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1 Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts Ram Subbaraman 1,2, Dusan Tripkovic 1, Kee-Chul Chang 1, Dusan Strmcnik 1, Arvydas P. Paulikas 1, Pussana Hirunsit 3, Maria Chan 3, Jeff Greeley 3, Vojislav Stamenkovic 1 and Nenad M. Markovic 1* 1 Materials Science Division, Argonne National Laboratory, Lemont, IL Nuclear Engineering Division, Argonne National Laboratory, Lemont, IL Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL XAS of 3d M-oxides on Pt(111) At least 3 sets of XAS measurements were performed for each of the Ni, Co, Fe, Mn oxides on Pt(111). All of the data sets show reproducible trends in the change of oxidation state with applied potential with slight variations in the onset potentials through the samples. These trends were also found to be reproducible by repeated sweeps of the applied potential. In this paper, we show a typical dataset for these systems. XAS measurements were carried out as a function of the electrode potential and here we will limit our discussion to the XAS at -0.1V (near HER) and at 1.4V (near OER) to better understand how the chemical nature of the 3d M-oxides is changing at the two potential regions of interest for the water electrolyzers. For this purpose, we employ XANES as a fingerprinting technique to understand the nature of the deposits on the Pt substrate (whether it is M 0 or M n+ ), as well as to demonstrate the electrochemical sensitivity, i.e., the clusters show potential dependent change in oxidation states as well as to understand about the nature/stoichiometry of these clusters. We are well aware that XAS analyses alone do not provide sufficient information regarding understanding reactivity trends which are dependent more on the energetic of interaction of these materials with oxygenated species, which are formed either as a reactant or intermediate during the water electrolyzer reactions. Due to the small size and the two dimensional character of the M-oxide clusters, we do not expect the XANES to exactly match the bulk phase. But since XANES should be most sensitive to the oxidation state, bond lengths and crystal symmetry of the oxides, position of the edges and the main peak features of the XANES were expected to remain unchanged, although the peak magnitudes may change. Figure S-1 shows the XANES spectra and the best candidate phases for the Ni, Fe and Mn oxides on Pt(111). We begin by describing the XANES analysis for the various systems given below. There are numerous in situ XAS studies of Ni(OH) because of its potential use as electrodes in rechargeable batteries and fuel cells. To briefly summarize a review on this system by McBreen 4, electrochemically prepared Ni(OH) 2 in highly alkaline electrolytes (>1M KOH) form α-ni(oh) 2, which has a disordered structure in the form of Ni-OH layers with interlayers of H 2 O. Within the Ni-OH layer, Ni is octahedrally coordinated by six oxygen atoms. This structure can be converted to the more ordered β-ni(oh) 2, which NATURE MATERIALS 1

2 form a brucite structure through the removal of the interlayers of H 2 O and ordering of the adjacent Ni-OH layers, by continuous discharging/charging or aging. Upon oxidation, Ni(OH) 2 is known to convert to NiOOH. The XANES of our Ni oxides on Pt(111) are similar to the Ni(OH) 2 in literature. We observe a decrease in the pre-edge intensity and an increase in the main peak intensity in the OER region, which is reversible as we go between the HER and the OER. This change in the XANES is attributed to the improvement of the octahedral symmetry around the Ni atom and is expected for α to β phase change 2. Within our potential windows of -0.1V to 1.4V, we observe very small edge shift in the Ni XANES, which shows that Ni remains in the +2 oxidation state. However, in situ XAS studies in the literature show that Ni(OH) 2 undergoes oxidation to NiOOH at high potentials. Kim and Kim 5 have shown that Ni(OH) 2 on gold films in 1M KOH starts to oxidize to NiOOH at 1.5V vs RHE for β-ni(oh) 2. Due to the relatively large OER currents at such potentials, which lead to excessive bubble formation on the surface, XANES measurements at such potentials could not be taken. We believe however, based on the observed evidence in literature, Ni will eventually transition through to +3 oxidation state in the OER region. Fe oxides on Pt(111) remain mostly in the FeOOH phase from comparison of the XANES to similar systems in literature 6,7. Due to the similarity of the XANES, Fe oxide is most likely to be in the FeOOH phase at OER. But at HER, we do not observe a complete reduction of the Fe to Fe(OH) 2. From the shift in the edge position and the position of the main peak, we conclude that only a partial reduction of Fe to an oxidation state between +2 and +3 occurs in the HER region. From our data, it is hard to determine whether the reduced FeOOH is a single phase or a combination of 2 phases. Given Fe exists at a lower oxidation state than +3 in the HER region but different from Fe(OH) 2 it is plausible to state that there are a combination of different species of Fe that are prevalent in the system, in line with what has been traditionally observed from Pourbaix diagrams 8. Fe(OH) 2 and other variants of Fe 2+ and Fe 3+ oxides and hydroxides are known to be present at this potential. The Fe transition to +3 oxidation state in the OER region is consistent with Co and Mn systems. The XANES analysis of Mn oxide on Pt(111) in the HER region reveals that this material is in the form of Mn(OH) 2 from comparisons to literature 9,10. The Mn XANES at OER shows that the edge position is between MnOOH (+3) and MnO 2 (+4), indicating that the Mn is in an oxidation state between +3 and +4. MnO 2 XANES was chosen for comparison due to the relative ease in acquiring such samples. From the XANES alone, we can not determine the phase of the Mn oxide. However, based on the behavior exhibited by the other 3d metal oxides, it is plausible to expect a combination of the MnOOH and a Mn +4 oxides. At this moment, the exact stoichiometry of this oxide is not known. 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION (a) (b) (c) Figure S-1. (a) Ni (b) Fe (c) Mn oxide/pt(111) XANES at HER (-0.1V) and OER (1.4V). 2. STM images of Fe 2+δ O δ (OH) 2-δ /Pt(111) and Ni 2+δ O δ (OH) 2-δ /Pt(111) Figure S4 shows the STM images for the Fe 2+δ O δ (OH) 2-δ /Pt(111) and Ni 2+δ O δ (OH) 2-δ /Pt(111). The Mn(OH) 2 /Pt(111) (not shown here) nanoclusters were found to exhibit qualitatively similar behavior to the other clusters. The images were recorded after 50 cycles between V. The nanoclusters were found to be stable and the particle sizes/shapes were similar to the ones observed for Co(OH) 2 clusters. We propose that in order to truly understand the role of a descriptor such as the one proposed in this work, OH ad ---M 2+δ across multiple systems, it is highly desirable to control the morphology, number and the coverage of the oxide particles on the substrate. From the analysis of the STM images of the nanoclusters these clusters are found to be ~2 atomic layers thick (5-6.5 Å in height) and ~7-8nm wide. While there are some differences in the shape of the clusters, for the given surface coverages (estimated from the H upd measurements), it is reasonable to assume the particles morphologies are comparable. This helps in treating the series of these oxides as a well characterized system. NATURE MATERIALS 3

4 h=5.8 Å D=8.2 nm D=7.06 nm h= 5.98 Å Fe 2+δ O δ (OH) 2-δ Ni(OH) 2 70 nm 35 nm Figure S-2: STM images of Fe(OH) 2 /Pt(111) (left) and Ni(OH) 2 /Pt(111) (right) 3. Density functional theory calculations The DFT energies in this work are calculated by employing fully periodic plane-wave Density Functional Theory (DFT) calculations, as implemented in the Vienna ab initio Simulation Program 11,12. The calculations use the GGA-PBE functional 13 and the Projector Augmented Wavefuction (PAW) 14,15 method. We also use the Hubbard U correction in the implementation of Dudarev et al 16 The applied U values for calculations involving M(OH) 2 films (see further description below), with M = Co and Mn, are 3.5 and 4.0 ev, respectively. These U values are similar to those which have been found to reproduce the oxidation energies of binary oxides accurately 17, and we checked that the formation energies of the bulk M(OH) 2 are also reasonably well reproduced. For all calculations reported herein, we use a 400 ev cutoff for the kinetic energy of the planewave basis-set and the Methfessel-Paxton smearing 18 of order one with a smearing parameter, σ, of 0.2 ev. Four layer, 2 2 unit cells are used to model the Pt(111) surface, and a vacuum region (not including the M(OH) 2 layers) of more than 10 Pt-layers (~20 Å) is employed. The surface Brillouin zone is sampled with a 4x4x1 Monkhorst-Pack k- point mesh 18. The bottom two layers of the Pt(111) slab are fixed, while the other layers, and the M(OH) 2 layers that are adsorbed on top of the Pt(111) slabs, are relaxed to their lowest energy configurations. The fixed Pt layers are set to the Pt bulk bond distance 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION according to its optimized lattice constant, which was determined to be 3.98 Å. The results were checked for convergence with respect to energy cutoff and number of k- points. The convergence criterion for the electronic self-consistent iteration was set to 10-7 ev, and the force criterion for ionic relaxations was set to ev/å. The calculated structures of bulk M(OH) 2, together with the structures of 2 monolayer (ML) M(OH) 2 bulk-like films on Pt(111), are shown in Figure S-3. For the 2 ML films, alternative stackings of the two layers were sampled by randomly initializing the positions of the M ions and OH groups in the second layer. Thirty such random initializations were relaxed into the lowest energy configurations. We find that the lowest-energy configurations of these 2ML films contain M layers that are AB-stacked, unlike the bulk hydroxides which are AA-stacked. The percentage difference between the calculated bulk lattice constant, a, of M(OH) 2, and the lateral M- M distance of the M(OH) 2 structure on Pt(111), are less than 5%, as shown in Table S-1. The M-M distances between layers, c, of the film and bulk M(OH) 2 are slightly larger, in the range of 5-9%. To calculate the OH binding energy on the M(OH) 2 /Pt(111) films, one OH group is removed from the top of the film to create an OH vacancy, and the OH binding energies are calculated as BE OH = E(M(OH) 2 /Pt) E(w/OH vacancy) [E(H 2 O) - 0.5E(H 2 )], where E(M(OH) 2 /Pt) is the total energy of the M(OH) 2 /Pt(111) film, E(w/OH vacancy) is the energy of M(OH) 2 /Pt(111) with an OH vacancy, and E(H 2 O) and E(H 2 ) are the energies of H 2 O and H 2 in gas phase. The more negative BE OH, the stronger the OH binding to the film. The structure, after removing the OH unit from the film, is shown in Figure S-4. The OH binding energies on Pt(111), Co(OH) 2 /Pt(111) and Mn(OH) 2 /Pt(111) are shown in Table S-2. We note that these binding energies are subject to some uncertainty due to the choice of U values. For example, assuming an uncertainty of ± 2 ev in the U values implies a corresponding uncertainty of ±0.25 ev in the calculated OH binding energies on Co(OH) 2 /Pt(111). A similar uncertainty is found for OH binding on Ni(OH) 2 /Pt(111) films (data not shown), and it is not, in fact, possible to conclusively distinguish between Co(OH) 2 /Pt(111) and Ni(OH) 2 /Pt(111) within the error bars of our calculations. In contrast, similar changes in U cause much smaller uncertainties in the OH binding energies to Mn(OH) 2 /Pt(111) (less than 0.05 ev) and to Fe(OH) 2 /Pt(111) (data not shown); again, it is not possible to distinguish between Mn(OH) 2 /Pt(111) and Fe(OH) 2 /Pt(111) within the error bars of our calculations. In spite of the uncertainty of binding energies on Ni, Co, Fe, and Mn based upon the change of U values, it is clear that OH always binds more strongly to Mn(OH) 2 /Pt(111) than to Co(OH) 2 /Pt(111), and OH binding to Pt(111) is the weakest of all. We further note that, although we expect this particular trend to be relatively independent of the detailed choice of hydroxide structural model, the quantitative values of the binding energies may vary substantially for models NATURE MATERIALS 5

6 with different oxidation states and for those containing edges of hydroxide islands rather than complete films. (a) (b) (c) Figure S-3. (a) M(OH) 2 bulk structure, (b) top view of M(OH) 2 /Pt(111), and (c) side-view of M(OH) 2 /Pt(111). Pt-grey, M-green, O-red, and H-pink Figure S-4. The structure of M(OH) 2 /Pt(111) after removing an OH unit. Table S-1. Calculated structural parameters of bulk M(OH) 2 and M(OH) 2 /Pt(111), in Å. M M-M in bulk (a) lateral M-M on Pt(111) M M between layers in bulk (c) M M between layers on Pt(111) % difference Co Mn Table S-2. Calculated OH binding energies on Pt(111) and M(OH) 2 /Pt(111) in ev. M BE OH Pt(111) 0.94 Co -0.48±0.25 Mn -1.35±0.05 % difference 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION 4. Monofunctional vs. Bi-functional catalysis The main distinction between the HER and OER reactions for the (oxy)hydroxide modified Pt(111) are the nature of the reactions namely: for the HER, the oxide clusters play a bi-functional role by providing water dissociation sites, whereas for the OER these sites are the sole catalytic centers. In order to further elucidate this point and the role of the oxides in these individual reactions, we compared the activities for HER and OER for different coverages of the oxide. 0 Hydrogen evolution reaction 10 Oxygen evolution reaction Current Density (ma/cm 2 ) -5 0% Co(OH) 2 62% Co(OH) 2 40% Co(OH) 2 0% 40 % % Co(OH) 2 62 % Current Density (ma/cm 2 ) 5 65% CoOOH 0% CoOOH E [V] E [V] (a) (b) Figure S-5. (a) HER activity as a function of three different coverages of Co(OH) 2 0% (bare Pt(111) surface), 40% (low) and 62% (high); (b) OER activity as a function of three different coverages of CoOOH: 0% (bare Pt(111) surface), 34% and 65% coverage on the Pt(111) surface. We once again, use Co(OH) 2 system as the model system, given the well defined nature of oxidation states as well as activities. Similar trends are also observed for other systems considered in this paper. For the HER, the activity for the reaction is found to increase with increasing coverage of Co(OH) 2, up to a value of ~40-45%, and after which it is found to decrease with increased coverages. This is shown schematically in the inset of the figure S-5 (a). Decreasing the number of H ad recombination sites (Pt sites), while increasing the water dissociation sites (Co(OH) 2 ) above the optimal levels, can lower the ability of the Co 2+δ O δ (OH) 2-δ /Pt(111) to perform the H ad recombination step and thus explaining the lower activity at high loadings of Co(OH) 2. Similarly, for very low coverages of the Co(OH) 2, there are insufficient number of water dissociation sites, which also leads to poorer activity. It must be pointed out that the oxides are still facilitating water dissociation steps faster than Pt evident from the observed HER activities being higher than those observed for bare Pt(111) surface. If the catalytic effect of Co(OH) 2 was indeed mono-functional then the reaction rates should be directly proportional to the number of active sites which is not the observed case. Thus, this result in conjunction with the results discussed in the manuscript for Au(111) vs. Pt(111) substrate, confirms the true necessity of a facile water dissociation agent (a 3d-oxide) and an efficient H-recombination species (Pt) to achieve optimal HER activities in alkaline NATURE MATERIALS 7

8 solution. Such behavior with respect to optimal composition of one component vs. the other is a classical means to identify bi-functional catalyst systems, as has often been used in the literature (e.g. PtSn systems for CO oxidation reaction 19 ). On the other hand, the OER activities are found to increase with increasing CoOOH coverage on the surface. This suggests that this reaction is directly influenced by the number of active sites of CoOOH present on the surface and the substrate, Pt in this case, has little to no effect on the observed catalytic activity. Furthermore, achieving the same activities for both Pt and Au substrates with similar coverages of CoOOH as shown in the manuscript (figure 3 inset) underlines this point that the OER is indeed a mono-functional reaction, primarily occurring on the CoOOH sites. The increase in activity with increasing coverages further emphasizes the need for well characterized systems with defined number, morphology and nature of active centers in order to derive true catalytic trends and therefore true descriptors for the reactions. 5. CO oxidation reaction 0.08 Pt(111) Pt(111)+Co 2+x (OH) 2+x i (ma/cm 2 ) E [V] Figure S-6. A closer look at the onset potentials for CO oxidation reaction; While the oxide covered surfaces exhibit significant activities at ~0.2V non zero currents are observed at much lower potentials such as ~0.1V. Similarly, for the Pt(111) surface which is known to have defects such as ad-islands in varying numbers, the activity for CO oxidation (onset) is at much lower potential than V typically seen from the CO oxidation polarization curves. This oxidation wave is typically known to disappear for a CO annealed surface which is known to be devoid of any such defects 20. References 1 Hu, Y., Bae, I. T., Mo, Y., Scherson, D. A. & Antonio, M. R. In situ X-ray absorption fine structure and optical reflectance studies of electrodeposited nickel hydrous oxide films in alkaline electrolytes. Canadian Journal of Chemistry 75, (1997). 2 McBreen, J. et al. In situ time-resolved x-ray absorption near edge structure study of the nickel oxide electrode. J. Phys. Chem. 93, (1989). 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION 3 Pandya, K. I., Hoffman, R. W., McBreen, J. & O'Grady, W. E. In Situ X-Ray Absorption Spectroscopic Studies of Nickel Oxide Electrodes. Journal of The Electrochemical Society 137, (1990). 4 McBreen, J. in Handbook of battery materials (John Wiley & Sons, 1999). 5 Kim, M.-S. & Kim, K.-B. A Study on the Phase Transformation of Electrochemically Precipitated Nickel Hydroxides Using an Electrochemical Quartz Crystal Microbalance. Journal of The Electrochemical Society 145, (1998). 6 Balasubramanian, M., Melendres, C. A. & Mini, S. X-ray Absorption Spectroscopy Studies of the Local Atomic and Electronic Structure of Iron Incorporated into Electrodeposited Hydrous Nickel Oxide Films. J. Phys. Chem. B 104, (2000). 7 Kim, S., Tryk, D. A., Antonio, M. R., Carr, R. & Scherson, D. In situ x-ray absorption fine structure studies of foreign metal ions in nickel hydrous oxide electrodes in alkaline electrolytes. J. Phys. Chem. 98, (1994). 8 Pourbaix, M. Atlas of electrochemical equilibria in aqueous solutions. M. Pourbaix, published 1974 by NACE, 644 (1974). 9 Lima, F. H. B., Calegaro, M. L. & Ticianelli, E. A. Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction. Electrochimica Acta 52, (2007). 10 Nam, K.-W., Kim, M. G. & Kim, K.-B. In Situ Mn K-edge X-ray Absorption Spectroscopy Studies of Electrodeposited Manganese Oxide Films for Electrochemical Capacitors. J. Phys. Chem. C 111, (2006). 11 Kresse, G. & Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Computational Materials Science 6, (1996). 12 Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Physical Review B 54, (1996). 13 Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized Gradient Approximation Made Simple. Physical Review Letters 77, (1996). 14 Blöchl, P. E. Projector augmented-wave method. Physical Review B 50, (1994). 15 Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Physical Review B 59, (1999). 16 Dudarev, S. L., Botton, G. A., Savrasov, S. Y., Humphreys, C. J. & Sutton, A. P. Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study. Physical Review B 57, (1998). 17 Wang, L., Maxisch, T. & Ceder, G. Oxidation energies of transition metal oxides within the GGA+U framework. Physical Review B 73, (2006). 18 Methfessel, M. & Paxton, A. T. High-precision sampling for Brillouin-zone integration in metals. Physical Review B 40, (1989). 19 Marković, N. M. & Ross Jr, P. N. Surface science studies of model fuel cell electrocatalysts. Surface Science Reports 45, (2002). NATURE MATERIALS 9

10 20 Strmcnik, D. S. et al. Unique activity of platinum adislands in the CO electrooxidation reaction. Journal of the American Chemical Society 130, (2008). 10 NATURE MATERIALS