Action at a distance: organic cation induced long range organization of interfacial water enhances hydrogen evolution and oxidation kinetics

Engineering efficient electrode–electrolyte interfaces for the hydrogen evolution and oxidation reactions (HOR/HER) is central to the growing hydrogen economy. Existing descriptors for HOR/HER catalysts focused on species that could directly impact the immediate micro-environment of surface-mediated reactions, such as the binding energies of adsorbates. In this work, we demonstrate that bulky organic cations, such as tetrapropyl ammonium, are able to induce a long-range structure of interfacial water molecules and enhance the HOR/HER kinetics even though they are located outside the outer Helmholtz plane. Through a combination of electrokinetic analysis, molecular dynamics and in situ spectroscopic investigations, we propose that the structure-making ability of bulky hydrophobic cations promotes the formation of hydrogen-bonded water chains connecting the electrode surface to the bulk electrolyte. In alkaline electrolytes, the HOR/HER involve the activation of interfacial water by donating or abstracting protons. The structural diffusion mechanism of protons in aqueous electrolytes enables water molecules and cations located at a distance from the electrode to influence surface-mediated reactions. The findings reported in this work highlight the prospect of leveraging the nonlocal mechanism to enhance electrocatalytic performance.


Electrochemical measurements
All electrochemical measurements were conducted with a Gamry Interface 1000 Potentiostat in a homemade Teflon cell to circumvent the contamination from the corrosion of glass.A graphite rod was used as the counter electrode.where ƞ is the overpotential, α is the transfer coefficient, T is the temperature, R is the gas constant (8.314J mol -1 K -1 ), ik is the kinetic current density normalized to the ECSA, and F is the Faraday constant (96485 C/mol).

Preparation of Pt film electrode for SEIRAS
Pt film electrodes were prepared based on previously reported deposition procedure 1 .The Si prism was first polished with 0.05 µm alumina slurry until the surface became totally hydrophobic.The polished Si prism was then ultrasonicated in water for 10 min and then rinsed with acetone and milli-Q water for multiple times.
Subsequently, the Si prism was immersed in NH4F (40%) for 360 s to obtain a hydrogen-terminated surface, after which the Si prism was immediately immersed in a mixture of 2 wt% aqueous HF solution and Au plating solution with a ratio of 1:4 at around 55 °C for 6 min to deposit an Au film on prism.The Au plating solution was prepared by mixing 5.75 mM NaAuCl4•2H2O (99%), 0.025 M NH4Cl (99.99%), 0.025 M Na2S2O3•5H2O (98%), 0.075 M Na2SO3 (98%), and 0.026 M NaOH (99%), all of which were purchased from Sigma-Aldrich.The Au-coated prism was used as the working electrode, on which Pt was electrodeposited in Ar saturated 4 mM H2PtCl6 + 0.7 M Na2HPO4 solution.The electrodeposition current density and time were set at -400 uA cm −2 and 360 s, respectively.The prepared Pt film was cleaned with water several times and used immediately.

In situ SEIRAS studies
The SEIRAS experiments were performed in a two-compartment spectroelectrochemical flow cell separated by a piece of anion exchange membrane.
The Pt thin film electrode deposited on the Si prism was employed as the working

Electrochemical impedance spectroscopy
EIS experiments were performed with a peak-to-peak amplitude of 5mV ranging from 10 kHz to 0.1Hz.The spectra were fitted to the equivalent electric circuit (EEC) as shown in Fig. S8 by the software Gamry Echem Analyst using the Levenberg-Marquardt Method for nonlinear least-squares regression.

Molecular dynamics simulations
Atomistic molecular dynamics (MD) simulations were carried out to study the hydration properties of single ions and the properties of electrolytes at the Pt-aqueous interface.Structure model was produced by XPONGE 2 .
For the system containing the Pt slab, the square slab consists of three layers of Pt(111) with an area of 30.932  29.223 Å 2 (1112 Pt atoms per layer).An aqueous solution system with 2100 water molecules, 20 cations and hydroxides was constructed above the Pt slab plane based on the surface coverage of the experiment, i.e., the reduction of the Hupd feature in CVs.Vacuum was placed above the bulk solution.
To study the properties of the hydration layer of free cations, for each type of cations, a single cation system was built.Each system consists of 5000-water cubic solvent box and one cation placed in the center of the box.
MD simulations were performed by the SPONGE 3 program and the force field parameters for TMA/TEA/TPA and hydroxide were taken from general AMBER force field 4 .The atomic point charges were calculated by Multiwfn using RESP charge model 5,6 .The TIP4P-EW model was chosen for water molecules 7 .The Lennard-Jones parameters for K + and Pt were taken from the literature 8,9 .10000-step gradient descent minimization was carried out before the systems were simulated.First, a 10 ns preequilibrium was carried out with the time step of 2 fs, in which the temperature of the system was set to 300 K by the middle scheme of Langevin dynamics 10 .For the single cation system, an additional 2 ns pressure-equilibrium step before pre-equilibrium step was carried out using NPT ensemble with the time step of 2 fs, in which the temperature of the system was set to 300 K by the middle scheme of Langevin dynamics and the pressure of the system was set to 1 atm using Monte-Carlo barostat 11 .The final production simulations were simulated for 30 ns in the canonical ensemble with the time step of 1 fs, in which the temperature of the system was set to 300 K by the middle scheme of Langevin dynamics.For the system with a Pt slab, the electric potential of the top surface was limited to -0.87 V relative to vacuum by FEM module in SPONGE 3 .The simulation trajectories were visualized by VMD software 12,13 .

Determination of hydrogen bond lifetimes
Identifying hydrogen bonds and calculating their lifetimes were performed using the Python package MDAnalysis 14,15,16 .hydrogen bond lifetime was calculated via the time autocorrelation function of the presence of a hydrogen bond: where the value of ℎ  represents whether there is a hydrogen bond between atoms  and .ℎ  = 1 means that there is a hydrogen bond between atoms i and j while ℎ  = 0 means there is no hydrogen bond between these two atoms.Criteria for identifying hydrogen bonds are: 1) the donor-acceptor distance is  3 Å, and 2) the donorhydrogen-acceptor angle is  150°.The average lifetime of the hydrogen bond was obtained by integrating the autocorrelation function:

Statistical length distribution of hydrogen bond chains
In order to count the length distribution of hydrogen-bonded water chains at the Pt(111)-organic cations interface, we selected the hydrogen-bonded water chains between the water with distance from the surface (z) < 4.4 Å (surface adsorbed water layer) and the z > 10.4 Å.We first counted water with z < 12 Å (slightly greater than 10.4 Å in order to identify water molecules located at just outside the cutoff distance of 10.4 Å from the surface) and total hydrogen bonds formed among these water molecules.Then we chose water molecules with z < 4.4 Å and z > 10.4 Å as the starting and ending points, respectively (vice versa).The horizontal distance between the starting and ending points was constrained within 3 Å.The shortest connecting path between the starting and ending points was was found by Floyd algorithm and selected as the length of the hydrogen bond chain for statistics.
electrolyte (Fig. S4).No appreciable difference was observed between polarization curves on Pt collected in 0.1 M TPAOH with and without the preelectrolysis treatment (Fig. S5), suggesting that no contaminants in the TPAOH electrolyte affect the measured activities.
A saturated calomel electrode (BASi) was used as the reference electrode.A 5 mm polycrystalline Pt rotating disk electrode (RDE) (Pine Instruments) was employed for electrochemical measurements.Before each experiment, the Pt disk was polished manually by 0.05 μm Al2O3 suspension and rinsed with Milli-Q water.The Pt electrode was subsequently electrochemically cleaned by cycling potential between 0.05 and 1.2 V vs RHE at a scan rate of 50 mV/s until reproducible cyclic voltammograms were achieved.All potentials reported in this work were corrected for the solution resistance (iR drop) before further analysis and referenced to the reversible hydrogen electrode (RHE).Before any electrochemical tests, the homemade Teflon cell was boiled in Milli-Q water for at least 3 h.The solution resistance was determined by potentiostatic electrochemical impedance spectroscopy.Cyclic voltammograms were measured in Ar saturated 0.1 M hydroxide solution with different cations by cycling between 0.05-1.0Vvs RHE at a scan rate of 50 mV/s.HER/HOR polarization curves were obtained with a potential range of -0.1-0.5 V vs RHE in H2 saturated electrolyte at a rotation rate of 1600 rpm.All HER/HOR polarization curves were normalized by the geometric area.The exchange current density was normalized by the electrochemical surface area (ECSA), which was calculated by the integrated area of the underpotential deposition hydrogen in CV with the assumption of 210 μC/cm 2 for the adsorption of one hydrogen monolayer on Pt.Kinetic current density for HOR was obtained using the Koutecky-Levich equation to remove the contribution of mass transport to the measured current density:1/ = 1/  + 1/  Where i is the measured current density, ilim is the limiting current density, and ik is the kinetic current density.The kinetic current density curves were fitted to the Butler-Volmer equation to get the exchange current density i0.
electrode.A graphite rod was used as the counter electrode.The electrolytes used in all spectroelectrochemical experiments were prepared by the same method as for electrochemical testing in the previous section.SEIRAS experiments were conducted on a Bruker INVENIO S FTIR spectrometer equipped with a liquid nitrogen-cooled MCT detector and a ATR accessory.A Bio-logic sp150e Potentiostat was used to control the electrode potential.An Ag/AgCl electrode (3.0 M KCl, BASi) was used as the reference electrode.All spectra were 64 co-added scans with 4 cm −1 resolution.
, finite element grids are first initialized, after which the electric potential distribution is obtained by solving Poisson's equation under a given boundary condition (In our work, the boundary condition is ϕ=-0.87V on the Pt surface and ϕ=0 V in the vacuum).The simulation with Pt slab was done in two dimensions with periodic boundary and double-Layer equipotential boundary conditions(The Pt slab system is periodic in the X & Y directions and has two equipotential boundary in the Z direction).

Fig. S2 .
Fig. S2.CV curve on pc Pt at 50 mV/s in Ar saturated 0.1 M TPAOH (black trace), and the electrolyte was switched to 0.05 M TPAOH + 0.05 M KOH (on the same Pt electrode without any cleaning) before the CV curve was collected (blue trace).CV curve in 0.1 M KOH (light grey race) collected on a fresh pc Pt electrode is included for comparison.

Fig. S3 .
Fig. S3.(a) Preelectrolysis at 0.01 V vs RHE in 0.1 M TPAOH for 8500 s.(b) The CV curves collected in 0.1 M TPAOH with and without the preelectrolysis.

Fig. S4 .
Fig. S4.50 CV scans in the range of 0.05-1.2V to remove any reducing species.Except for the the first CV scan (black trace), the rest of scans essentially overlap.

Fig. S6 .
Fig. S6.Measured and fitted (with the Butler-Volmer equation) HOR/HER kinetic current densities on pc-Pt in 0.1 M KOH and 0.1 M TPAOH at 10 mV/s.

Fig. S9 .
Fig. S9.(a), CO stripping plots on pc-Pt in 0.1 M hydroxide solution with different cations.The scanning rate is 20 mV/s.(b), Integrated ECSA based on the charge of CO oxidation.

Fig. S8 .
Fig. S8.Equivalent electric circuit for the Hupd region.Rsol represents the resistance of solution, while CDL and Cad stand for electric double layer capacitance and the pseudo-capacitance of the adsorbed hydrogen intermediate, respectively.

Fig. S10 .
Fig. S10.The potential dependent IR spectra for CO adsorption in the solutions of organic cations.

Fig. S11 .
Fig. S11.Distribution functions of water molecules on Pt (100) surface with TMA and TPA as electrolyte cation along the direction of surface normal.

Fig. S13 .
Fig. S13.The time autocorrelation function of hydrogen bonds in the MD simulations with TMA, TEA and TPA.

Fig. S14 .
Fig. S14.(a).Root-mean-square displacement of water in the TMA/TPA mixed system.(b).Rootmean-square displacement of TMA in the TMA/TPA mixed system.(c).Possibility of escape of water from Pt(111) surface in the TMA/TPA mixed system.(d).Extracted lifetime of hydrogen bonds from time autocorrelation functions in the TMA/TPA mixed system.

Fig. S15 .
Fig. S15.SEIRA spectra of C-H bending modes in Ar saturated 0.1 M hydroxide solution with various ratios of TPA and K + .The reference spectrum was obtained in 0.1 M TPAOH.