Simple Approach for Reconciling Cyclic Voltammetry with Hydrogen Adsorption Energy for Hydrogen Evolution Exchange Current.

Cyclic voltammetry (CV) is a standard technique to analyze the current-potential characteristics of the hydrogen evolution reaction (HER). Herein, we develop a computational quantum-scaled CV model for the HER building on the Butler-Volmer relation for a one-step, one-charge transfer process. Owing to a universal and absolute rate constant verified by fitting to experimental cyclic voltammograms of elemental metals, we show that the model quantifies the exchange current─the main analytical descriptor for HER activity─solely using the hydrogen adsorption free energy obtained from density functional theory calculations. Furthermore, the model resolves controversies over analytical studies for HER kinetics.

Reconciling the Volcano Trend with the Butler-Volmer Model for the Hydrogen Evolution Reaction.

The volcano trend has been widely utilized to forecast new optimum catalysts in computational chemistry while the Butler-Volmer relationship is the norm to explain current-potential characteristics from cyclic voltammetry in analytical chemistry. Herein, we develop an electrochemical model for hydrogen evolution reaction exchange currents that reconciles device-level chemistry, atomic-level volcano trend, and the Butler-Volmer relation. We show that the model is a function of the easy-to-compute hydrogen adsorption energy invariably obtained from first-principles atomic simulations. In addition, the model reproduces with high fidelity the experimental exchange currents for elemental metal catalysts over 15 orders of magnitude and is consistent with the recently proposed analytical model based on a data-driven approach. Our findings based on fundamental electrochemistry principles are general and can be applied to other reactions including CO2 reduction, metal oxidation, and lithium (de)intercalation reactions.

Revealing High-Temperature Reduction Dynamics of High-Entropy Alloy Nanoparticles via In Situ Transmission Electron Microscopy.

Understanding the behavior of high-entropy alloy (HEA) materials under hydrogen (H2) environment is of utmost importance for their promising applications in structural materials, catalysis, and energy-related reactions. Herein, the reduction behavior of oxidized FeCoNiCuPt HEA nanoparticles (NPs) in atmospheric pressure H2 environment was investigated by in situ gas-cell transmission electron microscopy (TEM). The reduction reaction front was maintained at the external surface of the oxide. During reduction, the oxide layer expanded and transformed into porous structures where oxidized Cu was fully reduced to Cu NPs while Fe, Co, and Ni remained in the oxidized form. In situ chemical analysis showed that the expansion of the oxide layer resulted from the outward diffusion flux of all transition metals (Fe, Co, Ni, Cu). Revealing the H2 reduction behavior of HEA NPs facilitates the development of advanced multicomponent alloys for applications targeting H2 formation and storage, catalytic hydrogenation, and corrosion removal.

In Situ Oxidation Studies of High-Entropy Alloy Nanoparticles.

Although high-entropy alloys (HEAs) have shown tremendous potential for elevated temperature, anticorrosion, and catalysis applications, little is known on how HEA materials behave under complex service environments. Herein, we studied the high-temperature oxidation behavior of Fe0.28Co0.21Ni0.20Cu0.08Pt0.23HEA nanoparticles (NPs) in an atmospheric pressure dry air environment by in situ gas-cell transmission electron microscopy. It is found that the oxidation of HEA NPs is governed by Kirkendall effects with logarithmic oxidation rates rather than parabolic as predicted by Wagner's theory. Further, the HEA NPs are found to oxidize at a significantly slower rate compared to monometallic NPs. The outward diffusion of transition metals and formation of disordered oxide layer are observed in real time and confirmed through analytical energy dispersive spectroscopy, and electron energy loss spectroscopy characterizations. Localized ordered lattices are identified in the oxide, suggesting the formation of Fe2O3, CoO, NiO, and CuO crystallites in an overall disordered matrix. Hybrid Monte Carlo and molecular dynamics simulations based on first-principles energies and forces support these findings and show that the oxidation drives surface segregation of Fe, Co, Ni, and Cu, while Pt stays in the core region. The present work offers key insights into how HEA NPs behave under high-temperature oxidizing environment and sheds light on future design of highly stable alloys under complex service conditions.

Revealing Sintering Kinetics of MoS2-Supported Metal Nanocatalysts in Atmospheric Gas Environments via Operando Transmission Electron Microscopy.

The decoration of two-dimensional (2D) substrates with nanoparticles (NPs) serve as heterostructures for various catalysis applications. Deep understanding of catalyst degradation mechanisms during service conditions is crucial to improve the catalyst durability. Herein, we studied the sintering behavior of Pt and bimetallic Au-core Pt-shell (Au@Pt core-shell) NPs on MoS2 supports at high temperatures under vacuum, nitrogen (N2), hydrogen (H2), and air environments by in situ gas-cell transmission electron microscopy (TEM). The key observations are summarized as effect of environment: while particle migration and coalescence (PMC) was the main mechanism that led to Pt and Au@Pt NPs degradation under vacuum, N2, and H2 environments, the degradation of MoS2 substrate was prominent under exposure to air at high temperatures. Pt NPs were less stable in H2 environment when compared with the Pt NPs under vacuum or N2, due to Pt-H interactions that weakened the adhesion of Pt on MoS2. Effect of NP composition: under H2, the stability of Au@Pt NPs was higher in comparison to Pt NPs. This is because H2 promotes the alloying of Pt-Au, thus reducing the number of Pt at the surface (reducing H2 interactions) and increasing Pt atoms in contact with MoS2. Effect of NP size: The alloying effect promoted by H2 was more pronounced in small size Au@Pt NPs resulting in their higher sintering resistance in comparison to large size Au@Pt NPs and similar size Pt NPs. The present work provides key insights into the parameters affecting the catalyst degradation mechanisms on 2D supports.

Graphene Activation Explains the Enhanced Hydrogen Evolution on Graphene-Coated Molybdenum Carbide Electrocatalysts.

Molybdenum carbides (MoxC) have shown high catalytic activities toward hydrogen evolution reaction (HER) when coupled with graphene. Herein, we use density functional theory (DFT) calculations in conjunction with ab initio thermodynamics and electrochemical modeling on γ-MoC supported graphene to determine the origin of the enhanced HER activities. In addition to previous claims that graphene's main role is to prevent agglomeration of MoxC nanoparticles, we show that the interplay between γ-MoC coupling and graphene defect chemistry activates graphene for the HER. For γ-MoC supported graphene systems, the HER mechanism follows the Volmer-Heyrovsky pathway with the Heyrovsky reaction as the rate-determining step. To simulate the electrochemical linear sweep voltammetry at the device level, we develop a computational current model purely from the thermodynamic and kinetics descriptors obtained using DFT. This model shows that γ-MoC supported graphene with divacancies is optimum for HER with an exchange current density of ∼1 × 10-4 A/cm2 and Tafel slope of ∼50 mV/dec-1, which are in good agreement with experimental results.

Tuning the hydrogen evolution activity of ß-Mo2C nanoparticles via control of their growth conditions.

The use of water electrocatalysis for hydrogen production is a promising, sustainable and greenhouse-gas-free process to develop disruptive renewable energy technologies. Transition metal carbides, in particular ß-phase Mo2C, are garnering increased attention as hydrogen evolution reaction (HER) catalysts due to their favourable synthesis conditions, stability and high catalytic efficiency. We use a thermodynamic approach in conjunction with density functional theory and a kinetic model of exchange current density to systematically study the HER activity of ß-Mo2C under different experimental conditions. We show that the (011) surface has the highest HER activity, which is rationalized by its lack of strong Mo-based hydrogen adsorption sites. Thus, the HER efficiency of ß-Mo2C can be tuned using nanoparticles (NPs) that expose larger fractions of this termination. We give definite maps between NP morphologies and experimental synthesis conditions, and show that the control of the carbon chemical potential during synthesis can expose up to 90% of the (011) surface, while ambient H2 has little effect on the NP morphology. The "volcano" plot shows that under these optimum conditions, the NP exchange current density is ∼10-5 A cm-2, that is only slightly smaller than that of Pt (111).