EDL structures of acid and alkaline interfaces at HER potentials. It is found that, for the interface models with a 4⋅4 orthogonal Pt(111) slab used in present study (Supplementary Figs. 1-3), the introduction of two H3O+ and four Na+ cations respectively into the water layers, and the same number of electrons into the electrode accordingly, can drive the electrode potentials (U) to values corresponding to the HER potentials for acid and alkaline systems, that are, ca. -0.20 V for pH=0 and ca. -0.32 V for pH=14 (unless stated, potentials in this paper are referenced to the reversible hydrogen electrode, RHE). The much higher electron density on electrode surface for the alkaline system is easy to understand when considering that the HER potentials are much further away from the potential of zero charge (PZC). As stated in the Methods section, the co-ions and the Gouy-Chapman diffusion layer can be neglected in the strong acid and alkaline systems. Therefore, we focus on the differences between the EDL structures at acid and alkaline interfaces associated with the profiles of cations. Figure 1a,b show the representative AIMD snapshots for the acid and alkaline interfaces, respectively. The statistic concentration profiles and trajectory analyses of H3O+ and Na+ distinctly demonstrate a layered distribution of cation in both acid and alkaline EDL (Supplementary Fig. 4).
The cations closest to the electrode surface constitute the outer Helmholtz planes (OHPs), which are at distances of ~4.26 Å and ~2.92 Å away from the electrode surface for acid and alkaline systems, respectively. The alkaline OHP bears much higher ion concentration than the acid one. The crowded cations at alkaline OHP lose considerably their solvation molecules, which can be seen from the Na-O radial distribution functions (gNa−O) shown in Supplementary Fig. 5. In addition, there hardly see water molecules within the alkaline OHP. At acid interface, in contrast, the protons are well solvated and the OHP is separated from the electrode surface by water molecules. Such differences between the cation and water distributions in the acid and alkaline EDLs should be ascribed to the much higher negative charge density of electrode surface at alkaline interface and higher mobility of protons. The position of the OHP in alkaline media agrees well with that measured through the surface X-ray scattering technology42,43, which rationalizes our simulated EDL structures.
The interfacial distributions of water molecules, represented by the oxygen concentration profiles along the surface normal direction, are further illustrated (Fig. 1c). It is apparent that at the distance of ~3.30 Å away from the Pt surface, both acid and alkaline interfaces exhibit a sharp peak of oxygen concentration, which is within the acid OHP while out of the alkaline OHP, again suggesting that there no water molecules being present between the cations and electrode surface at alkaline interface. As marked by the shadow, both the acid and alkaline interfaces possess a “gap zone” above the OHP, in which the water concentration is fairly low, and beyond which the water concentration fluctuates around the bulk value (0.056 mol/cm3). The “gap zone” at alkaline interface is apparently larger in width and more depleted in water concentration than that at acid interface. It is imaginable that the severe depletion of water molecule would reduce the connectivity of the H-bond networks at alkaline interface, which is verified by the statistical distributions of H-bond number along the surface normal direction (Fig. 1d). It is seen that there exists a region around ~4.2 Å away from the Pt surface (orange shadow) at alkaline interface, in which the H-bond number is significantly diminished as compared with the neighboring regions. Such diminishment in H-bond should be due to the large decrease in the numbers of solvated water molecules of the crowed OHP cations and the strong interaction between the OHP cations and their solvated water molecules, which significantly reduce the ability of these water molecules to form H-bond with others. It is conceivable that the greatly reduced connectivity of H-bond networks near the electrode surface would severely inhibit the hydrogen electrocatalytic reactions, since the H-bond networks constitute highways for delivering proton to surface, which, as will be shown in the following, is vital in hydrogen electrocatalysis. On the contrary, the H-bond number at acid interface is with little contrast to that in the bulk region.
Origin of the greatly different HER kinetics at acid and alkaline interfaces. The HER essentially involves hydrogen transfer (HT) processes, via the Volmer and/or Heyrovsky reaction. It has been argued that the great difference between the HER kinetics at acid and alkaline interfaces is due to the different hydrogen sources, namely, hydroniums (H3O+) at acid interface and water molecules at alkaline interface. The HT through water dissociation is traditionally believed to have high energy barrier than that through H3O+. As implied in the simulated EDL structures (Fig. 1a,b), the closest hydroniums (H3O+) at acid interface are separated from electrode surface by interfacial water molecules. Therefore, it should be the water molecules closely neighboring the electrode surface rather than the H3O+ that directly participate the HT processes in acid HER. Thus, the HER kinetics at acid and alkaline interfaces may be compared by investigating the HT processes of the water molecules closest to electrode surface. We consider the Volmer reaction for simplicity. The slow-growth method is used to evaluate the free energy barriers (ΔG≠) of the individual Volmer reaction, with each closest water molecule serving as the direct dissociative reactant to form Had (Supplementary Fig. 6). At acid interface, the collective variable (CV) is defined as the combination of several O-H distances connecting the hydrogen bond networks from H3O+ to closet water molecule, while the O-H bond length of interface water molecule is directly used as the CV in alkaline media (Supplementary Fig. 7). To ensure the accuracy and reliability of the free energy barriers comparison, the slow-growth simulation for each water molecule has been performed three times independently to obtain the error analysis. In acid Volmer reactions, the hydronium at OHP transfers H to the dissociated water molecules closet to the electrode surface through the H-bond networks in the EDL (Supplementary Fig. 8); while in alkaline, the dissociated water molecules closest to the surface gains H through the H-bond networks in the EDL and meanwhile a hydroxyl anion (OH−) is produced in the region out of OHP (Supplementary Fig. 9). As illustrated in Fig. 2a, surprisingly, the Volmer reactions of various water molecules at the acid interface have higher free energy barriers than those at the alkaline interface, which contradicts the long-established belief that the alkaline HT processes should be more energy-demanding than the acid ones through the dissociation of interfacial hydronium26. When considering the EDL structures, however, the lower free energy barrier of individual HT process in alkaline becomes reasonable.
As having been mentioned above, it is the water molecules closely neighboring the electrode surface rather than the H3O+ cations that directly act as the HT sources to form Had at acid interface. Therefore, the energy-limiting step of the HT processes at both the acid and alkaline interfaces should be the dissociation of the closest water molecules. At alkaline interface, these water molecules are strongly polarized by cations, which will significantly weaken the O-H bond strength and thereby lower the dissociation barrier. The facilitated reactivity by the interacting metal cations has been corroborated experimentally44. In addition, the electrode surface with much higher negative charge density should strongly attract the hydrogen atoms in water, which further facilitates the breakage of O-H bond of the nearby water molecules. Consequently, the ~2 orders of magnitude lowering in the alkaline HER activity cannot be simply attributed to the change of H source from hydronium ions to water molecules.
Besides the dissociation of the closest water molecules, the HT through the H-bond networks in EDL is also essentially involved in the HER process. As revealed in the simulated EDL structures (Fig. 1d), the H-bond number is significantly diminished in the region near alkaline OHP. The scarcity of H-bond would drastically reduce the HT channels, thereby leading to significant HT congestion at OHP. On the other hand, the insufficient supply of proton to the inner interface may result in the accumulation of OH− ions, which can further increase the free energy barrier of the water dissociation thereby. The present finding highlights immense opportunities to regulate the electrochemical reaction kinetics in alkaline environment by optimizing the interfacial water distribution and the connectivity of H-bond networks. It can explain well why bringing down the electrode PZC8,21 and/or adding the protic ionic liquids (or organic molecules) 39,41 as the electrolyte components can be sagacious strategies to improve the hydrogen electrocatalytic kinetics. The former can reduce the negative charge density on electrode surface, thereby decrease the cation concentration required to establish the EDL, and eventually improve the interfacial water distribution and H-bond networks8,21. The latter can construct and strengthen the interfacial H-bond networks and serve as the role of “proton pump” to efficiently transfer the proton in the EDL39,41.
We also evaluated the HBEs at acid and alkaline interfaces, through the calculation method introduced in the Supplementary Note 1 and Supplementary Fig. 10. Figure 2b shows that the hydrogen binding strength at acid interface is stronger than that in alkaline environment, agreeing well with the recent voltametric and spectroscopic experiments21–23. This demonstrates that the pH-dependent HER kinetics also cannot be described aptly by the difference in hydrogen binding strength.
Rationalization of the H-bond network gap in alkaline EDL with the results from in situ SEIRAS. To corroborate the H-bond network gap in the alkaline EDL, we have conducted in situ surface-enhanced IR spectroscopy (SEIRAS) measurements in attenuated total reflection (ATR) mode and compared the experimental vibration responses with the computational vibrational density of states (VDOS) of interfacial water molecules in the simulated EDLs. The ATR-SEIRAS given in the form of differential spectra usually has excellent surface enhancement effect and sensitivity within ~5-10 nm away from the electrode surface45, thus enabling accurate detection of the EDL structures at electrochemical interfaces.
Chemically deposited Pt thin film electrode is used as the working electrode in SEIRAS. The typical cyclic voltammograms of the Pt thin film electrode in 0.1 M HClO4 and NaOH solutions are shown in Supplementary Fig. 11, and both are identical to those of polycrystalline Pt in the corresponding environments22. The atomic force microscopy (AFM) images show that the Pt thin film electrode possesses rough surface with islands composed of Pt nanoparticles (Supplementary Fig. 12). In addition, the spectrum measured in CO-saturated 0.1 M HClO4 solution exhibits only two peaks centered at 2073 cm−1 and 1871 cm−1 (Supplementary Fig. 13), which are assigned to the C-O stretching modes of CO adsorbed on the atop and bridge sites of Pt surface, respectively46,47. This indicates that the Au substrate is completely covered by the deposited Pt film and thus has negligible interference on the IR spectra measurement.
Figure 3a,b show the in situ SEIRA spectra of interfacial water molecules on Pt at various potentials in Ar-saturated 0.1 M NaOH and HClO4 solutions, respectively. The spectra are given in the form of differential spectra with that collected at potentials close to the PZC of Pt electrode, namely, 0.9 V in alkaline and 0.5 V in acid. Using these reference spectra, we can obtain the spectral changes associated with the gradual formation of the EDL as the potential deviates from the PZC22,48,49. On the other hand, we have calculated the vibrational density of states (VDOSs) for the interfacial water molecules within ~6.6 Å from the Pt surface in the AIMD-simulated EDLs at alkaline and acid interfaces. As shown in Fig. 3c,d, the computational spectra possess very similar shapes to that of the experimental SEIRA spectra. Specifically, the O-H stretching peak of interfacial water displays a broader and symmetric shape in alkaline system while a more asymmetric shape extending toward the lower frequency region in acid system. In addition, it is noted that the experimental O-H stretching peak in NaOH solution can be deconvoluted into three distinct components through Gaussian fitting (Fig. 3e and Supplementary Fig. 14a); while in HClO4 solution, the O-H stretching band can only be resolved into two components (Fig. 3f and Supplementary Fig. 14b). The deconvolution of the computational VDOS of O-H stretching mode exhibits the nearly identical features (Fig. 3g,h). These consistencies not only indicate that the ATR-SEIRS signals were mainly derived from the first few layers of water molecules close to the electrode surface, but also confirm that the AIMD-simulated EDL structures reasonably represent that at the real electrochemical interfaces. Therefore, the existence of H-bond network gap at the alkaline interfaces can be convinced.
To further understand the experimental and computed vibrational spectra, the VDOS of interfacial water molecules at different distances from the electrode surface have been calculated. Figure 4a shows the schematic diagram of region division, in which the Region 2 is the gap region of water molecules and H-bond networks. As shown in Fig. 4b, the O-H stretching vibration peak of water molecules at alkaline interface exhibits a feature of first blue shift then red shift with the distance away from the electrode surface. The water molecules located in the gap region (denoted as H2O(gap)) have the highest O-H stretching vibration frequency, while those above the gap region (denoted as H2O(above−gap)) and those nearest to the electrode surface (denoted as Na.H2O(Pt)) have the second and lowest O-H stretching vibration frequencies, respectively. These features of theoretical spectra seem to agree well with the experimental observation that the O-H stretching spectra for water molecules at alkaline interface can be deconvoluted into three distinct components. Consequently, the high wavenumber component (green shadow), main component (yellow shadow) and low wavenumber component (blue shadow) in Fig. 3e and 3g are associated with H2O(gap), H2O(above−gap) and Na.H2O(Pt), respectively. In acid system, the H2O(gap) gives a slightly higher theoretical O-H stretching wavenumber, while the other interfacial water molecules (denoted as H2O(none−gap)) exhibit nearly the same O-H stretching vibration frequency (Supplementary Fig. 15), which seems to explain well why the experimenta spectra for acid system can only be deconvoluted into two distinct components (Fig. 3f and 3h). These results reveal that the H2O(gap) usually has a stronger O-H stretching vibration when comparing to the adjacent water molecules, which should further suggest the relative scarcity of H-bond networks in the gap region.
To further compare the degrees of the H-bond network gap at alkaline and acid interfaces, the potential dependence of O-H stretching vibration frequencies of each type of interfacial water molecules at alkaline and acid interfaces are compared (Supplementary Fig. 16). As well as the frequencies shift due to the vibrational Stark effect50, it is noted that the difference in the O-H stretching wavenumbers between H2O(gap) and H2O(above−gap) in alkaline system (~150 cm−1) is much higher than that between H2O(gap) and H2O(none−gap) in acid system (~72 cm−1), demonstrating the much more severe scarcity of H-bond networks in alkaline EDL. On the other hand, the potential-dependent proportions of the deconvoluted peak for H2O(gap) in alkaline and acid environments have been analyzed. It is found that the proportion of H2O(gap) molecules at alkaline interface gradually decreases with the potential decreasing (Fig. 4c), which should be due to the more serious depletion of water molecules and H-bond networks caused by the increased Na+ concentration in the EDL. By contrast, the proportion of H2O(gap) molecules at acid interface not only is much higher than that at alkaline interface, but also shows negligible change with potential (Fig. 4d), which suggests that the gap of H-bond networks is almost negligible in acid system. In addition, it’s encouraging that the proportions of H2O(gap) (marked by red circles) derived from the computational VDOS for both alkaline and acid systems reasonably agree with that calculated from the experimental SEIRAS spectra, further convincing the simulated EDL structures and the decreased connectivity of H-bond networks in the alkaline EDL.
Improving the connectivity of H-bond networks by OH adsorption. Numerous recent studies have revealed that alloying Pt with Ru can significantly enhance the HOR/HER kinetics in alkaline electrolyte, which has been ascribed to the facilitated water formation/dissociation due to the increased OH/H2O adsorption on the more oxophilic Ru atoms25,30,33. As shown in last section, the water dissociation may not be the major cause of the decreased activity of Pt for hydrogen electrocatalytic reaction in alkaline electrolyte, while the connectivity of H-bond networks near OHP may be a dominant factor of the kinetic pH effect. Therefore, we investigate the effect of OH adsorption on the connectivity of interfacial H-bond networks, using the Pt3Ru(111) as a model surface. Figure 5a,b show the AIMD-simulated atomic structures of alkaline interfaces on Pt3Ru(111) electrodes with and without OH adsorption, and the electrode potentials are driven to values corresponding to the HOR potentials for pH=14 by inserting three Na+ counter-ions, which are ~0.40 V and ~0.14 V, respectively. We consider HOR here because it has been shown the Pt-Ru alloys usually exhibit much more pronounced activity promotion for HOR25,30. Meanwhile, the interface on the Pt(111) electrode that contains the same number of Na+ cations is also simulated for comparison (Fig. 5c). As shown in Fig. 5d, the water concentration distributions in the interfacial structures of Pt3Ru(111) electrode without OH adsorption are very similar to that of the Pt(111) electrode, both exhibiting fairly low water concentrations within the green shaded region (viz. the “gap zone” above the OHP); while for the interface on the Pt3Ru(111) electrode with OH adsorbed on Ru site, the water concentration within the green shaded region is apparently higher. The increased number of water molecules in the gap zone is also manifested by the integral water concentration distributions (Supplementary Fig. 17) and the H-bond number profiles (Fig. 5e). This suggests that the OH adsorption can significantly increase the water connectivity and improve the H-bond networks in the EDL. To further understand this phenomenon, the EDL structures of the interfaces on Pt3Ru(111) electrodes with and without OH adsorption are carefully compared. First, one can find that on Pt3Ru(111) electrode with OH adsorption, the Na+ ions mainly gather around the adsorbed OH species due to the coordination interaction, rather than being more spread over the electrode surface as that on the bare Pt3Ru(111), as shown in Fig. 5a,b and Supplementary Fig. 18. The aggregation of cations around OH would increase the space for water molecules in the OHP region. On the other hand, it’s noted that the coordination between Na+ ions and adsorbed OH leaves more free water molecules (Supplementary Fig. 18), which possess stronger ability to form H-bond with others. These results suggest that the adsorbed OH species promote the alkaline HOR activity mainly through regulating the interfacial EDL structures.