First-Principles Design of New Electrodes for Proton-Conducting Solid-Oxide Electrochemical Cells: A‑Site Doped Sr₂Fe_1.5Mo_0.5O_6−δ Perovskite

Electrolyzer and fuel cells based on proton-conducting solid-oxide ceramics (PC-SOEC/FC) are gaining wide interest as promising green technologies for H₂ production and conversion. Despite major advances in PC electrolytes, large-scale deployment of PC-SOEC/FC has been hindered by severe limitations at electrodes, which must ensure catalytic activity, electronic conduction, and high proton diffusion rates. Designing electrodes with mixed proton and electron conduction capability represents a great challenge. Several attempts have been based on composite materials made of common electrocatalysts and PC electrolytes, but the resulting electrodes have often suffered stability and conductivity problems. Inspired by the good performance in PC regime of some perovskite oxides, here we propose an alternative approach by designing a new single-phase triple-conducting oxide (TCO) from the recently proposed and well-tested mixed ion-electron conductive electrocatalyst Sr₂Fe_1.5Mo_0.5O_6−δ (SFMO) double perovskite. We investigated with first-principles methods (DFT+U) the key processes that promote proton transport, i.e., oxygen vacancy formation, water dissociative incorporation into the defective lattice, and proton transfer along the oxide sublattice. We focused on SFMO and A-substituted derivatives with Ba or K cations. Both dopants lower the proton migration barrier of SFMO, thus improving proton transport effectiveness. In particular, we found K-doped SFMO to be the best candidate thanks to its peculiar and very favorable structural and electronic properties. Moreover, from our ab initio analysis, we identified a general design principle to enhance proton transport in perovskite oxides at the nanoscale. Our computational results can be easily implemented to develop and test new low-cost TCO-based electrodes for PC-SOEC/FC

Insufficient Hartree–Fock Exchange in Hybrid DFT Functionals Produces Bent Alkynyl Radical Structures

Density functional theory (DFT) is often used to determine the electronic and geometric structures of molecules. While studying alkynyl radicals, we discovered that DFT exchange-correlation (XC) functionals containing less than ∼22% Hartree–Fock (HF) exchange led to qualitatively different structures than those predicted from <i>ab initio</i> HF and post-HF calculations or DFT XCs containing 25% or more HF exchange. We attribute this discrepancy to rehybridization at the radical center due to electron delocalization across the triple bonds of the alkynyl groups, which itself is an artifact of self-interaction and delocalization errors. Inclusion of sufficient exact exchange reduces these errors and suppresses this erroneous delocalization; we find that a threshold amount is needed for accurate structure determinations. Below this threshold, significant errors in predicted alkyne thermochemistry emerge as a consequence

Unveiling the Role of Surface Ir-Oxo Species in O₂ Evolution at IrO₂ Electrocatalysts via Embedded Cluster Multireference Calculations

Understanding the mechanisms driving the oxygen evolution reaction (OER) on iridium oxide (IrO2)-based catalysts is essential to improving their performance and enabling an actual scale-up of water-splitting photoelectrochemical cells. The mechanistic pathways at IrO2 interfaces have been extensively investigated computationally using density functional theory (DFT), which predicts a high-energy barrier for the last step of the OER of molecular oxygen detachment and release from the catalyst surface. Nevertheless, surface O2 over- and under-binding results by standard generalized gradient approximation and hybrid density functionals, respectively, call for further analysis of this crucial step via multireference methods. Aiming at unveiling the nature of such a barrier, we hereby address the formation of O2 from the most-stable IrO2(110) surface with both periodic DFT and an electrostatic embedded cluster approach at the n-electron valence-state perturbation theory. With this multireference approach, we find a value for the aforementioned energy barrier that is much closer to experimental indications than DFT ones. An in-depth analysis of the involved molecular orbitals suggests that the origin of this barrier is related to the breaking of a π interaction between O2 and Ir surface atom and to a significant additional O2 interaction with adjacent electrophilic Ir-oxo species, which is present under experimental operating conditions. Besides shedding light on the mechanism of the OER on IrO2, these findings point out the importance of multireference methods for dissecting complex reactions at electrocatalytic interfaces and pave the route for further investigations with effective embedding approaches

First-Principles Study of Lanthanum Strontium Manganite: Insights into Electronic Structure and Oxygen Vacancy Formation

We characterize the structural, electronic, and defect behavior of La_1–<i>x</i>Sr_<i>x</i>MnO₃ (LSM) (<i>x</i>_Sr = 0.0, 0.25, and 0.5) by means of density functional theory + U (DFT+U) and hybrid DFT methods. Aliovalent substitution of Sr²⁺ for La³⁺ induces formation of holes in the LSM electronic structure. These holes affect electron and oxide ion transport, two key processes occurring within LSM when used as a solid oxide fuel cell (SOFC) cathode. To improve fundamental understanding of these processes, we investigated the atomic-scale effects of increasing Sr content and two different Mn magnetic moment alignments. In agreement with low-temperature experiments, we find a metallic, ferromagnetic (FM) electronic ground state with holes delocalized across the Mn and O sublattices. We also employ an antiferromagnetic (AFM) arrangement of Mn ions to model LSM’s high-temperature paramagnetic state. In contrast to FM LSM, the holes in AFM LSM localize to form Mn⁴⁺ ions, consistent with the observed high-temperature polaronic transport in LSM. The formation of oxygen vacancies governs oxide ion transport in bulk LSM. We find that the ease with which oxygen vacancies form is strongly influenced by the Sr content and the overall magnetic arrangement of Mn ions. These atomic-scale insights enable us to propose new guidelines for enhanced nanoscale LSM-based SOFC cathodes

Ab Initio DFT+U Analysis of Oxygen Vacancy Formation and Migration in La_1‑xSr_<i>x</i>FeO_3‑δ (<i>x</i> = 0, 0.25, 0.50)

Incorporating mixed oxygen-ion-electron conducting (MIEC) cathode materials is a promising strategy to make intermediate-temperature solid oxide fuel cells (IT-SOFCs) viable; however, a lack of fundamental understanding of oxygen transport in these materials limits their development. Density functional theory plus U (DFT+U) calculations are used to investigate how the Sr concentration affects the processes that govern oxygen ion transport in La_1‑<i>x</i>Sr_<i>x</i>FeO_3‑δ (LSF, <i>x</i> = 0, 0.25, and 0.50). Specifically, we show that oxygen vacancies compensate holes introduced by Sr and that this compensation facilitates oxygen vacancy formation in LSF. We also find that oxygen migration in LaFeO₃ is accompanied by electron transfer in the opposite direction. Our results explicitly identify and clarify the role of electron-deficient substitutions in promoting oxygen diffusion in LSF. This atomic level insight is important for enabling rational design of iron-based SOFC cathode materials

Unveiling Structure–Property Relationships in Sr₂Fe_1.5Mo_0.5O_6−δ, an Electrode Material for Symmetric Solid Oxide Fuel Cells

We characterize experimentally and theoretically the promising new solid oxide fuel cell electrode material Sr₂Fe_1.5Mo_0.5O_6−δ (SFMO). Rietveld refinement of powder neutron diffraction data has determined that the crystal structure of this material is distorted from the ideal cubic simple perovskite, instead belonging to the orthorhombic space group <i>Pnma</i>. The refinement revealed the presence of oxygen vacancies in the as-synthesized material, resulting in a composition of Sr₂Fe_1.5Mo_0.5O_5.90(2) (δ = 0.10(2)). DFT+U theory predicts essentially the same concentration of oxygen vacancies. Theoretical analysis of the electronic structure allows us to elucidate the origin of this nonstoichiometry and the attendant mixed ion–electron conductor character so important for intermediate temperature fuel cell operation. The ease with which SFMO forms oxygen vacancies and allows for facile bulk oxide ion diffusivity is directly related to a strong hybridization of the Fe d and O p states, which is also responsible for its impressive electronic conductivity

Structural Evolution of Air-Exposed Layered Oxide Cathodes for Sodium-Ion Batteries: An Example of Ni-doped Na_<i>x</i>MnO₂

Sodium-ion batteries have recently aroused the interest of industries as possible replacements for lithium-ion batteries in some areas. With their high theoretical capacities and competitive prices, P2-type layered oxides (NaxTMO2) are among the obvious choices in terms of cathode materials. On the other hand, many of these materials are unstable in air due to their reactivity toward water and carbon dioxide. Here, Na0.67Mn0.9Ni0.1O2 (NMNO), one of such materials, has been synthesized by a classic sol–gel method and then exposed to air for several weeks as a way to allow a simple and reproducible transition toward a Na-rich birnessite phase. The transition between the anhydrous P2 to the hydrated birnessite structure has been followed via periodic XRD analyses, as well as neutron diffraction ones. Extensive electrochemical characterizations of both pristine NMNO and the air-exposed one vs sodium in organic medium showed comparable performances, with capacities fading from 140 to 60 mAh g–1 in around 100 cycles. Structural evolution of the air-exposed NMNO has been investigated both with ex situ synchrotron XRD and Raman. Finally, DFT analyses showed similar charge compensation mechanisms between P2 and birnessite phases, providing a reason for the similarities between the electrochemical properties of both materials

Interface Modification for Energy Level Alignment and Charge Extraction in CsPbI₃ Perovskite Solar Cells

In perovskite solar cells (PSCs) energy level alignment and charge extraction at the interfaces are the essential factors directly affecting the device performance. In this work, we present a modified interface between all-inorganic CsPbI3 perovskite and its hole-selective contact (spiro-OMeTAD), realized by the dipole molecule trioctylphosphine oxide (TOPO), to align the energy levels. On a passivated perovskite film, with n-octylammonium iodide (OAI), we created an upward surface band-bending at the interface by TOPO treatment. This improved interface by the dipole molecule induces a better energy level alignment and enhances the charge extraction of holes from the perovskite layer to the hole transport material. Consequently, a Voc of 1.2 V and a high-power conversion efficiency (PCE) of over 19% were achieved for inorganic CsPbI3 perovskite solar cells. Further, to demonstrate the effect of the TOPO dipole molecule, we present a layer-by-layer charge extraction study by a transient surface photovoltage (trSPV) technique accomplished by a charge transport simulation