The growing hydrogen economy requires accelerating the hydrogen evolution reaction. The water dissociation step (Volmer step) has been proposed as a main kinetic limitation, but the mechanisms at play in the electrochemical double-layer are poorly understood. This is due to the dual role of water: it acts both as a reactant and as a solvent. Here we propose to confine water inside an organic liquid matrix in order to isolate the sole role of water as a reactant. We observed the formation of aqueous-rich nanodomains, whose size can be tuned by changing the supporting electrolyte and found that the reactivity of the system varies significantly with its nanostructure. Depending on the conditions, the reactivity is dominated by either the strength of short-range cation–water interactions or the formation of long chains of water molecules. Understanding this paves the way towards the development of more efficient and selective electrocatalysts for water, CO2, O2 or N2 reduction.
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Unconventional interfacial water structure of highly concentrated aqueous electrolytes at negative electrode polarizations
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Molecular dynamics inputs, initial and final configurations are available on a ChemRxiv depository (https://doi.org/10.26434/chemrxiv.11926293). Other experimental raw data or computational data are available from the corresponding authors on reasonable request.
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N.D. acknowledges the École Normale Supérieure for his PhD scholarship. We acknowledge the French National Research Agency for its support through the Labex STORE-EX project (ANR-10LABX-76-01). This project has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Grant No. 771294). We acknowledge SOLEIL for provision of synchrotron radiation facilities and we would like to thank Thomas Bizien for assistance in using beamline SWING.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Linear sweep voltammograms recorded in acetonitrile in presence of 100 mM LiTFSI with different water contents (1, 2.5, 5, 7.5 and 10% from light to dark blue) using a rotating disc (1,600 rpm) Pt electrode with a 50 mV s−1 sweeping rate. No modification of the onset potential at which water is reduced was observed, independently of the water content, confirming that the trend previously measured for LiClO4 is independent on the nature of the anion.
Potential of mean force between Li+ cations and Owater (a) and between Owater and Hwater (b) derived from the g(r) functions. 𝑝𝑚𝑓(𝑟)=−𝑘𝐵𝑇 ln𝑔(𝑟), where 𝑘𝐵 is the Boltzmann constant and 𝑇 the temperature (300K) for different water contents (1% in light blue, 5% in blue and 10% in dark blue). Since the largest values are obtained for Li-Owater, independently on the water content, this indicates that it is more difficult to separate one Li+ cation from a water molecule than a water molecule from another one.
Coordination number between Li+ cation (blue) or Na+ cations (pink) and oxygen atoms from ClO4− anions (a), nitrogen atoms from acetonitrile molecules (b) and oxygen atoms from water molecules (c) in the presence of 100 mM of AClO4 in acetonitrile containing 10% of water in mass. The simulation box for NaClO4 was similar to the LiClO4 one (Supplementary Table 1) and the Lennard-Jones parameters for Na+ were chosen from Åqvist56. The number of water molecules and ClO4- anions present in the first solvation shell of the Na+ and Li+ cations is similar, while more acetonitrile is found in the case of Na+.
Potential of mean force between Li+ or Na+ cations and Owater (derived from the g(r) functions (pmf(𝑟)=−𝑘𝐵𝑇 ln𝑔(𝑟), where 𝑘𝐵 is the Boltzmann constant and 𝑇 the temperature (300K)) in acetonitrile containing 10% of water in mass and 100 mM of LiClO4 or NaClO4 salt. Since a smaller value is predicted in the presence of Na+ cations, this indicates that the water-Na+ interaction is weaker when compared to the water-Li+ interaction.
Coordination number between Li+ cations and nitrogen atoms from acetonitrile molecules (a) and oxygen atoms from ClO4- anions (b) and oxygen atoms from water molecules (c) in acetonitrile containing 10% of water in mass at different H2O/Li ratio (44 yellow, 8.8 orange, 4.4 red). The increase of the LiClO4 concentration results in an increased amplitude of the first plateau of the Li-O(ClO4-) coordination number, which suggests more ion-pairing, accompanied by a small increase of the acetonitrile in the first solvation shell of Li+ cations and a small decrease of the water content in the first solvation shell.
a, Evolution of the electrolyte (acetonitrile with 10% of water in mass) conductivity and b, 7Li chemical shift with the LiClO4 concentration. The indicated concentrations correspond to the electrolytes previously studied with different Li/H2O ratios (Supplementary Table 3). The bell shape for the conductivity and the downfield of the 7Li suggest that ion-pairs are formed when the salt concentration is increased. The lines are guides for the eyes.
X-Ray weighted contributions from the oxygen-oxygen pairs to the global X-ray-weighted structure factor obtained from the MD simulations for acetonitrile electrolytes containing 10% of water in mass at different H2O/Li ratio (44 yellow, 8.8 orange, 4.4 red).
Extended Data Fig. 8 Impact of the concomitant increase of water and salt on the water reactivity and electrolyte structure.
a, Linear sweep voltammograms recorded in acetonitrile with a H2O/Li+ ratio of 4.4 at different water concentration (from 1% light grey, to 10% red) on a rotating disc (1,600 rpm) Pt electrode with a 50 mV s-1 sweeping rate. b, Number of LiClO4-H2O domains and their average volume depending on the LiClO4/H2O ratio at 10% of water concentration extracted from the MD simulations. c, SAXS intensity in the low-q range of acetonitrile for the electrolytes with 1% (grey), 5% (rose) or 10% of added H2O at constant H2O/Li+ ratio of 4.4 and d, corresponding coordination numbers between the Li+ cations and acetonitrile molecules, ClO4- anions and water molecules. The simulation box for the 5% H2O electrolyte contains 966 acetonitrile molecules, 110 water molecules and 25 LiClO4 ion pairs with a box length of 44.32 Å. Since the H2O/Li ratio is not modified in this set of experiments, a similar short-range environment for the Li+ cations is maintained in this third series of electrolytes. The linear-sweep voltammograms recorded with this series of electrolytes show a gradual reduction of the electrode passivation and a gradual shift of the onset potential for reduction toward less negative values when the concentration is increased. Logically, this behavior directly results from a combination of the two extreme trends previously measured (Figs. 2 and 5 in the main text). Furthermore, a significant increase in the SAXS intensity at low-q with the Li-H2O concentrations is observed, experimentally confirming the growth of these long-range LiClO4-H2O domains in this third series of electrolytes. These observations confirm the impact of the long-range organization of the water-rich nanodomains on the water reduction potential.
a, Linear sweep voltammograms recorded over a platinum electrode in acetonitrile with TBAClO4 (left panel) at 100 mM with 1% (grey) or 10% (dark blue) in mass of water and with LiClO4 (right panel) at 100 mM with 1% of water (grey) or 10% of water (dark blue) or 2M LiClO4 with 10% of water (brown). b, Exchange current densities for the HER over platinum and gold electrodes compared with the computed free energy for the formation of the adsorbed hydrogen intermediate (Data taken from Nørskov, J. K. et al.8). c, Linear sweep voltammograms recorded over a gold electrode in acetonitrile with TBAClO4 (left panel) at 100 mM with 1% (grey) or 10% (dark blue) in mass of water and with LiClO4 (right panel) at 100 mM with 1% of water (grey) or 10% of water (dark blue) or 2M LiClO4 with 10% of water (brown) The electrolyte effects (short- and long-range) are observed whatever the position of the catalyst on the Volcano plot made using the hydrogen binding energy as descriptor. Briefly, in presence of TBA cations the water can be reduced only at large water concentrations on both Au and Pt electrodes. Regarding the lithium case, increasing only the water concentration does not shift the onset potential for the water reduction on gold and platinum electrodes. Nevertheless, at high water concentration, increasing the lithium salt concentration shifts the reduction of water toward less negative potentials.
Extended Data Fig. 10 Onset potential for the water reduction as a function of the cation-water interaction or the size of the aqueous-rich domains.
a, Onset potential for the water reduction (at j = 1 mA cm-2) in acetonitrile electrolyte with 5% in mass of water and 100 mM of TBA, Na or Li perchlorate expressed in function of the activation energy for the cation-water separation obtained from the cation-Owater potential of mean force. b, Onset potential for the water reduction (at j = 1 mA cm-2) in acetonitrile electrolyte with 10% in mass of water at different Li/H2O ratios expressed in function of the aqueous domain volume obtained from the domain analysis of the molecular dynamics simulations. At the short range, the strength of the cation-water molecule interaction that can be assessed by the activation energy for the cation-water separation extracted from the cation-water potential of mean force may be considered as a potential descriptor describing the water reduction. Nevertheless, this physical parameter, which is not strongly modified when for instance the salt concentration in the electrolyte is increased, cannot capture the HER kinetics when entering a second regime where long-range parameters, such as the volume of the H2O-rich nanodomains, can be considered as a physical descriptor.
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Dubouis, N., Serva, A., Berthin, R. et al. Tuning water reduction through controlled nanoconfinement within an organic liquid matrix. Nat Catal 3, 656–663 (2020). https://doi.org/10.1038/s41929-020-0482-5
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