Electronic structure and thermodynamic approaches to the prospect of super abundant vacancies in δ-Pu.

Super abundant vacancies (SAVs) have been suggested to form in the fcc phase of plutonium, δ-Pu, under a low-pressure hydrogen environment. Under these conditions, the vacancy concentration is proposed to reach 10-3 at% due to H trapping in vacancies lowering the effective vacancy formation energy. Previous density functional theory (DFT) results suggest that seven H atoms can be trapped in a single vacancy when a collinear special quasirandom magnetic structure is used to stabilize the δ phase, suggesting SAVs are a possible source of H stored in plutonium. In this report, we present DFT results for δ-Pu in the noncollinear 3Q magnetic state to study the formation of SAVs in mechanically stable δ-Pu. Together with these new simulations, we use publicly available computational and experimental data to provide further constraints on the physical conditions needed to thermodynamically stabilize SAVs in δ-Pu. Using several thermodynamic models, we estimate the vacancy concentrations in δ-Pu and discuss the limits of hydrogen driven formation of vacancies in δ-Pu. We find that, when hydrogen in the lattice is equilibrated with gaseous H2, the formation of SAVs in δ-Pu is unlikely and any excess vacancy concentration beyond thermal vacancies would need to occur by a different mechanism.

Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells.

Modern electrochemical energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhydrous proton conductors for fuel cells. However, the anhydride formation of phosphonic acid functional groups lowers proton conductivity and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic conductivity above 200 °C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. This synergistically integrated fuel cell reached peak power densities of 1,130 mW cm-2 at 160 °C and 1,740 mW cm-2 at 240 °C under H2/O2 conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.

Nitrogen electroreduction and hydrogen evolution on cubic molybdenum carbide: a density functional study.

We report herein a density functional theory study of the nitrogen electroreduction and hydrogen evolution reactions on cubic molybdenum carbide (MoC) in order to investigate the viability of using this material as an electro-catalyst for ammonia synthesis. Free energy diagrams for associative and dissociative Heyrovsky mechanisms showed that nitrogen reduction on cubic MoC(111) can proceed via an associative mechanism and that small negative potentials of -0.3 V vs. standard hydrogen electrode can onset the reduction of nitrogen to ammonia. Kinetic volcano plots for hydrogen evolution showed that the MoC[110] surface is expected to have a high rate for the hydrogen evolution reaction, which could compete with the reduction of nitrogen on cubic MoC. The comparison between the adsorption energies of H-adatoms and N-adatoms also shows that at low potentials adsorption of hydrogen atoms competes with nitrogen adsorption on all the MoC surfaces except the MoC(111) surface. The hydrogen evolution and accumulation of H-adatoms can be mitigated by introducing carbon vacancies i.e. increasing the ratio of metal to carbon atoms, which will significantly increase the affinity of the catalytic surface for both nitrogen molecules and N-adatoms.

Functional interfaces for biomimetic energy harvesting: CNTs-DNA matrix for enzyme assembly.

The development of 3D structures exploring the properties of nano-materials and biological molecules has been shown through the years as an effective path forward for the design of advanced bio-nano architectures for enzymatic fuel cells, photo-bio energy harvesting devices, nano-biosensors and bio-actuators and other bio-nano-interfacial architectures. In this study we demonstrate a scaffold design utilizing carbon nanotubes, deoxyribose nucleic acid (DNA) and a specific DNA binding transcription factor that allows for directed immobilization of a single enzyme. Functionalized carbon nanotubes were covalently bonded to a diazonium salt modified gold surface through carbodiimide chemistry creating a brush-type nanotube alignment. The aligned nanotubes created a highly ordered structure with high surface area that allowed for the attachment of a protein assembly through a designed DNA scaffold. The enzyme immobilization was controlled by a zinc finger (ZNF) protein domain that binds to a specific dsDNA sequence. ZNF 268 was genetically fused to the small laccase (SLAC) from Streptomyces coelicolor, an enzyme belonging to the family of multi-copper oxidases, and used to demonstrate the applicability of the developed approach. Analytical techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and enzymatic activity analysis, allowed characterization at each stage of development of the bio-nano architecture. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

Air Breathing Cathodes for Microbial Fuel Cell using Mn-, Fe-, Co- and Ni-containing Platinum Group Metal-free Catalysts.

The oxygen reduction reaction (ORR) is one of the major factors that is limiting the overall performance output of microbial fuel cells (MFC). In this study, Platinum Group Metal-free (PGM-free) ORR catalysts based on Fe, Co, Ni, Mn and the same precursor (Aminoantipyrine, AAPyr) were synthesized using identical sacrificial support method (SSM). The catalysts were investigated for their electrochemical performance, and then integrated into an air-breathing cathode to be tested in "clean" environment and in a working microbial fuel cell (MFC). Their performances were also compared to activated carbon (AC) based cathode under similar conditions. Results showed that the addition of Mn, Fe, Co and Ni to AAPyr increased the performances compared to AC. Fe-AAPyr showed the highest open circuit potential (OCP) that was 0.307 ± 0.001 V (vs. Ag/AgCl) and the highest electrocatalytic activity at pH 7.5. On the contrary, AC had an OCP of 0.203 ± 0.002 V (vs. Ag/AgCl) and had the lowest electrochemical activity. In MFC, Fe-AAPyr also had the highest output of 251 ± 2.3 µWcm-2, followed by Co-AAPyr with 196 ± 1.5 µWcm-2, Ni-AAPyr with 171 ± 3.6 µWcm-2, Mn-AAPyr with 160 ± 2.8 µWcm-2 and AC 129 ± 4.2 µWcm-2. The best performing catalyst (Fe-AAPyr) was then tested in MFC with increasing solution conductivity from 12.4 mScm-1 to 63.1 mScm-1. A maximum power density of 482 ± 5 µWcm-2 was obtained with increasing solution conductivity, which is one of the highest values reported in the field.

Role of Quinones in Electron Transfer of PQQ-Glucose Dehydrogenase Anodes­Mediation or Orientation Effect.

In this study, the influence of two quinones (1,2- and 1,4-benzoquinone) on the operation and mechanism of electron transfer in PQQ-dependent glucose dehydrogenase (PQQ-sGDH) anodes has been determined. Benzoquinones were experimentally explored as mediators present in the electrolyte. The electrochemical performance of the PQQ-sGDH anodes with and without the mediators was examined and for the first time molecular docking simulations were used to gain a fundamental understanding to explain the role of the mediator molecules in the design and operation of the enzymatic electrodes. It was proposed that the higher performance of the PQQ-sGDH anodes in the presence of 1,2- and 1,4-benzoquinones introduced in the solution is due to the shorter distance between these molecules and PQQ in the enzymatic molecule. It was also hypothesized that when 1,4-benzoquinone is adsorbed on a carbon support, it would play the dual role of a mediator and an orienting agent. At the same time, when 1,2-benzoquinone and ubiquinone are adsorbed on the electrode surface, the enzyme would transfer the electrons directly to the support, and these molecules would primarily play the role of an orienting agent.

Predicting Electrocatalytic Properties: Modeling Structure-Activity Relationships of Nitroxyl Radicals.

Stable nitroxyl radical-containing compounds, such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and its derivatives, are capable of electrocatalytically oxidizing a wide range of alcohols under mild and environmentally friendly conditions. Herein, we examine the structure-function relationships that determine the catalytic activity of a diverse range of water-soluble nitroxyl radical compounds. A strong correlation is described between the difference in the electrochemical oxidation potentials of a compound and its electrocatalytic activity. Additionally, we construct a simple computational model that is able to accurately predict the electrochemical potential and catalytic activity of a wide range of nitroxyl radical derivatives.

Bio-inspired design of electrocatalysts for oxalate oxidation: a combined experimental and computational study of Mn-N-C catalysts.

We report a novel non-platinum group metal (non-PGM) catalyst derived from Mn and amino- antipyrine (MnAAPyr) that shows electrochemical activity towards the oxidation of oxalic acid comparable to Pt with an onset potential for oxalate oxidation measured to be 0.714 ± 0.002 V vs. SHE at pH = 4. The material has been synthesized using a templating Sacrificial Support Method with manganese nitrate and 4-aminoantipyrine as precursors. This catalyst is a nano-structured material in which Mn is atomically dispersed on a nitrogen-doped graphene matrix. XPS studies reveal high abundance of pyridinic, Mn-Nx, and pyrrolic nitrogen pointing towards the conclusion that pyridinic nitrogen atoms coordinated to manganese constitute the active centers. Thus, the main features of the MnAAPyr catalyst are it exhibits similarity to the active sites of naturally occurring enzymes that are capable of efficient and selective oxidation of oxalic acid. Density functional theory in plane wave formalism with Perdew, Burke and Ernzerhof functional was further used to study the stability and activity of different one-metal active centers that could exist in the catalyst. The results show that the stability of the Mn-Nx sites changes in the following order: MnN4 > MnN3C > MnN2C2 > MnN3. Based on the overpotentials of 0.64 V and 0.71 V vs. SHE, calculated using the free energy diagrams for the oxalate oxidation mechanism, we could conclude that the MnN3C and MnN2C2 sites are most probable Mn-Nx sites responsible for the reported catalytic activity of the new catalyst.

Applicability of density functional theory in reproducing accurate vibrational spectra of surface bound species.

The structural equilibrium parameters, the adsorption energies, and the vibrational frequencies of the nitrogen molecule and the hydrogen atom adsorbed on the (111) surface of rhodium have been investigated using different generalized-gradient approximation (GGA), nonlocal correlation, meta-GGA, and hybrid functionals, namely, Perdew, Burke, and Ernzerhof (PBE), Revised-RPBE, vdW-DF, Tao, Perdew, Staroverov, and Scuseria functional (TPSS), and Heyd, Scuseria, and Ernzerhof (HSE06) functional in the plane wave formalism. Among the five tested functionals, nonlocal vdW-DF and meta-GGA TPSS functionals are most successful in describing energetics of dinitrogen physisorption to the Rh(111) surface, while the PBE functional provides the correct chemisorption energy for the hydrogen atom. It was also found that TPSS functional produces the best vibrational spectra of the nitrogen molecule and the hydrogen atom on rhodium within the harmonic formalism with the error of -2.62 and -1.1% for the N-N stretching and Rh-H stretching frequency. Thus, TPSS functional was proposed as a method of choice for obtaining vibrational spectra of low weight adsorbates on metallic surfaces within the harmonic approximation. At the anharmonic level, by decoupling the Rh-H and N-N stretching modes from the bulk phonons and by solving one- and two-dimensional Schrödinger equation associated with the Rh-H, Rh-N, and N-N potential energy we calculated the anharmonic correction for N-N and Rh-H stretching modes as -31 cm(-1) and -77 cm(-1) at PBE level. Anharmonic vibrational frequencies calculated with the use of the hybrid HSE06 function are in best agreement with available experiments.

Electro-reduction of nitrogen on molybdenum nitride: structure, energetics, and vibrational spectra from DFT.

We used density functional theory to study the electrochemical conversion of nitrogen to ammonia on the (001), (100/010), (101), and (111) surfaces of γ-Mo2N. Based on the calculated free energy profiles for the reduction of nitrogen by the associative and dissociative mechanisms, reactivity was found to decrease in the order (111) > (101) > (100/010) ≈ (001). Namely, the cell potentials needed to drive the reduction to ammonia increase in the following order: -0.7 V on (111), -1.2 V on (101), and -1.4 V on (100/010) and (001) surfaces. The (111) surface was found to be the most reactive for nitrogen due to (i) its ability to adsorb the N2 in the side-on position which activates N-N bonding and (ii) its high affinity for N-adatoms which also prevents accumulation of H-adatoms on the catalytic surface at low cell potentials. We have also calculated vibrational frequencies of different NxHy species adsorbed on various γ-Mo2N surfaces. The frequencies are found to depend strongly on the type of the binding sites available on the crystal facets. A comparison of the calculated frequencies with the frequencies of the corresponding species in transition metal complexes and other metal surfaces shows that the frequencies of several signature modes fall in a similar region and might be used to assign the spectra of hydrogen and nitrogen containing surface species on different metal surfaces.

Effect of enzymatic orientation through the use of syringaldazine molecules on multiple multi-copper oxidase enzymes.

The effect of proper enzyme orientation at the electrode surface was explored for two multi-copper oxygen reducing enzymes: Bilirubin Oxidase (BOx) and Laccase (Lac). Simultaneous utilization of "tethering" agent (1-pyrenebutanoic acid, succinimidyl ester; PBSE), for stable enzyme immobilization, and syringaldazine (Syr), for enzyme orientation, of both Lac and BOx led to a notable enhancement of the electrode performance. For Lac cathodes tested in solution it was established that PBSE-Lac and PBSE-Syr-Lac modified cathodes demonstrated approximately 6 and 9 times increase in current density, respectively, compared to physically adsorbed and randomly oriented Lac cathodes. Further testing in solution utilizing BOx showed an even higher increase in achievable current densities, thus BOx was chosen for additional testing in air-breathing mode. In subsequent air-breathing experiments the incorporation of PBSE and Syr with BOx resulted in current densities of 0.65 ± 0.1 mA cm(-2); 2.5 times higher when compared to an unmodified BOx cathode. A fully tethered/oriented BOx cathode was combined with a NAD-dependent Glucose Dehydrogenase anode for the fabrication of a complete enzymatic membraneless fuel cell. A maximum power of 1.03 ± 0.06 mW cm(-2) was recorded for the complete fuel cell. The observed significant enhancement in the performance of "oriented" cathodes was a result of proper enzyme orientation, leading to facilitated enzyme/electrode interface interactions.

Rapid Postgrafting Reaction to Prepare Quaternized Polycarbazoles.

We report a rapid postgrafting reaction to prepare alkyl ammonium functionalized polycarbazoles from a commercially available monomer. This novel synthetic approach provides benefit to preparing the high molecular weight quaternized polycarbazoles within 1 h of Friedel-Crafts polycondensation, avoiding the synthesis and purification step to prepare a functionalized monomer. The postgrafting reaction produces hexyl alkyl ammonium functionalized polycarbazole with 100% grafting degree. However, the postgrafting reaction produced only 60% grafting with propyl alkyl ammonium due to the competitive elimination reaction because of the higher acidity of ß-hydrogen in the propyl alkyl group resulting from the proximity of the bromide and ammonium groups. The hexyl alkyl ammonium functionalized polycarbazole has a high hydroxide conductivity of 103 mS cm-1 at 80 °C and showed excellent alkaline stability with less than 3% loss of ion group after 1 M NaOH treatment at 80 °C for 500 h. This study highlights that the postgrafting reaction provides a pathway for the scale-up synthesis of quaternized aryl ether-free polyaromatics.

Elucidating electrochemical nitrate and nitrite reduction over atomically-dispersed transition metal sites.

Electrocatalytic reduction of waste nitrates (NO3-) enables the synthesis of ammonia (NH3) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts demonstrate a high catalytic activity and uniquely favor mono-nitrogen products. However, the reaction fundamentals remain largely underexplored. Herein, we report a set of 14; 3d-, 4d-, 5d- and f-block M-N-C catalysts. The selectivity and activity of NO3- reduction to NH3 in neutral media, with a specific focus on deciphering the role of the NO2- intermediate in the reaction cascade, reveals strong correlations (R=0.9) between the NO2- reduction activity and NO3- reduction selectivity for NH3. Moreover, theoretical computations reveal the associative/dissociative adsorption pathways for NO2- evolution, over the normal M-N4 sites and their oxo-form (O-M-N4) for oxyphilic metals. This work provides a platform for designing multi-element NO3RR cascades with single-atom sites or their hybridization with extended catalytic surfaces.

Hydrogen adsorbed in a metal organic framework-5: coupled translation-rotation eigenstates from quantum five-dimensional calculations.

We report rigorous quantum five-dimensional (5D) calculations of the coupled translation-rotation (T-R) eigenstates of a H(2) molecule adsorbed in metal organic framework-5 (MOF-5), a prototypical nanoporous material, which was treated as rigid. The anisotropic interactions between H(2) and MOF-5 were represented by the analytical 5D intermolecular potential energy surface (PES) used previously in the simulations of the thermodynamics of hydrogen sorption in this system [Belof et al., J. Phys. Chem. C 113, 9316 (2009)]. The global and local minima on this 5D PES correspond to all of the known binding sites of H(2) in MOF-5, three of which, α-, ß-, and γ-sites are located on the inorganic cluster node of the framework, while two of them, the δ- and ε-sites, are on the phenylene link. In addition, 2D rotational PESs were calculated ab initio for each of these binding sites, keeping the center of mass of H(2) fixed at the respective equilibrium geometries; purely rotational energy levels of H(2) on these 2D PESs were computed by means of quantum 2D calculations. On the 5D PES, the three adjacent γ-sites lie just 1.1 meV above the minimum-energy α-site, and are separated from it by a very low barrier. These features allow extensive wave function delocalization of even the lowest translationally excited T-R eigenstates over the α- and γ-sites, presenting significant challenges for both the quantum bound-state calculations and the analysis of the results. Detailed comparison is made with the available experimental data.

Fluxionality of hydrogen ligands in Fe(H)2(H2)(PEtPh2)3.

Extensive computational investigations along with additional quasielastic neutron scattering data were used to obtain a consistent picture of the extensive fluxionality of hydride and dihydrogen ligands in Fe(H)(2)(H(2))(PEtPh(2))(3) over a wide range of temperatures from 1.5 to 320 K. We were able to identify three different regimes in the dynamical processes based on activation energies obtained from line spectral broadening. The rotational tunneling lines (coherent exchange of the two hydrogens of the H(2) ligand) are broadened with increasing temperature by incoherent exchange up to about 80 K at which point they merge into a quasielastic spectrum from 100 K to about 225 K. The effective activation energies for the two regions are 0.14 and 0.1 kcal mol(-1), respectively. A third dynamical process with a higher activation energy of 0.44 kcal mol(-1) dominates above 225 K, which we attribute to a quantum dynamical exchange of dihydrogen and hydride ligands. Our detailed density functional theory (DFT) structural calculations involving the three functionals (B3LYP, TPSS, and wB97XD) provide a good account of the experimental structure and rotational barriers when only the hydrogen ligands are relaxed. Full relaxation of the "gas-phase" molecule, however, appears to occur to a greater degree than what is possible in the crystal structure. The classical dihydrogen-hydride exchange path involves a cis-dihydrogen and tetrahydride structure with energies of 6.49 and 7.38 kcal mol(-1), respectively. Experimental observation of this process with much lower energies would seem to suggest involvement of translational tunneling in addition to the rotational tunneling. Dynamics of this type may be presumed to be important in hydrogen spillover from metal particles, and therefore need to be elucidated in an effort to utilize this phenomenon.

Protocol for rapid ammonia detection via surface-enhanced Raman spectroscopy.

As a key industrial nitrogenous product and a critical environmental pollutant, ammonia broadly affects our daily lives. Rapid and sensitive detection of ammonia is essential to both environmental monitoring and process control for industrial manufacturing. Here, we present a protocol for rapid detection of low amounts of ammonia in the aqueous phase, via surface-enhanced Raman spectroscopy. We believe the mechanism and speed of the approach demonstrate its potential toward applications in operando electrochemical catalysis and in situ ammonia detection. For complete details on the use and execution of this protocol, please refer to Liu et al. (2020).

Sulfur ylide promoted synthesis of N-protected aziridines: a combined experimental and computational approach.

A range of N-protected aziridines [N-Tosyl (N-Ts), N-2-trimethylsilylethanesulfonamide (N-SES), N-tert-butoxycarbonylamido (N-Boc), and N-o-nitrobenzenesulfonamide (oNs)] were prepared in moderate to good yield and with high enantiomeric excess of both isomers starting from N-protected imines, using a sulfonium salt derived from Eliel's oxathiane. The diastereoselectivities of the reactions are influenced by the imine N-protecting group, the imine substituent, and the sulfide structure. An unusual cis selectivity was observed in the formation of N-tosyl-2-phenyl-3-tert-butylaziridine and N-o-trimethylsilylethanesulfonamide-2-phenyl-3-tert-butylaziridine, which was explained by using computational models. The analysis suggests that betaine formation in the case of N-tosyl-tert-butylaldimine aziridination using oxathiane benzyl sulfonium ylide 1' is reversible and that the selectivity is determined at the rotation step, which is unusual for semistabilized ylide aziridination. We have shown herein that the steric bulk of an imine substituent, in combination with a sterically demanding sulfonium ylide, can also affect the reversibility of the reaction. This is the first example of this sort involving aziridinations using semistabilized ylides.

Bergman cyclization of acyclic amino acid derived enediynes leads to the formation of 2,3-dihydrobenzo[f]isoindoles.

Enediyne-peptide conjugates are recently recognized as useful tools in targeting various proteins, while the mechanism underlying the observed activity remains somewhat unclear. Addressing these issues, we have prepared acyclic amino acid derived enediynes and disclosed a novel thermally induced cyclization-elimination pathway. Initial formation of 1,4-benzene diradical and H-atom abstraction from an external donor is followed by S(N)2 substitution leading to 2,3-dihydrobenzo[f]isoindoles. The proposed mechanism is supported by experimental and computational data. Additionally, we showed that amino acid side chains, although placed three bonds away from acetylene terminuses, have an appreciable influence on the reactivity of studied enediynes. These results demonstrate that amino acid or peptide parts of enediyne-peptide conjugates cannot be considered as recognition elements exclusively but may also participate in various reactions through amine functionality.

Energetics of Base-Acid Pairs for the Design of High-Temperature Fuel Cell Polymer Electrolytes.

The interaction energy of base-acid plays a key role in acid retention of phosphoric acid (PA)-doped polymer electrolytes under fuel cell operating conditions. Here, we investigate the energetics of proton-accepting and hydroxide-donating organic bases using density functional theory calculations. Because of their weak basicity, proton-accepting organic bases such as benzimidazole have relatively low interaction energy with the acid in the absence of water (15.3-28.0 kcal mol-1). Energetics of the proton-accepting base-PA complex increases by adding water, indicating that the interactions in the base-acid complex strengthen in the presence of water. On the other hand, hydroxide-donating organic bases, such as tetramethylammonium hydroxide, have high interaction energy with PA (∼110 kcal mol-1), which remains high in the presence of water. The chemical shifts of 31P NMR support the energetics of the base-acid complexes. This study further discusses the benefit of incorporating hydroxide-donating organic bases into the polymeric structure over proton-accepting bases as a way to increase acid retention.

Facile All-Optical Method for In Situ Detection of Low Amounts of Ammonia.

As a key precursor for nitrogenous compounds and fertilizer, ammonia affects our lives in numerous ways. Rapid and sensitive detection of ammonia is essential, both in environmental monitoring and in process control for industrial production. Here we report a novel and nonperturbative method that allows rapid detection of ammonia at low concentrations, based on the all-optical detection of surface-enhanced Raman signals. We show that this simple and affordable approach enables ammonia probing at selected regions of interest with high spatial resolution, making in situ and operando observations possible.