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.

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.

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.

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.

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.

Revealing the Nature of Active Sites on Pt-Gd and Pt-Pr Alloys during the Oxygen Reduction Reaction.

For large-scale applications of hydrogen fuel cells, the sluggish kinetics of the oxygen reduction reaction (ORR) have to be overcome. So far, only platinum (Pt)-group catalysts have shown adequate performance and stability. A well-known approach to increase the efficiency and decrease the Pt loading is to alloy Pt with other metals. Still, for catalyst optimization, the nature of the active sites is crucial. In this work, electrochemical scanning tunneling microscopy (EC-STM) is used to probe the ORR active areas on Pt5Gd and Pt5Pr in acidic media under reaction conditions. The technique detects localized fluctuations in the EC-STM signal, which indicates differences in the local activity. The in situ experiments, supported by coordination-activity plots based on density functional theory calculations, show that the compressed Pt-lanthanide (111) terraces contribute the most to the overall activity. Sites with higher coordination, as found at the bottom of step edges or concavities, remain relatively inactive. Sites of lower coordination, as found near the top of step edges, show higher activity, presumably due to an interplay of strain and steric hindrance effects. These findings should be vital in designing nanostructured Pt-lanthanide electrocatalysts.

Metamorphosis of Heterostructured Surface-Mounted Metal-Organic Frameworks Yielding Record Oxygen Evolution Mass Activities.

Materials derived from surface-mounted metal-organic frameworks (SURMOFs) are promising electrocatalysts for the oxygen evolution reaction (OER). A series of mixed-metal, heterostructured SURMOFs is fabricated by the facile layer-by-layer deposition method. The obtained materials reveal record-high electrocatalyst mass activities of ≈2.90 kA g-1 at an overpotential of 300 mV in 0.1 m KOH, superior to the benchmarking precious and nonprecious metal electrocatalysts. This property is assigned to the particular in situ self-reconstruction and self-activation of the SURMOFs during the immersion and the electrochemical treatment in alkaline aqueous electrolytes, which allows for the generation of NiFe (oxy)hydroxide electrocatalyst materials of specific morphology and microstructure.

Oxygen Reduction Activities of Strained Platinum Core-Shell Electrocatalysts Predicted by Machine Learning.

Core-shell nanocatalyst activities are chiefly controlled by bimetallic material composition, shell thickness, and nanoparticle size. We present a machine learning framework predicting strain with site-specific precision to rationalize how strain on Pt core-shell nanocatalysts can enhance oxygen reduction activities. Large compressive strain on Pt@Cu and Pt@Ni induces optimal mass activities at 1.9 nm nanoparticle size. It is predicted that bimetallic Pt@Au and Pt@Ag have the best mass activities at 2.8 nm, where active sites are exposed to weak compressive strain. We demonstrate that optimal strain depends on the nanoparticle size; for instance, strengthening compressive strain on 1.92 nm sized Pt@Cu and Pt@Ni, or weakening compressive strain on 2.83 nm sized Pt@Ag and Pt@Au, can lead to further enhanced mass activities.

Prospects of Value-Added Chemicals and Hydrogen via Electrolysis.

Cost is a major drawback that limits the industrial-scale hydrogen production through water electrolysis. The overall cost of this technology can be decreased by coupling the electrosynthesis of value-added chemicals at the anode side with electrolytic hydrogen generation at the cathode. This Minireview provides a directory of anodic oxidation reactions that can be combined with cathodic hydrogen generation. The important parameters for selecting the anodic reactions, such as choice of catalyst material and its selectivity towards specific products are elaborated in detail. Furthermore, various novel electrolysis cell architectures for effortless separation of value-added products from hydrogen gas are described.

Revealing the nature of active sites in electrocatalysis.

Heterogeneous electrocatalysis plays a central role in the development of sustainable, carbon-neutral pathways for energy provision and the production of various chemicals. It determines the overall efficiency of electrochemical devices that involve catalysis at the electrode/electrolyte interface. In this perspective, we discuss key aspects for the identification of active centers at the surface of electrocatalysts and important factors that influence them. The role of the surface structure, nanoparticle shape/size and the electrolyte composition in the resulting catalytic performance is of particular interest in this work. We highlight challenges that from our point of view need to be tackled, and provide guidelines for the design of "real life" electrocatalysts for renewable energy provision systems as well as for the production of industrially important compounds.

Top-Down Synthesis of Nanostructured Platinum-Lanthanide Alloy Oxygen Reduction Reaction Catalysts: Pt xPr/C as an Example.

The oxygen reduction reaction (ORR) is of great interest for future sustainable energy conversion and storage, especially concerning fuel cell applications. The preparation of active, affordable, and scalable electrocatalysts and their application in fuel cell engines of hydrogen cars is a prominent step toward the reduction of air pollution, especially in urban areas. Alloying nanostructured Pt with lanthanides is a promising approach to enhance its catalytic ORR activity, whereby the development of a simple synthetic route turned out to be a nontrivial endeavor. Herein, for the first time, we present a successful single-step, scalable top-down synthetic route for Pt-lanthanide alloy nanoparticles, as witnessed by the example of Pr-alloyed Pt nanoparticles. The catalyst was characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and photoelectron spectroscopy, and its electrocatalytic oxygen reduction activity was investigated using a rotating disk electrode technique. Pt xPr/C showed ∼3.5 times higher [1.96 mA/cm2Pt, 0.9 V vs reversible hydrogen electrode (RHE)] specific activity and ∼1.7 times higher (0.7 A/mgPt, 0.9 V vs RHE) mass activity compared to commercial Pt/C catalysts. On the basis of previous findings and characterization of the Pt xPr/C catalyst, the activity improvement over commercial Pt/C originates from a lattice strain introduced by the alloying process.

Oxygen Electroreduction at High-Index Pt Electrodes in Alkaline Electrolytes: A Decisive Role of the Alkali Metal Cations.

Currently, platinum group metals play a central role in the electrocatalysis of the oxygen reduction reaction (ORR). Successful design and synthesis of new highly active materials for this process mainly rely on understanding of the so-called electrified electrode/electrolyte interface. It is widely accepted that the catalytic properties of this interface are only dependent on the electrode surface composition and structure. Therefore, there are limited studies about the effects of the electrolyte components on electrocatalytic activity. By now, however, several key points related to the electrolyte composition have become important for many electrocatalytic reactions, including the ORR. It is essential to understand how certain "spectator ions" (e.g., alkali metal cations) influence the electrocatalytic activity and what is the contribution of the electrode surface structure when, for instance, changing the pH of the electrolyte. In this work, the ORR activity of model stepped Pt [n(111) × (111)] surfaces (where n is equal to either 3 or 4 and denotes the atomic width of the (111) terraces of the Pt electrodes) was explored in various alkali metal (Li+, Na+, K+, Rb+, and Cs+) hydroxide solutions. The activity of these electrodes was unexpectedly strongly dependent not only on the surface structure but also on the type of the alkali metal cation in the solutions with the same pH, being the highest in potassium hydroxide solutions (i.e., K+ ≫ Na+ > Cs+ > Rb+ ≈ Li+). A possible reason for the observed ORR activity of Pt [n(111) × (111)] electrodes is discussed as an interplay between structural effects and noncovalent interactions between alkali metal cations and reaction intermediates adsorbed at active catalytic sites.

Influence of the Nature of the Alkali Metal Cations on the Electrical Double-Layer Capacitance of Model Pt(111) and Au(111) Electrodes.

Understanding the properties of the electrical double layer (EDL) is one of the interdisciplinary topics that plays a key role in the investigation of numerous natural and artificial systems. We present experimental evidence about the influence of the nature of the alkali metal cations on the EDL capacitance for two model electrodes, Pt(111) and Au(111), in 0.05 M AMClO4 ( AM: Li+, Na+, K+, Rb+, Cs+) electrolytes using impedance spectroscopy measurements. Our data show that counterintuitively the differential EDL capacitance of both electrodes measured close to their potentials of zero charge increased linearly in the presence of alkali metal cations as Li+ < Na+ < K+ < Rb+ < Cs+. We also estimated the effective concentrations of these cations at the EDL, which appeared ∼80 times higher than their bulk concentrations. We believe that these findings should be of importance for theoretical modeling of the EDL and better understanding and faster design of new functional systems for numerous applications.

Reconsidering Water Electrolysis: Producing Hydrogen at Cathodes Together with Selective Oxidation of n-Butylamine at Anodes.

Electrocatalysis for the oxygen evolution reaction (OER) is of great interest for improving the effectiveness of water splitting devices. Decreasing the anodic overpotential and simultaneously changing the anodic reaction selectively to produce valuable chemicals instead of O2 would be a major improvement of the overall cost efficiency. Some amines, when present in aqueous electrolytes, were recently shown to change the selectivity of the anodic process to generate H2 O2 rather than O2 on MnOx at pH 10. This results in unusually high apparent "anodic activities". In this work, industrially relevant OER catalysts, oxyhydroxides of cobalt (CoOx ), nickel-iron (NiFeOx ), and nickel (NiOx ) all show more pronounced effects. Moreover, as anodes they also selectively catalyzed the production of nbutyronitrile from n-butylamine at higher pH as an easily retrievable valuable product. The pH dependence of the activity was investigated at pH values closer those at which alkaline electrolyzers operate. The highest activities were observed for NiOx thin-film electrodes at pH 12 in the presence of 0.4 m n-butylammonium sulfate, without poisoning the active sites of Pt electrocatalysts at the hydrogen evolution electrode. 1 H NMR spectroscopy showed that n-butylamine is selectively oxidized to n-butyronitrile, an organic chemical with numerous applications. However, measurements using rotating ring-disk electrodes indicated that some H2 O2 is also generated at the surface of the oxide anodes.

Electronic and vibrational dynamics of hollow au nanocages embedded in cu2 o shells.

We have synthesized hollow Au nanocages embedded within thick porous shells of cuprous oxide (Cu2 O). The shell causes a significant redshift of the localized surface plasmon resonance of Au into the near-IR. Electron-phonon coupling in the Au nanocage is 3-6 times faster in the core-shell structure due to the higher thermal conductivity of Cu2 O compared to water. Coherent phonon oscillations within the Au lattice are characterized by a breathing mode of the entire structure for both bare and core-shell nanocages, an assignment made through the use of structural mechanics simulations. The experimental frequencies are obtained through simulations by selectively applying a force to the shell of the core-shell structure. We interpret this as rapid thermal expansion of the gold leading to a mechanical force that acts on the shell.

Controlling the Catalytic Efficiency on the Surface of Hollow Gold Nanoparticles by Introducing an Inner Thin Layer of Platinum or Palladium.

The efficiency of heterogeneous catalysis of electron-transfer reactions on the surface of gold nanoshells was changed by adding an inner platinum or palladium nanoshell in the double-shell nanocatalysts. The reduction of 4-nitrothiophenol (4NTP) by borohydride was studied as a model reaction. To confirm the heterogeneous catalytic mechanism, the nanocatalysts were assembled into a monolayer on the surface of a quartz substrate using the Langmuir-Blodgett technique, and the 4NTP was allowed to bind to the surface of gold through a strong thiol bond. The stages of the reduction reaction of 4NTP on the surface of gold were successfully followed by time-resolved surface-enhanced Raman spectroscopy. Palladium was found to increase the catalytic efficiency of the gold surface due to the presence of a new Fermi level of the palladium-gold alloy, while platinum decreased its catalytic efficiency due to the electron-withdrawing effect of platinum atoms, which resulted from the difference in their electrochemical reduction potentials.