Effect of the molecular functionalization on the OER activity
In the first set of experiments, enantiopure (M)- and (P)-thiadiazole-helicenes, previously reported by some of us in the frame of our general interest in chiral molecular materials, have been used for the chiral molecular functionalization of metal electrodes.
Fig. 1 shows the effect of the adsorbed thiadiazole-helicene enantiomers on the OER activity of two types of electrodes, namely Au(111) with monolayer NiOx islands and bare Au(111), in O2-saturated 0.1 M aqueous KOH solution. Farhat et al. reported that the OER activity of ultrathin NiOx and NiFeOx films under reaction conditions decreases over time in Fe-free KOH but stays more constant in unpurified (i.e., containing trace Fe impurities) KOH after Fe incorporation. Therefore, an unpurified KOH solution has been used to ensure all activity measurements are reproducible.
A clear enhancement in the OER current can be seen after the helicene molecules were deposited on NiOx samples, as shown in Fig. 1 a and b. Deposition of (P)- and (M)-thiadiazole-helicene enhances the OER current at 1.65 V vs. RHE approximately 85 % and 74 %, respectively. Electrochemical impedance spectroscopy (EIS) measurements were conducted before and after the molecule deposition to determine if the current enhancement could be just due to an increment in the ECSA during the molecule deposition process. The ECSA (proportional to the active surface) was obtained by determining the specific adsorption capacitance around the ‘‘onset’’ potential of the OER (Supplementary Figure 1 a). The OER currents normalized by the ECSA values still uphold the enhancement effect, i.e., ca. 61 % increase in the current density at 1.65 V vs. RHE and ca. 33.4 mV reduction in the overpotential at the current density of 10 mA cm-2 in the case of (P)-thiadiazole-helicene (Supplementary Figure 1 b). It is likely that the increase in the specific adsorption capacitance in Supplementary Figure 1 a was mainly induced by the adsorbed helicene molecules,, and not by the increment of the ECSA. Nevertheless, the normalized current reinforces the conclusion that the thiadiazole-helicene enantiomers are able to enhance the OER at NiOx islands on Au surfaces.
Although the effectiveness of the helicene molecules on OER enhancement at NiOx is validated, it is likely that molecules were not directly adsorbed on the NiOx islands as no metallic Ni sites were accessible during the measurements. Direct thiadiazole bonding on metal oxide surfaces is difficult under the current conditions. Therefore, the molecules were mostly adsorbed on the Au substrate. To test the effect of direct bonding between the helicene molecule and the catalyst surface on the OER, bare Au(111) electrodes were used for analogous OER experiments (Fig. 1 c). Although such chirally functionalized Au(111) has lower OER activity compared to NiOx, we anticipate that the activity measurements on Au(111) help to elucidate the effect of direct adsorption of helicene molecules at OER active sites. In contrast to the results from NiOx samples, the presence of (P)-thiadiazole-helicene molecules reduced the OER at the Au(111) surface. This difference can be ascribed to the different catalyst-helicene molecule configurations. At the NiOx decorated Au substrates, the molecules were mostly adsorbed on the Au surfaces, and the active centers at NiOx islands were not directly affected. At bare Au samples, the OER was taking place at the Au surface, which was partially blocked by the molecules after deposition. The exposed surface area of the Au samples was determined by the stripping of Pb atoms deposited by the underpotential deposition (UPD) method (see Supplementary Figure 2 a and b for the Pb stripping results and the normalized current density). The results show that a submonolayer of helicene molecules was on the sample surface after the deposition. The decrease in the OER current is not proportional to the decrease in the surface area of Au. The helicene molecules even reduced the specific current density. As shown in Supplementary Figure 2 a, peak 1, which can be assigned to low-coordination and step sites31,, decreased and shifted towards more negative potentials. Zwaschka et al. found that OER activity on a polycrystalline Au surface is dominated by < 1 % of the surface consisting of defects. Therefore, the molecules were preferentially adsorbed on surface defects, or the molecule-metal bond at defect sites was more stable during the reactions.
Molecular functionalization vs. Fe doping effect on OER
Trotochaud et al. studied the effect of Fe impurities in the electrolyte and Fe doping in the catalysts on the OER activity, showing that Fe doping is efficient to improve the OER activity. However, this improvement still adheres to the volcano-plot limits,. In this work, Fe impurities from the electrolyte had been incorporated in the NiOx samples during activity measurements before chiral molecular functionalization. A comparison of the activity enhancement caused by the (P)-thiadiazole-helicene and the Fe doping in Ni-based catalysts is shown in Fig. 2. The Ni9FeOx catalyst was deposited on Ni foils using a combustion method introduced elsewhere. The ECSA was obtained from the determination of the specific adsorption capacitance27. The helicene molecules further increased the OER activity of highly active Ni-based catalysts modified by Fe doping. The enhancement was higher on NiOx than on Ni9FeOx, e.g., ca. 93.8 % on NiOx and ca. 25.9 % on Ni9FeOx at the potential of 1.6 V vs. RHE, respectively. It is presumably caused by the different amounts of helicene molecules that can be embedded into different electrodes. On the NiOx electrodes, helicene molecules were stably bonded to the Au substrates. However, the Ni9FeOx electrode surface only consists of metal oxides, which do not form strong interactions with the helicene molecules. The Ni9FeOx electrodes have much higher roughness compared to the NiOx electrodes. The helicene molecules were probably inserted into the porous structures of the Ni9FeOx electrodes. Nevertheless, the effect caused by the chiral molecules is independent and compatible with the Fe doping effect.
To confirm the enhancement is owing to the chirality of the adsorbed helicene molecules instead of other neglected properties of thiadiazole-helicene and, at the same time, to investigate the influence of the helicene length on the OER, we synthesized bis(thiadiazole)-helicene, which contains an additional thiadiazole ring at the opposite end of the helix, expected to stabilize the Au-helicene-NiOx sandwich structure. We produced the racemic form of bis(thiadiazole)-helicene and resolved the (P) and (M) enantiomers by chiral HPLC on Chiralpak IF (see Methods and the Supplementary Information for the synthesis, separation procedures and crystal structures). Suitable single crystals for the X-ray diffraction analysis have been obtained for both the enantiopure and also the racemic helicenes. The first eluted enantiomer was the dextrorotatory one, having a specific optical rotation of = + (8100 ± 1 %)°, corresponding, as expected, to the clockwise (P) enantiomer, in agreement with the single crystal X-ray analysis. Mirror-image circular dichroism (CD) spectra were obtained for the two enantiomers (see Supplementary Figure 12). In the solid state, the helical curvature, characterizing the dihedral angle between the two terminal thiadiazole rings, amounts at 29° for the racemate and 35° for the enantiopure compounds, thus smaller than 45° observed for the thiadiazole-helicenes27. Then, with the new bis(thiadiazole)-helicene in our hands, we set out to the chiral molecular functionalization of our electrodes.
Fig. 3 a shows that the activity of NiOx on Au was improved by the presence of (M)-bis(thiadiazole)-helicene molecules on the surface, e.g., ca. 131.5 % at the potential of 1.65 V vs. RHE. The (M)-bis(thiadiazole)-helicene yields a sensibly greater improvement in the OER current than the thiadiazole-helicenes. The much more intense OER activity enhancement by the bis(thiadiazole)-helicene functionalized electrodes compared to the thiadiazole-helicene functionalized ones is in favor of this sandwich-type structure of the electrodes, with the helicene lying between the Au substrate and the NiOx islands, since it can be hypothesized that the former strongly interacts with both the substrate and the catalyst thanks to the presence of the functional thiadiazole rings on both sides of the helical connector.
So far, the results show that chiral helicene molecules can enhance the OER activity. However, the compound in the molecules that directly bonds to the electrode surface is the thiadiazole group. Thus, it is essential to evaluate the role of thiadiazole. Accordingly, we used 2,1,3-benzothiadiazole, a simple achiral molecule containing a thiadiazole cycle fused to a benzene ring. It has the same molecular footprint bonds to the Au(111) surface as the helicene molecules used in this work. As shown in Fig. 3 b, the presence of the achiral molecules did not noticeably influence the OER activity of NiOx islands on Au. Therefore, the interaction between the thiadiazole compound and the electrode surface is not the origin of the OER enhancement activity.
On Au(111) surfaces, the helicene molecules formed self-assembled monolayer (SAM) structures. The STM images of (P)-thiadiazole-helicene and (M)-bis(thiadiazole)-helicene molecules on Au(111) are shown in Fig. 4 a and b, respectively (see Supplementary Figure 3 for larger scale images showing different domains of assemblies). The former formed trimeric structures, while the latter formed rows of dimers. In the SAM of (M)-bis(thiadiazole)-helicene molecules, the existence of three types of domains (60° to each other) is observed, as shown in Supplementary Figure 3 b. These types of molecular assemblies are common to other functionalized helicenes on Au(111)18,19,20,22. The molecular coverage of functionalized helicenes on the substrates determines the assembly, going from trimeric structures at low coverage to dimeric rows at high coverage19,20,21,22. Therefore, the (M)-bis(thiadiazole)-helicene reached a higher coverage than the (P)-thiadiazole-helicene under similar conditions, indicating that the (M)-bis(thiadiazole)-helicene established stronger interactions with the Au substrate likely due to the added thiadiazole group.
STM images were taken on the helicene molecule functionalized NiOx samples after the OER measurements, as shown in Supplementary Figure 4. The NiOx islands were roughened due to the OER, and the surfaces of the islands were likely not covered by the molecules. However, the structures on the Au surface suggest that the helicene molecule assembly on Au was stable during the OER. Bare Au surfaces can be seen in several areas in the images, which is in line with the Pb stripping measurement results in Supplementary Figure 2.
The activity enhancement using chiral molecules appears to depend on the anchoring of the molecules regarding to the catalysts. Based on our results, the dependence of the OER activity enhancement on the molecule-catalyst configuration is presented in Fig. 4 c. Helicene molecules on top of the catalyst surface block the active centers and consequently reduce the overall catalytic activity. Therefore, electrode manufacturing is vital to the performance of helicene molecule functionalized catalysts. A rationally designed electrode requires the catalytically active centers free of blockage and the electron transfer through the spin polarizers. For an optimal activity enhancement, the chiral molecules should be between the catalytic material and the substrate. A substrate that can strongly bond the chiral molecules (for example, Au in this work) is preferred for fabricating a stable electrode.
Chiral molecular functionalization
In this work, we show that chiral molecular functionalization can improve the OER activity of metal oxide catalysts. A specific catalyst-chiral helicene molecule-Au substrate configuration (as shown in Fig. 4c) shows the optimal catalytic activity. By comparing the effect of chiral helicene molecules and the achiral 2,1,3-benzothiadiazole, it is evident that such activity enhancement is directly related to the chirality of the molecules, rather than ligand effects introduced by the bonding of the molecules on the catalyst surface. The bonding between the catalyst and the chiral molecule alone does not cause any improvement in the catalytic activity. Contrarily, the bonding can reduce the activity when the chiral molecules are directly deposited at the catalyst active sites. The results of this comprehensive study are in agreement with the interpretation from Ref. 8 that a CISS-induced effect can efficiently enhance the OER.
Although often considered a spin filter, chiral molecules are better described as spin polarizers as the chirality does not reduce the transition probability. In electron scattering, the spin polarization is induced by symmetry breaking. This can be expanded to any process that can be described by a transition matrix, for example, photoemission, where the spin polarization is induced by the broken experimental symmetry also for light elements,. In electron transport, all contributions from the complex transition matrix elements are typically symmetric unless a chiral element is introduced. In this case, the transmitted electrons will acquire a spin polarization, similar to a paramagnetic material in an external magnetic field. Garcés-Pineda et al.9 reported on the external magnetic field enhancement of the OER at various magnetic transition metal oxide-based catalysts. Although an activity enhancement can be caused by both chiral molecular functionalization and an external magnetic field, the enhancement magnitudes appear to be different. At the potential of 1.65 V vs. RHE, the OER current density (scan rate: 5 mV/s) on NiOx-based catalysts in 0.1 M KOH increased ca. 61 % by using (P)-thiadiazole-helicene, which is significantly larger than the ca. 10 % in 1 M KOH (scan rate: 5 mV/s) caused by an external magnetic field (≤450 mT)39. Thus, chiral molecular functionalization may be relatively more effective. It has also been well established that external magnetic fields can accelerate mass transfer processes through a magnetohydrodynamic effect ,,. The local magnetic field at the catalyst surface introduced by the chiral molecules and its effect on the local mass transfer should be further probed.
It was suggested that the electron spin at the catalyst surface strongly influences the bonding strength of the catalyst to oxygen species and the charge transfer between the catalyst and the oxygen adsorbates9,,,. Electron spin polarization at catalyst surfaces with specifically modulated ferromagnetic properties leads to advanced OER kinetics10,,,, and a ferromagnetic catalyst can promote the spin polarization under an external magnetic field and thus enhances its OER activity. However, not all practical OER catalysts are ferromagnetic. Additionally, metal oxide-based catalysts, especially highly active 2D nanostructured catalysts, undergo severe surface reconstruction processes under OER conditions,,. The preservation of the well-prepared ferromagnetic properties under surface reconstruction is questionable. The chiral molecular functionalization provides a more versatile and sustainable electron spin polarization effect (Fig. 4 c). It can be applied to most catalysts independent of their electronic (magnetic) properties and under different reaction conditions, i.e., less affected by the catalyst composition and surface reconstruction.
Compared to chiral molecular functionalization, Fe doping is a verified way to improve the OER activity of Ni-based catalysts. It is revealed that the OER activity of Ni-based catalysts in non-specifically treated electrolytes (containing a trace amount of Fe) is affected by the Fe doping, and the catalysts effectively turn into NiFe dual catalysts34. Our results show that chiral molecular functionalization enhancement can coexist with the Fe-doping effect and other alloying methods since the helicene molecules do not interact directly with the metal oxide active centers. Therefore, it is compatible with most metal oxide-based catalysis systems and independent of the chemical composition of the catalysts.
Moreover, the newly introduced (M)-bis(thiadiazole)-helicene brings a higher enhancement to the OER than (M)-thiadiazole-helicene, i.e., ca. 131.5 % vs. ca. 74 % in the overall current at the potential of 1.65 V vs. RHE in 0.1 M KOH. This finding suggests the enhancement is related to the structure of the helicene molecules, for instance, molecular length and functional groups that determine the interactions with the catalyst and the substrates. Further enhancement through modification of the helicene molecules can be anticipated.