For electrochemical energy conversion, highly efficient and inexpensive electrocatalysts are required, which are principally designed and synthesized by virtue of structural regulations. Herein, we propose a rational linker scission approach to induce lattice strain in metal–organic framework (MOF) catalysts by partially replacing multicoordinating linkers with nonbridging ligands. Strained NiFe-MOFs with 6% lattice expansion exhibit a superior catalytic performance for the oxygen evolution reaction (OER) under alkaline conditions; the overpotential is reduced to 230 mV (86.6 mV dec–1) from 320 mV (164.9 mV dec–1) for the unstrained NiFe-MOFs at a current density of 10 mA cm–2. Operando studies by using synchrotron radiation X-ray absorption and infrared spectroscopy identified the emergence of a key *OOH intermediate on Ni3+/4+ sites during OER, providing strong evidence that the Ni3+/4+ sites are the active sites and the formation of *OOH is the rate-limiting step. The first-principles calculations were performed to reveal the strain-induced electronic structure changes of the NiFe-MOFs and the Gibbs free energy profile during OER. It is found that the optimized Ni 3d eg-orbital facilitates the formation of *OOH, thus enhancing the OER performance of the strained MOFs.

Transition-metal sulfides are investigated as promising electrocatalysts for oxygen evolution reaction (OER) in alkaline media; however, the real active species remain elusive and the development of oxyhydroxides reconstructed from sulfides delivering stable large current density at low applied potentials is a great challenge. Here, we report a synergistic hybrid catalyst, composed of nanoscale heterostructures of Co9S8 and Fe3O4, that exhibits only a low potential of 350 mV and record stability of 120 h at the 500 mA cm–2 in 1.0 M KOH. Voltage-dependent soft X-ray absorption spectroscopy (XAS) and Operando Raman spectroscopy demonstrate that the initial Co9S8@Fe3O4 is reconstructed into CoOOH/CoO x @Fe3O4 and further to complete CoOOH@Fe3O4. Operando XAS and electron microscopy imaging analyses reveal that the completely reconstructed CoOOH acts as active species and Fe3O4 components prevent the aggregation of CoOOH. Operando infrared spectroscopy indicates cobalt superoxide species (*OOH) as the active intermediates during the OER process. Density functional theory calculations demonstrate the formation of *OOH as the rate-determining step of OER and CoOOH@Fe3O4 exhibits a lower energy barrier for OER. Our results provide an in-depth understanding of the dynamic surface structure evolutions of sulfide electrocatalysts for alkaline OER and insights into the design of excellent nanocatalysts for stable large current density.

Dissipaton-equation-of-motion (DEOM) theory [Y. J. Yan, J. Chem. Phys. 140, 054105 (2014)] is an exact and nonperturbative many-particle method for open quantum systems. The existing dissipaton algebra treats also the dynamics of hybrid bath solvation coordinates. The dynamics of conjugate momentums remain to be addressed within the DEOM framework. In this work, we establish this missing ingredient, the dissipaton algebra on solvation momentums, with rigorous validations against necessary and sufficient criteria. The resulted phase-space DEOM theory will serve as a solid ground for further developments of various practical methods toward a broad range of applications. We illustrate this novel dissipaton algebra with the phase-space DEOM-evaluation on heat current fluctuation.

Bismuth is doped to lanthanum strontium ferrite to produce ferrite-based perovskites with a composition of La(0.8-x)Bi(x)Sr0.2FeO(3-δ) (0 ≤ x ≤ 0.8) as novel cathode material for intermediate-temperature solid oxide fuel cells. The perovskite properties including oxygen nonstoichiometry coefficient (δ), average valence of Fe, sinterability, thermal expansion coefficient, electrical conductivity (σ), oxygen chemical surface exchange coefficient (K(chem)), and chemical diffusion coefficient (D(chem)) are explored as a function of bismuth content. While σ decreases with x due to the reduced Fe(4+) content, D(chem) and K(chem) increase since the oxygen vacancy concentration is increased by Bi doping. Consequently, the electrochemical performance is substantially improved and the interfacial polarization resistance is reduced from 1.0 to 0.10 Ω cm(2) at 700 °C with Bi doping. The perovskite with x = 0.4 is suggested as the most promising composition as solid oxide fuel cell cathode material since it has demonstrated high electrical conductivity and low interfacial polarization resistance.

Ceria and doped ceria materials are often used as the oxide components for solid oxide fuel cell (SOFC) anodes to enhance their catalytic activity, especially those using Cu as the electronic conductors. In this work, the surface process of doped ceria reduction, i.e. the chemical surface exchange process in reduced atmospheres, is studied using electrical conductivity relaxation (ECR) technique to characterize their catalytic activity for fuel oxidation. The oxygen surface exchange coefficient of Gd 0.1 Ce 0.9 O 2-δ is comparable to that obtained by thermogravimetric measurement, demonstrating the feasibility of ECR method in determining the oxygen transport in doped ceria under reducing conditions. The relaxation process is limited by the surface exchange step and almost independent on the bulk oxygen ion diffusion kinetics. Among various materials of R 0.2 Ce 0.8 O 2-δ (R = Y, Gd, Sm, La) and Sm x Ce 1-x O 2-δ (x = 0, 0.05, 0.1, 0.2, 0.3), Sm 0.2 Ce 0.8 O 2-δ exhibits the highest surface exchange coefficient, thus should be promise as the anode component. Moreover, it is found that, at temperature below 700 • C, surface exchange kinetics at the grain boundary is significantly faster than on the grain, suggesting additional advantage of developing SOFCs by low-temperature sintering.Ceria-based materials have been investigated as various parts of solid oxide fuel cells (SOFCs) including 1) electrolytes for lowtemperature SOFCs operated below 600 • C, 2) inter-layers between yttria-stabilized zirconia (YSZ) electrolytes and La 0.6 Sr 0.4 CoO 3-δ based cathodes, and 3) oxide components in composite cathodes and anodes. 1-4 When it is used as the anodic component, its higher ionic conductivity over typical YSZ extends the electrochemical reaction zone beyond the electrolyte-electrode interface, and consequently, enhancing the electrode performance. 5 In addition, its reversible Ce 3+ -Ce 4+ transition in reducing atmospheres may cause excellent catalytic activity for the electrochemical oxidation of the fuels, 6-8 which permits a particular application in the composite anodes; replacing Ni with Cu that is not catalytically active for carbon deposition. In the Cuceria composite anodes, Cu serves as the electronic conductor and ceria as both oxygen ionic conductor and catalyst. While the conductivities have been extensively measured for ceria and doped ceria (DCO) as a function of temperature and oxygen partial pressure, 9,10 only a few experiments have been conducted to characterize its catalytic activity, i.e. the surface properties for anodic reaction. The surface properties are critical for Cu-ceria anodes, and also very important for the Ni-ceria anodes. 11 It is demonstrated that H 2 oxidation pathway is dominated by electrocatalysis at the ceria/gas interfaces with minimal contributions from the ceria/metal/gas triple-phase boundaries. 11,12 That is to say, the surface properties of ceria are critical for the anode reactions.In an SOFC with a ceria-based anode, oxygen ions are generated at the cathode, transpo...

An efficient low-frequency logarithmic discretization (LFLD) scheme for the decomposition of fermionic reservoir spectrum is proposed for the investigation of quantum impurity systems. The scheme combines the Padé spectrum decomposition (PSD) and a logarithmic discretization of the residual part in which the parameters are determined based on an extension of the recently developed minimum-dissipaton ansatz [J. J. Ding et al., J. Chem. Phys. 145, 204110 (2016)]. A hierarchical equations of motion (HEOM) approach is then employed to validate the proposed scheme by examining the static and dynamic system properties in both the Kondo and noninteracting regimes. The LFLD scheme requires a much smaller number of exponential functions than the conventional PSD scheme to reproduce the reservoir correlation function and thus facilitates the efficient implementation of the HEOM approach in extremely low temperature regimes.

This work presents a unified dissipaton-equation-of-motion (DEOM) theory and its evaluations on the Helmholtz free energy change due to the isotherm mixing of two isolated subsystems. One is a local impurity, and the other is a nonlocal Gaussian bath. DEOM constitutes a fundamental theory for such open quantum mixtures. To complete the theory, we also construct the imaginary-time DEOM formalism via an analytical continuation of dissipaton algebra, which would be limited to equilibrium thermodynamics. On the other hand, the real-time DEOM deals with both equilibrium structural and nonequilibrium dynamic properties. Its combination with the thermodynamic integral formalism would be a viable and accurate means to both equilibrium and transient thermodynamics. As illustrations, we report the numerical results on a spin-boson system, with elaborations on the underlying anharmonic features, the thermodynamic entropy vs the von Neumann entropy, and an indication of “solvent-cage” formation. Beside the required asymptotic equilibrium properties, the proposed transient thermodynamics also supports the basic spontaneity criterion.

In this work, we establish a so-called "system-bath entanglement theorem", for arbitrary systems coupled with Gaussian environments. This theorem connects the entangled system-bath response functions in the total composite space to those of local systems, as long as the interacting bath spectral densities are given. We validate the theorem with the direct evaluation via the exact dissipaton-equation-of-motion approach. Therefore, this work enables various quantum dissipation theories, which originally describe only the reduced system dynamics, for their evaluations on the system-bath entanglement properties. Numerical demonstrations are carried out on the Fano interference spectroscopies of spin-boson systems.