Vacuum annealed samples were placed in a AJA inc. (Scituate, MA) sputter system and heated up to 400 C. The pressure in this machine was always kept below 5x10-8 torr during vacuum annealing. The in-situ 100 nm TiO 2 /5 nm Ti/n + p Si electrodes were made by taking an n + p Si wafer and heating it to 400 C in an AJA int. sputter machine. Since the sputter machine was not in a clean room, the n + p Si surface could not be kept completely clean of carbon at the surface. To account for this, the sample was first Argon sputtered to remove any adventitious carbon at 35 W for 120 s. Then 5 nm of Ti was deposited followed by 100 nm of TiO 2 at an O 2 partial pressure of 3 mbar. The Ti was deposited at 200W using a 2 diameter target. Electrodes were produced from these silicon wafers in the same manner as in previous work 1. Before testing all samples were cleaned with a fresh piranha solution (3:1 H 2 SO 4 :H 2 O 2 ) and then rinsed with Milipore (18MΩ) water and dried. They were then coated with Teflon tape with a 5 mm diameter hole punched out of the middle, thus yielding an active area of cm 2. It should be noted that Pt/n + p Si electrodes were made in 2 different ways. In the 1 st method n + p Si wafers were first prepared using previous methods 1. n + p Si wafers where H- terminated by cleaning them with 1% HF for 1 minute. They were washed, and then 250 ng (Pt basis) of a dinitrosulphatoplatinate were dropcast onto their surface. This had been our standard way of producing electrodes. This method was used for Figure 1, and is denoted as Pt/n+p Si. In the 2 nd method 5 nm of Pt was sputter deposited on H-terminated n+p Si samples. These were S1

2 used in all other Figures and are denoted as 5 nm Pt/n + p Si. The sputtered method gave more durable electrodes and had a slightly higher onset potential (0.51 V vs. RHE versus 0.50 V vs. RHE). Photoelectrochemical Testing All H 2 evolution and Fe(II)/Fe(III) redox couples experiments were done in an H-cell design with an added compartment for a reference electrode. There was a glass frit in-between the 2 sections of the H-cell to prevent crossover from the working and counter electrode. For H 2 testing the electrolyte was an aqueous 1M HClO 4 (Aldrich 99.99%). The counter electrode was a Pt mesh and the reference was a saturated Hg/HgSO 4 electrode (VWR International). For Fe(II)/Fe(III) experiments 10 mm of both Fe(II) perchlorate and Fe(III) perchlorate (Aldrich) were added to the aqueous 1 M HClO 4 (Aldrich 99.99%) solution. The solution was purged with either argon or hydrogen gas 30 minutes before the start of any experiment, and during the entire duration of the experiment. A 1000 W xenon lamp (Oriel) was used with a 635 nm cut-off filter and an AM1.5 filter to simulate the red part of the solar spectrum. The light intensity reaching the sample was monitored via a spectrophotometer, and the light intensity was adjusted to match that of the total light intensity of red light (λ>635 nm, 38.6 µw/cm 2 ±1 µw/cm 2 ). For electrochemical measurements a Bio-Logic (France) VSP potentiostat was used using EC Lab software. For all CV s the electrodes were scanned at a sweep rate of 50 mv/s. For Tafel analysis a scan rate of 5 mv/s was used. Long Term Stability with a Titanium Support A 9 nm Ti/n + p Si electrode with amorphous MoS x was produced using the procedure described in our previous work 1. The electrode was put in a H 2 purged 1 M HClO 4 solution and photoirradiated with the red part (λ>635 nm) of the AM1.5 spectrum. The electrode was then held at +0.2 V vs. RHE for 24 hours. There is a distinct drop-off in photocurrent after 5 hours. Figure S1 is only one example of many that showed this trend. Looking at the electrodes after failure showed that all the titanium had been removed, allowing the underlying silicon to be oxidized. Due to the stability issues with the titanium, long term durability tests (>2 hour) could not be done on the amorphous MoS x catalyst. S2

3 Fig. S1: Durability test of an A-MoS x /9 nmti/n + p Si electrode which was photoirradiated with the red part of the AM1.5 spectrum and held at potential of +0.2 V vs. RHE in an H 2 saturated 1 M ClO 4 electrolyte. Incident Photon to Current Efficiency (IPCE) Measurements To determine the photon to electron efficiency of the Pt / 100 nm TiO 2 / 5 nm Ti/ n + p Si samples IPCE measurement were employed. A 1000 W xenon lamp (Oriel) was used as an excitation and an Oriel monochromator was used to give monochromatic light. The full width half maximum peak of the monochromatic light was 20 nm. To remove low wavelength resonance peaks a cut-off filter was employed on wavelengths of 660 nm and above. At 640 nm and below the resonance peak (e.g. 320 nm and below) had too low an intensity to contribute any proportionally significant photocurrent. The light intensity was measured via a spectrophotometer. To determine the photocurrent, the electrodes were run at 0.0 V vs. RHE using the same set-up and conditions as in Figure 1 & 2 of the main paper. Since Figure 2 in the main paper showed that the vacuum annealed and the in-situ sputter annealed electrodes were the most durable, these 2 electrodes were chosen to do IPCE. Figure S2 shows the IPCE data for each of these electrodes. S3

4 a) b) Fig. S2: IPCE of Pt on 100 nm TiO 2 / 5 nm Ti/ n + p Si that was prepared by either a) Sputtering at room temperature and then vacuum annealing at 400 C or b) in-situ annealing by sputtering the TiO 2 at 400 C. Figure S2 clearly shows that there is a high IPCE for both of the electrodes. It can be seen though that at lower wavelengths the IPCE does drop off significantly. This is of little significance though, since it is expected that Si will only absorb long wavelength light (~ λ > 635 nm) in a 2-photon water splitting device 2. (It is expected the photoanode will absorb the short wavelength light). Cyclic Voltammograms of Various TiO 2 Deposition Procedures Figure S3 shows is a cyclic voltammogram comparison of Pt/100 nm TiO 2 /5 nm Ti/n + p Si electrodes with either no annealing, vacuum annealing, air annealing, or annealing while depositing the TiO 2 (e.g. in-situ annealing). The conditions used were the same as in Figure 1 of the main paper. S4

5 Fig. S3: CV scans of photoelectrodeposited Pt/100 nm TiO 2 / 5 nmti/n + p Si electrodes with various treatments. The samples were irradiated with the red part (λ > 635 nm) of a simulated AM1.5 spectrum and scanned at 50 mv/s in a H 2 saturated 1 M HClO 4 electrolyte. While there is no difference between the unannealed and annealed sample, there is a slight decrease in performance from the in-situ annealed sputtered sample and a much larger decrease for the air annealed sample. Both the in-situ sputtered and air annealed samples appear to have a larger ohmic resistance which can most likely be attributed to varying degrees of oxidation of the silicon. It should be noted that the in-situ annealed sample was not done in a cleanroom, thus the Si was less clean before Ti/TiO 2 deposition. To compensate for the lack of cleanliness, the in-situ samples were argon sputtered to clean the surface. However it was found that this procedure slightly oxidizes the Si. This different deposition procedure during in-situ annealing TiO 2 deposition may have contributed to the in-situ sample s slightly worse performance. S5

6 Long Term Stability with sputtered Pt on n + p Si An n + p Si electrode with 5 nm of sputtered Pt was tested for photocatalytic H 2 evolution for 24 hours at a potential of +0.3V vs. RHE using the conditions mentioned in Figure 2 of the main paper. Below are cyclic voltammograms initially and after 24 hours of testing. The chronoampometry results are shown in the main paper. Fig. S4: CV scans of 5 nm sputtered Pt on an n + p Si electrodes both initially and after 24 hours of testing at +0.3 V vs. RHE. The samples were irradiated with the red part (λ > 635 nm) of a simulated AM1.5 spectrum and scanned at 50 mv/s in a H 2 saturated 1 M HClO 4 electrolyte. S6

7 Longer Term Durability Tests with In-Situ Sputtered TiO 2 Electrodes The in-situ sputtered Pt/100 nm TiO 2 /5 nm Ti/n + p Si electrodes tested in Figure 2 were continued for an additional 2 days. Figure S5a shows the chronoampometry of the entire 3 day test, while Figure S5b shows CV scans taken in 24 hour intervals. Fig. S5: a) 72 hour chronoampometry durability tests of in-situ annealed of Pt/100 nm TiO 2 /5 nm Ti/n + p Si electrodes. The samples were irradiated with the red part of the AM1.5 spectra (λ>635 nm) in 1M HClO 4 and held at a potential of +0.3 V vs. RHE. B) cyclic voltammograms during the 72 hour run. CV s were taken in 24 hour intervals at 50 mv/s scan rate. Figure S5a shows that the photoelectrode is consistent for 72 hours. The random spikes that occur approximately every 10 hours are believed to be an inconsistency within the potentiostat or light source. Figure S5b shows that after the first 24 hours there is negligible change in the CV scans. S7

8 SEM Images Before and After Durability Test SEM images were taken of vacuum annealed and in-situ annealed Pt/100 nm TiO2/5 nm Ti/n+p Si electrodes. The first images (Figure S6a and S6c) of the electrodes are after 5 minutes of testing. Since the Pt salt was photoelectrochemically reduced to Pt metal, the electrodes needed to be run for at least a short while to reduce the Pt and accurately show how the electrodes looked when they started producing H2. For this reason it was decided that the initial SEM s should be after 5 minutes of testing rather than before the Pt salt was reduced. Figure S6c and S6d are SEM images after durability tests for the vacuum annealed and in-situ annealed samples respectively. While the vacuum annealed sample was only tested for 24 hours, the in-situ samples were tested for 72 hours. Fig. S6: SEM images of vacuum annealed (a, b) and In-situ annealed (c,d) samples of Pt/100 nm TiO2/5 nm Ti/n+p Si electrodes. The images on the left (a,c) correspond to how the electrodes looked initially, and the images on the right (b,d) correspond to how the electrodes looked after durability testing. S8

9 In Figure S6a it is apparent that there are a large number of well dispersed Pt particles across the surface initially on the vacuum annealed sample. Using ImageJ software, it was determined that the Pt coverage was ~20% initially for the vacuum annealed sample. This coverage is high enough for the efficient H 2 evolution catalysis, but low enough for the pinch-off effect to still take place. After the 24 hour testing, Figure 5b shows small nanoparticles. Using back scattered SEM, it was determined that these were not Pt particles, but TiO 2 particles. From the AFM results it is known that the underlying TiO 2 support is initially relatively flat. It appears as if the TiO 2 substrate is slowly forming larger particles over the 24 hour period. It s also interesting to note that a large portion of the Pt had fallen off the surface. Figure S6c shows an initial SEM of the in-situ annealed electrode. While the Pt particles are more agglomerated in Figure S6c compared to Figure S6a, this is probably just a function of variations in drop-casting and photo-reduction rather than an effect of the in-situ annealed surface. Figure S6d shows an in-situ electrode after 72 hours of testing. Again this sample shows that a fair amount of Pt had fallen off the surface with the exception of a notable agglomerate on the left side of the image. Unlike in the vacuum annealed sample, the in-situ electrode retains its original surface morphology. This difference in surface morphology changes between the vacuum annealed and in-situ annealed samples may account for the varying degrees on durability between the samples. This furthers our point that optimizing deposition procedures are extremely important in creating a durable protective layer. XRD Analysis XRD patterns were taken on n + p Si wafers both with and without the 5 nm Ti/ 100nm TiO 2 layer. The 5 nm Ti/ 100 nm TiO 2 samples were either unannealed, annealed in vacuum at 400 C for 90 minutes with a ramp rate of 10 C/min, or annealed in air at 400 C for 90 minutes with a ramp rate of 10 C/min. All data was taken on a Panalytical X Pert Pro X-ray diffractometer using a glancing angle XRD technique at an incident angle of 1.5. The results of this data are shown in Figure S7. S9

10 Counts 5000 n + p Si with the following: 5nm Ti/ 100nm TiO 2 Unannealed nm Ti/ 100nm TiO 2 Vac Ann. 5nm Ti/ 100nm TiO 2 Air Ann. plain Theta ( ) Fig. S7: XRD peaks of n + p Si with various protection layers and annealing conditions. The XRD results show a large number of peaks however most of these are a function of the single crystal nature of the Si. Having particles aligned on a single crystal allows for some diffraction peaks of some very high index planes. By turning the electrode to a different angle, these peaks will disappear, however other high index peaks will arise. The angle in which the silicon was placed in Figure S7 was used because the silicon wafer did not have any peaks near the main peak for either anatase (25.3) or rutile. The peaks at 23, 31 35, 38,42, 52, 57, 62, and 68 all arise from the Si single crystal as shown by the standard. This figure shows that for the unannealed, and vacuum annealed samples there are no additional peaks, thus indicating that this is amorphous TiO 2. For the air annealed sample, there are distinct peaks at 25, 48, and 54.5, all of which can be attributed to anatase. The 25 and 48 peaks correspond to the (101) and (200) peaks, respectively. The peak at 54.5 is probably a combination of the (105) peak at 54 and the (211) peak at 55. Both the thinness of the TiO 2 layer and the single crystal nature of the silicon make it hard to distinguish minute details in the XRD graph, which is most probably why some of the lesser intense peaks of anatase TiO 2 cannot be seen. However the point that the sputtered TiO 2 is amorphous both initially and after vacuum annealing should be clear from this figure. By using a different sputtering machine, the in-situ samples were able to be made much larger for XRD testing. This increased area allowed for increased counts, which allowed for the use of more precise XRD techniques. For this reason, the in-situ annealed samples used the Panalytical X Pert Pro X-ray beam alignment hardware made for glancing angle XRD. Figure S8 shows these results. S10

11 Fig. S8: XRD peaks of in-situ sputtered 100 nm TiO 2 /5 nm Ti/n + p Si. The peak at 25 was assigned to anatase (101) while the peak at 40 was ascribed to Anatase (004). The peak at 48 and 49 was ascribed to the anatase (102) and (200) respectively, while the peak at 62 was ascribed to the anatase (204). The peak at 22 and 32 are unassigned and probably related to a contaminant or impurity. AFM imaging of TiO 2 substrates A Veeco Nanoscope III AFM was used via tapping mode to take AFM images. Nanosurf (Boston, MA) software was used to process the images. Figure S9 shows an AFM of the surface of both the vacuum annealed and in-situ annealed 100 nm TiO 2 /5 nm Ti/n + p Si. Looking at Figure S9, there is a clear difference in surface morphology between the vacuum annealed TiO 2 and the in-situ sputtered annealed TiO 2. While the vacuum annealed surface has surface variations on the order of 12 nm, the in-situ sample has surface variations on the order of 20 nm. Large surface height variations in the in-situ sample would lead for deeper a) b) S11 Fig. S9: AFM images of the surface of the a) vacuum annealed and b) in-situ annealed 100 nm TiO 2 /5 nm Ti/n + p Si.

12 penetration of any oxidative species (e.g. oxygen). At certain spots this penetration of oxygen may be deep enough to attack the underlying Ti or even Si, which may partially account for the slightly worse CV of the in-situ sample compared to the vacuum annealed sample. This may be able to be resolved by simply increasing the thickness of the TiO 2 layer. Determination of Flat Band To study solely the TiO 2 and mitigate any erroneous effects that may come from the photovoltage, 100 nm TiO 2 /5 nm Ti was sputtered on n + silicon rather than n + p silicon. These were then tested electrochemically in a hydrogen saturated solution in 1 M HClO 4 solution using the Mott-Schottky analysis on the Biologic Potentiostat. For these experiments a modulation frequency of 10 khz, a perturbation amplitude of 35 mv, while the potential was scanned from 0.25 V vs. RHE to -0.3 V vs. RHE taking 100 points between the 2 potentials. Figure S10 shows the results of this experiment. The two slopes are typical of a thin sputtered TiO 2 on a semiconducting support 3. The dilute peak can be attributed to the semiconducting support, whereas the sharp peak is attributed to the TiO 2. The Mott-Shottky equation (Equation 1) is shown below, Equation 1 where C is the measured differential capacitance per area, e is the elementary charge, ε is the dielectric constant (55) 3, and ε 0 is the permittivity of vacuum ( F/m) N d is the dopant density, E is the applied potential, E FB is the flat band potential of the TiO 2, k is Boltzmann s constant ( m 2 kg s -2 K -1 ) and T is the temperature (298K). It should be noted that the dielectric constant used in these calculations was for anatase, because no dielectric constant for amorphous TiO 2 could be found in literature. While this has no effect on the flat band potential, the accuracy of the dopant density may suffer. Applying this equation results in a flat band potential of V vs. RHE and a dopant density of cm -3. (Figure S10 also shows what the dopant density would be if ε is slightly varied.) It was found that the addition of Fe(II)/Fe(III) did not affect the flat band potential. The conduction band can also be derived using Equation 2. ln Equation 2 Where N c is the effective density of states for the TiO 2 conduction band (10 20 cm -3 ). Applying this equation yields a conduction band being located at V vs. RHE. S12

13 Fig. S10: Mott-Schottky plot of sputtered 100 nm TiO 2 /5 nm Ti on n + Si which had been vacuum annealed at 400 C. Using Equation 3 the depletion depth can be calculated. / / Equation 3 The Fe(II)/Fe(III) redox potential is 0.77V vs. RHE, which would correspond to a depletion layer of 138 nm. This is typically too thick to tunnel through, thus explaining why negligible current is seen at the redox potential. Figures of CV scans used in Figure 3 S13

14 Figure S11a and S11b show the full cyclic voltammogram scans of Figures 3. These figures were not used in the main paper because the large H 2 evolution current hides the details of the Fe(II)/Fe(III) redox couple. Fig. S11: CV scans of a Pt wire, sputtered 5 nm Pt/n + p Si, dropcast Pt/100 nm TiO 2 /5 nm Ti/n + p Si, and sputtered Pt /100 nm TiO 2 /5 nm Ti/n + p Si electrode in a 10 mm each Fe(III)/Fe(II) electrolyte with 1 M HClO 4. The scan rate was 20 mv/s and the samples were irradiated with the red part (λ>635 nm) of the AM1.5 spectrum. Explanation of Pt film/100 nm TiO 2 /5 nm Ti/n + p Si In the main paper it was mentioned that the small H 2 oxidation peak of the Pt film/100 nm TiO 2 /5 nm Ti/n + p Si electrode could be attributed to morphological issues. It was noted that when the hydrogen evolution started to occur on these samples, that the H 2 bubbles would adhere to the surface. This did not occur on the 5 nm Pt/n + p Si, thus denoting a change in the surface of the electrode. The sputtered TiO 2 is a rougher surface to sputter Pt on than the pure single crystal Si, which may account for this variation in adhesion. This adhesion of hydrogen bubbles provided a reservoir of H 2 at the electrode surface. S14

15 When cycling, these photoelectrodes should reduce protons on the cathodic sweep and then oxidize the produced hydrogen on the anodic sweep. The anodic sweep should produce a sharp onset due to H 2 oxidation. Once all the H 2 near the surface is used up, the process becomes mass transfer limited, thus resulting in peak and sharp decrease. This is quite clearly shown in the case for 5 nm Pt/n + p Si. However the Pt film/100 nm TiO 2 /5 nm Ti/n + p Si has adhered H 2 bubbles which provides a reservoir to continually allow for H 2 oxidation. The second scan in Figure S10C shows the results of having this reservoir. While the H 2 oxidation does slowly decrease over time, it is clearly not all removed by the time the electrode is scanned to the Fe(III)/Fe(II) redox potential. 1 The 1 st scan shows where the redox potential should be if not taken to H 2 evolution conditions. The 2 nd scan shows the Fe(II) oxidation does not happen at that potential indicating that the H 2 oxidation has an effect on the Fe(III)/Fe(II) redox potential. (It should be noted that if all the H 2 is oxidized/purged from the system, the Fe(III)/Fe(II) redox reaction reverts back to its initial onset voltage.) To compensate for the H 2 adhesion issue at the Pt film/100 nm TiO 2 /5 nm Ti/n + p Si, the electrode was cycled to only slightly H 2 evolution potentials in Figure 3 of the main paper (Figure S10b). This allowed for enough H 2 to be removed from the surface for the H 2 current to be minimal. This also resulted in a lower H2 oxidation peak. However even this minimal H 2 in solution was enough to shift the onset potential similar to that shown in the 2 nd scan of Figure S11c. Rather than explain and discuss the onset shift in the main paper, scans were simply started at a potential right before the Fe(III) oxidation and the scans were ended after the H 2 oxidation peak. This allowed for the elimination of the shifting of the Fe(III) oxidation due to H 2 in solution. Properly analyzing and explaining this shift is beyond the scope of this work. Figures of CV scans used in Figure 5 Figure S12 show the full cyclic voltammogram scans of Figures 6. This figure was not used in the main paper because the large H 2 evolution current hides the details of the H 2 oxidation reaction. 1 The scan rate could be slowed to account for this, but this would dilute the sharpness of the Fe(II)/Fe(III) peak to a point where these features are unrecognizable. It was found it took approximately 30 minutes to purge enough H 2 out to where it did not affect the Fe(II)/Fe(III) redox potential. S15

16 Fig. S12: CV scans of a red light (λ>635 nm) irradiated Pt/100 nm TiO 2 /5 nm Ti/n + p Si electrode in an electrolyte with 1 M HClO 4 at 50 mv/s. The sample was first bubbled in argon and then in H 2 to show the effect of the H 2 oxidation reaction. Explanation of Electrodes in a solution of both Fe(II)/Fe(III) and H + /H 2 redox couples To investigate how these electrodes behave in solution with multiple redox couples, various electrodes were scanned to highly oxidative potentials in the presence of both Fe(II)/Fe(III) and H + using the same photoelectrochemical set-up as in Figure 3. The results of this experiment are shown in Figure S13. In the Pt/100 nm TiO 2 /5 nm Ti/n + p Si sample no increase in current is seen at potentials near 1.3 V vs. RHE due to Fe(II) oxidation. With the 5 nm sputtered Pt sample the material is not limited by a conduction band/bandgap thus it can easily oxidize Fe(II) as shown by the peak at 1.35 V vs. RHE. As mentioned in the main paper, Pt itself can oxidize to form platinum oxide, which is unreactive to H 2 oxidation. This can help explain the great decrease in current at highly oxidative potentials on the 5nm Pt/n + p Si sample. The reduction of Pt oxide back to metallic Pt appears to start at 1.4 V vs. RHE. This potential is near the same potential that Fe(III) reduces back to Fe(II). The combination of these two reactions going on simultaneously results in a strange, but reproducible cathodic curve in the range of V vs. RHE. No literature could be found on how Fe(II)/Fe(III) interacts with platinum oxide, and we felt it beyond the scope of this work to investigate that. For these reasons we can t accurately predict what type of currents we would expect on the 5 nm Pt/n + p Si sample. S16

17 Another interesting point is that the current for the Pt/100 nm TiO 2 /5 nm Ti/n + p Si does not drop due to Pt oxidation as in the case with the 5nm Pt/n + p Si sample. This helps to reaffirm the analysis of Figure 6 in the main paper. Figure S13 helps to confirm this. In should be noted that the 5 nm Pt/n + p Si sample in Figure S13 has a more dramatic drop in H 2 oxidation current than Figure 6/Figure S12, but this is simply due to hydrogen bubbling rates and mass transfer, which were not consistent between the two tests. Fig. S13: CV scans of various electrodes in 1M HClO 4 electrolyte with 10 mm each of Fe(III) and Fe(II). The scan rate was 20 mv/s and the samples were irradiated with the red part (λ>635 nm) of the AM1.5 spectrum. An Hg/HgSO 4 reference and a Pt counter were used. (1) Seger, B.; Laursen, A. B.; Vesborg, P. C. K.; Pedersen, T.; Hansen, O.; Dahl, S.; Chorkendorff, I. Angewandte Chemie (International ed. in English) 2012, 51. (2) Laursen, A. B.; Kegnaes, S.; Dahl, S.; Chorkendorff, I. Energy & Environmental Science 2012, 5. (3) vandekrol, R.; Goossens, A.; Schoonman, J. Journal of the Electrochemical Society 1997, 144. S17