Emerging Atomistic Modeling Methods for Heterogeneous Electrocatalysis.

Heterogeneous electrocatalysis lies at the center of various technologies that could help enable a sustainable future. However, its complexity makes it challenging to accurately and efficiently model at an atomic level. Here, we review emerging atomistic methods to simulate the electrocatalytic interface with special attention devoted to the components/effects that have been challenging to model, such as solvation, electrolyte ions, electrode potential, reaction kinetics, and pH. Additionally, we review relevant computational spectroscopy methods. Then, we showcase several examples of applying these methods to understand and design catalysts relevant to green hydrogen. We also offer experimental views on how to bridge the gap between theory and experiments. Finally, we provide some perspectives on opportunities to advance the field.

Synergistic Modulation of Multiple Sites Boosts Anti-Poisoning Hydrogen Electrooxidation Reaction with Ultrasmall (Pt0.9Rh0.1)3V Ternary Intermetallic Nanoparticles.

Promoting the hydrogen oxidation reaction (HOR) activity and poisoning tolerance of electrocatalysts is crucial for the large-scale application of hydrogen-oxygen fuel cell. However, it is severely hindered by the scaling relations among different intermediates. Herein, lattice-contracted Pt-Rh in ultrasmall ternary L12-(Pt0.9Rh0.1)3V intermetallic nanoparticles (~2.2 nm) were fabricated to promote the HOR performances through an oxides self-confined growth strategy. The prepared (Pt0.9Rh0.1)3V displayed 5.5/3.7 times promotion in HOR mass/specific activity than Pt/C in pure H2 and dramatically limited activity attenuation in 1000 ppm CO/H2 mixture. In situ Raman spectra tracked the superior anti-CO* capability as a result of compressive strained Pt, and the adsorption of oxygen-containing species was promoted due to the dual-functional effect. Further assisted by density functional theory calculations, both the adsorption of H* and CO* on (Pt0.9Rh0.1)3V were reduced compared with that of Pt due to lattice contraction, while the adsorption of OH* was enhanced by introducing oxyphilic Rh sites. This work provides an effective tactic to stimulate the electrocatalytic performances by optimizing the adsorption of different intermediates severally.

Visualization of Electrooxidation on Palladium Single Crystal Surfaces via In Situ Raman Spectroscopy.

The electrooxidation of catalyst surfaces is across various electrocatalytic reactions, directly impacting their activity, stability and selectivity. Precisely characterizing the electrooxidation on well-defined surfaces is essential to understanding electrocatalytic reactions comprehensively. Herein, we employed in situ Raman spectroscopy to monitor the electrooxidation process of palladium single crystal. Our findings reveal that the Pd surface's initial electrooxidation process involves forming *OH intermediate and ClO4- ions facilitate the deprotonation process, leading to the formation of PdOx. Subsequently, under deep electrooxidation potential range, the oxygen atoms within PdOx contribute to creating surface-bound peroxide species, ultimately resulting in oxygen generation. The adsorption strength of *OH and the coverage of ClO4- can be adjusted by the controllable electronic effect, resulting in different oxidation rates. This study offers valuable insights into elucidating the electrooxidation mechanisms underlying a range of electrocatalytic reactions, thereby contributing to the rational design of catalysts.

Pt Nanoparticle Assisted Homogeneous Surface Engineering of Polymer-Based Bulk-Heterojunction Photocathodes for Efficient Charge Extraction and Catalytic Hydrogen Evolution.

To fabricate a high-efficiency bulk-heterojunction (BHJ)-based photocathode, introducing suitable interfacial modification layer(s) is a crucial strategy. Surface engineering is especially important for achieving high-performance photocathodes because the photoelectrochemical (PEC) reactions at the photocathode/electrolyte interface are the rate-limiting process. Despite its importance, the influence of interfacial layer morphology regulation on PEC activity has attracted insufficient attention. In this work, RuO2 , with excellent conductivity, capacity and catalytic properties, is utilized as an interfacial layer to modify the BHJ layer. However, the homogeneous coverage of hydrophilic RuO2 on the hydrophobic BHJ surface is challenging. To address this issue, a Pt nanoparticle-assisted homogeneous RuO2 layer deposition method is developed and successfully applied to several BHJ-based photocathodes, achieving superior PEC performance compared to those prepared by conventional interface engineering strategies. Among them, the fluorine-doped tin oxide (FTO)/J71:N2200(Pt)/RuO2 photocathode generates the best photocurrent density of -9.0 mA cm-2 at 0 V with an onset potential of up to 1.0 V under AM1.5 irradiation.

Molecular understanding of the Helmholtz capacitance difference between Cu(100) and graphene electrodes.

Unraveling the origin of Helmholtz capacitance is of paramount importance for understanding the interfacial structure and electrostatic potential distribution of electric double layers (EDL). In this work, we combined the methods of ab initio molecular dynamics and classical molecular dynamics and modeled electrified Cu(100)/electrolyte and graphene/electrolyte interfaces for comparison. It was proposed that the Helmholtz capacitance is composed of three parts connected in series: the usual solvent capacitance, water chemisorption induced capacitance, and Pauling repulsion caused gap capacitance. We found the Helmholtz capacitance of graphene is significantly lower than that of Cu(100), which was attributed to two intrinsic factors. One is that graphene has a wider gap layer at interface, and the other is that graphene is less active for water chemisorption. Finally, based on our findings, we provide suggestions for how to increase the EDL capacitance of graphene-based materials in future work, and we also suggest that the new understanding of the potential distribution across the Helmholtz layer may help explain some experimental phenomena of electrocatalysis.

Modeling stepped Pt/water interfaces at potential of zero charge with ab initio molecular dynamics.

It is worth understanding the potentials of zero charge (PZCs) and structures of stepped metal/water interfaces, because for many electrocatalytic reactions, stepped surfaces are more active than atomically flat surfaces. Herein, a series of stepped Pt/water interfaces are modeled at different step densities with ab initio molecular dynamics. It is found that the structures of Pt/water interfaces are significantly influenced by the step density, particularly in regard to the distribution of chemisorbed water. The step sites of metal surfaces are more preferred for water chemisorption than terrace sites, and until the step density is very low, water will chemisorb on the terrace. In addition, it is revealed that the PZCs of stepped Pt/water interfaces are generally smaller than that of Pt(111), and the difference is mainly attributed to the difference in their work function, providing a simple way to estimate the PZCs of stepped metal surfaces. Finally, it is interesting to see that the Volta potential difference is almost the same for Pt/water interfaces with different step densities, although their interface structures and magnitude of charge transfer clearly differ.

Direct In Situ Raman Spectroscopic Evidence of Oxygen Reduction Reaction Intermediates at High-Index Pt(hkl) Surfaces.

The study of the oxygen reduction reaction (ORR) at high-index Pt(hkl) single crystal surfaces has received considerable interest due to their well-ordered, typical atomic structures and superior catalytic activities. However, it is difficult to obtain direct spectral evidence of ORR intermediates during reaction processes, especially at high-index Pt(hkl) surfaces. Herein, in situ Raman spectroscopy has been employed to investigate ORR processes at high-index Pt(hkl) surfaces containing the [011̅] crystal zone-i.e., Pt(211) and Pt(311). Through control and isotope substitution experiments, in situ spectroscopic evidence of OH and OOH intermediates at Pt(211) and Pt(311) surfaces was successfully obtained. After detailed analysis based on the Raman spectra and theoretical simulation, it was deduced that the difference in adsorption of OOH at high-index surfaces has a significant effect on the ORR activity. This research illuminates and deepens the understanding of the ORR mechanism on high-index Pt(hkl) surfaces and provides theoretical guidance for the rational design of high activity ORR catalysts.

In situ probing electrified interfacial water structures at atomically flat surfaces.

Solid/liquid interfaces are ubiquitous in nature and knowledge of their atomic-level structure is essential in elucidating many phenomena in chemistry, physics, materials science and Earth science1. In electrochemistry, in particular, the detailed structure of interfacial water, such as the orientation and hydrogen-bonding network in electric double layers under bias potentials, has a significant impact on the electrochemical performances of electrode materials2-4. To elucidate the structures of electric double layers at electrochemical interfaces, we combine in situ Raman spectroscopy and ab initio molecular dynamics and distinguish two structural transitions of interfacial water at electrified Au single-crystal electrode surfaces. Towards negative potentials, the interfacial water molecules evolve from structurally 'parallel' to 'one-H-down' and then to 'two-H-down'. Concurrently, the number of hydrogen bonds in the interfacial water also undergoes two transitions. Our findings shed light on the fundamental understanding of electric double layers and electrochemical processes at the interfaces.

In Situ Raman Study of CO Electrooxidation on Pt(hkl) Single-Crystal Surfaces in Acidic Solution.

The adsorption and electrooxidation of CO molecules at well-defined Pt(hkl) single-crystal electrode surfaces is a key step towards addressing catalyst poisoning mechanisms in fuel cells. Herein, we employed in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) coupled with theoretical calculation to investigate CO electrooxidation on Pt(hkl) surfaces in acidic solution. We obtained the Raman signal of top- and bridge-site adsorbed CO* molecules on Pt(111) and Pt(100). In contrast, on Pt(110) surfaces only top-site adsorbed CO* was detected during the entire electrooxidation process. Direct spectroscopic evidence for OH* and COOH* species forming on Pt(100) and Pt(111) surfaces was afforded and confirmed subsequently via isotope substitution experiments and DFT calculations. In summary, the formation and adsorption of OH* and COOH* species plays a vital role in expediting the electrooxidation process, which relates with the pre-oxidation peak of CO electrooxidation. This work deepens knowledge of the CO electrooxidation process and provides new perspectives for the design of anti-poisoning and highly effective catalysts.

Early Stages of Electrochemical Oxidation of Cu(111) and Polycrystalline Cu Surfaces Revealed by in Situ Raman Spectroscopy.

Investigating the chemical nature of the adsorbed intermediate species on well-defined Cu single crystal substrates is crucial in understanding many electrocatalytic reactions. Herein, we systematically study the early stages of electrochemical oxidation of Cu(111) and polycrystalline Cu surfaces in different pH electrolytes using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). On Cu(111), for the first time, we identified surface OH species which convert to chemisorbed "O" before forming Cu2O in alkaline (0.01 M KOH) and neutral (0.1 M Na2SO4) electrolytes; while at the Cu(poly) surface, we only detected the presence of surface hydroxide. Whereas, in a strongly acidic solution (0.1 M H2SO4), sulfate replaces the hydroxyl/oxy species. This results improves the understanding of the reaction mechanisms of various electrocatalytic reactions.

In†situ Spectroscopic Insight into the Origin of the Enhanced Performance of Bimetallic Nanocatalysts towards the Oxygen Reduction Reaction (ORR).

It is vital to understand the oxygen reduction reaction (ORR) mechanism at the molecular level for the rational design and synthesis of high activity fuel-cell catalysts. Surface enhanced Raman spectroscopy (SERS) is a powerful technique capable of detecting the bond vibrations of surface species in the low wavenumber range, however, using it to probe practical nanocatalysts remains extremely challenging. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to investigate ORR processes on the surface of bimetallic Pt3 Co nanocatalyst structures. Direct spectroscopic evidence of *OOH suggests that ORR undergoes an associative mechanism on Pt3 Co in both acidic and basic environments. Density functional theory (DFT) calculations show that the weak *O adsorption arise from electronic effect on the Pt3 Co surface accounts for enhanced ORR activity. This work shows SHINERS is a promising technique for the real-time observation of catalytic processes.

Theoretical insight into the vibrational spectra of metal-water interfaces from density functional theory based molecular dynamics.

Understanding the structures of electrochemical interfaces at the atomic level is key to developing efficient electrochemical cells for energy storage and conversion. Spectroscopic techniques have been widely used to investigate the structures and vibrational properties of the interfaces. The interpretation of these spectra is however not straightforward. In this work, density functional theory based molecular dynamics simulations were performed to study the vibrational properties of the Pt(111)- and Au(111)-water interfaces. It was found that the specific adsorption of some surface water on Pt(111) leads to a partial charge transfer to the metal, and strong hydrogen bonding with neighbouring water molecules, which resolves the interpretation of the elusive O-H stretching peak at around 3000 cm-1 observed in some experiments.

Determining Potentials of Zero Charge of Metal Electrodes versus the Standard Hydrogen Electrode from Density-Functional-Theory-Based Molecular Dynamics.

We develop a computationally efficient scheme to determine the potentials of zero charge (PZC) of metal-water interfaces with respect to the standard hydrogen electrode. We calculate the PZC of Pt(111), Au(111), Pd(111) and Ag(111) at a good accuracy using this scheme. Moreover, we find that the interface dipole potentials are almost entirely caused by charge transfer from water to the surfaces, the magnitude of which depends on the bonding strength between water and the metals, while water orientation hardly contributes at the PZC conditions.

Molecular understanding of cation effects on double layers and their significance to CO-CO dimerization.

Cation effects have been shown in numerous experiments to play a significant role in electrocatalysis. To understand these effects at the molecular level, we systematically investigate the structures and capacitances of electric double layers with a variety of cations as counter charges at Pt(111)-COad/water interfaces with ab initio molecular dynamics. It is encouraging to find that the computed Helmholtz capacitances for different cations are in quantitative agreement with experiments, and that the trend of cation effects on capacitances shows clear correlation with the structures of interface cations of differing sizes and hydration energies. More importantly, we demonstrate the Helmholtz capacitance as the key descriptor for measuring the activity of CO-CO dimerization, the rate-determining step for C2+ formation in electroreduction of CO and CO2. Our work provides atomistic insights into cation effects on electric double layers and electrocatalysis that are crucial for optimizing electrode and electrolyte materials.

Understanding the Effects of Electrode Material, Single Crystal Facet, and Electrolyte Ion on the Helmholtz Capacitance of Metal/Aqueous Solution Interfaces.

The comprehensive interpretation of the measured differential Helmholtz capacitance curve is vital for advancing our understanding of the interfacial structure. While several possible physical effects contributing to the Helmholtz capacitance have been proposed theoretically, combining those factors to explain the experimentally observed potential-dependent capacitance profile remains a significant challenge. In this study, we employ ab initio molecular dynamics simulations to model various metal/solution interfaces. Our investigation primarily emphasizes the substantial effect of water chemisorption on the potential-dependent behavior of the Helmholtz capacitance. Additionally, we identify other critical factors that profoundly impact the Helmholtz capacitance: (1) Ions with low hydration energy hinder the availability of surface sites for water adsorption, resulting in a diminished enhancement of capacitance from water chemisorption. (2) Using large-sized ions leads to an expansion of the Helmholtz layer, causing a decrease in the Helmholtz capacitance. (3) Metal surfaces with higher affinity for water attract water adsorption at lower potentials, resulting in a lower peak potential for the differential Helmholtz capacitance curve.

Renewable galactomannan-based biogums with structure regulation to protect zinc metal anodes via blocking and confinement effect.

Galactomannan-based biogums were derived from fenugreek, guar, tara, and carob and consisted of mannose and galactose with different ratios, as well as the implementation of high-value utilization was very significant for sustainable development. In this work, renewable and low-cost galactomannan-based biogums were designed and developed as functional coatings protected on the Zn metal anodes. The molecule structure of galactomannan-based biogums were explored on the effect of anticorrosion ability and uniform deposition behavior through the introduction of fenugreek gum, guar gum, tara gum, and carob gum with different ratios of mannose to galactose as 1.2:1, 2:1, 3:1, and 4:1. The existence of biogum protective layers can reduce the contact area between Zn anodes and aqueous electrolyte to enhance the anticorrosion ability of Zn anodes. Rich oxygen-containing groups in galactomannan-based biogums can coordinate with Zn2+ and Zn atoms to form ion conductivity gel layer and adsorb closely on the surface of Zn metal, which can induce uniform deposition of Zn2+ to avoid dendrite growth. Zn electrodes protected by biogums can cycle impressively for 1980 h with 2 mA cm-2 and 2 mAh cm-2. This work can provide a novel strategy to enhance Zn metal anodes' electrochemical performance, as well as implement the high-value application of biomass-based biogums as functional coatings.

Surface Control Behavior toward Crystal Regulation and Anticorrosion Capacity for Zinc Metal Anodes.

The commercial application of high-safety aqueous zinc (Zn) secondary batteries is hindered by the poor cycling life of Zn metal anodes. Here we propose a dendrite growth and hydrogen evolution corrosion reaction mechanism from the binding energy of the deposited crystal plane on the Zn surface and the adsorption energy of H2O molecules on different crystal planes as well as the binding energy of H2O molecules with Zn2+ ions. The biomass-based alkyl polyglucoside (APG) surfactant is adopted as an electrolyte additive of 0.15% to regulate the preferential growth of a parallel Zn(002) plane and enhance the anticorrosion ability of Zn metal anodes. The robust binding and adsorption energies of APG with Zn2+ ions in the aqueous electrolyte and the Zn(002) plane on Zn surface generate a synergistic effect to increase the concentration of Zn2+ ions on the APG-adsorbed Zn(002) plane, endowing the continuous growth of the preferential parallel Zn(002) plane and the excellent anticorrosion capacity. Accordingly, the long-term cycle stability of 4000 h can be achieved for Zn anodes with APG additives, which is better than that with pure ZnSO4 electrolyte. With the addition of APG in the anolyte electrolyte, Zn-I2 full cells display excellent cycling performance (70 mAh g-1 after 20000 cycles) as compared with that without APG additives.

Revealing the role of interfacial water and key intermediates at ruthenium surfaces in the alkaline hydrogen evolution reaction.

Ruthenium exhibits comparable or even better alkaline hydrogen evolution reaction activity than platinum, however, the mechanistic aspects are yet to be settled, which are elucidated by combining in situ Raman spectroscopy and theoretical calculations herein. We simultaneously capture dynamic spectral evidence of Ru surfaces, interfacial water, *H and *OH intermediates. Ru surfaces exist in different valence states in the reaction potential range, dissociating interfacial water differently and generating two distinct *H, resulting in different activities. The local cation tuning effect of hydrated Na+ ion water and the large work function of high-valence Ru(n+) surfaces promote interfacial water dissociation. Moreover, compared to low-valence Ru(0) surfaces, high-valence Ru(n+) surfaces have more moderate adsorption energies for interfacial water, *H, and *OH. They, therefore, facilitate the activity. Our findings demonstrate the regulation of valence state on interfacial water, intermediates, and finally the catalytic activity, which provide guidelines for the rational design of high-efficiency catalysts.

Polishing the Lead-Poor Surface for Efficient Inverted CsPbI3 Perovskite Solar Cells.

Triiodide cesium lead perovskite (CsPbI3 ) has promising prospects in the development of efficient and stable photovoltaics in both single-junction and tandem structures. However, achieving inverted devices that provide good stability and are compatible to tandem devices remains a challenge, and the deep insights are still not understood. This study finds that the surface components of CsPbI3 are intrinsically lead-poor and the relevant traps are of p-type with localized states. These deep-energy-level p traps induce inferior transfer or electrons and serious nonradiative recombination at the CsPbI3 /PCBM interface, leading to the considerable open-circuit voltage (Voc ) loss and reduction of fill factor (FF). Compared to molecular passivation, polishing treatment with 1,4-butanediamine can eliminate the nonstoichiometric components and root these intrinsically lead-poor traps for superior electron transfer. The polishing treatment significantly improves the FF and Voc of the inverted CsPbI3 photovoltaics, creating an efficiency promotion from 12.64% to 19.84%. Moreover, 95% of the initial efficiency of the optimized devices is maintained after the output operation for 1000 h.