Tianbiao Liu

TEMPO-Based Catholyte for High-Energy Density Nonaqueous Redox Flow Batteries

A TEMPO-based non-aqueous electrolyte with the TEMPO concentration as high as 2.0 m is demonstrated as a high-energy-density catholyte for redox flow battery applications. With a hybrid anode, Li|TEMPO flow cells using this electrolyte deliver an energy efficiency of ca. 70% and an impressively high energy density of 126 W h L(-1) .

Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All‐Organic Redox Flow Battery

Nonaqueous redox flow batteries hold the promise of achieving higher energy density because of the broader voltage window than aqueous systems, but their current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. We report a new nonaqueous all-organic flow battery based on high concentrations of redox materials, which shows significant, comprehensive improvement in flow battery performance. A mechanistic electron spin resonance study reveals that the choice of supporting electrolytes greatly affects the chemical stability of the charged radical species especially the negative side radical anion, which dominates the cycling stability of these flow cells. This finding not only increases our fundamental understanding of performance degradation in flow batteries using radical-based redox species, but also offers insights toward rational electrolyte optimization for improving the cycling stability of these flow batteries.

Highly reversible Mg insertion in nanostructured Bi for Mg ion batteries

Rechargeable magnesium batteries have attracted wide attention for energy storage. Currently, most studies focus on Mg metal as the anode, but this approach is still limited by the properties of the electrolyte and poor control of the Mg plating/stripping processes. This paper reports the synthesis and application of Bi nanotubes as a high-performance anode material for rechargeable Mg ion batteries. The nanostructured Bi anode delivers a high reversible specific capacity (350 mAh/gBi or 3430 mAh/cm(3)Bi), excellent stability, and high Coulombic efficiency (95% initial and very close to 100% afterward). The good performance is attributed to the unique properties of in situ formed, interconnected nanoporous bismuth. Such nanostructures can effectively accommodate the large volume change without losing electric contact and significantly reduce diffusion length for Mg(2+). Significantly, the nanostructured Bi anode can be used with conventional electrolytes which will open new opportunities to study Mg ion battery chemistry and further improve its properties.

Series of mixed valent Fe(II)Fe(I) complexes that model the Hox state of [FeFe]hydrogenase: redox properties, density-functional theory investigation, and reactivities with extrinsic CO.

A series of asymmetrically disubstituted models of the active site of [FeFe]-hydrogenase, (mu-pdt)[Fe(CO) 2PMe 3][Fe(CO) 2NHC] (pdt = 1,3-propanedithiolate, NHC = IMes, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene IMes ( 1), IMesMe, 1-methyl,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene ( 2) or IMe, 1,3-bis(methyl)imidazol-2-ylidene ( 3)), have been synthesized and characterized. The one-electron oxidation of these complexes to generate mixed valent models of the H ox state of [FeFe]-hydrogenase, such as the previously reported (mu-pdt)(mu-CO)[Fe(CO) 2PMe 3][Fe(CO)IMes] (+) ( 1 ox ) (Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008-7009) has been examined to explore the steric and electronic effects of different N-atom substituents on the stability and structure of the mixed valent cations. The differences in spectroscopic properties, structures, and relative stabilities of 1 ox , (mu-pdt)[Fe(CO) 2PMe 3][Fe(CO) 2IMesMe] (+) ( 2 ox ), and (mu-pdt)[Fe(CO) 2PMe 3]-[Fe(CO) 2IMe] (+) ( 3 ox ) are discussed in the context of both experimental and theoretical data. Of the three derivatives, only that with greatest steric bulk on the NHC ligand, 1 ox , shows a clear indication of a mu-CO by solution nu(CO) IR and yields to crystallization as a rotated form, commensurate with the two-Fe subsite of H ox. In addition, the reactivity of the complexes with extrinsic CO to form CO adducts and/or exchange with (13)CO is explored by experiment and by using density-functional theory calculations.

Heterolytic cleavage of hydrogen by an iron hydrogenase model: an Fe-H⋅⋅⋅H-N dihydrogen bond characterized by neutron diffraction.

Hydrogenase enzymes in nature use hydrogen as a fuel, but the heterolytic cleavage of H-H bonds cannot be readily observed in enzymes. Here we show that an iron complex with pendant amines in the diphosphine ligand cleaves hydrogen heterolytically. The product has a strong Fe-H⋅⋅⋅H-N dihydrogen bond. The structure was determined by single-crystal neutron diffraction, and has a remarkably short H⋅⋅⋅H distance of 1.489(10) Å between the protic N-H(δ+) and hydridic Fe-H(δ-) part. The structural data for [Cp(C5F4N)FeH(P(tBu)2N(tBu)2H)](+) provide a glimpse of how the H-H bond is oxidized or generated in hydrogenase enzymes. These results now provide a full picture for the first time, illustrating structures and reactivity of the dihydrogen complex and the product of the heterolytic cleavage of H2 in a functional model of the active site of the [FeFe] hydrogenase enzyme.

Two pathways for electrocatalytic oxidation of hydrogen by a nickel bis(diphosphine) complex with pendant amines in the second coordination sphere.

A nickel bis(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni(P(Cy)2N(t-Bu)2)2](BF4)2 (P(Cy)2N(t-Bu)2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni(II) complex gives three isomers of the doubly protonated Ni(0) complex [Ni(P(Cy)2N(t-Bu)2H)2](BF4)2. Using the pKa values and Ni(II/I) and Ni(I/0) redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(P(Cy)2N(t-Bu)2)2](2+) was determined to be -7.9 kcal mol(-1). The catalytic rate observed in dry acetonitrile for the oxidation of H2 depends on base size, with larger bases (NEt3, t-BuNH2) resulting in much slower catalysis than n-BuNH2. The addition of water accelerates the rate of catalysis by facilitating deprotonation of the hydrogen addition product before oxidation, especially for the larger bases NEt3 and t-BuNH2. This catalytic pathway, where deprotonation occurs prior to oxidation, leads to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes proton movement. Under the optimal conditions of 1.0 atm H2 using n-BuNH2 as a base and with added water, a turnover frequency of 58 s(-1) is observed at 23 °C.

Synthesis, Characterization, and Reactivity of Fe Complexes Containing Cyclic Diazadiphosphine Ligands: The Role of the Pendant Base in Heterolytic Cleavage of H2

The iron complexes CpFe(P(Ph)(2)N(Bn)(2))Cl (1-Cl), CpFe(P(Ph)(2)N(Ph)(2))Cl (2-Cl), and CpFe(P(Ph)(2)C(5))Cl (3-Cl)(where P(Ph)(2)N(Bn)(2) is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P(Ph)(2)N(Ph)(2) is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane, and P(Ph)(2)C(5) is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction. These chloride derivatives are readily converted to the corresponding hydride complexes [CpFe(P(Ph)(2)N(Bn)(2))H (1-H), CpFe(P(Ph)(2)N(Ph)(2))H (2-H), CpFe(P(Ph)(2)C(5))H (3-H)] and H(2) complexes [CpFe(P(Ph)(2)N(Bn)(2))(H(2))]BAr(F)(4), [1-H(2)]BAr(F)(4), (where BAr(F)(4) is B[(3,5-(CF(3))(2)C(6)H(3))(4)](-)), [CpFe(P(Ph)(2)N(Ph)(2))(H(2))]BAr(F)(4), [2-H(2)]BAr(F)(4), and [CpFe(P(Ph)(2)C(5))(H(2))]BAr(F)(4), [3-H(2)]BAr(F)(4), as well as [CpFe(P(Ph)(2)N(Bn)(2))(CO)]BAr(F)(4), [1-CO]Cl. Structural studies are reported for [1-H(2)]BAr(F)(4), 1-H, 2-H, and [1-CO]Cl. The conformations adopted by the chelate rings of the P(Ph)(2)N(Bn)(2) ligand in the different complexes are determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for [1-CO]BAr(F)(4) is 2.848 Å, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes [1-H(2)](+), [2-H(2)](+), and [3-H(2)](+) carried out using H(2) and D(2) indicate that the relatively rapid H/D exchange observed for [1-H(2)](+) and [2-H(2)](+) compared to [3-H(2)](+) is consistent with intramolecular heterolytic cleavage of H(2) mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H(2). These mononuclear Fe(II) dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe]-hydrogenase required for H-H bond formation and cleavage.

Synthesis and Mössbauer characterization of octahedral iron(II) carbonyl complexes FeI2(CO)3L and FeI2(CO)2L2: developing models of the [Fe]-H(2)ase active site.

A series of mono- and disubstituted complexes, FeI(2)(CO)(x)L(4-x), x = 2 or 3, is conveniently accessed from simple mixing of N-heterocyclic carbenes, phosphines, and aromatic amines with FeI(2)(CO)(4), first reported by Hieber in 1928. The highly light sensitive complexes yield to crystallization and X-ray diffraction studies for six complexes showing them to be rudimentary structural models of the monoiron hydrogenase, [Fe]-H(2)ase or Hmd, active site in native (Fe(II)(CO)(2)) or CO-inhibited (Fe(II)(CO)(3)) states. Diatomic ligand (nu(CO)) vibrational and Mossbauer spectroscopies are related to those reported for the Hmd active site. The importance of a serial approach for relating such parameters in model compounds to low spin Fe(II) in the diverse ligation of enzyme active sites is stressed.

A facile approach using MgCl2 to formulate high performance Mg2+ electrolytes for rechargeable Mg batteries

Rechargeable Mg batteries have been regarded as a viable battery technology for grid scale energy storage and transportation applications. However, the limited performance of Mg2+ electrolytes has been a primary technical hurdle to develop high energy density rechargeable Mg batteries. In this study, MgCl2 is demonstrated as a non-nucleophilic and cheap Mg2+ source in combination with Al Lewis acids (AlCl3, AlPh3 and AlEtCl2) to formulate a series of Mg2+ electrolytes, representing the simplest method to prepare Mg2+ conductive electrolytes (no precursor synthesis, free of recrystallization and giving quantitative yield). These electrolytes are characterized by high oxidation stability (up to 3.4 V vs. Mg), improved electrophile compatibility and electrochemical reversibility (up to 100% coulombic efficiency). Three electrolyte systems (MgCl2–AlCl3, MgCl2–AlPh3, and MgCl2–AlEtCl2) were fully characterized by multinuclear NMR (1H, 27Al{1H} and 25Mg{1H}) spectroscopies and electrochemical analysis. Single crystal X-ray diffraction and NMR studies consistently established molecular structures of the three electrolytes sharing a common Mg2+-dimer mono-cation, [(μ-Cl)3Mg2(THF)6]+, along with an anion (AlCl4−, AlPh3Cl− and AlEtCl3− respectively). Clean and dendrite free Mg bulk plating and viable battery performance were validated through representative studies using the MgCl2–AlEtCl2 electrolyte. The reaction mechanism of MgCl2 and the Al Lewis acids in THF is discussed to highlight the formation of the electrochemically active [(μ-Cl)3Mg2(THF)6]+ dimer mono-cation in these electrolytes and their improved performance compared to reported electrolytes using nucleophilic Mg2+ sources.

Electrochemically stable cathode current collectors for rechargeable magnesium batteries

Rechargeable magnesium (Mg) batteries are attractive energy storage systems that could yield cost-effective energy solutions. Currently, however, no practical cathode current collector that can withstand more than 2.0 V in Mg2+ electrolytes has been identified; this greatly hinders cathode research. Here we identified that molybdenum (Mo) and tungsten (W) are electrochemically stable (>2.8 V) through formation of passive surface layers. The presented results could have a significant impact on the development of high voltage Mg batteries.