Yuanyue Liu

Charged defect scattering of electrons plays a critical role in determining a wide range of material properties. However, there is a lack of an accurate method to calculate the scattering, as all the existing methods require assuming a specific distribution for unscreened (i.e., bare) excess charge carried by the defect, which limits their accuracy. Here we develop a first-principles method to determine the bare charge distribution, which thus enables accurate calculations of the scattering. Using this method, we accurately calculate the electron mobility of 2D MoS_{2} under the scattering of charged sulfur vacancies and compare with that of charge-neutral oxygen substitutes of sulfur. Interestingly, we find that the charged defect-limited mobility exhibits a strong temperature dependence at low temperature and carrier concentration, in contrast to the neutral defect and the conventional belief that the defect-limited mobility is less sensitive to the temperature. This behavior arises from the free carrier screening of the long-range scattering potential of the charged defect. Our work offers new insights into the transport mechanism, and opens access to accurate calculation, understanding, and prediction of charged defect scattering and a broad range of associated properties.

Better understanding and prediction of the electrochemical interface require large-scale atomistic simulations. Machine learning force fields (MLFFs) have proven to be an effective approach. However, current MLFFs typically do not account for the effect of electrode potential, which requires treating interface electrons with a grand canonical ensemble. Here, we develop a constant potential MLFF (CP-MLFF) based on an equivariant graph neural network and implement it into MACE. Specifically, we design an architecture that can take the number of electrons as the input and accurately predict the Fermi level. The CP-MLFF allows us to examine the convergency of the electrochemical barrier with respect to sampling, which we demonstrate through the example of CO₂ reduction on the Ni-N-C catalyst. Our work provides a useful method and tool enabling accurate and efficient large-scale simulation of the electrochemical interface.

The electrochemical reduction of carbon dioxide (CO₂) to carbon monoxide (CO) is challenged by a selectivity decline at high current densities. Here we report a class of indigo-based molecular promoters with redox-active CO₂ binding sites to enhance the high-rate conversion of CO₂ to CO on silver (Ag) catalysts. Theoretical calculations and in situ spectroscopy analyses demonstrate that the synergistic effect at the interface of indigo-derived compounds and Ag nanoparticles could activate CO₂ molecules and accelerate the formation of key intermediates (*CO₂^– and *COOH) in the CO pathway. Indigo derivatives with electron-withdrawing groups further reduce the overpotential for CO production upon optimizing the interfacial CO₂ binding affinity. By integrating the molecular design of redox-active centres with the defect engineering of Ag structures, we achieve a Faradaic efficiency for CO exceeding 90% across a current density range of 0.10 − 1.20 A cm^–2. The Ag mass activity toward CO increases to 174 A mg^–1_Ag. This work showcases that employing redox-active CO₂ sorbents as surface modification agents is a highly effective strategy to intensify the reactivity of electrochemical CO₂ reduction.

Single metal atoms embedded in nitrogen-doped graphene (M-N-C) have emerged as a promising catalyst for a wide variety of reactions. In addition to the pyridinic site, there is another site responsible for the catalytic activity, but its structure is under debate. Here, we resolve its structure using first-principles calculations. Using Fe-N-C as a representative example, we systematically explore numerous possible structures and discover a new moiety with comparable energy to the pyridinic. This moiety features a hybrid coordination environment between pyridinic and porphyrinic and is located at the edge of graphene sheets or pores. We further calculate its X-ray absorption spectrum, catalytic thermodynamics for oxygen reduction reaction (ORR), and stability under ORR conditions, all of which support its existence. Lastly, we show that this site also exists in other M-N-C with different M elements. This study uncovers a new and important structure in M-N-C and paves a critical step toward site engineering for improved catalytic performance.

Electron-surface scattering is important for many transport phenomena and practical applications. Particularly, the downscaling of microelectronics demands higher electrical conductivity for interconnects, which are currently based on Cu, which suffers from strong surface scattering. However, much is still unclear, such as which surface orientation causes stronger scattering. Existing theories require phenomenological parameters whose values are unknown unless fitted to experimental data or based on assumptions, thereby limiting their accuracy and predictive power. Here we present an accurate, parameter-free approach that enables an accurate calculation of electronic transport with surface scattering. Then we apply it to study the conductivities of Cu films with different surface orientations. Contrary to the common belief that a more compact surface should have higher conductivity, we find that (111) is less conductive than (001). This can be explained by the symmetry of the electronic structure. Furthermore, we propose a phenomenological model that has a better fit to the first-principles results than the conventional one. Our work offers insights into electronic transport and enables accurate calculation, understanding, and prediction for a broad range of systems where surface scattering matters.

Electrocatalytic CO₂ reduction is garnering significant interest due to its potential applications in mitigating CO₂ and producing fuel. However, the scaling up of related catalysis is still hindered by several challenges, including the cost of the catalytic materials, low selectivity, small current densities to maintain desirable selectivity. In this study, Fluorine (F) atoms were introduced into an N-doped carbon-supported single nickel (Ni) atom catalyst via facile polymer-assisted pyrolysis. This method not only maintains the high atom utilization efficiency of Ni in a cost-effective and sustainable manner but also effectively manipulates the electronic structure of the active Ni-N₄ site through F doping. The catalyst has also been further optimized by controlling the F states, including convalent and semi-ionic states, by adjusting the fluorine sources involved. Consequently, this catalyst with unique structure exhibited comparable electrocatalytic performance for CO₂-to-CO conversion, achieving a Faradaic efficiency (FE) of over 99% across a wide potential range and an exceptional CO evolution rate of 9.5 × 10⁴ h^-1 at -1.16 V vs reversible hydrogen electrode (RHE). It also delivered a practical current of 400 mA cm^-2 while maintaining more than 95% CO FE. Experimental analysis combined with density functional theory (DFT) calculations have also shown that F-doping modifies the electron configuration at the central Ni-N₄ sites. This modification lowers the energy barrier for CO₂ activation, thereby facilitating the production of the crucial *COOH intermediate.

Given the growing emphasis on energy efficiency, environmental sustainability, and agricultural demand, there’s a pressing need for decentralized and scalable ammonia production. Converting nitrate ions electrochemically, which are commonly found in industrial wastewater and polluted groundwater, into ammonia offers a viable approach for both wastewater treatment and ammonia production yet limited by low producibility and scalability. Here we report a versatile and scalable solution-phase synthesis of high-entropy single-atom nanocages (HESA NCs) in which Fe and other five metals-Co, Cu, Zn, Cd, and In-are isolated via cyano-bridges and coordinated with C and N, respectively. Incorporating and isolating the five metals into the matrix of Fe resulted in Fe-C₅ active sites with a minimized symmetry of lattice as well as facilitated water dissociation and thus hydrogenation process. As a result, the Fe-HESA NCs exhibited a high selectivity toward NH₃ from the electrocatalytic reduction of nitrate with a Faradaic efficiency of 93.4% while maintaining a high yield rate of 81.4 mg h⁻¹ mg⁻¹.