Remarkable stability of a molecular ruthenium complex in PEM water electrolysis

The dinuclear Ru diazadiene olefin complex, [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2], is an active catalyst for hydrogen evolution in a Polymer Exchange Membrane (PEM) water electrolyser. When supported on high surface area carbon black and at 80 °C, [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2]@C evolves hydrogen at the cathode of a PEM electrolysis cell (400 mA cm−2, 1.9 V). A remarkable turn over frequency (TOF) of 7800 molH2 molcatalyst−1 h−1 is maintained over 7 days of operation. A series of model reactions in homogeneous media and in electrochemical half cells, combined with DFT calculations, are used to rationalize the hydrogen evolution mechanism promoted by [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2].


Figures S9: Eelectrolysis cell set up
IX. Figure S9: Electriysis cell set up
XIII. Figure  XIV. Figure S14: Mechanism of hydrogen evolution in neutral solution calculated by DFT.
3 XV. Figure S15: Electrochemical characterization of a glassy carbon electrode coated with 2@C k and 3a@C k .
XVI. Figure S16 Electrochemical characterization of 2@C k for the OER.
XVIII. Figure S18: Electrochemical characterization of 2@C k for HER in homogeneous media.
Exhaust solutions were directly injected in the instrument. For calculating the ruthenium content in the 2@C k catalyst, 5 mg of the sample were dissolved in a PTFE vessel in 8 mL of aqua regia (6 mL of HCl 37% a and 2 mL of HNO3 conc.). The resulting mixture was then digested in a microwawe oven and diluted to 50 mL adding bidistilled water.
GC-TCD was measured on an Agilent Technology 7890A GC System on a HP-Molsieve (19091P) column.

5
The scanning transmission electron microscopy (STEM) investigations were performed on the aberration-corrected HD-2700CS (Hitachi; cold-field emitter) or a Jeol F200 equipped with a cold field emission gun and a 16 MPixel camera, both operated at an acceleration potential of 200 kV. On the HD-2700CS, a probe corrector (CEOS) is incorporated in the microscope column between the condenser lens and the probe-forming objective lens providing excellent high-resolution capability (beam diameter ca. 0.1 nm in the selected ultra-high resolution mode). Images (1024 x 1024 pixels) were recorded with a high-angle annular dark field (HAADF) detector with frame times of ca 15 s.
These imaging conditions give rise to atomic number (Z) contrast, a highly sensitive method to detect even atoms of strongly scattering elements (high Z) on light supports.
The mixture was stirred at room temperature for 30 minutes before the solvent was evaporated at reduced pressure. The obtained brown solid was washed with diethylether:dimethoxyethane 9:1 (3x 3 mL). The leftover solid was dissolved in acetonitrile, filtered through a syringe filter (pore size 0.2 µM) and precipitated by layering with diisopropyl ether to obtain a red-brown powder.
The mixture was stirred for additional 15 minutes allowing it to warm to room temperature which caused a further color change to a red-brown solution, before the solvent was evaporated at reduced pressure. The obtained red-brown solid was washed with toluene (3x 2 mL) and 1,2-difluorobenzene (1x 2 mL). Subsequently, the solid was dissolved in acetonitrile and layered with diisopropyl ether.
After 24 hours, the product was obtained as a red-brown powder. Yield: 167 mg, 69 %.

e. Synthesis of IrO2
The IrO2 anode for the complete electrolysis cell was synthetized as described in [S2] . In brief, 247 mg of IrCl3*3H2O (0.7 mmol) were dissolved in 50 mL of ethylene glycol in presence of 700 mg of polyvinyl-pirrolidone (PVP). The solution was heated at 120°C for one hour to evaporate residual water and then was refluxed for 60 minutes under magnetic stirring. The solution was concentrated, cooled down and poured in a crucible, which was placed in a muffle furnace heated at 400°C for one hour (heating ramp: 7°C min -1 ). The resulting solid was milled in an agate mortar. Suitable crystals were selected, protected by polybutene oil and mounted under a cold nitrogen stream. The crystals were kept at 100 K during data collection. The data reduction was performed using CrysAlisPro and Apex3, respectively.. Using Olex2 [S3] , the structures were solved with SHELXT [S4] , followed by least-squares refinement against full matrix (versus F2) with SHELXL. [S5] All non-hydrogen atoms were refined anisotropically. The crystal structures of [Ru(OH2)(µ-H) figure 1 in the main text, crystal data and further crystallographic details are reported in table S1 and S2.

g. DFT caluclations
All calculations were carried out with ORCA 4.2.0. [S6,S7] Geometry optimizations were performed at the PBE0-D3BJ/def2-SVP/def2-TZVP(Ru) level of theory. [S7-S13] Solvent effects were taken into account implicitly by using the cpcm model with water as solvent and a gaussian charge scheme. [S14,S15] Numerical frequency calculations were carried out to confirm the nature of stationary points found by geometry optimizations. The RIJCOSX approximation was used for density functional theory (DFT) calculations. [S16,S17] Approximate transition states were generated using the nudged elastic band (NEB) method implemented in ORCA, followed by a saddle-point optimization.
Cartesian coordinates of optimized molecular structures are reported in table S4. h. XPS XPS experiments were carried out in an UHV chamber with a base pressure lower than 10 −10 mbar.
The chamber was equipped with non-monochromatized Al (hν = 1486.6 eV) radiation and with a hemispherical electron/ion energy analyser (VSW mounting a 16-channel detector). The operating power of the X-ray source was 150 W (15 kV and 10 mA). Photoelectrons were collected normal to the sample surface and the analyser maintaining as well the angle between the analyser axis and the X-ray source fixed at 54.5°. All the samples were drop cats on gold on mica and all the XPS spectra were measured in fixed analyser transmission mode with pass energy of 44 eV. The binding energy (BE) was calibrated setting the Au4f7/2 peak at 83.9 eV.

II. Complex impregnation on carbon black
The impregnation procedure was adapted from previous works. [S18-S20]

e. Ink preparation
The ink was prepared in a 5 mL glass vial suspending the milled catalyst in 600 mg of Milli-Q water, 600 mg of ethanol (purity 99%) and 12 mg of a 5% wt Nafion ® solution in 2-propanol. The mixture was treated for 30 minutes with ultrasounds (59 Hz, 100 W) and then was stored under magnetic stirring. A fresh ink was prepared before each set of measurements.

a. General techniques
Electrolysis experiments were performed in a Scribner cell (Scribner ass.) modified in our laboratory with stainless steel collector plates stable in acidic environment at potentials higher than 1.2 V. Cell temperature was controlled with a Scribner 850c testing station (Scribenr ass.) and the anodic and cathodic compartments were fed respectively with 50 mL of Milli-Q water and 50 mL of a 0.1 M H2SO4 solution or H2O by means of a multichannel peristaltic pump (Gilson minipulse 3) with 1 mL min -1 flow rate; exhaust fuels were collected in a closed vessel and recirculated in the cell. The cathodic vessel head was equipped with a flow meter to quantify the hydrogen produced during the electrolysis experiments.
The electrochemical measurements were performed with an Arbin LBT21084 multichannel potentiostat-galvanostat and the hydrogen amount produced during electrolysis was quantified with a Brockhorst El-Flow (3 mL min -1 or 1 L min -1 ) flow meter.

b. MEA (Membrane Electrode Assembly) fabrication
The Membrane Electrode Assembly was obtained sandwiching a Nafion 117 membrane between the anode and the cathode in a 5 cm 2 Scribner cell fixture using a 4 Nm screwing torque.
The anode was realized mixing in an agate mortar 40 mg of IrO2 with 20 mg of C k and 100 mg of a Nafion 5%wt ionomer solution in low aliphatic alcohols; the resulting paste was spread onto a wovennon-woven titanium web conductive support (5 cm 2 , Beakert). The electrode has the 66.6% wt IrO2 loading (c.a. 7 mgcatalyst cm -2 ) and the 3% wt of pure Nafion amount.

V. Hydrogen and energy consumption quantification
The electrolysis efficiency in hydrogen production was calculated evaluating the hydrogen evolution reaction faradic efficiency FE as described in Eq.1.
(%) = 2 2 ℎ * 100 Eq.1 The real hydrogen amount produced during the electrolysis was calculated with the flow meter placed at the cell outlet while the theoretic amount was calculated electrochemically applying the Faraday's law (Eq.2) assuming 100% of coulombic efficiency for hydrogen evolution reaction (HER): I is the 12 current load applied to the cell during the experiment, F is the Faraday's constant (96485.3 C mol -1 ) and 2 is the number of electrons moles involved in HER.
The cell energy consumption required for electrolysis was calculated integrating over time the charging energy and dividing this value with the total hydrogen amount produced (expressed in kgH 2 ) as described in Eq.3. Eq.3

b. Verification of Stoichiometric H2 Production by GC-TCD
In a 10 mL Schlenk tube capped with a rubber septum, [RuH(µ-H)(Me2(dad))(dbcot)2] (30 mg, 43 µmol, 1 eq.) was dissolved in THF (3 mL). Degassed sulfuric acid (50%, 0.2 mL, 10 eq.) was added and the stopcock closed. The mixture was heated to 80 °C for 1 hour, before a sample was taken from the headspace via syringe. The sample was subjected to GC-TCD, which allowed the identification of H2 by its retention time.

Figures S1-S3: NMR spectra of compounds 3a and 3c
In less coordinating solvents like THF, the terminal coordination site is partially occupied by solvent and partially by triflate (2 and 3b in equilibrium) or water (3aand 3b in equilibrium), as described in the literature for 3b. [S1] Acetonitrile replaces the triflat anion or water molecule from the terminal coordination site. The NMR spectra in acetonitrile show only one species which is the same for 2, 3a and 3b and thus is assumed to be [Ru(µ-H)(NCCD3)(Me2(dad))(dbcot)2] + (3c).

Figures S5-S7: Thermal Gravimetry Analysis
The TGA data suggest that in the solid, H2O is indeed coordinated to the complex and not just cocrystallised (vide infra).