Impact of Pt(hkl) Electrode Surface Structure on the Electrical Double Layer Capacitance.

The classical theory of the electrical double layer (EDL) does not consider the effects of the electrode surface structure on the EDL properties. Moreover, the best agreement between the traditional EDL theory and experiments has been achieved so far only for a very limited number of ideal systems, such as liquid metal mercury electrodes, for which it is challenging to operate with specific surface structures. In the case of solid electrodes, the predictive power of classical theory is often not acceptable for electrochemical energy applications, e.g., in supercapacitors, due to the effects of surface structure, electrode composition, and complex electrolyte contributions. In this work, we combine ab initio molecular dynamics (AIMD) simulations and electrochemical experiments to elucidate the relationship between the structure of Pt(hkl) surfaces and the double-layer capacitance as a key property of the EDL. Flat, stepped, and kinked Pt single crystal facets in contact with acidic HClO4 media are selected as our model systems. We demonstrate that introducing specific defects, such as steps, can substantially reduce the EDL capacitances close to the potential of zero charge (PZC). Our AIMD simulations reveal that different Pt facets are characterized by different net orientations of the water dipole moment at the interface. That allows us to rationalize the experimentally measured (inverse) volcano-shaped capacitance as a function of the surface step density.

Nanosized and metastable molybdenum oxides as negative electrode materials for durable high-energy aqueous Li-ion batteries.

The development of inherently safe energy devices is a key challenge, and aqueous Li-ion batteries draw large attention for this purpose. Due to the narrow electrochemical stable potential window of aqueous electrolytes, the energy density and the selection of negative electrode materials are significantly limited. For achieving durable and high-energy aqueous Li-ion batteries, the development of negative electrode materials exhibiting a large capacity and low potential without triggering decomposition of water is crucial. Herein, a type of a negative electrode material (i.e., Li x Nb2/7Mo3/7O2) is proposed for high-energy aqueous Li-ion batteries. Li x Nb2/7Mo3/7O2 delivers a large capacity of ∼170 mA ⋅ h ⋅ g-1 with a low operating potential range of 1.9 to 2.8 versus Li/Li+ in 21 m lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) aqueous electrolyte. A full cell consisting of Li1.05Mn1.95O4/Li9/7Nb2/7Mo3/7O2 presents high energy density of 107 W ⋅ h ⋅ kg-1 as the maximum value in 21 m LiTFSA aqueous electrolyte, and 73% in capacity retention is achieved after 2,000 cycles. Furthermore, hard X-ray photoelectron spectroscopy study reveals that a protective surface layer is formed at the surface of the negative electrode, by which the high-energy and durable aqueous batteries are realized with Li x Nb2/7Mo3/7O2 This work combines a high capacity with a safe negative electrode material through delivering the Mo-based oxide with unique nanosized and metastable characters.

Direct instrumental identification of catalytically active surface sites.

The activity of heterogeneous catalysts-which are involved in some 80 per cent of processes in the chemical and energy industries-is determined by the electronic structure of specific surface sites that offer optimal binding of reaction intermediates. Directly identifying and monitoring these sites during a reaction should therefore provide insight that might aid the targeted development of heterogeneous catalysts and electrocatalysts (those that participate in electrochemical reactions) for practical applications. The invention of the scanning tunnelling microscope (STM) and the electrochemical STM promised to deliver such imaging capabilities, and both have indeed contributed greatly to our atomistic understanding of heterogeneous catalysis. But although the STM has been used to probe and initiate surface reactions, and has even enabled local measurements of reactivity in some systems, it is not generally thought to be suited to the direct identification of catalytically active surface sites under reaction conditions. Here we demonstrate, however, that common STMs can readily map the catalytic activity of surfaces with high spatial resolution: we show that by monitoring relative changes in the tunnelling current noise, active sites can be distinguished in an almost quantitative fashion according to their ability to catalyse the hydrogen-evolution reaction or the oxygen-reduction reaction. These data allow us to evaluate directly the importance and relative contribution to overall catalyst activity of different defects and sites at the boundaries between two materials. With its ability to deliver such information and its ready applicability to different systems, we anticipate that our method will aid the rational design of heterogeneous catalysts.

Elucidation of Structure-Activity Relations in Proton Electroreduction at Pd Surfaces: Theoretical and Experimental Study.

The structure-activity relationship is a cornerstone topic in catalysis, which lays the foundation for the design and functionalization of catalytic materials. Of particular interest is the catalysis of the hydrogen evolution reaction (HER) by palladium (Pd), which is envisioned to play a major role in realizing a hydrogen-based economy. Interestingly, experimentalists observed excess heat generation in such systems, which became known as the debated "cold fusion" phenomenon. Despite the considerable attention on this report, more fundamental knowledge, such as the impact of the formation of bulk Pd hydrides on the nature of active sites and the HER activity, remains largely unexplored. In this work, classical electrochemical experiments performed on model Pd(hkl) surfaces, "noise" electrochemical scanning tunneling microscopy (n-EC-STM), and density functional theory are combined to elucidate the nature of active sites for the HER. Results reveal an activity trend following Pd(111) > Pd(110) > Pd(100) and that the formation of subsurface hydride layers causes morphological changes and strain, which affect the HER activity and the nature of active sites. These findings provide significant insights into the role of subsurface hydride formation on the structure-activity relations toward the design of efficient Pd-based nanocatalysts for the HER.

Correlative Electrochemical Microscopy for the Elucidation of the Local Ionic and Electronic Properties of the Solid Electrolyte Interphase in Li-Ion Batteries.

The solid-electrolyte interphase (SEI) plays a key role in the stability of lithium-ion batteries as the SEI prevents the continuous degradation of the electrolyte at the anode. The SEI acts as an insulating layer for electron transfer, still allowing the ionic flux through the layer. We combine the feedback and multi-frequency alternating-current modes of scanning electrochemical microscopy (SECM) for the first time to assess quantitatively the local electronic and ionic properties of the SEI varying the SEI formation conditions and the used electrolytes in the field of Li-ion batteries (LIB). Correlations between the electronic and ionic properties of the resulting SEI on a model Cu electrode demonstrates the unique feasibility of the proposed strategy to provide the two essential properties of an SEI: ionic and electronic conductivity in dependence on the formation conditions, which is anticipated to exhibit a significant impact on the field of LIBs.

Dual In Situ Laser Techniques Underpin the Role of Cations in Impacting Electrocatalysts.

Understanding the electrode/electrolyte interface is crucial for optimizing electrocatalytic performances. Here, we demonstrate that the nature of alkali metal cations can profoundly impact the oxygen evolution activity of surface-mounted metal-organic framework (SURMOF) derived electrocatalysts, which are based on NiFe(OOH). In situ Raman spectroscopy results show that Raman shifts of the Ni-O bending vibration are inversely proportional to the mass activities from Cs+ to Li+ . Particularly, a laser-induced current transient technique was introduced to study the cation-dependent electric double layer properties and their effects on the activity. The catalytic trend appeared to be closely related to the potential of maximum entropy of the system, suggesting a strong cation impact on the interfacial water layer structure. Our results highlight how the electrolyte composition can be used to maximize the performance of SURMOF derivatives toward electrochemical water splitting.

Spotlight on the Effect of Electrolyte Composition on the Potential of Maximum Entropy: Supporting Electrolytes Are Not Always Inert.

The influence of electrolyte pH, the presence of alkali metal cations (Na+ , K+ ), and the presence of O2 on the interfacial water structure of polycrystalline gold electrodes has been experimentally studied in detail. The potential of maximum entropy (PME) was determined by the laser-induced current transient (LICT) technique. Our results demonstrate that increasing the electrolyte pH and introducing O2 shift the PME to more positive potentials. Interestingly, the PME exhibits a higher sensitivity to the pH change in the presence of K+ than Na+ . Altering the pH of the K2 SO4 solution from 4 to 6 can cause a drastic shift in the PME. These findings reveal that, for example, K2 SO4 and Na2 SO4 cannot be considered as equal supporting electrolytes: it is not a viable assumption. This can likely be extrapolated to other common "inert" supporting electrolytes. Beyond this, knowledge about the near-ideal electrolyte composition can be used to optimize electrochemical devices such as electrolyzers, fuel cells, batteries, and supercapacitors.

Review on physical impedance models in modern battery research.

Electrochemical impedance spectroscopy (EIS) is a versatile tool to understand complex processes in batteries. This technique can investigate the effects of battery components like the electrode and electrolyte, electrochemical reactions, interfaces, and interphases forming in the electrochemical systems. The interpretation of the EIS data is typically made using models expressed in terms of the so-called electrical equivalent circuits (EECs) to fit the impedance spectra. Therefore, the EECs must unambiguously represent the electrochemistry of the system. EEC models with a physical significance are more relevant than the empirical ones with their inherent imperfect description of the ongoing processes. This review aims to present the readers with the importance of physical EEC modeling within the context of battery research. A general introduction to EIS and EEC models along with a brief description of the mathematical formalism is provided, followed by showcasing the importance of physical EEC models for EIS on selected examples from the research on traditional, aqueous, and newer all-solid-state battery systems.

Monitoring the active sites for the hydrogen evolution reaction at model carbon surfaces.

Carbon is ubiquitous as an electrode material in electrochemical energy conversion devices. If used as a support material, the evolution of H2 is undesired on carbon. However, recently, carbon-based materials have aroused significant interest as economic and eco-conscious alternatives to noble metal catalysts. The targeted design of improved carbon electrode materials requires atomic scale insight into the structure of the sites that catalyse H2 evolution. This work shows that electrochemical scanning tunnelling microscopy under reaction conditions (n-EC-STM) can be used to monitor the active sites of highly oriented pyrolytic graphite for the hydrogen evolution reaction. With down to atomic resolution, the most active sites in acidic medium are pinpointed near edge sites and defects, whereas the basal planes remain inactive. Density functional theory calculations support these findings and reveal that only specific defects on graphite are active. Motivated by these results, the extensive usage of n-EC-STM on doped carbon-based materials is encouraged to locate their active sites and guide the synthesis of enhanced electrocatalysts.

Enhancing the Hydrogen Evolution Reaction Activity of Platinum Electrodes in Alkaline Media Using Nickel-Iron Clusters.

Herein, we demonstrate an easy way to improve the hydrogen evolution reaction (HER) activity of Pt electrodes in alkaline media by introducing Ni-Fe clusters. As a result, the overpotential needed to achieve a current density of 10 mA cm-2 in H2 -saturated 0.1 m KOH is reduced for the model single-crystal electrodes down to about 70 mV. To our knowledge, these modified electrodes outperform any other reported electrocatalysts tested under similar conditions. Moreover, the influence of 1) Ni to Fe ratio, 2) cluster coverage, and 3) the nature of the alkali-metal cations present in the electrolyte on the HER activity has been investigated. The observed catalytic performance likely originates from both the improved water dissociation at the Ni-Fe clusters and the subsequent optimal hydrogen adsorption and recombination at Pt atoms present at the Ni-Fe/Pt boundary.

Advanced Bifunctional Oxygen Reduction and Evolution Electrocatalyst Derived from Surface-Mounted Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) and their derivatives are considered as promising catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are important for many energy provision technologies, such as electrolyzers, fuel cells and some types of advanced batteries. In this work, a "strain modulation" approach has been applied through the use of surface-mounted NiFe-MOFs in order to design an advanced bifunctional ORR/OER electrocatalyst. The material exhibits an excellent OER activity in alkaline media, reaching an industrially relevant current density of 200 mA cm-2 at an overpotential of only ≈210 mV. It demonstrates operational long-term stability even at a high current density of 500 mA cm-2 and exhibits the so far narrowest "overpotential window" ΔEORR-OER of 0.69 V in 0.1 m KOH with a mass loading being two orders of magnitude lower than that of benchmark electrocatalysts.

Unprecedented High Oxygen Evolution Activity of Electrocatalysts Derived from Surface-Mounted Metal-Organic Frameworks.

The oxygen evolution reaction (OER) is a key process for renewable energy storage. However, developing non-noble metal OER electrocatalysts with high activity, long durability and scalability remains a major challenge. Herein, high OER activity and stability in alkaline solution were discovered for mixed nickel/cobalt hydroxide electrocatalysts, which were derived in one-step procedure from oriented surface-mounted metal-organic framework (SURMOF) thin films that had been directly grown layer-by-layer on macro- and microelectrode substrates. The obtained mass activity of ∼2.5 mA·µg-1 at the defined overpotential of 300 mV is 1 order of magnitude higher than that of the benchmarked IrO2 electrocatalyst and at least 3.5 times higher than the mass activity of any state-of-the-art NiFe-, FeCoW-, or NiCo-based electrocatalysts reported in the literature. The excellent morphology of the SURMOF-derived ultrathin electrocatalyst coating led to a high exposure of the most active Ni- and Co-based sites.

Optimizing the Size of Platinum Nanoparticles for Enhanced Mass Activity in the Electrochemical Oxygen Reduction Reaction.

High oxygen reduction (ORR) activity has been for many years considered as the key to many energy applications. Herein, by combining theory and experiment we prepare Pt nanoparticles with optimal size for the efficient ORR in proton-exchange-membrane fuel cells. Optimal nanoparticle sizes are predicted near 1, 2, and 3 nm by computational screening. To corroborate our computational results, we have addressed the challenge of approximately 1 nm sized Pt nanoparticle synthesis with a metal-organic framework (MOF) template approach. The electrocatalyst was characterized by HR-TEM, XPS, and its ORR activity was measured using a rotating disk electrode setup. The observed mass activities (0.87±0.14 A mgPt -1 ) are close to the computational prediction (0.99 A mgPt -1 ). We report the highest to date mass activity among pure Pt catalysts for the ORR within similar size range. The specific and mass activities are twice as high as the Tanaka commercial Pt/C catalysis.

Elucidation of the Oxygen Reduction Volcano in Alkaline Media using a Copper-Platinum(111) Alloy.

The relationship between the binding of the reaction intermediates and oxygen reduction activity in alkaline media was experimentally explored. By introducing Cu into the 2nd surface layer of a Pt(111) single crystal, the surface reactivity was tuned. In both 0.1 m NaOH and 0.1 m KOH, the optimal catalyst should exhibit OH binding circa 0.1 eV weaker than Pt(111), via a Sabatier volcano; this observation suggests that the reaction is mediated via the same surface bound intermediates as in acid, in contrast to previous reports. In 0.1 m KOH, the alloy catalyst at the peak of the volcano exhibits a maximum activity of 101±8 mA cm-2 at 0.9 V vs. a reversible hydrogen electrode (RHE). This activity constitutes a circa 60-fold increase over Pt(111) in 0.1 m HClO4 .

Elucidation of adsorption processes at the surface of Pt(331) model electrocatalysts in acidic aqueous media.

The Pt(331) surface has long been known to be the most active pure metal electrocatalyst for the oxygen reduction reaction (ORR) in acidic media. Its activity is often higher than those known for the Pt-based alloys towards ORR, being comparable with the most active Pt3Ni(111), Pt3Y or Pt5Gd, and being more active than e.g. polycrystalline Pt3Ni. Multiple active sites at this surface offer adsorption energies which are close to the optimal binding energy with respect to the main ORR intermediates; nevertheless, the exact location of these sites is still not clear. Taking into account the unique surface geometry of Pt(331), some adsorbates (including some oxygenated ORR-intermediates) should also contribute to the electronic structure of the neighbouring catalytic centres. However, the experimental elucidation of the specific adsorption of oxygenated species at this surface appears to be a non-trivial task. Such information holds the keys to the understanding of the high activity of this material and would enable the rational design of nanostructured ORR catalysts even without alloying. In this work, the electrified Pt(331)/electrolyte interface has been characterised using cyclic voltammetry (CV) combined with potentiodynamic electrochemical impedance spectroscopy (PDEIS) in 0.1 M HClO4 solutions. The systems were studied in the potential region between 0.05 V and 1.0 V vs. RHE, where the adsorption of *H, *OH and *O species is possible in both O2-free and O2-saturated electrolytes. Our CV and PDEIS results support the hypothesis that in contrast to Pt(111), many Pt(331) surface sites are likely blocked by *O species at the polymer electrolyte membrane fuel cell benchmark potential of 0.9 V (RHE). We propose a model illustrated by simplified adsorbate structures at different electrode potentials, which is, however, able to explain the voltammetric and impedance data, and which is in good agreement with previously reported electrocatalytic measurements.

Non-covalent interactions in water electrolysis: influence on the activity of Pt(111) and iridium oxide catalysts in acidic media.

Electrolyte components, which are typically not considered to be directly involved in catalytic processes at solid-liquid electrified interfaces, often demonstrate a significant or even drastic influence on the activity, stability and selectivity of electrocatalysts. While there has been certain progress in the understanding of these electrolyte effects, lack of experimental data for various important systems frequently complicates the rational design of new active materials. Modern proton-exchange membrane (PEM) electrolyzers utilize Pt- and Ir-based electrocatalysts, which are among the very few materials that are both active and stable under the extreme conditions of water splitting. We use model Pt(111) and Ir-oxide films grown on Ir(111) electrodes and explore the effect of alkali metal cations and sulfate-anions on the hydrogen evolution and the oxygen evolution reactions in acidic media. We demonstrate that sulfate anions decrease the activity of Ir-oxide towards the oxygen evolution reaction while Rb(+) drastically promotes hydrogen evolution reaction at the Pt(111) electrodes as compared to the reference HClO4 electrolytes. Issues related to the activity benchmarking for these catalysts are discussed.

Techniques and methodologies in modern electrocatalysis: evaluation of activity, selectivity and stability of catalytic materials.

The development and optimisation of materials that promote electrochemical reactions have recently attracted attention mainly due to the challenge of sustainable provision of renewable energy in the future. The need for better understanding and control of electrode-electrolyte interfaces where these reactions take place, however, implies the continuous need for development of efficient analytical techniques and methodologies capable of providing detailed information about the performance of electrocatalysts, especially in situ, under real operational conditions of electrochemical systems. During the past decade, significant efforts in the fields of electrocatalysis and (electro)analytical chemistry have resulted in the evolution of new powerful methods and approaches providing ever deeper and unique insight into complex and dynamic catalytic systems. The combination of various electrochemical and non-electrochemical methods as well as the application of quantum chemistry calculations has become a viable modern approach in the field. The focus of this critical review is primarily set on discussion of the most recent cutting-edge achievements in the development of analytical techniques and methodologies designed to evaluate three key constituents of the performance of electrocatalysts, namely, activity, selectivity and stability. Possible directions and future challenges in the design and elaboration of analytical methods for electrocatalytic research are outlined.

Elucidating the activity of stepped Pt single crystals for oxygen reduction.

The unexpectedly high measured activity of Pt[n(111) × (111)] and Pt[n(111) × (100)] stepped single crystal surfaces towards the oxygen reduction reaction (ORR) is explained utilizing the hydroxyl binding energy as the activity descriptor. Using this descriptor (estimated using experimental data obtained by different groups), a well-defined Sabatier-type volcano is observed for the activities measured for the Pt[n(111) × (111)] and Pt[n(111) × (100)] stepped single crystals, in remarkable agreement with earlier theoretical studies. We propose that the observed destabilisation of *OH species at these surfaces is due to the decreased solvation of the adsorbed hydroxyl intermediates on adjacent terrace sites.

Tailoring the catalytic activity of electrodes with monolayer amounts of foreign metals.

During the past decade, electrocatalysis has attracted significant attention primarily due to the increased interest in the development of new generations of devices for electrochemical energy conversion. This has resulted in a progress in both fundamental understanding of the complex electrocatalytic systems and in the development of efficient synthetic schemes to tailor the surface precisely at the atomic level. One of the viable concepts in electrocatalysis is to optimise the activity through the direct engineering of the properties of the topmost layers of the surface, where the reactions take place, with monolayer and sub-monolayer amounts of metals. This forms (bi)metallic systems where the electronic structure of the active sites is optimised using the interplay between the nature and position of the atoms of solute metals at the surface. In this review, we focus on recent theoretical and experimental achievements in designing efficient (bi)metallic electrocatalysts with selective positioning of foreign atoms to form a variety of active catalytic sites at the electrode surface. We summarize recent results published in the literature and outline challenges for computational and experimental electrocatalysis to engineer active and selective catalysts using atomic layers.

Influence of Alkali Metal Cations on the Oxygen Reduction Activity of Pt5Y and Pt5Gd Alloys.

Electrolyte species can significantly influence the electrocatalytic performance. In this work, we investigate the impact of alkali metal cations on the oxygen reduction reaction (ORR) on active Pt5Gd and Pt5Y polycrystalline electrodes. Due to the strain effects, Pt alloys exhibit a higher kinetic current density of ORR than pure Pt electrodes in acidic media. In alkaline solutions, the kinetic current density of ORR for Pt alloys decreases linearly with the decreasing hydration energy in the order of Li+ > Na+ > K+ > Rb+ > Cs+, whereas Pt shows the opposite trend. To gain further insights into these experimental results, we conduct complementary density functional theory calculations considering the effects of both electrode surface strain and electrolyte chemistry. The computational results reveal that the different trends in the ORR activity in alkaline media can be explained by the change in the adsorption energy of reaction intermediates with applied surface strain in the presence of alkali metal cations. Our findings provide important insights into the effects of the electrolyte and the strain conditions on the electrocatalytic performance and thus offer valuable guidelines for optimizing Pt-based electrocatalysts.