Single Crystal Electrodes Research Articles

Electrochemical Hydrogenation of Aromatic Hydrocarbons

Electrochemical hydrogenation (ECH) of bio-mass derived compounds is an attractive alternative to traditional thermochemical hydrogenation (TCH) methods that are used in the U.S. chemical and petroleum industries to produce value-added fuels and chemicals. TCH uses high pressures and temperatures, along with an external source of hydrogen gas typically produced via methane reformation, in order to achieve hydrogenation; these requirements make it an energy intensive process. ECH has the advantages of operating near ambient conditions and sourcing the participating hydrogen from the aqueous electrolyte solution, resulting in reduced energy costs and CO2 emissions compared to TCH. Furthermore, the applied potential provides an additional parameter for controlling selectivity, making ECH more suitable to handle the wide chemical variability of biomass-derived feedstocks.Aromatic hydrocarbons are of interest as model compounds for lignocellulosic bio-oil [1]. The ECH of aromatic hydrocarbons proceeds with high faradaic efficiency (FE) at low overpotentials, however, the ECH turnover rate remains low at these potentials. There is no simple fix to this, as any increase in overpotential in an attempt to increase the degree hydrogenation will reduce the reaction FE, due to competition with the hydrogen evolution reaction (HER). In this work we address the need for a more fundamental understanding of ECH systems to promote high turnover rates at low overpotentials. In phenol TCH, it is well known that the hydrogenation is limited by the formation/addition of surface adsorbed hydrogen. The current ECH literature draws on these TCH studies and assumes that phenol ECH is also limited by surface adsorbed hydrogen [2]. Thus, most ECH work is done is acidic media, where the kinetics of surface adsorbed hydrogen formation are fastest on platinum [3]. HER, however, is also faster on platinum in acidic media [4], making it more difficult to achieve high faradaic efficiency.We have investigated the ECH of phenol in aqueous electrolyte at varying pH (1, 3, 11, and 13) on low-index single crystal platinum electrodes using rotating disk electrode voltammetry. We have also included the ECH of acetophenone and benzaldehyde to compare the effect of different functionalities on the phenyl ring. The conversion and faradaic efficiency have been determined for these molecules at pH 1 and pH 11 at selected potentials. In this poster we will present new insights on the ECH reaction mechanism of aromatic hydrocarbons along with methods to promote higher ECH rates at low overpotentials.[1] J. Yan et al., "Characterizing Variability in Lignocellulosic Biomass: A Review," ACS Sustainable Chemistry & Engineering, vol. 8, no. 22, pp. 8059-8085, 2020, doi: 10.1021/acssuschemeng.9b06263.[2] N. Singh et al., "Aqueous phase catalytic and electrocatalytic hydrogenation of phenol and benzaldehyde over platinum group metals," Journal of Catalysis, vol. 382, pp. 372-384, 2020, doi: 10.1016/j.jcat.2019.12.034.[3] S. Intikhab, J. D. Snyder, and M. H. Tang, "Adsorbed Hydroxide Does Not Participate in the Volmer Step of Alkaline Hydrogen Electrocatalysis," ACS Catalysis, vol. 7, no. 12, pp. 8314-8319, 2017, doi: 10.1021/acscatal.7b02787.[4] I. Ledezma-Yanez, W. D. Z. Wallace, P. Sebastián-Pascual, V. Climent, J. M. Feliu, and M. T. M. Koper, "Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes," Nature Energy, vol. 2, no. 4, 2017, doi: 10.1038/nenergy.2017.31.

(Invited) Improved Thermal Stability of Low-Probe Density DNA SAMs Prepared with Electrodeposition

DNA self-assembled monolayer (SAM) on gold electrodes has been widely used for biosensing.1, 2 The sensor stability and performance determine the reliability of the test results in early disease detection and point-of-care use.3, 4 We have carefully examined the thermal stability of a mixed SAMs composed of a short alkylthiol and alkylthiol modified DNA prepared with electrodeposition vs. those prepared at OCP (Open Circuit Potential) on a single crystal gold bead electrode. The nature of these two types of SAMs were carefully studied in buffer after exposure to different temperatures using in-situ fluorescence microscopy and electrochemical measurements. The analysis showed that surfaces with hexagonal symmetry were thermally less stable than those with rectangular or square symmetry, suggesting that the crystallography of the gold substrate plays an important role in sensor stability and performance in addition to the DNA density.5 We also found that OCP deposition creates a surface with high DNA coverage (>1012 DNA/cm2) but with higher level of irreproducibility. Even though this kind of surface can be more thermally stable, it is not optimal for effective biosensing due to its high coverage.6 It also seems to be challenging to make DNA SAMs with both low coverage and high thermal stability by using current SAM preparation protocols (e.g., OCP). In this study, we show that electrodeposition can prepare DNA SAMs with both higher thermal stability than those made at OCP and with good control of low surface coverage. This study provides valuable insights on making high quality DNA SAMs with better stability and functionality and improved reproducibility.

(Keynote) Effect of the Surface Charge on the Oxygen and Hydrogen Peroxide Reduction Reactions

The electrochemical behavior of hydrogen peroxide, which is a possible intermediate of the oxygen reduction reaction (ORR), has been investigated on platinum single crystal electrodes containing terraces with (111) symmetry separated by monoatomic steps with either (110) or (100) symmetry in different pH solutions [1]. On these electrodes, a decrease, in absolute value, of the reduction current at low potentials is observed. The potential at which the decrease is observed, can be correlated with the potential of maximum entropy (pme) of the double layer formation on the terraces. In addition, it is possible to relate the peak appearing at potentials below 0.3 V (RHE) to the local pme in the steps.By performing experiments at different pH values, up to pH ≈ 5, it is observed that the inhibition potential on the terraces shifts 59 mV per pH unit towards more positive potentials, in line with what was observed for the pme in laser-induced temperature jump experiments. For the possible explanation of this phenomenon, it must be taken into account that OH adsorption may influence this trend when the pme values are close to the potentials corresponding to OH adsorption. From results obtained in alkaline medium, it is clear that the surface charge plays an important role in this inhibition of the limiting diffusion current.Finally, the comparison of the results obtained under similar conditions for the hydrogen peroxide and oxygen reduction reactions suggests that the formation of hydrogen peroxide as an ORR intermediate is less favored with increasing pH. As a consequence, there must be a bifurcation in the ORR mechanism prior to peroxide formation. These results agree with previous results [2], which indicate that ORR involves different steps in which several bifurcation points may exist. Depending on the experimental conditions, the reaction may follow a different reaction pathway [3]. References. (1) Briega-Martos, V.; Herrero, E.; Feliu, J.M. Electrochimica Acta 2020 , 334, 135452.(2) Gomez-Marin, A.M.; Feliu, J.M. ChemSusChem 2013, 6, 172-176.(3) Briega-Martos, V.; Herrero, E.; Feliu, J.M. Chinese J. Catal. 2020, 41, 732-738.

(Invited) Towards a FRET Based DNA SAM Biosensor for Detection of Nucleic Acids

Rapid and sensitive detection of nucleic acids is an essential tool in many medical and clinical settings, that allows for fast diagnosis of bacterial and viral infections. Here, we present a DNA FRET-based biosensor to detect a hybridization event on a DNA SAM on gold surface. The monolayer was prepared by a potential assisted thiol exchange deposition, which provides better control over the surface coverage and its uniformity1. Our approach relies on a change in FRET (Förster Resonance Energy Transfer) signal caused by variations in distance between fluorophores. FRET imaging is a well-known, very sensitive technique, used commonly in many areas of chemistry and microbiology. FRET signal depends strongly on the distance between an acceptor and a donor fluorophore. It is an excellent tool to analyze small changes in DNA distance between adjacent strands and the surface2. Metal quenching also plays a major role, since fluorescence is strongly dependent on the distance from the metal surface3, which is an efficient quencher. Fluorophores, both acceptor and donor, are quenched significantly by the gold surface, this however, decreases substantially as the distance increases between the surface and fluorophore. As DNA is binding its complementary strands, the distance between the surface and the fluorophore labelled end increases.Using a self-assembly property of thiols on gold surface, we show that mixed ssDNA SAM composed of two strands of thiolated ssDNA, each labelled with a different FRET fluorophore, gives an 8-to-10-fold signal enhancement, upon DNA hybridization (see figure 1). Binding of a complementary strand rigidifies the adsorbed DNA structure, and as a result we observe the increase in signal due to weaker gold quenching and shorter distance between FRET participating fluorophores. Moreover, by utilizing a single crystal electrode, we can study different surface concentration regimes and fluorescence signal increases, based on surface crystallography and the electrode potential.(1) Leung, K. K.; Gaxiola, A. D.; Yu, H.-Z.; Bizzotto, D. Tailoring the DNA SAM Surface Density on Different Surface Crystallographic Features Using Potential Assisted Thiol Exchange. Electrochim. Acta 2018, 261, 188–197. https://doi.org/https://doi.org/10.1016/j.electacta.2017.12.114.(2) Verhaven, A.; Doneux, T.; Bizzotto, D. Application of FRET Microscopy to the Study of the Local Environment and Dynamics of DNA SAMs on Au Electrodes. Langmuir 2018, 34 (49), 14802–14810. https://doi.org/10.1021/acs.langmuir.8b02131.(3) Casanova-Moreno, J.; Yu, Z. L.; Massey-Allard, J.; Ditchburn, B.; Young, J. F.; Bizzotto, D. Luminescence in Electrochemistry; Springer International Publishing: Cham, 2017; pp 21– 77. Figure 1

Oxygen reduction reaction on platinum single-crystal electrodes modified with protonic ionic liquid

The oxygen reduction reaction (ORR) on the low- and high-index planes of Pt modified with a protonic ionic liquid ([7-methyl-1,5,7-triazabicyclo(4.4.0)dec-5-ene][bis(pentafluoroethylsulfonyl)imide], referred to here as [MTBD][beti]) was studied in 0.1 M perchloric acid solution. The specific activity of the ORR on Pt(111) modified with [MTBD][beti] was 3.1 times higher than that on unmodified Pt(111), while the ORR was deactivated on modified Pt(100). [MTBD][beti] slightly enhanced the ORR activity of the Pt n(111)–(111) planes (where n = 2–9). These results show that a larger (111) terrace is essential for a marked increase in the ORR activity on Pt electrodes modified with [MTBD][beti]. Although the ratio of the increase in the activity of n(111)–(111) was lower than that of Pt(111), Pt(331) = 3(111)–(111) showed the highest ORR activity after [MTBD][beti] modification. The effects of MTBD and beti were also studied separately. MTBD provided the main contribution to the increase in ORR activity, giving the highest activity at a concentration of 2.0–3.5 × 10–5 M, whereas beti affected the ORR activity only slightly.

Cover Feature: An Interfacial Study of Au(111) Electrodes in Deep Eutectic Solvents (ChemElectroChem 14/2022)

The Cover Feature illustrates the interface between an Au(111) single crystal electrode surface with deep eutectic solvents (DES) of different types and composition. The different systems were studied with cyclic voltammetry as well as electrochemical impedance spectroscopy, revealing important and interesting new insights into the electrode/electrolyte-interfaces. More information can be found in the Research Article by S. J. Zeller et al.

Self-assembled monolayers of O-(2-Mercaptoethyl)-O′-methyl-hexa(ethylene glycol) (EG7-SAM) on gold electrodes. Effects of the nature of solution/electrolyte on formation and electron transfer blocking characteristics

The self-assembly of O-(2-Mercaptoethyl)-O’-methyl-hexa(ethylene glycol) (EG7) on gold substrates produces monolayers whose structure should depend primarily on the solvents used for their formation. Although this should be also the case for any self-assembled monolayer (SAM) system, the presence of oxygen atoms in the EG7 chains brings about specific interactions with the solvent molecules other than just the van der Waals interactions taking place with alkanethiols. In this work we investigate the effects of the nature of the solutions for EG7-SAM formation in the reductive desorption processes using gold substrates either polycrystalline or single crystal electrodes. The patterns obtained in polycrystalline substrates are compared to the peaks observed at gold single crystal electrodes and it has been found that the main peaks correspond to the molecules desorbed from the different gold facets contained in the polycrystalline substrate. These single peaks are in fact composed of at least two contributions that can be explained as the presence of domains where the EG7 molecules are organized with different orientations, giving place to intermolecular interactions of different magnitude. The blocking properties of these films are strongly determined by the nature of the electrolyte used for the analysis and not by the solvent used in their formation. Although no experimental evidence of the specific interaction and/or retention inside the film of Na+ or phosphate or both ions is obtained, a specific effect that accounts for an electron transfer rate constant of an order of magnitude lower than the obtained with other electrolytes is observed. These results can contribute to increase the understanding of the relationship between the EG7-SAMs structure and functionalities.

Screening Surface Structure–Electrochemical Activity Relationships of Copper Electrodes under CO2 Electroreduction Conditions

Understanding how crystallographic orientation influences the electrocatalytic performance of metal catalysts can potentially advance the design of catalysts with improved efficiency. Although single crystal electrodes are typically used for such studies, the one-at-a-time preparation procedure limits the range of secondary crystallographic orientations that can be profiled. This work employs scanning electrochemical cell microscopy (SECCM) together with co-located electron backscatter diffraction (EBSD) as a screening technique to investigate how surface crystallographic orientations on polycrystalline copper (Cu) correlate to activity under CO2 electroreduction conditions. SECCM measures spatially resolved voltammetry on polycrystalline copper covering low overpotentials of CO2 conversion to intermediates, thereby screening the different activity from low-index facets where H2 evolution is dominant to high-index facets where more reaction intermediates are expected. This approach allows the acquisition of 2500 voltammograms on approximately 60 different Cu surface facets identified with EBSD. The results show that the order of activity is (111) < (100) < (110) among the Cu primary orientations. The collection of data over a wide range of secondary orientations leads to the construction of an “electrochemical–crystallographic stereographic triangle” that provides a broad comprehension of the trends among Cu secondary surface facets rarely studied in the literature, [particularly (941) and (741)], and clearly shows that the electroreduction activity scales with the step and kink density of these surfaces. This work also reveals that the electrochemical stripping of the passive layer that is naturally formed on Cu in air is strongly grain-dependent, and the relative ease of stripping on low-index facets follows the order of (100) > (111) > (110). This allows a procedure to be implemented, whereby the oxide is removed (to an electrochemically undetectable level) prior to the kinetic analyses of electroreduction activity. SECCM screening allows for the most active surfaces to be ranked and prompts in-depth follow-up studies.

Evaluating single-crystal and polycrystalline NMC811 electrodes in lithium-ion cells via non-destructive EIS alone

Evaluating single-crystal and polycrystalline NMC811 electrodes in lithium-ion cells via non-destructive EIS alone

Influence of water on the electrochemical characteristics and nanostructure of Bi(hkl)│Ionic liquid interface

In electrochemical applications of ionic liquids (ILs), the presence of residual amounts of water can influence the performance of ILs as electrolytes when developing novel electrochemical devices. In this work, the influence of water additive on the electrical double layer formation and adsorption of ions from 1-ethyl-3-methylimidazolium trifluromethanesulfonate IL at semi-metallic bismuth single crystal electrodes has been studied via cyclic voltammetry, electrochemical impedance spectroscopy, and in situ scanning tunneling microscopy methods. Major differences in water-adsorbed IL mixtures are observed at bismuth single crystal electrodes within a wide potential region: increased current densities and anomalous capacitance maxima, especially at positive polarization conditions. The kinetics of adsorption layer formation significantly are strongly affected by the adsorption processes of both cations and anions under polarization conditions, shown by the characteristic relaxation time constant results. Interfacial distribution of ions and surface nanostructure is visualized at the atomic resolution under in situ scanning tunneling microscopy study. These results provide a detailed understanding of the Bi(hkl) surface structure and the ion adsorption processes dependence on electrode polarization.

Crystal facet effect induced by different pretreatment of Cu2O nanowire electrode for enhanced electrochemical CO2 reduction to C2+ products

Electrocatalytic CO2 conversion has been considered as a promising way to recycle CO2 and produce sustainable fuels and chemicals. However, the efficient and highly selective electrochemical reduction of CO2 directly into multi-carbon (C2+) products remains a great challenge. Herein, we synthesized three type catalysts with different morphologies based on Cu2O nanowires, and studied their morphology and crystal facet reconstruction during the pre-reduction process. Benefiting from abundant exposure of Cu (100) crystal facet, the nanosheet structure derived Cu catalyst showed a high faradaic efficiency (FE) of 67.5% for C2+ products. Additionally, electrocatalytic CO2 reduction studies were carried out on Cu(100), Cu(110), and Cu(111) single crystal electrodes, which verified that Cu(100) crystal facets are favorable for the C2+ products in electrocatalytic CO2 reduction. Our work showed that catalysts would reconstruct during the CO2 reduction process and the importance in morphology and crystal facet control to obtain desired products.

Structural effects of the oxygen reduction reaction on the high index planes of Pt3Fe

In this study, the oxygen reduction reaction (ORR) on n(111)–(111), n(111)–(100), and n(100)–(111) series of Pt3Fe in 0.1 M HClO4 was studied. The specific activity of the ORR (jk) at 0.90 V(RHE) reached an upper limit at 4 ≤ n on the n(111)–(111) series. This tendency differed from those observed for the same series of Pt, Pt3Co, and Pt3Ni. In the n(111)–(100) series, Pt, Pt3Co, and Pt3Ni plots, as well as the values of jk plotted against step atom density (dS), exhibited a volcano-shape: Pt3Fe(544) n = 9 provided the maximum activity. In the n(100)–(111) series, the values of jk were constant regardless of dS, as occurred for the same series of Pt, Pt3Co, and Pt3Ni. The ORR activity of Pt3Fe(544) = 9(111)–(100) was 5.8 times higher than that of Pt(331) = 3(111)–(111), which has the highest activity among single-crystal Pt electrodes.

Effects of Surface Structures and Hydrophobic Species on the Oxygen Reduction Reaction Activity of Pt3Fe Single-Crystal Electrodes

Effects of Surface Structures and Hydrophobic Species on the Oxygen Reduction Reaction Activity of Pt3Fe Single-Crystal Electrodes

How palladium inhibits CO poisoning during electrocatalytic formic acid oxidation and carbon dioxide reduction

Development of reversible and stable catalysts for the electrochemical reduction of CO2 is of great interest. Here, we elucidate the atomistic details of how a palladium electrocatalyst inhibits CO poisoning during both formic acid oxidation to carbon dioxide and carbon dioxide reduction to formic acid. We compare results obtained with a platinum single-crystal electrode modified with and without a single monolayer of palladium. We combine (high-scan-rate) cyclic voltammetry with density functional theory to explain the absence of CO poisoning on the palladium-modified electrode. We show how the high formate coverage on the palladium-modified electrode protects the surface from poisoning during formic acid oxidation, and how the adsorption of CO precursor dictates the delayed poisoning during CO2 reduction. The nature of the hydrogen adsorbed on the palladium-modified electrode is considerably different from platinum, supporting a model to explain the reversibility of this reaction. Our results help in designing catalysts for which CO poisoning needs to be avoided.

SO2 electrooxidation reaction on Pt single crystal surfaces in acidic media: Electrochemical and in situ FTIR studies

SO2 is a poisonous and anthropogenic gas generated by fossil fuel burning due to the common presence of S-containing contaminants in oil and coal. As a strategy to handle this pollutant and to increase H2 generation, the electrocatalytic SO2 oxidation reaction (SO2OR) is considered. In this way, SO2 is converted to H2SO4, while hydrogen gas is generated at the cathode. To better understand the mechanism and its dependence on the surface crystallography and electrolyte composition, electrochemical and in situ spectroscopic measurements using Pt single crystal electrodes were performed. The crystallographic orientation of the electrode is shown to be critical not only to the SO2OR activity but also to the resulting mechanism. The Pt(100) face is found to be the most active electrode in both H2SO4 and HClO4 aqueous electrolytes, and S2O62− appears as a key intermediate for the mechanism. Volcano plots, drawn considering the Stark tuning slope as a quantity proportional to the adsorption energy, show the importance of S2O62− in the electrode catalytic activity.

Improved Thermal Stability and Homogeneity of Low Probe Density DNA SAMs Using Potential-Assisted Thiol-Exchange Assembly Methods.

Methods for producing DNA SAM-based sensors with improved thermal stability and control over the homogeneity of low DNA probe density will enable advanced sensor development. The thermal stability of low-coverage DNA SAMs was studied for surfaces prepared using potential-assisted thiol exchange (Edep) and compared to DNA SAMs prepared without control over the substrate potential (OCPdep). Both surface preparation methods were studied using in situ fluorescence microscopy and electrochemistry with fluorophore or redox-modified DNA SAMs on a single-crystal gold bead electrode. Fluorescence microscopy showed that the influence of the underlying surface crystallography was important in both cases. The highest thermal stability was realized for square or rectangular surface atomic structure (e.g., surfaces from 110 to 100). The 111 and related surfaces were the least thermally stable. The low DNA coverage surfaces prepared by Edep had better thermal stability and higher DNA probe mobility as compared to OCPdep-prepared surfaces with the similar coverage. These results were correlated with methylene blue redox-tagged DNA probes, which directly measured the average DNA coverage. Both methods indicated that Edep DNA SAMs were more uniformly distributed across the electrode surface, while the surfaces prepared via OCPdep assembled into clusters with reduced mobility. The potential-assisted thiol-exchange approach to preparing low-coverage DNA SAMs was shown to quickly create modified surfaces that were consistent, had mobility characteristics which should yield superior DNA hybridization efficiencies, and having greater thermal stability which will translate into a longer shelf-life.

Voltammetric investigation of anodic and cathodic processes at Au(hkl)|ionic liquid interfaces

• The electrochemical behaviors of 8 ionic liquids (ILs) were studied on Au( hkl ). • Pyrrolidinium-based ILs had the widest electrical double-layer region. • The potential windows depended on Au( hkl ) and the ions comprising the ILs. • The desorption/re-adsorption of iodine for 2 iodide-based ILs depended on Au( hkl ). • In terms of the peak potential for iodine desorption: Au(100) < Au(111) < Au(110) . In this work, the effect of the crystallographic orientation of gold on the interfacial processes of six non-halide-based ionic liquids (ILs) and two iodide-based ILs was systematically investigated by linear sweep voltammetry (LSV), cyclic voltammetry (CV), and double potential step chronocoulometry (DPSCC). The cation and anion of the IL affected both the width of the electrical double-layer (EDL) region ( E dl ) and the electrochemical potential window ( E pw ) of the ILs. The shape of the CV profile in the EDL region was unique for each Au( hkl )|IL interface. The reductive desorption and re-adsorption of iodine adatoms on gold in 1-alkyl-3-methylimidazolium iodide ([C n mim][I], n = 4 or 6) were also surface-sensitive. The reduction peak current density was proportional to the scan rate, suggesting that the reaction involves adsorbed iodine. The amount of iodine adatoms desorbed during the initial cathodic scan was identical to the monolayer coverage of iodine on gold single crystal electrodes. The peak potential for reductive desorption was estimated to follow the ensuing order from negative to positive: [C 6 mim][I] < [C 4 mim][I] and Au(100) < Au(111) < Au(110) . Following reductive desorption of iodine, the cations were partly attached to the gold surface. The adsorption strength of [C n mim] + followed an identical order: Au(100) < Au(111) < Au(110) .

(Invited) Enhancement of the ORR Activity By Hydrophobic Species Analyzed Using Single Crystal Electrodes

Introduction Enhancement of the oxygen reduction reaction (ORR) is important for the development of fuel cells. Pt oxides are blocking species of the ORR on Pt electrodes.1,2 Theoretical calculation predicts that the change of adsorbed water structure inhibits the formation of Pt oxides on n(111)−(111) series of Pt, resulting in the high ORR activity of this series.3 In the notation of high index planes n(hkl)−(mno), n, (hkl) and (mno) show the number of terrace atomic rows, structures of terrace and step, respectively.Hydrophobic species also changes water structure around electrode surfaces. Modification with alkyl amines (OA/PA) and melamine enhances the ORR activity of Pt nanoparticles.4,5 This paper summarized the structural effects on the enhancement of the ORR by hydrophobic species on single crystal electrodes of Pt6 and Pt x Pd y Co z . Experimental Linear sweep voltammograms were measured using rotating disk electrode (RDE) in 0.1 M HClO4 saturated with O2. Potential was scanned positively from 0.05 V(RHE) at scanning rate of 0.010 V s−1. The ORR activity was estimated using the specific activity at 0.90 V(RHE). Hydrophobic species examined were alkyl amines (OA/PA), tetraalkyl ammonium cation (THA+) and melamine. The structure formulae and the hard sphere models of single crystal electrodes are shown in Fig. 1. Results and Discussion <Pt single crystal electrodes> Alkyl amines (OA/PA) enhance the ORR activity on n(111)-(111) series of Pt with 7 ≤ n.7 This fact indicates that wide (111) terrace (7 ≤ n) is necessary for the ORR activation by OA/PA. The ORR activity on flat Pt(111) is most activated by OA/PA. However, OA/PA markedly deactivates the ORR on flat Pt(100). Wide flat hexagonal terrace plays a key role in the activation by OA/PA. Modification with OA/PA decreases the charges of Pt oxide formation of voltammograms of all the single crystal electrodes, even on the surfaces of which ORR activity decreases after the modification. Infrared reflection absorption spectroscopy (IRAS) shows that the formation of ice-like water with smaller cluster size enhances the ORR activity after OA/PA modification.8 The ORR activity of Pt(111) is enhanced with the increase of the alkyl chain length of tetraalkyl ammonium cation. The ORR activity of Pt(111) modified with THA+ is 8 times as high as that of bare Pt(111).9 IRAS band intensity of PtOH decreases markedly after THA+ modification. This fact indicates that the ORR activity of Pt(111) is extremely high without Pt oxides. THA+ slightly enhanced the activity of Pt(331) = 3(111)-(111) that has the highest ORR activity in single crystal Pt electrodes. Melamine enhances the ORR activity on all the n(111)-(111) series of Pt.10 The activity on Pt(111) is enhanced most markedly again after melamine modification. Melamine also increases the activity of Pt(331) twice, giving the highest activity in the Pt single crystal electrodes examined. IRAS spectra show that the enhancement of the activity was attributed to the decrease of the coverage of PtOH. <Pt x Pd y Co z single crystal electrodes> Pt x Pd y Co z single crystal electrodes have higher ORR activity than Pt and Pt3Co single crystal electrodes.11 THA+ enhances the ORR activity of Pt x Pd y Co z (111) electrodes. The ORR activity of PtPd0.1Co0.2(111) modified with THA+ is 3.7 and 19 times as high as that of bare PtPd0.1Co0.2(111) and Pt(111), respectively. However, THA+ deactivates the ORR on Pt x Pd y Co z (100) and Pt x Pd y Co z (110) electrodes. Melamine increases the ORR activity of all the Pt x Pd y Co z single crystal electrodes. The enhancement ratio of PtPd0.1Co0.2(111) (2.1) is the highest in Pt x Pd y Co z single crystal electrodes examined. Acknowledgements This study was supported by New Energy and Industrial Technology Development Organization (NEDO). References N. M. Marković, H. A. Gasteiger, B. N. Grgur, and P. N. Ross, J. Electroanal. Chem., 467, 157 (1999). T. Ueno, H. Tanaka, S. Sugawara, K. Shinohara, A. Ohma, N. Hoshi, and M. Nakamura, J. Electroanal. Chem., 800, 162 (2017). R. Jinnouchi, K. Kodama, and Y. Moromoto, J. Electroanal. Chem., 716, 31 (2014).K. Miyabayashi, H. Nishihara, and M. Miyake, Langmuir, 30, 2936 (2014). M. Asahi, S. Yamazaki, N. Taguchi, and T. Ioroi, J. Electrochem. Soc., 166, F498 (2019). N. Hoshi, and M. Nakamura, Chem. Lett., 50, 72 (2021). K. Saikawa, M. Nakamura, and N. Hoshi, Electrochem. Commun., 87, 5 (2018). N. Hoshi, K. Saikawa, and M. Nakamura, Electrochem. Cummun., 106, 106536 (2019). T. Kumeda, H. Tajiri, O. Sakata, N. Hoshi, and M. Nakamura, Nat. Commun., 9, 4378 (2018). N. Wada, M. Nakamura, and N. Hoshi, Electrocatalysis, 11, 275 (2020). M. Torihata, M. Nakamura, N. Todoroki, T. Wadayama, and N. Hoshi, Electrochem. Commun., 125, 107007 (2021). Figure 1

Evidence for the Lack of Caffeine Specific Adsorption and Its Impact on Water Structure to Increase HOR/HER Activity on Pt

As the cornerstone for much of our electrochemical knowledge, the inexplicable 2 orders of magnitude decrease in the hydrogen oxidation and evolution reactions (HOR/HER) in moving from pH 1 to 14 on precious metals continues to undermine our understanding of and ability to better design electrochemical systems.1 Several theories have been proposed for this anomalous change, from electric field effects, the influence of cations, co-adsorption of H and OH, and the orientation of interfacial water, driving innovations in in situ electrochemical techniques, such as potential changes with Fourier Transform Infrared Spectroscopy (FTIR) or X-ray Absorption Spectroscopy (XAS).2–6 Recently, it was found that including small concentrations of caffeine in pH 13 was able to drastically increase the HOR/HER catalytic activity of platinum through what was postulated as surface specific adsorption.7 In this work, we challenge this notion through an in situ surface sensitive technique, termed Attenuated Total Reflection-Surface Enhanced Infrared Reflection Absorption Spectroscopy (ATR-SEIRAS) while simultaneously analyzing the underlying theory of water reorganization. Due to SEIRAS’ s surface sensitivity, we can distinguish between specific electrochemical surface adsorption and molecules merely located in the outer Helmholtz plane (OHP) through stark tuning, where species located in the inner Helmholtz plane (IHP) experience the strongest electric fields, and demonstrate a potential dependence of their vibrational frequency.8 Further, trends in caffeine induced water structure changes support our hypothesis that surface bound water layer changes observed in our in situ spectra are responsible for the “apparent pH dependence” of HOR/HER.The electrochemical interface is traditionally viewed as a double layer model, with specific electrochemical adsorbates existing in the Inner Helmholtz Plane and the first layer of non-adsorbates at the Outer Helmholtz Plane. ATR-SEIRAS enhanced surface sensitivity allows for strong signals to be obtained within 5 nm of the surface, selectively capturing both the IHP and OHP with decreasing strength into the diffuse layer and bulk electrolyte. In situ surface sensitive IR shows strong peaks corresponding to caffeine on addition of 10-4 M to basic electrolyte, but a lack of stark tuning suggests no specific adsorption of caffeine on the Pt surface. Furthermore, SEIRAS spectra reveal subtle changes to the interfacial water with addition of caffeine, namely losses of certain types of interfacial water and changes in the water-water hydrogen bonding network. These spectra compared with similar samples collected in traditional 0.1 M HClO4 electrolyte point toward the “apparent pH dependence” being a function of pH induced changes to the interfacial water structure. 1. Durst, J., Siebel, A., Simon, C., Hasche, f., Herranz, J., & Gasteiger, H. A. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014). Chen, X., McCrum, I. T., Schwarz, K. A., Janik, M. J. & Koper, M. T. M. Co-adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chemie - Int. Ed. 56, 15025–15029 (2017).Yang, X., Nash, J., Oliveira, N. J., Yan, Y. & Xu, B. Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 58, 17718–17723 (2019).Sarabia, F. J., Sebastián-Pascual, P., Koper, M. T. M., Climent, V. & Feliu, J. M. Effect of the Interfacial Water Structure on the Hydrogen Evolution Reaction on Pt(111) Modified with Different Nickel Hydroxide Coverages in Alkaline Media. ACS Appl. Mater. Interfaces 11, 613–623 (2019).Liu, E. Jiao, L., Li, J., Stracensky, T., Sun, Q., Mukerjee, S., & Jia, Q. Interfacial water shuffling the intermediates of hydrogen oxidation and evolution reactions in aqueous media. Energy Environ. Sci. 13, 3064–3074 (2020).McCrum, I. T. & Koper, M. T. M. The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum. Nat. Energy 5, 891–899 (2020).Intikhab, S. et al. Caffeinated Interfaces Enhance Alkaline Hydrogen Electrocatalysis. ACS Catal. 10, 6798–6802 (2020).Lambert, D. K. Vibrational Stark effect of CO on Ni(100), and CO in the aqueous double layer: Experiment, theory, and models. J. Chem. Phys. 89, 3847–3860 (1988). Figure 1

Electrocatalytic activity and volatile product selectivity for nitrate reduction at tin-modified Pt(100), Pd(100) and Pd–Pt(100) single crystal electrodes in acidic media

We prepared Sn-modified Pt(100), Pd(100) and Pd–Pt(100) single crystal electrodes and investigated the nitrate reduction reaction (NO3RR) activity and the product selectivity for them using online electrochemical mass spectroscopy (OLEMS), also known as differential electrochemical mass spectroscopy (DEMS). OLEMS measurements allowed us to quantify volatile products of N2, N2O and NO and confirm the production of N2 at Sn/Pd(100) but not at Sn/Pt(100). Pd-doping to Pt(100) with a 3 atomic % increased the product selectivity for the NO3RR to N2. These results indicate that the presence of Pd in the (100) surface is the key to produce N2, which seems to be related to the hydrogen adsorption energy to the metal surface. The suppression of hydrogenation of intermediate species at the electrode surface could lead to the production of N2. This work will guide us to understand N2 production mechanism for the NO3RR and develop highly selective electrocatalysts for denitrification.