Mechanical modulation of reaction rates in electrocatalysis

Tuning Electrocatalytic Activities of Dealloyed Nanoporous Catalysts by Macroscopic Strain Engineering

It is technically challenging to quantitatively apply strains to tune catalysis because most heterogeneous catalysts are nanoparticles, and lattice strains can only be applied indirectly via core-shell structures or crystal defects. Herein, we report quantitative relations between macroscopic strains and hydrogen evolution reaction (HER) activities of dealloyed nanoporous gold (NPG) by directly applying macroscopic strains upon bulk NPG. It was found that macroscopic compressive strains lead to a decrease, while macroscopic tensile strains improve the HER activity of NPG, which is in line with the d-band center model. The overpotential and onset potential of HER display approximately a linear relation with applied macroscopic strains, revealing an ∼2.9 meV decrease of the binding energy per 0.1% lattice strains from compressive to tensile. The methodology with the high strain sensitivity of electrocatalysis, developed in this study, paves a new way to investigate the insights of strain-dependent electrocatalysis with high precision.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Non-precious transition metal single-atom catalysts for the oxygen reduction reaction: progress and prospects

The massive exploitation and use of fossil resources have created many negative issues, such as energy shortage and environmental pollution. It prompts us to turn our attention to the development of new energy technologies. This review summarizes the recent research progress of non-precious transition metal single-atom catalysts (NPT-SACs) for the oxygen reduction reaction (ORR) in Zn-air batteries and fuel cells. Some commonly used preparation methods and their advantages/disadvantages have been summarized. The factors affecting the ORR performances of NPT-SACs have been focused upon, such as the substrate type, coordination environment and nanocluster effects. The loading mass of a metal atom has a direct effect on the ORR performances. Some general strategies for stabilizing metal atoms are included. This review points out some existing challenges of NPT-SACs, and also provides ideas for designing and synthesizing NPT-SACs with excellent ORR performances. The large-scale preparation and commercialization of NPT-SACs with excellent ORR properties are prospected.

High-frequency and rapid response tungsten sulfide nano onion-based electrochemical actuators

Poor rate capability, the biggest barrier to potential applications of electrochemical actuators (ECAs), is primarily resulted from symmetric electrochemical reactions. This makes it extremely difficult for ECAs to actuate above 1 Hz while maintaining sufficient displacement retainability compared with their actuations at relatively low frequencies, particularly when working in liquids. Here, tungsten trisulfide (WS₃) assisted tungsten disulfide nano onions are synthesized through a one-step laser-assisted strategy. Using the irreversibility of WS₃ in adsorbing hydrogen in an acidic solution, the electrochemical reaction of tungsten sulfide nano onions is tailored to realize an asymmetric redox reaction for breaking the symmetry of the electrical double layer and battery-like process. Experiments demonstrate that the ECA's response rate (0.24 mm^-1 s^-1) is at least 10 times faster than that of the previously reported ECAs. Moreover, this ECA can actuate at 30 Hz and reaches top performance in liquids at 4 Hz with long-term durability (>90% after 23 000 cycles), which is comparable to that of electromagnetic and electrothermal actuators. To understand the electrochemical actuation of tungsten sulfide from the atomic scale to the macroscopic scale, density functional theory calculations are conducted and an electrochemomechanical coupling model is proposed. A new generation of subvolt electric-driven actuators used in underwater robotics can be developed by modulating the electrochemical response and chemomechanical coupling effect.

Noble Metal-Based Catalysts with Core-Shell Structure for Oxygen Reduction Reaction: Progress and Prospective

With the deterioration of the ecological environment and the depletion of fossil energy, fuel cells, representing a new generation of clean energy, have received widespread attention. This review summarized recent progress in noble metal-based core-shell catalysts for oxygen reduction reactions (ORRs) in proton exchange membrane fuel cells (PEMFCs). The novel testing methods, performance evaluation parameters and research methods of ORR were briefly introduced. The effects of the preparation method, temperature, kinds of doping elements and the number of shell layers on the ORR performances of noble metal-based core-shell catalysts were highlighted. The difficulties of mass production and the high cost of noble metal-based core-shell nanostructured ORR catalysts were also summarized. Thus, in order to promote the commercialization of noble metal-based core-shell catalysts, research directions and prospects on the further development of high performance ORR catalysts with simple synthesis and low cost are presented.

Electrocatalyst Design for Conversion of Energy Molecules: Electronic State Modulation and Mass Transport Regulation

Electrocatalytic conversions of energy molecules are involved in many energy conversion processes. Improving the activity of electrocatalyst is critical for increasing the efficiency of these energy conversion processes. However, the...

In situ lattice tuning of quasi-single-crystal surfaces for continuous electrochemical modulation

The ability to control the atomic-level structure of a solid represents a straightforward strategy for fabricating high-performance catalysts and semiconductor materials. Herein we explore the capability of the mechanically controllable surface strain method in adjusting the surface structure of a gold film. Underpotential deposition measurements provide a quantitative and ultrasensitive approach for monitoring the evolution of surface structures. The electrochemical activities of the quasi-single-crystalline gold films are enhanced productively by controlling the surface tension, resulting in a more positive potential for copper deposition. Our method provides an effective way to tune the atom arrangement of solid surfaces with sub-angstrom precision and to achieve a reduction in power consumption, which has vast applications in electrocatalysis, molecular electronics, and materials science.

We reported a new method capable of adjusting the lattice structure of solid surfaces with sub-angstrom precision and achieved in situ and continuous control over electrochemical activity.

Activating the hydrogen evolution activity of Pt electrode via synergistic interaction with NiS2

Electrocatalytic hydrogen evolution reaction (HER) is a green approach to produce high-quality hydrogen fuel. Developing efficient electrocatalyst is the key to realize cost-effective HER. Pt is the state-of-the-art HER catalyst so far. However, the use of Pt for HER is limited by its high cost. Thus, it is essential to lower down the usage of Pt in the electrocatalyst by improving the intrinsic activity of Pt. In this work, we propose to achieve this goal by introducing synergistic interaction between Pt and substrate material (NiS₂). The favorable synergy interaction can modify the d band structure of Pt (111) facet and modulate the hydrogen adsorption on Pt (111), which enhances the intrinsic electrocatalytic activity of Pt. The effectiveness of this strategy is demonstrated with both experimental and theoretical investigations.

The Mechanical Effect of MnO2 Layers on Electrochemical Actuation Performance of Nanoporous Gold

This study investigated the electrochemical actuation behavior of nanoporous material during the capacitive process. The length change of nanoporous gold (npg) was in situ investigated in a liquid environment using the dilatometry technique. The mechanical effect of MnO₂ layers was introduced in this work to improve the actuation characteristics of the npg samples. Our work found that the actuation behavior of npg sample could be significantly modulated with a covering of MnO₂ layers. The electrochemical actuation amplitude was efficiently improved and strongly dependent on the thickness of MnO₂ layers covered. Aside from the amplitude, the phase relation between the length change and the electrode potential was inverted when covering the MnO₂ layer on the npg samples. This means the expansion of the npg samples and the contraction of samples covered with the MnO₂ layer when electrochemical potential sweeps positively. A simple finite element model was built up to understand the effect of the MnO₂ layer. The agreement between the simulation result and the experimental data indicates that the sign-inverted actuation-potential response of nanoporous gold contributes to the mechanical effect of MnO₂. It is believed that our work could offer a deep understanding on the effect of the MnO₂ layer on the electrochemical actuation and then provide a useful strategy to modulate the actuation performance of nanoporous metal materials.

Strain Influences the Hydrogen Evolution Activity and Absorption Capacity of Palladium

Strain engineering can increase the activity and selectivity of an electrocatalyst. Tensile strain is known to improve the electrocatalytic activity of palladium electrodes for reduction of carbon dioxide or dioxygen, but determining how strain affects the hydrogen evolution reaction (HER) is complicated by the fact that palladium absorbs hydrogen concurrently with HER. We report here a custom electrochemical cell, which applies tensile strain to a flexible working electrode, that enabled us to resolve how tensile strain affects hydrogen absorption and HER activity for a thin film palladium electrocatalyst. When the electrodes were subjected to mechanically-applied tensile strain, the amount of hydrogen that absorbed into the palladium decreased, and HER electrocatalytic activity increased. This study showcases how strain can be used to modulate the hydrogen absorption capacity and HER activity of palladium.

Surface Strain-Induced Collective Switching of Ensembles of Molecules on Metal Surfaces

A central difficulty in the design of molecular electronics is poor control of the contact state between the molecule and metal electrode, which may induce instability and noise in logic and memory devices and even destroy the intrinsic functionality of the device. Here, we theoretically propose a simple and effective strategy for realizing full control of the contact state of organic molecules coated on the metal surface by applying homogeneous surface strain. As exemplified by pyrazine molecules on Cu(111), application of compressive (tensile) strain causes the molecules to uniformly adopt the physisorbed (chemisorbed) state. Within the framework of non-equilibrium Green's function calculations, we show that the two distinct contact states yield simultaneous rectification and switching behaviors. Because the contact states of all surface-bound molecules are transformed uniformly via surface strain perturbations, fully controlled collective switching and rectification effects can be simultaneously achieved in this contact system.

Advances in dynamically controlled catalytic reaction engineering

Dynamically forced input oscillations exhibit ability to surpass classical thermodynamic barriers through reactor operation and surface resonance.

Dynamic Tuning of a Thin Film Electrocatalyst by Tensile Strain

We report the ability to tune the catalytic activities for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by applying mechanical stress on a highly n-type doped rutile TiO₂ films. We demonstrate through operando electrochemical experiments that the low HER activity of TiO₂ can reversibly approach those of the state-of-the-art non-precious metal catalysts when the TiO₂ is under tensile strain. At 3% tensile strain, the HER overpotential required to generate a current density of 1 mA/cm² shifts anodically by 260 mV to give an onset potential of 125 mV, representing a drastic reduction in the kinetic overpotential. A similar albeit smaller cathodic shift in the OER overpotential is observed when tensile strain is applied to TiO₂. Results suggest that significant improvements in HER and OER activities with tensile strain are due to an increase in concentration of surface active sites and a decrease in kinetic and thermodynamics barriers along the reaction pathway(s). Our results highlight that strain applied to TiO₂ by precisely controlled and incrementally increasing (i.e. dynamic) tensile stress is an effective tool for dynamically tuning the electrocatalytic properties of HER and OER electrocatalysts relative to their activities under static conditions.

A Simple Mechanical Method to Modulate the Electrochemical Electrosorption Processes at Metal Surfaces

The coupling of electrochemical processes and surface strain has been widely investigated in the past. The present work briefly introduces a simple method to modulate the electrochemical process at metal surfaces by mechanical bending. In this way, the static strain at the metal layer can reach the order of 1%. The cyclic voltammogram was used to study the electrosorption process of oxygen species at sputtered metal surfaces under different strain states. The experimental results show that the desorption peak potential of oxygen at the Au surface shifted positively by tensile strain, whereas the desorption peak potential at the Pt surface shifted negatively. This phenomenon indicates that tensile strain has an opposite effect on the electrosorption process for Au and Pt surfaces. Our results agree with the previous reports on the potential variation induced by dynamic strain. This work thus offers a simple method to modulate the electrosorption process at metal surfaces and then to enhance the reactivity of metal electrodes.

Probing the Surface of Noble Metals Electrochemically by Underpotential Deposition of Transition Metals

The advances in material science have led to the development of novel and various materials as nanoparticles or thin films. Underpotential deposition (upd) of transition metals appears to be a very sensitive method for probing the surfaces of noble metals, which is a parameter that has an important effect on the activity in heterogeneous catalysis. Underpotential deposition as a surface characterization tool permits researchers to precisely determine the crystallographic orientations of nanoparticles or the real surface area of various surfaces. Among all the work dealing with upd, this review focuses specifically on the main upd systems used to probe surfaces of noble metals in electrocatalysis, from poly‒ and single-crystalline surfaces to nanoparticles. Cuupd is reported as a tool to determine the active surface area of gold‒ and platinum‒based bimetallic electrode materials. Pbupd is the most used system to assess the crystallographic orientations on nanoparticles’ surface. In the case of platinum, Bi and Ge adsorptions are singled out for probing (1 1 1) and (1 0 0) facets, respectively.

Tuning the oxygen evolution reaction on a nickel-iron alloy via active straining

We report that one can gain active control of the electrocatalytic oxygen evolution reaction (OER) on Ni3Fe thin films via externally applied strains. The combination of theory and experiments shows that an elastic strain on the surface can tune the OER activity in a predictable way that is consistent with the d-band model. The OER overpotential can be lowered by uniaxial tensions and increased by compressions in a linear manner.

Strain-induced changes to the methanation reaction on thin-film nickel catalysts

We investigate how mechanical strain can directly manipulate the catalytic rate of a purely thermochemical reaction.

Confining nano-sized platinum in nitrogen doped ordered mesoporous carbon: An effective approach toward efficient and robust hydrogen evolution electrocatalyst

Despite recent progress in the development of earth abundant electrochemical catalyst for hydrogen evolution reaction (HER), Pt based materials still stand as the state of the art HER catalyst. Due to the high cost of Pt, it is desirable to increase the utilization efficiency of Pt in practical HER process to a realize cost effective hydrogen production. Herein, we repot a novel nitrogen doped ordered mesoporous carbon supported Pt (Pt@NOMC-A) catalyst with a low Pt loading of 7.2 wt% and show that the synergy between Pt nanoparticles and carbon support, as well as the physical confinement offered by the carbon support enhance the electrochemical performance of the novel catalyst. Pt@NOMC-A exhibits a low HER overpotential comparable with commercial 20 wt% Pt/C catalyst under acidic, neutral and alkaline condition. Furthermore, Pt@NOMC-A shows a superior electrochemical stability under working conditions suppressing that of commercial Pt/C catalyst.

Electrocapillary coupling at rough surfaces

The surface roughness of an electrode has a strong impact on the apparent value of electrocapillary coupling coefficient, ς_eff, which relates the response of electrode potential to tangential strain.

Electrocapillary coupling during electrosorption

The electrocapillary coupling coefficient, ς, measures the response of the electrode potential, E, to tangential elastic strain at the surface of an electrode. Using dynamic electro-chemo-mechanical analysis, we study ς(E) simultaneously with cyclic voltammetry. The study covers extended potential intervals on Au, Pt, and Pd, including the electrosorption of oxygen species and of hydrogen. The magnitude and sign of ς vary during the scans, and quite generally the graphs of ς(E) emphasize details which are less obvious or missing in the cyclic voltammograms (CVs). Capacitive processes on the clean electrode surfaces exhibit ς < 0, whereas capacitive processes on oxygen-covered surfaces are characterized by ς < 0 on Au but ς > 0 on Pt and Pd. The findings of ς < 0 during the initial stages of oxygen species adsorption and ς > 0 for hydrogen electrosorption agree with the trend that tensile strain makes surfaces more binding for adsorbates. However, the large hysteresis of oxygen electrosorption on all electrodes raises the question: is the exchange current associated with that process sufficient for its measurement by potential response during small cyclic strain?