SUPPLEMENTARY INFORMATION

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1 In the format provided by the authors and unedited. Polyoxometalate electrocatalysts based on earth-abundant metals for efficient water oxidation in acidic media SUPPLEMENTARY INFORMATION DOI: /NCHEM.2874 Marta Blasco-Ahicart 1, Joaquín Soriano-López 1,2, Jorge J. Carbó 2, Josep M. Poblet 2, J. R. Galan- Mascaros 1,3 * Index Page Materials 2 Synthesis 2 Physical methods 2 Active sites calculation 3 Turnover number (TON) estimation 4 Figure 1 6 Figure 2 6 Figure 3 7 Figure 4 7 Figure 5 8 Figure 6 9 Figure 7 10 Figure 8 11 Figure 9 11 Figure Figure Figure Figure Table 1 13 Table 2 13 Table 3 14 Table 4 14 NATURE CHEMISTRY Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

2 Materials. All reagents and solvents were used of commercial grade, without further purification. Carbon paste (ALS, CPO Carbon Paste Oil). Synthesis. Cs 15 K[Co 9 (H 2 O) 6 (OH) 3 (HPO 4 ) 2 (PW 9 O 34 ) 3 ] 28H 2 O (Cs[Co-POM]) was prepared by metathesis: An excess of CsCl was added to a solution of 1 (500 mg, mmol) in 15 ml distilled water. Cs[Co-POM] precipitated immediately out of solution. After 15 minutes, the product was filtered (Nylon Filter-Lab, pore size 0.45 µm), washed with 10 ml water, and 10 ml acetone, and left to dry in air overnight. Digestion of this material in acid, under heating, yields insoluble products, liberating the metal content only partially. This problem makes difficult the proper elemental analysis. ICP-OES only gave accurate Cs, Co and P ratios. The presence of the [Co 9 (H 2 O) 6 (OH) 3 (HPO 4 ) 2 (PW 9 O 34 ) 3 ] 16 anion is confirmed by its signature IR spectra (Supplementary Fig. 7), identical to 1. In POM chemistry, the IR bands represent a reliable fingerprint for the molecular structure. Cation content was estimated by ICP-OES and EDX. Water content was determined by TGA analysis (Supplementary Fig. 10). Cs[Co-POM].28 H 2 O M w = Elemental analyses, Calc.: Cs, 19.73; K, Exp.: Cs, 19.51; K, Ba 8 [Co 9 (H 2 O) 6 (OH) 3 (HPO 4 ) 2 (PW 9 O 34 ) 3 ] 55H 2 O (Ba[Co-POM]) was prepared by the same method as Cs[Co-POM], substituting CsCl by BaCl 2. Elemental analyses also presented the same problems. The presence of the [Co 9 (H 2 O) 6 (OH) 3 (HPO 4 ) 2 (PW 9 O 34 ) 3 ] 16 anion was also confirmed by its signature IR spectra (Supplementary Fig. 7), identical to 1. Countercation content was estimated by ICP-OES and EDX. Water content was determined by TGA analysis (Supplementary Fig. 11). Ba[Co-POM].55H 2 O M w = Elemental Analyses, Calc.: Ba, 11.37; K, Exp.: Ba, 11.51; K, <0.1. Physical methods. Thermogravimetric analysis (TGA) was performed under N 2 flow with powder samples using a TGA/SDTA851 Mettler Toledo with a MT1 microbalance. Powder X-ray diffraction data were collected with a Bruker D8 Advance Series equipped with a VANTEC-1 PSD 2

3 detector. IR spectra were collected with a FTIR Bruker spectrometer model Alpha equipped with an ATR accessory. Raman measurements were acquired using a Renishaw invia Reflex Raman confocal microscope (Gloucester- Shire, UK), equipped with a diode laser emitting at 785 nm at a nominal power of 300 mw, and a Peltier-cooled CCD detector ( 70 ºC) coupled to a Leica DM microscope. Calibration was carried out daily by recording the Raman spectrum of an internal Si standard. Rayleigh scattered light was appropriately rejected by using edge-type filters. Laser power was used at nominal 1% to avoid sample damage. Spectra were recorded with the accumulation of at least 3 scans with a 30 s scan time each one. Energy-dispersive X-ray spectroscopy (EDX) was collected with a JEOL- JMS6400 environmental scanning electron microscope equipped with an Oxford Instruments X-ray elemental analyzer. X-ray photoelectron spectroscopy (XPS, K-ALPHA, Thermo Scientific, SSTTI at University of Alicante) was used to analyze sample surfaces. All spectra were collected using Al-K α radiation ( ev), monochromatized by a twin crystal monochromator, yielding a focused X-ray spot with a diameter of 400 µm, at 3 ma 12 kv. The alpha hemispherical analyzer was operated in the constant energy mode with survey scan pass energies of 200 ev to measure the whole energy band and 50 ev in a narrow scan to selectively measure the particular elements. Charge compensation was achieved with the system flood gun that provides low energy electrons and low energy argon ions from a single source. Elemental analysis was carried out with an Agilent 725-ES inductively coupled plasma optical emission spectrometer (ICP-OES) at University of Valladolid. Active sites calculation. The electrode pocket where the carbon paste is inserted has a nominal surface area of 0.07 cm 2, and a depth of 4 mm. This makes very difficult to estimate the actual active layer. In order to quantify the number of possible active sites for comparison, we followed this procedure: The electrode is weighted before and after the incorporation of a homogeneous mixture of the desired catalyst/cp ratio. Taking into account the catalyst/cp ratio, the exact amount of catalyst can be determined, as well as, the total content (mols) of metal present. Although not all the catalyst/cp 3

4 is active, estimating the total catalyst content gives us the maximum active sites limit (and minimum activity per active site). We think this is a fair approximation. For each catalyst, we identified all active metal (Co, or Ir) as a viable active site. We need to note, in addition, that in the case of Co 9, only six Co positions are viable active sites (those directly bound to water molecules). Still, we are accounting for all nine Co positions in these calculations. Turnover number (TON) estimation. As before, it is not easy to determine the active layer thickness in these electrodes. So, we introduced an approximation to calculate the corresponding TON in our systems. The amount of O 2 generated can be calculated using the following equation, considering 100% Faradaic efficiency (true at least for our Co-POM catalysts): n "# = Q n & 'F where Q (C) is the charge passed through the system, n & '= 4 is the number of electrons needed to generate one molecule of O 2 and F is the Faraday constant (96485 C/mol). To determine the mols of active catalyst, we limited to 1/8 of the total electrode pocket volume as maximum limit really in contact with the solution. This implies a penetration layer of 0.5 mm thickness, which seems reasonable. Even if this TON calculation is arbitrary regarding total numbers, the results are still good for relative comparisons between catalysts. The results of our TON estimations are in Tables 3 and 4 of this Supplementary data document. 4

5 a b Figure 1 Details of electrochemistry data (LSV) in H2SO4 (1 M) solution with 40% catalyst/cp blends. a, Estimation of the water oxidation onset potential. b, Pre-catalytic region for Ba[Co-POM]/CP and Co3O4/CP electrodes, showing the precatalytic event that appears exclusively in the case of cobalt oxide. a b Figure 2 a, Adsorption and desorption isotherms of Ba[Co-POM], Cs[Co-POM], Co 3 O 4 and IrO 2 previously degassed at 120ºC during 12 h. b, Plot of Dj (ja-jc) at 0.85 V vs Ag/AgCl (3.5 M KCl) against the scan rate, where the slope is twice the Cdl. We selected this range, V vs Ag/AgCl 3.5 M KCl, because the current response does not arise from any redox or catalytic process, so it must be due to the charging of the double layer. The values obtained for 30% catalyst/cp are mf/cm 2 for Ba[Co-POM], 6.2 mf/cm 2 for Cs[Co-POM], 8.7 mf/cm 2 for Co3O4 and 0.6 mf/cm 2 for IrO2. 5

6 µmols O µmols O time (min) time (min) Figure 3 Oxygen evolution (blue trace) and maximum Faradaic oxygen production (red line) during water electrolysis in H 2 SO 4 (1 M) with a 30% Cs[Co-POM]/CP (left) and Ba[Co-POM]/CP (right) electrodes at a constant current of 1 ma cm 2. The arrows indicate initial and final electrolysis times. The delay between electrochemical data and oxygen detection is due to the temporary trapping of the evolving gas in the carbon paste. Figure 4 Oxygen evolution (blue trace) and maximum Faradaic oxygen production (red line) during water electrolysis in H 2 SO 4 (1 M) with a Co 3 O 4 electrode at a constant current of 1 ma cm 2. The arrows indicate initial and final electrolysis times. The delay between electrochemical data and oxygen detection is due to the temporary trapping of the evolving gas in the carbon paste. 6

7 Figure 5 Raman spectra for Cs[Co-POM] and Ba[Co-POM] before and after 2 h electrocatalytic water oxidation at a constant current density of 1 ma cm 2 in H 2 SO 4 (1 M). Comparison with the corresponding spectra of Co 3 O 4. None of the cobalt oxide strong Raman peaks between 400 and 700 cm 1 could be identified in the fresh or used materials. 7

8 Figure 6 XPS data (Co, O, W and P edges) for Cs[Co-POM] before and after 2 h electrocatalytic water oxidation at a constant current density of 1 ma cm 2 in H 2 SO 4 (1 M). Co and O edges are compared with the corresponding XPS data for Co 3 O 4. In all cases, there are no signals that could be assigned to the adventitious presence of cobalt oxide impurities, since CoO x typical XPS peaks appear at ev (Co2p) and (O1s). The Co-POM used materials maintain all Co2p peaks above 781 ev and all O1s peaks above 530 ev, missing any new peak that could be assigned to lattice oxygen coming from cobalt oxide-type structures (529.8 ev). 8

9 Figure 7 XPS data for Ba[Co-POM] before and after 2 h electrocatalytic water oxidation at a constant current density of 1 ma cm 2 in H 2 SO 4 (1 M). O edges are compared with the corresponding XPS data for Co 3 O 4. In all cases, there are no signals that could be assigned to the adventitious presence of cobalt oxide impurities. Due to the strong overlap between Ba3d and Co2p XPS absorptions, we checked the Co2s peaks instead. The results are analogous to the Cs[Co- POM] case. Again, all O1s peaks appear above 530 ev, missing any new peak that could be assigned to lattice oxygen coming from cobalt oxide-type structures (529.8 ev). 9

10 Figure 8 IR spectra for K[Co-POM] (left), Cs[Co-POM] (center) and Ba[Co-POM] (right) before and after 2 h electrocatalytic water oxidation at a constant current density of 1 ma cm 2 in H 2 SO 4 (1 M). KCo 9 is soluble in water (or acid), and cannot be used for this electrocatalytic experiments. Figure 9 X-ray powder diffraction pattern for Cs[Co-POM] (top) and Ba[Co-POM] (bottom) before and after 2 h electrocatalytic water oxidation at a constant current density of 1 ma cm 2 in H 2 SO 4 (1 M). 10

11 j (ma cm 2 ) Figure 10 Intermittent chronoamperometry in H 2 SO 4 (1 M) solution at a constant anodic overpotential of 250 mv with 30 % Ba[Co-POM]/CP blends: Anodic potential was applied for 1 hour, and then open circuit voltage was maintained for 20 minutes. This cycle was repeated five times showing identical response. t (h) Figure 11 Thermogravimetric analysis for Cs[Co-POM]. Figure 12 Thermogravimetric analysis for Ba[Co-POM]. 11

12 Figure 13 Chronoamperometry data in H2SO4 (1 M) solution at a constant anodic current of 10 ma cm 2 with 30 % catalyst/cp blends. Table 1. Elemental analyses (ICP-OES) of the liquid reaction media after 2 h electrocatalytic water oxidation at a constant current density of 1 ma cm 2 in H 2 SO 4 (1 M). mother liquor Cs Ba K P Co W Cs[Co-POM] 37 ppm - 7 ppm 1 ppm 1 ppm 1 ppm Ba[Co-POM] - 1 ppm 1 ppm 1 ppm 1 ppm 1 ppm Co 3 O ppm - Table 2 Elemental analyses (ICP-OES) for the fresh, and recovered catalyst, including also the supernatant fractions after 2 h electrocatalytic water oxidation at a constant current density of 1 ma cm 2 in H 2 SO 4 (1 M). Data was normalized to the W content to facilitate comparison. Sample Cs Ba K P Co W Cs[Co-POM] fresh (theoretical) Cs[Co-POM]_fresh Cs[Co-POM]_recovered Cs[Co-POM]_carbon paste_supernatant * Ba[Co-POM] fresh (theoretical) Ba[Co-POM]_fresh Ba[Co-POM]_recovered Ba[Co-POM]_carbon paste_supernatant * *This increment in P content can only be assigned to a problem with the analytical method, probably due to the carbon/binder predominant content in these samples. 12

13 Table 3. TON estimation after 24 h chronoamperometry at h=250 mv BaCo 9 CsCo 9 Co 3 O 4 IrO 2 CoP i NiFe Mols catalyst (30% CP/cat) (1/8) Mols catalyst Q 24h (C) Mols O TON Table 4. TON calculation after 2 h chronopotentiometry at 1mA cm -2 BaCo 9 CsCo 9 Mols catalyst (30% CP/cat) (1/8) Mols catalyst Q 2h (C) Mols O TON