Boris V. Merinov

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

An activity lift for platinum Platinum is an excellent but expensive catalyst for the oxygen reduction reaction (ORR), which is critical for fuel cells. Alloying platinum with other metals can create shells of platinum on cores of less expensive metals, which increases its surface exposure, and compressive strain in the layer can also boost its activity (see the Perspective by Stephens et al.). Bu et al. produced nanoplates—platinum-lead cores covered with platinum shells—that were in tensile strain. These nanoplates had high and stable ORR activity, which theory suggests arises from the strain optimizing the platinum-oxygen bond strength. Li et al. optimized both the amount of surface-exposed platinum and the specific activity. They made nanowires with a nickel oxide core and a platinum shell, annealed them to the metal alloy, and then leached out the nickel to form a rough surface. The mass activity was about double the best reported values from previous studies. Science, this issue p. 1410, p. 1414; see also p. 1378 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.

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.

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

Significance The growing demand for energy storage urges the development of alternative cation batteries, which calls for a systematic understanding of binding energetics. We discover a general phenomenon for binding of alkali and alkaline earth metal atoms with substrates, which is explained in a unified picture of chemical bonding. This allows us to solve the long-standing puzzle of low Na capacity in graphite and predict the trends of battery voltages, and also forms a basis for analyzing the binding of alkali and alkaline earth metal atoms over a broad range of systems. 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.

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.

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.

Proton diffusion pathways and rates in Y-doped BaZrO3 solid oxide electrolyte from quantum mechanics.

We carried out quantum mechanical calculations (Perdew-Becke-Ernzerhof flavor of density functional theory) on 12.5% Y-doped BaZrO(3) (BYZ) periodic structures to obtain energy barriers for intraoctahedral and interoctahedral proton transfers. We find activation energy (E(a)) values of 0.48 and 0.49 eV for the intraoctahedral proton transfers on O-O edges (2.58 and 2.59 A) of ZrO(6) and YO(6) octahedra, respectively, and E(a) = 0.41 eV for the interoctahedral proton transfer at O-O separation of 2.54 A. These results indicate that both the interoctahedral and intraoctahedral proton transfers are important in the BYZ electrolyte. Indeed, the calculated values bracket the experimental value of E(a) = 0.44 eV. Based on the results obtained, the atomic level proton diffusion mechanism and possible proton diffusion pathways have been proposed for the BYZ electrolyte. The thermal librations of BO(6) octahedra and uncorrelated thermal vibrations of the two oxygen atoms participating in the hydrogen bond lead to a somewhat chaotic fluctuation in the distances between the O atoms involved in the hydrogen bonding. Such fluctuations affect the barriers and at certain O-O distances allow the hydrogen atoms to move within the hydrogen bonds from one potential minimum to the other and between the hydrogen bonds. Concertation of these intra- and inter-H-bond motions results in continuous proton diffusion pathways. Continuity of proton diffusion pathways is an essential condition for fast proton transport.

Structural study of Cs5H3(SO4)4.xH2O-alkali metal sulfate proton conductor

Abstract An X-ray diffraction single crystal study of the hexagonal phase of Cs5H3(SO4)4·xH2O, which occurs at room temperature, has been performed with the aim of determining the atomic structure and the mechanism of proton transport in this phase. M r =1051.79, λ (Mo Kα)=0.71076 A , hexagonal, P6 3 /mmc, a h =6.2455(8), c h =29.690(3) A , V=1003.0 A 3 , Z=2, D x =3.51 g·cm−3, μ = 96.1 cm−1, F(000) = 950, room temperature, R(F) = 0.039, R(F)w = 0.038 for 531 unique reflections. The results show that a Dynamically Disordered Hydrogen Bond Network (DDHBN), which is responsible for the high protonic conductivity, exists in the hexagonal phase.

Crystal structure and mechanism of proton transport in the hexagonal phase of Cs5H3(SeO4)4·H2O

Abstract Am X-ray diffraction single crystal study of the hexagonal high-temperature phase of Cs 5 H 3 (SeO 4 O 4 ·H 2 O has been performed with the aim of determining the atomic structure and the mechanism of proton transport in this phase. M r =1257.39, λ (Mo Kα)=0.71076 A, hexagonal, P6 3 /mmc, a h =6.400(3), c h =30.03(1) A, V =1065.2 A 3 , Z =2, D x =3.92 g·cm −3 , μ =160.2 cm −1 , F (000)=1104, T =360 K, R ( F )=0.042, R ( F ) w =0.040 for 231 unique reflections. The results show that a dynamically disordered hydrogen bond network (DDHBN), which is responsible for the high protonic conductivity, exists in the hexagonal phase. In addition, some aspects of the high-temperature phase transition II-I were studied.

Composite protonic electrolytes in the system (NH4)3H(SO4)2–SiO2

Abstract Transport, thermal and structural properties of (1− x )(NH 4 ) 3 H(SO 4 ) 2 – x SiO 2 composite solid electrolytes ( x =0–0.8) were investigated, and an increase in (NH 4 ) 3 H(SO 4 ) 2 (II) low temperature phase conductivity was shown. The dependence of the low temperature conductivity on x has a smooth maximum with values five to six times higher than that of the pure polycrystalline salt and two orders of magnitude higher than that of single crystals. The (NH 4 ) 3 H(SO 4 ) 2 phase transition temperature in the composites decreases by 20 K with an increase of SiO 2 content from 0 to 0.7. The observed jump in conductivity that coincides with the phase transition becomes more diffuse and disappears at x =0.8. The thermal stability of (NH 4 ) 3 H(SO 4 ) 2 in such composites increases markedly as compared with the pure salt.