Active-Site Computational Screening: Role of Structural and Compositional Diversity for the Electrochemical CO2 Reduction at Mo Carbide Catalysts

Understanding the Curvature Effect on the Structure and Bonding of MoCy Nanoparticles on Carbon Supports

The interaction between molybdenum carbide (MoCy) nanoparticles and both flat and curved graphene surfaces, serving as models for carbon nanotubes, was investigated by means of density functional theory. A variety of MoCy nanoparticles with different sizes and stoichiometries have been used to explore different adsorption sites and modes across models with different curvature degrees. On flat graphene, off-stoichiometric MoCy featuring more low-coordinated Mo atoms exhibits stronger interaction and increased electron transfers from the carbide to the carbon substrate. This preferentially occurs through support C and Mo atoms leading to the formation of additional Mo-C bonds. Notably, the MoCy adsorption strength increases on concave surfaces and decreases on convex surfaces, showing a strong linear correlation with the surface curvature. This curvature-dependent behavior alters the charge state of the nanoparticles, making them more/less positively charged in concave/convex regions. The present results demonstrate that the interaction strength can be effectively tuned by manipulating the carbide stoichiometry, the substrate curvature, and the local concave/convex environments, providing valuable guidelines for the rational design of MoCy/C-based catalysts.

Quantification and optimization of platinum-molybdenum carbide interfacial sites to enhance low-temperature water-gas shift reaction

Pt/α-MoC1-x catalysts exhibit exceptional activity in low-temperature water-gas shift reactions. However, quantitatively identifying and fine-tuning the active sites has remained a significant challenge. In this study, we reveal that fully exposed monolayer Pt nanoclusters on molybdenum carbides demonstrate mass activity that exceeds that of bulk molybdenum carbide catalysts by one to two orders of magnitude at 100-200 °C for low-temperature water-gas shift reactions. This advancement is driven by the precise quantification and elucidation of active sites along the Pt-molybdenum carbide interfacial perimeter. By combining sacrificial CO adsorption per Pt atom, Density Functional Theory calculations, and CO chemisorption measurements, we establish a direct correlation between the monolayer Pt nanocluster size and the number of interfacial perimeters on Pt/α-MoC1-x catalysts. In this work, these findings provide key insights into the active site configuration of Pt/α-MoC1-x catalysts and open pathways for innovative catalyst design, with the interfacial perimeter identified as a crucial factor in enhancing catalytic performance.

Two-dimensional phosphorus carbides (β-PC) as highly efficient metal-free electrocatalysts for lithium-sulfur batteries: a first-principles study

Li-S batteries are considered as the next-generation batteries due to their exceptional theoretical capacity. However, their practical application is hampered by the shuttling effects of lithium polysulfides (LiPSs) and the sluggish Li2S decomposition, particularly the slow conversion from Li2S2 to Li2S. Addressing these challenges, the quest for effective catalysts that can accelerate the conversion of LiPSs and enhance the performance of Li-S batteries is crucial. In this study, we explored the electrocatalytic activity of two-dimensional phosphorus carbides (β0-PC and β1-PC) in Li-S batteries based on first-principles calculations. Our findings reveal that these materials demonstrate optimal binding strengths (ranging from 1.09 to 1.83 eV) with long-chain LiPSs, effectively preventing them from dissolving into the electrolyte. Additionally, they show remarkable catalytic activity during the sulfur redox reaction (SRR), with ΔG being only 0.37 eV for β0-PC and 0.13 eV for β1-PC. The low energy barrier induced by β-PC enhances ion migration barrier and significantly expedites the charge/discharge cycles of Li-S batteries. Furthermore, we investigated the conversion dynamics of Li2S2 to Li2S, employing the computational lithium electrode (CLE) model. The excellent performance in these aspects underscores the potential of these materials as electrocatalysts for Li-S batteries, paving the way for advanced high-efficiency energy storage solutions.

Machine Learning Big Data Set Analysis Reveals C-C Electro-Coupling Mechanism

Carbon-carbon (C-C) coupling is essential in the electrocatalytic reduction of CO2 for the production of green chemicals. However, due to the complexity of the reaction network, there remains controversy regarding the underlying reaction mechanisms and the optimal direction for catalyst material design. Here, we present a global perspective to establish a comprehensive data set encompassing all C-C coupling precursors and catalytic active site compositions to explore the reaction mechanisms and screen catalysts via big data set analysis. The 2D-3D ensemble machine learning strategy, developed to target a variety of adsorption configurations, can quickly and accurately expand quantum chemical calculation data, enabling the rapid acquisition of this extensive big data set. Analyses of the big data set establish that (1) asymmetric coupling mechanisms exhibit greater potential efficiency compared to symmetric coupling, with the optimal path involving the coupling CHO with CH or CH2, and (2) C-C coupling selectivity of Cu-based catalysts can be enhanced through bimetallic doping including CuAgNb sites. Importantly, we experimentally substantiate the CuAgNb catalyst to demonstrate actual boosted performance in C-C coupling. Our finding evidence the practicality of our big data set generated from machine learning-accelerated quantum chemical computations. We conclude that combining big data with complex catalytic reaction mechanisms and catalyst compositions will set a new paradigm for accelerating optimal catalyst design.

Electroreduction of CO2 in a Non-aqueous Electrolyte-The Generic Role of Acetonitrile

Transition metal carbides, especially Mo2C, are praised to be efficient electrocatalysts to reduce CO2 to valuable hydrocarbons. However, on Mo2C in an aqueous electrolyte, exclusively the competing hydrogen evolution reaction takes place, and this discrepancy to theory was traced back to the formation of a thin oxide layer at the electrode surface. Here, we study the CO2 reduction activity at Mo2C in a non-aqueous electrolyte to avoid such passivation and to determine products and the CO2 reduction reaction pathway. We find a tendency of CO2 to reduce to carbon monoxide. This process is inevitably coupled with the decomposition of acetonitrile to a 3-aminocrotonitrile anion. Furthermore, a unique behavior of the non-aqueous acetonitrile electrolyte is found, where the electrolyte, instead of the electrocatalyst, governs the catalytic selectivity of the CO2 reduction. This is evidenced by in situ electrochemical infrared spectroscopy on different electrocatalysts as well as by density functional theory calculations.

Selective CO2 electroreduction to methanol via enhanced oxygen bonding

The reduction of carbon dioxide using electrochemical cells is an appealing technology to store renewable electricity in a chemical form. The preferential adsorption of oxygen over carbon atoms of intermediates could improve the methanol selectivity due to the retention of C-O bond. However, the adsorbent-surface interaction is mainly related to the d states of transition metals in catalysts, thus it is difficult to promote the formation of oxygen-bound intermediates without affecting the carbon affinity. This paper describes the construction of a molybdenum-based metal carbide catalyst that promotes the formation and adsorption of oxygen-bound intermediates, where the sp states in catalyst are enabled to participate in the bonding of intermediates. A high Faradaic efficiency of 80.4% for methanol is achieved at -1.1 V vs. the standard hydrogen electrode.

A novel PdC monolayer with fully dispersed Pd atoms and a rigid carbon backbone: an intrinsic versatile electrocatalyst for overall water splitting and the corresponding reverse reaction

The electrocatalytic overall water splitting and the corresponding reverse reaction play a vital role in future renewable energy systems and, hence, are frontiers of catalysis research. In this work, we identify a heretofore unknown two-dimensional palladium carbide using the structure swarm intelligence algorithm. The proposed monolayer, named α-PdC, consists of fully dispersed Pd atoms and a rigid carbon backbone, exhibiting high mechanical, dynamical, and thermal stability with desirable electrical conductivity. Further calculations show that the proposed monolayer is an intrinsic multifunctional electrocatalyst. It possesses an excellent catalytic performance toward the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) with low overpotentials. Specifically, the overpotential for the HER is only -0.01 V, and the significantly low activation energy barrier (0.16 eV) on α-PdC elucidates the fast kinetics. Moreover, α-PdC could also be highly active towards the OER and ORR with comparable overpotentials (0.38 and 0.27 V, respectively). This study identifies an intrinsic versatile electrocatalyst with potential applications in the fields of energy conversion and storage.

Implicit Solvation Methods for Catalysis at Electrified Interfaces

Implicit solvation is an effective, highly coarse-grained approach in atomic-scale simulations to account for a surrounding liquid electrolyte on the level of a continuous polarizable medium. Originating in molecular chemistry with finite solutes, implicit solvation techniques are now increasingly used in the context of first-principles modeling of electrochemistry and electrocatalysis at extended (often metallic) electrodes. The prevalent ansatz to model the latter electrodes and the reactive surface chemistry at them through slabs in periodic boundary condition supercells brings its specific challenges. Foremost this concerns the difficulty of describing the entire double layer forming at the electrified solid–liquid interface (SLI) within supercell sizes tractable by commonly employed density functional theory (DFT). We review liquid solvation methodology from this specific application angle, highlighting in particular its use in the widespread ab initio thermodynamics approach to surface catalysis. Notably, implicit solvation can be employed to mimic a polarization of the electrode’s electronic density under the applied potential and the concomitant capacitive charging of the entire double layer beyond the limitations of the employed DFT supercell. Most critical for continuing advances of this effective methodology for the SLI context is the lack of pertinent (experimental or high-level theoretical) reference data needed for parametrization.

Density functional theory studies of transition metal carbides and nitrides as electrocatalysts

Transition metal carbides and nitrides are interesting non-precious materials that have been shown to replace or reduce the loading of precious metals for catalyzing several important electrochemical reactions. The purpose of this review is to summarize density functional theory (DFT) studies, describe reaction pathways, identify activity and selectivity descriptors, and present a future outlook in designing carbide and nitride catalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (N₂RR), CO₂ reduction reaction (CO₂RR) and alcohol oxidation reactions. This topic is of high interest to scientific communities working in the field of electrocatalysis and this review should provide theoretical guidance for the rational design of improved carbide and nitride electrocatalysts.

True Nature of the Transition-Metal Carbide/Liquid Interface Determines Its Reactivity

Compound materials, such as transition-metal (TM) carbides, are anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) to useful chemicals. This expectation is nurtured by density functional theory (DFT) predictions of a break of key adsorption energy scaling relations that limit CO2RR at parent TMs. Here, we evaluate these prospects for hexagonal Mo2C in aqueous electrolytes in a multimethod experiment and theory approach. We find that surface oxide formation completely suppresses the CO2 activation. The oxides are stable down to potentials as low as -1.9 V versus the standard hydrogen electrode, and solely the hydrogen evolution reaction (HER) is found to be active. This generally points to the absolute imperative of recognizing the true interface establishing under operando conditions in computational screening of catalyst materials. When protected from ambient air and used in nonaqueous electrolyte, Mo2C indeed shows CO2RR activity.