Effects of Zr dopants on properties of PtNi nanoparticles for ORR catalysis: A DFT modeling.

Pt-based alloys, such as Pt3Ni, are among the best electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. Doping of PtNi alloys with Zr was shown to enhance the durability of the operating ORR catalysts. Rationalizing these observations is hindered by the absence of atomic-level data for these tri-metallic materials, even when not exposed to the fuel cell operation conditions. This study aims at understanding structure-property relations in Zr-doped PtNi nanoparticles as a key to their ORR function. In particular, we calculated, using a method based on density functional theory, the most stable chemical orderings of pristine and Zr-doped Pt3Ni particles containing over 400 atoms. We thus clarify (i) preferential location and charge states of Zr atoms in the Pt3Ni NPs; (ii) effect of doping Zr atoms on the stability of the Pt skin of the Pt3Ni NPs; (iii) charge redistribution induced by Zr dopants; (iv) layer-by-layer atomic ordering in the Pt3Ni/Zr NPs with the increasing Zr content; and (v) effect of Zr atoms on the adsorption energies of O and OH species as indicators of the ORR activity.

Proton transport mechanism and pathways in the superprotonic phase of M3H(AO4)2 solid acids from ab initio molecular dynamics simulations.

The proton transport mechanism in superprotonic phases of solid acids has been a subject of experimental and theoretical studies for a number of years. Despite this, details of the mechanism still need further clarification. In particular in the M3H(AO4)2 family of crystals, where M = NH4, K, Rb, Cs, and A = S, Se, the proton diffusion is mostly considered in the (001) plane, whereas it is relatively high in the [001] direction as well. In this paper, we report the results of our ab initio molecular dynamics simulations of the Cs3H(SeO4)2 superprotonic phase and propose an atomic-level mechanism of proton transport and pathways both in the (001) plane and along the [001] direction. It turned out that structural configurations formed by hydrogen-bonded tetrahedral anions during the proton diffusion are more complicated and diverse than those considered so far in the literature. Our predicted values of the proton conductivity and activation energy agree well with available experimental data. This validates the reliability of the computational results obtained.

Li-diffusion at the interface between Li-metal and [Pyr14][TFSI]-ionic liquid: Ab initio molecular dynamics simulations.

We previously reported comprehensive density functional theory-molecular dynamics (DFT-MD) at 400 K to determine the composition and structure of the solid electrolyte interface (SEI) between a Li anode and [Pyr14][TFSI] ionic liquid. In this paper, we examined diffusion rates in both the Li-electrode region and SEI compact layer in smaller 83Li/2[TFSI] and larger 164Li/4[TFSI] systems. At 400 K, the Li-diffusion constant in the Li-region is 1.35 × 10-10 m2/s for 83Li/2[TFSI] and 5.64 × 10-10 m2/s for 164Li/4[TFSI], while for the SEI it is 0.33 × 10-10 m2/s and 0.22 × 10-10 m2/s, thus about one order slower in the SEI compared to the Li-region. This Li-diffusion is dominated by hopping from the neighbor shell of one F or O to the neighbor shell of another. Comparing the Li-diffusion at different temperatures, we find that the activation energy is 0.03 and 0.11 eV for the Li-region in the smaller and larger systems, respectively, while for the SEI it is 0.09 and 0.06 eV.

Interface Structure in Li-Metal/[Pyr14][TFSI]-Ionic Liquid System from ab Initio Molecular Dynamics Simulations.

Ionic liquids (ILs) are promising materials for application in a new generation of Li batteries. They can be used as electrolyte or interlayer or incorporated into other materials. ILs have the ability to form a stable solid electrochemical interface (SEI), which plays an important role in protecting the Li-based electrode from oxidation and the electrolyte from extensive decomposition. Experimentally, it is hardly possible to elicit fine details of the SEI structure. To remedy this situation, we have performed a comprehensive computational study (density functional theory-based molecular dynamics) to determine the composition and structure of the SEI compact layer formed between the Li anode and [Pyr14][TFSI] IL. We found that the [TFSI] anions quickly reacted with Li and decomposed, unlike the [Pyr14] cations which remained stable. The obtained SEI compact layer structure is nonhomogeneous and consists of the atomized S, N, O, F, and C anions oxidized by Li atoms.

Role of solvent-anion charge transfer in oxidative degradation of battery electrolytes.

Electrochemical stability windows of electrolytes largely determine the limitations of operating regimes of lithium-ion batteries, but the degradation mechanisms are difficult to characterize and poorly understood. Using computational quantum chemistry to investigate the oxidative decomposition that govern voltage stability of multi-component organic electrolytes, we find that electrolyte decomposition is a process involving the solvent and the salt anion and requires explicit treatment of their coupling. We find that the ionization potential of the solvent-anion system is often lower than that of the isolated solvent or the anion. This mutual weakening effect is explained by the formation of the anion-solvent charge-transfer complex, which we study for 16 anion-solvent combinations. This understanding of the oxidation mechanism allows the formulation of a simple predictive model that explains experimentally observed trends in the onset voltages of degradation of electrolytes near the cathode. This model opens opportunities for rapid rational design of stable electrolytes for high-energy batteries.

Factors affecting cyclic durability of all-solid-state lithium batteries using poly(ethylene oxide)-based polymer electrolytes and recommendations to achieve improved performance.

A detailed experimental analysis of the factors affecting cyclic durability of all-solid-state lithium batteries using poly(ethylene oxide)-based polymer electrolytes was published in EES by Nakayama et al. We use quantum mechanics to interpret these results, identifying processes involved in the degradation of rechargeable lithium batteries based on polyethylene oxide (PEO) polymer electrolyte with LiTFSI. We consider that ionization of the electrolyte near the cathode at the end of the recharge step is probably responsible for this degradation. We find that an electron is likely removed from PEO next to a TFSI anion, triggering a sequence of steps leading to neutralization of a TFSI anion and anchoring of another TFSI to the PEO. This decreases the polymer conductivity near the cathode, making it easier to ionize additional PEO and leading to complete degradation of the battery. We refer to this as the Cathode Overpotential Driven Ionization of the Solvent (CODIS) model. We suggest possible ways to confirm experimentally our interpretation and propose modifications to suppress or reduce electrolyte degradation.

Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH.

Hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) are both 2 orders slower in alkaline electrolyte than in acidic electrolyte, but no explanation has been provided. The first step toward understanding this dramatic pH-dependent HOR/HER performance is to explain the pH-dependent hydrogen binding to the electrode, a perplexing behavior observed experimentally. In this work, we carried out Quantum Mechanics Molecular Dynamics (QMMD) with explicit considerations of solvent and applied voltage ( U) to in situ simulate water/Pt(100) interface in the condition of under-potential adsorption of hydrogen ( HUPD). We found that as U is made more negative, the electrode tends to repel water, which in turn increases the hydrogen binding. We predicted a 0.13 eV increase in hydrogen binding from pH = 0.2 to pH = 12.8 with a slope of 10 meV/pH, which is close to the experimental observation of 8 to 12 meV/pH. Thus, we conclude that the changes in water adsorption are the major causes of pH-dependent hydrogen binding on a noble metal. The new insight of critical role of surface water in modifying electrochemical reactions provides a guideline in designing HER/HOR catalyst targeting for the alkaline electrolyte.

Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction.

Improving the platinum (Pt) mass activity for the oxygen reduction reaction (ORR) requires optimization of both the specific activity and the electrochemically active surface area (ECSA). We found that solution-synthesized Pt/NiO core/shell nanowires can be converted into PtNi alloy nanowires through a thermal annealing process and then transformed into jagged Pt nanowires via electrochemical dealloying. The jagged nanowires exhibit an ECSA of 118 square meters per gram of Pt and a specific activity of 11.5 milliamperes per square centimeter for ORR (at 0.9 volts versus reversible hydrogen electrode), yielding a mass activity of 13.6 amperes per milligram of Pt, nearly double previously reported best values. Reactive molecular dynamics simulations suggest that highly stressed, undercoordinated rhombus-rich surface configurations of the jagged nanowires enhance ORR activity versus more relaxed surfaces.

Origin of low sodium capacity in graphite and generally weak substrate binding of Na and Mg among alkali and alkaline earth metals.

It is well known that graphite has a low capacity for Na but a high capacity for other alkali metals. The growing interest in alternative cation batteries beyond Li makes it particularly important to elucidate the origin of this behavior, which is not well understood. In examining this question, we find a quite general phenomenon: among the alkali and alkaline earth metals, Na and Mg generally have the weakest chemical binding to a given substrate, compared with the other elements in the same column of the periodic table. We demonstrate this with quantum mechanics calculations for a wide range of substrate materials (not limited to C) covering a variety of structures and chemical compositions. The phenomenon arises from the competition between trends in the ionization energy and the ion-substrate coupling, down the columns of the periodic table. Consequently, the cathodic voltage for Na and Mg is expected to be lower than those for other metals in the same column. This generality provides a basis for analyzing the binding of alkali and alkaline earth metal atoms over a broad range of systems.

Annealing kinetics of electrodeposited lithium dendrites.

The densifying kinetics of lithium dendrites is characterized with effective activation energy of Ea ≈ 6 - 7 kcal mol(-1) in our experiments and molecular dynamics computations. We show that heating lithium dendrites for 55 °C reduces the representative dendrites length λ¯(T,t) up to 36%. NVT reactive force field simulations on three-dimensional glass phase dendrites produced by our coarse grained Monte Carlo method reveal that for any given initial dendrite morphology, there is a unique stable atomic arrangement for a certain range of temperature, combined with rapid morphological transition (∼10 ps) within quasi-stable states involving concurrent bulk and surface diffusions. Our results are useful for predicting the inherent structural characteristics of lithium dendrites such as dominant coordination number.

Thermal relaxation of lithium dendrites.

The average lengths λ̅ of lithium dendrites produced by charging symmetric Li(0) batteries at various temperatures are matched by Monte Carlo computations dealing both with Li(+) transport in the electrolyte and thermal relaxation of Li(0) electrodeposits. We found that experimental λ̅(T) variations cannot be solely accounted by the temperature dependence of Li(+) mobility in the solvent but require the involvement of competitive Li-atom transport from metastable dendrite tips to smoother domains over ΔE(++)(R) ∼ 20 kJ mol(-1) barriers. A transition state theory analysis of Li-atom diffusion in solids yields a negative entropy of activation for the relaxation process: ΔS(++)(R) ≈ -46 J mol(-1) K(-1) that is consistent with the transformation of amorphous into crystalline Li(0) electrodeposits. Significantly, our ΔE(++)(R) ∼ 20 kJ mol(-1) value compares favorably with the activation barriers recently derived from DFT calculations for self-diffusion on Li(0)(001) and (111) crystal surfaces. Our findings suggest a key role for the mobility of interfacial Li-atoms in determining the morphology of dendrites at temperatures above the onset of surface reconstruction: TSR ≈ 0.65 TMB (TMB = 453 K: the melting point of bulk Li(0)).

Dynamics of Lithium Dendrite Growth and Inhibition: Pulse Charging Experiments and Monte Carlo Calculations.

Short-circuiting via dendrites compromises the reliability of Li-metal batteries. Dendrites ensue from instabilities inherent to electrodeposition that should be amenable to dynamic control. Here, we report that by charging a scaled coin-cell prototype with 1 ms pulses followed by 3 ms rest periods the average dendrite length is shortened ∼2.5 times relative to those grown under continuous charging. Monte Carlo simulations dealing with Li(+) diffusion and electromigration reveal that experiments involving 20 ms pulses were ineffective because Li(+) migration in the strong electric fields converging to dendrite tips generates extended depleted layers that cannot be replenished by diffusion during rest periods. Because the application of pulses much shorter than the characteristic time τc ∼ O(∼1 ms) for polarizing electric double layers in our system would approach DC charging, we suggest that dendrite propagation can be inhibited (albeit not suppressed) by pulse charging within appropriate frequency ranges.

ReaxFF Reactive Force-Field Modeling of the Triple-Phase Boundary in a Solid Oxide Fuel Cell.

In our study, the Ni/YSZ ReaxFF reactive force field was developed by combining the YSZ and Ni/C/H descriptions. ReaxFF reactive molecular dynamics (RMD) were applied to model chemical reactions, diffusion, and other physicochemical processes at the fuel/Ni/YSZ interface. The ReaxFF RMD simulations were performed on the H2/Ni/YSZ and C4H10/Ni/YSZ triple-phase boundary (TPB) systems at 1250 and 2000 K, respectively. The simulations indicate amorphization of the Ni surface, partial decohesion (delamination) at the interface, and coking, which have indeed all been observed experimentally. They also allowed us to derive the mechanism of the butane conversion at the Ni/YSZ interface. Many steps of this mechanism are similar to the pyrolysis of butane. The products obtained in our simulations are the same as those in experiment, which indicates that the developed ReaxFF potential properly describes complex physicochemical processes, such as the oxide-ion diffusion, fuel conversion, water formation reaction, coking, and delamination, occurring at the TPB and can be recommended for further computational studies of the fuel/electrode/electrolyte interfaces in a SOFC.

Mechanism for degradation of Nafion in PEM fuel cells from quantum mechanics calculations.

We report results of quantum mechanics (QM) mechanistic studies of Nafion membrane degradation in a polymer electrolyte membrane (PEM) fuel cell. Experiments suggest that Nafion degradation is caused by generation of trace radical species (such as OH(●), H(●)) only when in the presence of H(2), O(2), and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H(2) (or H(+)) and O(2) in the presence of the Pt catalyst. We find a barrier of 0.53 eV for OH radical formation from HOOH chemisorbed on Pt(111) and of 0.76 eV from chemisorbed OOH(ad), suggesting that OH might be present during the ORR, particularly when the fuel cell is turned on and off. Based on the QM, we propose two chemical mechanisms for OH radical attack on the Nafion polymer: (1) OH attack on the S-C bond to form H(2)SO(4) plus a carbon radical (barrier: 0.96 eV) followed by decomposition of the carbon radical to form an epoxide (barrier: 1.40 eV). (2) OH attack on H(2) crossover gas to form hydrogen radical (barrier: 0.04 eV), which subsequently attacks a C-F bond to form HF plus carbon radicals (barrier as low as 1.00 eV). This carbon radical can then decompose to form a ketone plus a carbon radical with a barrier of 0.86 eV. The products (HF, OCF(2), SCF(2)) of these proposed mechanisms have all been observed by F NMR in the fuel cell exit gases along with the decrease in pH expected from our mechanism.

ReaxFF reactive force field for the Y-doped BaZrO3 proton conductor with applications to diffusion rates for multigranular systems.

Proton-conducting perovskites such as Y-doped BaZrO 3 (BYZ) are promising candidates as electrolytes for a proton ceramic fuel cell (PCFC) that might permit much lower temperatures (from 400 to 600 degrees C). However, these materials lead to relatively poor total conductivity ( approximately 10 (-4) S/cm) because of extremely high grain boundary resistance. In order to provide the basis for improving these materials, we developed the ReaxFF reactive force field to enable molecular dynamics (MD) simulations of proton diffusion in the bulk phase and across grain boundaries of BYZ. This allows us to elucidate the atomistic structural details underlying the origin of this poor grain boundary conductivity and how it is related to the orientation of the grains. The parameters in ReaxFF were based entirely on the results of quantum mechanics (QM) calculations for systems related to BYZ. We apply here the ReaxFF to describe the proton diffusion in crystalline BYZ and across grain boundaries in BYZ. The results are in excellent agreement with experiment, validating the use of ReaxFF for studying the transport properties of these membranes. Having atomistic structures for the grain boundaries from simulations that explain the overall effect of the grain boundaries on diffusion opens the door to in silico optimization of these materials. That is, we can now use theory and simulation to examine the effect of alloying on both the interfacial structures and on the overall diffusion. As an example, these calculations suggest that the reduced diffusion of protons across the grain boundary results from the increased average distances between oxygen atoms in the interface, which necessarily leads to larger barriers for proton hopping. Assuming that this is the critical issue in grain boundary diffusion, the performance of BYZ for multigranular systems might be improved using additives that would tend to precipitate to the grain boundary and which would tend to pull the oxygens atoms together. Possibilities might be to use a small amount of larger trivalent ions, such as La or Lu or of tetravalent ions such as Hf or Th. Since ReaxFF can also be used to describe the chemical processes on the anode and cathode and the migration of ions across the electrode-membrane interface, ReaxFF opens the door to the possibility of atomistic first principles predictions on models of a complete fuel cell.

ReaxFF reactive force field for solid oxide fuel cell systems with application to oxygen ion transport in yttria-stabilized zirconia.

We present the ReaxFF reactive force field developed to provide a first-principles-based description of oxygen ion transport through yttria-stabilized zirconia (YSZ) solid oxide fuel cell (SOFC) membranes. All parameters for ReaxFF were optimized to reproduce quantum mechanical (QM) calculations on relevant condensed phase and cluster systems. We validated the use of ReaxFF for fuel cell applications by using it in molecular dynamics (MD) simulations to predict the oxygen ion diffusion coefficient in yttria-stabilized zirconia as a function of temperature. These values are in excellent agreement with experimental results, setting the stage for the use of ReaxFF to model the transport of oxygen ions through the YSZ electrolyte for SOFC. Because ReaxFF descriptions are already available for some catalysts (e.g., Ni and Pt) and under development for other high-temperature catalysts, we can now consider fully first-principles-based simulations of the critical functions in SOFC, enabling the possibility of in silico optimization of these materials. That is, we can now consider using theory and simulation to examine the effect of materials modifications on both the catalysts and transport processes in SOFC.