exceptional design fl exibility, excellent scalability and modularity, and high energy effi ciency. These technical merits underline RFBs a well-suitable choice to stabilize the power grid and overcome the intermittency of renewable energy sources (e.g., solar, and wind).Aqueous redox fl ow batteries (ARFBs), because of safety operation and high power density, have attracted both governmental and industrial investments for technical development and applications. Currently, vanadium (V) based ARFBs are the most populated systems and have been commercialized by numerous companies. [ 1a,b ] However, high cost and volatile supply of V 2 O 5 raw material lead to high system capital cost and thus limit wide implementation of the vanadium ARFBs. [ 2 ] Current capital cost for V-ARFB is ≈$450/kWh according to a cost analysis [ 3 ] while DOE target cost is below $150/kWh. Therefore, there are increasing efforts to identify new fl ow battery systems with affordable material cost and high electrochemical performance to replace vanadium ARFBs. Hybrid nonaqueous redox fl ow batteries (NRFBS) using Li metal as solid-state anode [ 4 ] and various solution catholytes (e.g., anthraquinone, [ 5 ] ferrocene, [ 6 ] TEMPO, [ 7 ] ployhalides, [ 8 ] and polysulfi des [ 9 ] have emerged. Fullly organic NRFBs were also reported. [ 10 ] These NRFBs, albeit potential high energy densities, encounter a number of challenges for practical applications: safety issues associated the use of highly reactive Li metal and fl ammable organic solvents, low current density due to Li dendrite formation, and limited cycling life.To overcome the cost and sustainable issues and retain safety features and high power density of vanadium ARFBs, redox active quinonoid molecules have been employed in several acidic ARFBs in the last few years. In 2009, Xu et al. fi rst reported the concept of the organic ARFB by adopting 1,2-dihydrobenzoquinone-3,5-disulfonic acid (BQDS) or 1,4-dihydrobenzoquinone-2-sulfonic acid (BQS) as cathode and conventional PbSO 4 as anolyte in an acid ARFB. In 2014, Aziz and co-workers reported an ARFB study using anthraquinon-2,7-disulfonic acid (AQDS) as anolyte and bromine as catholyte. [ 12 ] The AQDS/Br 2 ARFB can be operated at impressively high current densities (>0.5 A cm −2 ), highlighting the possibility of using organic redox active materials to generate high power output. However, the use of Br 2 is concerned with Increasing worldwide energy demands and rising CO 2 emissions have motivated a search for new technologies to take advantage of renewables such as solar and wind energies. Redox fl ow batteries (RFBs) with their high power density, high energy effi ciency, scalability (up to MW and MWh), and safety features are one suitable option for integrating such energy sources and overcoming their intermittency. However, resource limitation and high system costs of current RFB technologies impede wide implementation. Here, a total organic aqueous redox fl ow battery (OARFB) is reported, using low-cost and sustainable met...

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.

The increasing energy needs of society have led to a search for technologies that can tap carbon-neutral and sustainable energy sources, such as solar and wind. Using properly designed catalysts, such sources can also be used to create fuels such as hydrogen; however, a significant barrier to the use of hydrogen as an energy carrier is the need for an inexpensive and efficient catalyst for its oxidation. The oxidation of hydrogen is the process by which electricity is produced in low-temperature fuel cells, and the best catalyst for this is platinum-a precious metal of low abundance. Here we report a molecular complex of iron (an abundant and inexpensive metal) as a rationally designed electrocatalyst for the oxidation of H(2) at room temperature, with turnover frequencies of 0.66-2.0 s(-1) and low overpotentials of 160-220 mV. This iron complex, Cp(C(6)F(5))Fe(P((t)Bu)(2)N(Bn)(2))(H), has pendent amines in the diphosphine ligand that function as proton relays.

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.

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.

This work identified that Mo and W are two electrochemically stable metals for using as current collectors and cell cases for rechargeable magnesium batteries.

A series of iron hydride complexes featuring PRNR′ PR (PRNR′ PR = R2PCH2N(R′)CH2PR2 where R = Ph, R′ = Me; R = Et, R′ = Ph, Bn, Me, t Bu) and cyclopentadienide (CpX = C5H4X where X = H, C5F4N) ligands has been synthesized; characterized by NMR spectroscopy, X-ray diffraction, and cyclic voltammetry; and examined by quantum chemistry calculations. Each compound was tested for the electrocatalytic oxidation of H2, and the most active complex, (CpC5F4N)Fe(PEtNMePEt)(H), exhibited a turnover frequency of 8.6 s–1 at 1 atm of H2 with an overpotential of 0.41 V, as measured at the potential at half of the catalytic current and using N-methylpyrrolidine as the exogenous base to remove protons. Control complexes that do not contain pendant amine groups were also prepared and characterized, but no catalysis was observed. The rate-limiting steps during catalysis are identified through combined experimental and computational studies as the intramolecular deprotonation of the FeIII hydride by the pendant amine and the subsequent deprotonation by an exogenous base.

The [Ni(P R 2 N R′ 2 ) 2 ] 2+ complexes (where P R 2 N R′ 2 is 1,5-R′-3,7-R-1,5-diaza-3,7-diphosphacyclooctane) are fast electrocatalysts for H 2 production and oxidation. Binding of a fifth ligand (CH 3 CN or BF 4 − ) or chair/boat isomerization has the potential to slow catalysis by blocking the addition of H 2 or by incorrectly positioning the pendant amines. We report the structural dynamics of a series of nickel complexes characterized by NMR spectroscopy and theoretical modeling to examine the effects of the fifth ligand for the Ni(II) complexes, including CH 3 CN, BF 4 − , Cl − , and H − , as well as the differences in dynamics between the Ni(II) and Ni(0) oxidation states. A fast exchange process was observed for the [Ni(CH 3 CN)(P R 2 N R′ 2 ) 2 ] 2+ complexes, with rates ranging from 10 4 to 10 7 s −1 depending on the phosphorus and nitrogen substituents on the P R 2 N R′ 2 ligand. This exchange process was identified to occur through a multistep mechanism, which consists of dissociation of the acetonitrile, boat/chair isomerization of each of the four rings (including nitrogen inversion), and reassociation of an acetonitrile on the opposite side of the complex. The rate of the chair/boat inversion was found to be influenced by varying the substituent on the nitrogen atom, but the rate of the overall exchange process is at least an order of magnitude faster than the catalytic rate in acetonitrile, demonstrating that the structural dynamics of the [Ni(CH 3 CN)-(P R 2 N R′ 2 ) 2 ] 2+ complexes do not hinder catalysis. Possible catalytic implications of the coordination of a fifth ligand to the Ni(II) complex are discussed.