Microkinetic Model to Rationalize the Lifetime of Electrocatalysis: Trade-off between Activity and Stability

Electrocatalysts which can operate for several years are required to produce hydrogen and commodity chemicals in an environmentally friendly manner. However, designing materials with long operational lifetimes is challenging, due to the lack of a conceptual framework to predict catalytic lifetimes quantitatively. Here, we report a microkinetic equation which quantifies the lifetime of an electrocatalyst undergoing dissolution. This equation was obtained by taking advantage of the fact that catalysis is much faster than deactivation, which allows the ordinary differential equations to be solved via the quasi steady-state approximation. All chemical reactions were modeled as irreversible, first-order elementary reactions. Under this assumption, the catalytic rate correlates linearly with the deactivation rate, leading to a trade-off relationship between activity and stability. Our model was supported by the correlation between theoretical and experimental lifetimes (r2 = 0.86) of a manganese oxide electrocatalyst during the oxygen evolution reaction.

Best practices of modeling complex materials in electrocatalysis, exemplified by oxygen evolution reaction on pentlandites

Pentlandites are natural ores with structural properties comparable to that of [FeNi] hydrogenases. While this class of transition-metal sulfide materials - (Fe,Ni)9S8 - with a variable Fe : Ni ratio has been proven to be an active electrode material for the hydrogen evolution reaction, it is also discussed as electrocatalyst for the alkaline oxygen evolution reaction (OER), corresponding to the bottleneck of anion exchange membrane electrolyzers for green hydrogen production. Despite the experimental evidence for the use of (Fe,Ni)9S8 as an OER catalyst, a detailed investigation of the elementary reaction steps, including consideration of adsorbate coverages and limiting steps under anodic polarizing conditions, is still missing. We address this gap in the present manuscript by gaining atomistic insights into the OER on an Fe4.5Ni4.5S8(111) surface through density functional theory calculations combined with a descriptor-based analysis. We use this system to introduce best practices for modeling this rather complex material by pointing out hidden pitfalls that can arise when using the popular computational hydrogen electrode approach to describe electrocatalytic processes at the electrified solid/liquid interface for energy conversion and storage.

Theoretical investigation of electrocatalytic activity of Pt-free dual atom-doped graphene for O2 reduction in an alkaline solution

Non-precious electrocatalysts as the alternative to Pt have become a hot research area in the last decade due to the suitable catalytic activity in Oxygen reduction reaction (ORR) in electrochemical systems. In this work, the density functional theory calculations were investigated to explore the activity of Fe, Cu, and Fe-Cu atoms supported by N-doped graphene as the ORR electrocatalyst for Oxygen-depolarized cathodes (ODCs). To this end, the ORR mechanism was surveyed in detail in the gas and solvent phases. The results show that the solvent phase leads to a higher overpotential and thermodynamic limiting potential. According to the density of states curves, there are strong interactions between metal atom and substrate that can effectively tune the electronics of catalysts. Bader's analysis confirms that, in addition to the single metal atoms, nitrogen atoms have also played a critical role in charge transfer between substrates and oxygen molecules in ORR. It is also predicted that Fe-Cu@NC SAC exhibits the highest catalytic activity which is consistent with thermodynamic limiting potential and theoretical overpotential of  - 0.26 and  0.66 (V vs. SHE), respectively, indicating that this type of catalyst may be a suitable candidate instead of precious metals in oxygen-depolarized cathodes in electrochemical devices.

Theoretical Study of Electrocatalytic CO2 Reduction Mechanism on Typical MXenes under Realistic Conditions

MXenes are a revolutionary class of two-dimensional materials that have been recently demonstrated to exhibit promising capability of electrocatalytic CO2 reduction reaction (CO2RR) in theory and experiment. In electrocatalytic reactions, the active phases, the mechanism, and the performance can be greatly influenced by electrochemical conditions such as applied electrode potential, pH, and electrolyte. Therefore, in this first-principles study, the stable surface structures of three typical MXenes (V2C, Mo2C, and Ti3C2) with variation of electrocatalytic conditions were determined by the Pourbaix phase diagrams. Additionally, the reaction mechanism for CO2RR toward C1 products was investigated based on the thermal dynamically stable phases. The computation revealed that surfaces of all three MXenes are dominated by H* termination throughout the practical CO2RR electrochemical condition ranges. Meanwhile, the bicarbonate ions, which serve as the major electrolyte in CO2RR, show thermal dynamic unfavorability to adsorb on the surfaces. Among the three types of MXenes, V2CH exhibits higher activity in generating CO and HCOOH through the CO2RR, while Mo2CH exhibits higher activity in producing HCHO, CH3OH, and CH4. This comprehensive study provides crucial insights into the mechanism of electrocatalytic CO2RR on MXenes under realistic electrochemical conditions.

Unveiling the Role of Termination Groups in Stabilizing MXenes in Contact with Water

MXenes are versatile 2D materials demonstrating outstanding electrochemical and physical properties, but their practical use is limited, because of fast degradation in an aqueous environment. To prevent the degradation of MXenes, it is essential to understand the atomistic details of the reaction and to identify active sites. In this letter, we provided a computational analysis of the degradation processes at the interface between MXene basal planes and water using enhanced sampling ab initio molecular dynamics simulations and symbolic regression analysis. Our results indicate that the reactivity of Ti sites toward the water attack reaction depends on both local coordination and chemical composition of the MXene surfaces. Decreasing the work function of the Ti3C2Tx surfaces and avoiding Ti sites that are loosely anchored to the subsurface (e.g., O-coordinated) can improve surface stability. The developed computational framework can be further used to investigate other possible culprits of the degradation reaction, including the role of defects and edges.

Importance of broken geometric symmetry of single-atom Pt sites for efficient electrocatalysis

Platinum single-atom catalysts hold promise as a new frontier in heterogeneous electrocatalysis. However, the exact chemical nature of active Pt sites is highly elusive, arousing many hypotheses to compensate for the significant discrepancies between experiments and theories. Here, we identify the stabilization of low-coordinated PtII species on carbon-based Pt single-atom catalysts, which have rarely been found as reaction intermediates of homogeneous PtII catalysts but have often been proposed as catalytic sites for Pt single-atom catalysts from theory. Advanced online spectroscopic studies reveal multiple identities of PtII moieties on the single-atom catalysts beyond ideally four-coordinated PtII-N4. Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated PtII species from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction. This study may afford general guidelines for achieving a high electrocatalytic performance of carbon-based single-atom catalysts based on other d8 metal ions.

Single-Atom Anchored g-C3N4 Monolayer as Efficient Catalysts for Nitrogen Reduction Reaction

Electrochemical N₂ reduction reaction (NRR) is a promising approach for NH₃ production under mild conditions. Herein, the catalytic performance of 3d transition metal (TM) atoms anchored on s-triazine-based g-C₃N₄ (TM@g-C₃N₄) in NRR is systematically investigated by density functional theory (DFT) calculations. Among these TM@g-C₃N₄ systems, the V@g-C₃N₄, Cr@g-C₃N₄, Mn@g-C₃N₄, Fe@g-C₃N₄, and Co@g-C₃N₄ monolayers have lower ΔG(*NNH) values, especially the V@g-C₃N₄ monolayer has the lowest limiting potential of -0.60 V and the corresponding limiting-potential steps are *N2+H++e-=*NNH for both alternating and distal mechanisms. For V@g-C₃N₄, the transferred charge and spin moment contributed by the anchored V atom activate N₂ molecule. The metal conductivity of V@g-C₃N₄ provides an effective guarantee for charge transfer between adsorbates and V atom during N₂ reduction reaction. After N₂ adsorption, the p-d orbital hybridization of *N₂ and V atoms can provide or receive electrons for the intermediate products, which makes the reduction process follow acceptance-donation mechanism. The results provide an important reference to design high efficiency single atom catalysts (SACs) for N₂ reduction.