Friday, January 10, 2014

Organic mega flow battery promises breakthrough for renewable energy

Cambridge, Mass. – January 8, 2014 – A team of Harvard scientists and engineers has demonstrated a new type of battery that could fundamentally transform the way electricity is stored on the grid, making power from renewable energy sources such as wind and solar far more economical and reliable.
Michael J. Aziz (pictured) and others at Harvard University have developed a metal-free flow battery that relies on the electrochemistry of naturally abundant, small organic molecules to store electricity generated from renewable, intermittent energy sources. 
Utilities would love to be able to store the power that wind farms generate at night—when no one wants it—and use it when demand is high during the day. But conventional battery technology is so expensive that it only makes economic sense to store a few minutes of electricity, enough to smooth out a few fluctuations from gusts of wind.
The novel battery technology is reported in a paper published in Nature on January 9. Under the OPEN 2012 program, the Harvard team received funding from the U.S. Department of Energy’s Advanced Research Projects Agency–Energy (ARPA-E) to develop the innovative grid-scale battery and plans to work with ARPA-E to catalyze further technological and market breakthroughs over the next several years.  
The paper reports a metal-free flow battery that relies on the electrochemistry of naturally abundant, inexpensive, small organic (carbon-based) molecules called quinones, which are similar to molecules that store energy in plants and animals.
The mismatch between the availability of intermittent wind or sunshine and the variability of demand is the biggest obstacle to getting a large fraction of our electricity from renewable sources. A cost-effective means of storing large amounts of electrical energy could solve this problem.
Liquid energy: 
Novel energy storage materials flow from the white containers shown here into a fuel-cell like device in the foreground, where they generate electricity.
Harvard University researchers say they’ve developed a new type of battery that could make it economical to store a couple of days of electricity from wind farms and other sources of power. The new battery, which is described in the journal Nature, is based on an organic molecule—called a quinone—that’s found in plants such as rhubarb and can be cheaply synthesized from crude oil. The molecules could reduce, by two-thirds, the cost of energy storage materials in a type of battery called a flow battery, which is particularly well suited to storing large amounts of energy.

A metal-free organic–inorganic aqueous flow battery
Brian Huskinson, Michael P. Marshak, Changwon Suh, Süleyman Er,Michael R. Gerhardt, Cooper J. Galvin, Xudong Chen,Alán Aspuru-Guzik, Roy G. Gordon & Michael J. Aziz

a, Cell schematic. Discharge mode is shown; the arrows are reversed for electrolytic/charge mode. AQDSH2 refers to the reduced form of AQDS.b, Cell potential versus current density at five different states of charge (SOCs; average of three runs); inset shows the cell open circuit potential versus SOC with best-fit line superimposed (Eeq = (0.00268 × SOC) + 0.670; R2 = 0.998). c, Galvanic power density versus current density for the same SOCs. d, Electrolytic power density versus current density. All data here were collected at 40 °C using a 3 M HBr + 0.5 M Br2 solution on the positive side and a 1 M AQDS + 1 M H2SO4 solution on the negative side.
a, Constant-current cycling at 0.2 A cm−2 at 40 °C using a 2 M HBr + 0.5 M Br2 solution on the positive side and a 0.1 M AQDS + 2 M H2SO4 solution on the negative side; current efficiency is indicated for each complete cycle. b, Constant-current cycling at 0.5 A cm−2 at 40 °C using a 3 M HBr + 0.5 M Br2 solution on the positive side and a 1 M AQDS + 1 M H2SO4 solution on the negative side (same solution used in Fig. 1); discharge capacity retention is indicated for each cycle.
a, Rotating disk electrode (RDE) measurements of AQDS using a glassy carbon electrode in 1 M H2SO4 at 11 rotation rates ranging from 200 r.p.m. (red) to 3,600 r.p.m. (black). b, Koutecký–Levich plot (current−1 versus rotation rate−1/2) derived from a at seven different AQDS reduction overpotentials, η. c, Calculated reduction potentials of AQDS substituted with –OH groups (black), calculated AQDS and DHAQDS values (blue), and experimental values for AQDS and DHAQDS (red squares). d, Cyclic voltammogram of AQDS and DHAQDS (1 mM) in 1 M H2SO4 on a glassy carbon electrode (scan rate = 25 mV s−1).
a, Levich plot (limiting current versus square root of rotation rate ω) of 1 mM AQDS in 1 M H2SO4 (the fitted line has a slope of 4.53(2) µA s1/2 rad−1/2, giving D = 3.8(1) × 10−6 cm2 s−1). Data are an average of three runs; error bars indicate the standard deviation. b, As a but for DHAQDS in 1 M H2SO4 (slope of 3.94(6) µA s1/2 rad−1/2 gives D = 3.19(7) × 10−6 cm2 s−1). c, Koutecky–Levich plot (i−1 versus ω−1/2) of 1 mM DHAQDS in 1 M H2SO4. The current response, i, is shown for seven different AQDS reduction overpotentials η.
Constructed using the current response in the absence of mass transport at low AQDS reduction overpotentials; iK is the current extrapolated from the zero-intercept of Fig. 3b and Extended Data Fig. 2c (infinite rotation rate). Data are an average of three runs; error bars indicate the standard deviation. a, AQDS: best-fit line has the equation y = 62(x + 4.32). This yields α = 0.474(2) and k0 = 7.2(5) × 10−3 cm s−1. b, DHAQDS: best-fit line is the function y = 68(x + 3.95). This yields α = 0.43(1) and k0 = 1.56(5) × 10−2 cm s−1.
Data are fitted to three solid lines indicating slopes of −59 mV pH−1, −30 mV pH−1 and 0 mV pH−1, corresponding to two-, one- and zero-proton processes, respectively. Dashed lines linearly extrapolate the one- and zero-proton processes to give E0 values of 18 mV (2e−/1H+) and −296 mV (2e−/0H+).
a, Spectrum of AQDS: chemical shift δ = 7.99 p.p.m. versus tetramethylsilane (TMS) (doublet (d), coupling constant J = 2 Hz, 1,8 C–H), 7.79 p.p.m. (doublet of doublets, J = 2 and 8 Hz, 4,5 C–H), 7.50 p.p.m. (d, J = 8 Hz, 3,6 C–H). b, The same sample, 20 h after addition of Br2. c, 1H NMR of AQDS treated with 2 M HBr and Br2 and heated to 100 °C for 48 h. The peaks are shifted due to presence of trace HBr, which shifted the residual solvent peak due to increased acidity. Coupling constants for each peak are identical to a.
a, AQDS, δ = 181.50 p.p.m. versus TMS (C 9), 181.30 p.p.m. (C 10), 148.51 p.p.m. (C 2,7), 133.16 p.p.m. (C 11), 132.40 p.p.m. (C 12), 130.86 p.p.m. (C 3,6), 128.59 p.p.m. (C 4,5), 124.72 p.p.m. (C 1,8). b, The same sample, 24 h after addition of Br2. c, 13C NMR of AQDS treated with 2 M HBr and Br2 and heated to 100 °C for 48 h.
This shows a linear relationship (red dashed line; R2 = 0.97) between calculated ΔHf (this work) and experimental E0 (from the literature) of six quinones in aqueous solutions: BQ, benzoquinone; NQ, naphthoquinone; AQ, anthraquinone; and PQ, phenanthraquinone.
Black curve, obtained for a 1 mM solution of AQDS in 1 M H2SO4 on a stationary glassy carbon working electrode. Red curve, obtained for a crude anthraquinone sulphonation solution containing a mixture of AQDS, 9,10-anthraquinone-2,6-disulphonic acid and 9,10-anthraquinone-2-sulphonic acid diluted to 1 mM total anthraquinone in 1 M H2SO4.
Data collected by cycling the current at 0.2 A cm−2 at 40 °C using a 2 M HBr + 0.5 M Br2 solution on the positive side and a 2 M HBr + 0.1 M AQDS solution on the negative side; cell potential versus time performance is comparable to data in Fig. 2.
A metal-free organic–inorganic aqueous flow battery
As the fraction of electricity generation from intermittent renewable sources—such as solar or wind—grows, the ability to store large amounts of electrical energy is of increasing importance. Solid-electrode batteries maintain discharge at peak power for far too short a time to fully regulate wind or solar power output 1, 2. In contrast, flow batteries can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all of the electro-active species in fluid form 3, 4, 5. Wide-scale utilization of flow batteries is, however, limited by the abundance and cost of these materials, particularly those using redox-active metals and precious-metal electrocatalysts 6, 7. Here we describe a class of energy storage materials that exploits the favourable chemical and electrochemical properties of a family of molecules known as quinones. The example we demonstrate is a metal-free flow battery based on the redox chemistry of 9,10-anthraquinone-2,7-disulphonic acid (AQDS). AQDS undergoes extremely rapid and reversible two-electron two-proton reduction on a glassy carbon electrode in sulphuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the Br2/Br− redox couple, yields a peak galvanic power density exceeding 0.6 W cm−2 at 1.3 A cm−2. Cycling of this quinone–bromide flow battery showed >99 per cent storage capacity retention per cycle. The organic anthraquinone species can be synthesized from inexpensive commodity chemicals 8. This organic approach permits tuning of important properties such as the reduction potential and solubility by adding functional groups: for example, we demonstrate that the addition of two hydroxy groups to AQDS increases the open circuit potential of the cell by 11% and we describe a pathway for further increases in cell voltage. The use of π-aromatic redox-active organic molecules instead of redox-active metals represents a new and promising direction for realizing massive electrical energy storage at greatly reduced cost.
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By the end of the three-year development period, Connecticut-based Sustainable Innovations, LLC, a collaborator on the project, expects to deploy demonstration versions of the organic flow battery contained in a unit the size of a horse trailer. The portable, scaled-up storage system could be hooked up to solar panels on the roof of a commercial building, and electricity from the solar panels could either directly supply the needs of the building or go into storage and come out of storage when there’s a need. Sustainable Innovations anticipates playing a key role in the product’s commercialization by leveraging its ultra-low cost electrochemical cell design and system architecture already under development for energy storage applications.

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