Patent ID: 12191545

FIGURES AND EXAMPLES

FIGS.1and2depict redox flow batteries according to the present disclosure. The operation of the batteries in bothFIGS.1and2is similar and the same reference numerals are adopted to describe the function of components performing the same function, below. Differences between the function of the two batteries are discussed below.

In the power delivery mode, the liquid catholyte containing a power delivery/energy storage species is pumped by a pump (11) from a chamber of a catholyte storage container (12A), through a conduit (12B) and into the catholyte chamber (9), where it is reduced at a cathode (2) according to the half reaction:

Mnn+1+e-⇌Mnn

The catholyte containing the spent electrolyte species is then carried away from the catholyte chamber through a second conduit (1) to the catholyte storage container (12A), where it is stored in a chamber separate from the fresh catholyte chamber.

The anode and at least part of the anolyte chamber (8) are formed by a porous gas flow electrode (4) and hydrogen is supplied from a pressurised gas source vessel (7) through a conduit (13), to the anode/anode chamber (8), where the hydrogen is oxidised to protons (H+) according to the half reaction:

H2⇌2⁢H++2⁢e-

and the current is collected by a current collector (also labelled4). A proton exchange membrane (3) separates the anolyte and catholyte chambers (8&9) and selectively passes the protons from the anolyte to the catholyte side of the membrane (3) to balance the charge, thereby completing the electrical circuit. Any unreacted hydrogen is carried away from the anolyte chamber (8) by a second conduit (5) and returned to the pressurised gas source vessel (7) via compressor (6).

In the energy storage mode, the system is reversed so that the power delivery/energy storage species Xnis pumped from the catholyte storage container (12A), through the conduit (1) to the catholyte chamber (9), where the spent electrolyte species Xnis oxidised at the cathode (2) to form the redox active species Xn+2. The resulting regenerated electrolyte is transferred away from the catholyte container (9) by the pump (11), through the second conduit (12B) to the catholyte storage container (12A). Meanwhile, protons at the anolyte side of the proton exchange membrane (3) are catalytically reduced at the porous gas anode (4) to hydrogen gas; the hydrogen is transferred away from the porous anode (4) through the conduit (5) and compressed by the compressor (6) before being stored in the pressurised gas source vessel (7).

It will be appreciated that the above system is illustrated with a power delivery/energy storage species that undergoes a two-electron reduction (Xn+2+2e−→Xn). However, the power delivery/energy storage species could be one which undergoes a single-electron reduction). Moreover, although the discussion above is formulated in the context of a manganese power delivery/energy storage species, it will be appreciated that the procedure is analogous for a flow cell employing a vanadium power delivery/energy storage species and electrolyte comprising same.

During power delivery mode, MnO2builds up over time as described herein. The redox flow battery can then be operated in a precipitate removal mode to remove the oxide build-up.

The RFB fixture is purchased from Scribner Associates. The cell comprises two POCO graphite bipolar plates with a machined flow field in contact with gold-plated copper current collectors that are held together utilizing anodized aluminum end plates. Commercially available 0.32 mm thick untreated carbon paper (SGL group, Germany, Sigracet SGL 10AA, typically 3 layers) or 4.6 mm thick untreated graphite felt (SGL group, Germany, Sigracell GFD4,6 EA) was used as the positive electrode. The hydrogen negative electrode was obtained from Fuel Cell Store, 0.4 mgPt cm-2 loading on Carbon Paper or 0.03 mgPt cm−2 loading on Carbon Cloth). The membrane was Nafion 212 (nominal thickness 52 μm). A peristaltic pump (for example, Masterflex easy-load, Cole-Palmer) and a platinum-cured silicone tubing (L/S 14, 25 ft) (for example, Masterflex platinum-cured silicone tubing) were used to pump the manganese electrolyte through the cell at flow rate of 25-100 mL min−1. Hydrogen was provided by a fuel cell test station (850e, Scribner Associates), passing through the negative side at a flow rate of 35-150 mL min−1. Due to the current range, polarization curves were recorded using a fuel cell test station (850e, Scribner Associates) whereas galvanostatic charge and charge experiments were conducted with a Gamry potentiostat 3000.

In-Situ Generation of Redox Active Species

In the first embodiment shown inFIG.1, precipitate removal is achieved with generation of redox active precipitate removal species in-situ in the catholyte chamber (9).

This embodiment employs a catholyte comprising manganese and Ti4+species in sulphuric acid solution. The manganese species functions as the power delivery/energy storage species while the titanium species functions as the precursor species for conversion into a redox active species.

The catholyte was prepared by initially adding sulphuric acid to a solution of Ti(SO4)2or TiOSO4. A corresponding amount of MnCO3or MnSO4was then slowly added. Effervescence of CO2was observed as a result, facilitating metal solubility.

The catholyte was exposed to a cell voltage between 0 and 0.1 V to effect generation of Ti3+redox active precipitate removal species from the Ti4+precursor species. Reduction of the precursor species was achieved with no power input.

Precipitate removal mode involves reducing the oxide precipitate with the Ti3+redox active precipitate removal species.

The redox active precipitate removal species/oxide precipitate reduction reaction may proceed as follows:

2⁢Ti(III)+Mn(IV)⇌2⁢Ti(IV)+Mn(II)

Mn(II), such as Mn2+, is soluble in aqueous electrolyte and hence the reduction reaction solubilises the precipitate.

After precipitate removal, the catholyte was exposed to a cell voltage between 0 and 0.1 V again to re-generate Ti3+redox active precipitate removal species for further precipitate removal, as required.

Independent Generation of Redox Active Species

In the second embodiment shown inFIG.2, the redox flow battery comprises an independent electrochemical stack (14) and conduits (15) fluidly connecting the electrochemical stack (14) to the catholyte chamber (9). The electrochemical stack (14) includes a liquid catholyte chamber with associated cathode and a gaseous (hydrogen) anode chamber and associated anode (not labelled or illustrated). The function of these components is similar to that described above and will not be explained in detail.

The electrochemical stack comprises Ti4+redox active precipitate removal species in the liquid catholyte side thereof. Spent catholyte from the catholyte chamber (9) is pumped to the electrochemical stack (14) and is mixed with the Ti4+species. Energy input to the electrochemical stack (14) produces Ti3+and O2according to:

Gas⁢⁢side⁢:⁢⁢H2⁢O⇌O2+4⁢H++4⁢e-E0=1.23⁢⁢VMn⁢⁢containing⁢⁢side⁢:⁢⁢Ti(IV)+e-⇌Ti(III)E0=0.1⁢

The gas side reaction used an IrO2metal catalyst and runs at a stack cell voltage of 1.6-1.7 V.

Once produced, the catholyte containing redox active Ti3+species was pumped back to the catholyte chamber (9) and oxide precipitate was reduced to effect removal thereof, in a similar manner to that described in the first embodiment.

Capacity Loss

Capacity loss and amount of precipitate (e.g. MnO2) which is produced can be calculated by comparison of discharge time (RFB Capacity) during the first cycle with discharge time of subsequent cycles, as follows (e.g. with reference toFIG.3):

Capacity⁢⁢(A×s)=current⁢⁢(A)×time⁢⁢(s)Capacity⁢⁢cycle⁢⁢1-capacity⁢⁢cycle⁢⁢2=capacity⁢⁢Loss⁢⁢(As=Coulomb)Capacity⁢⁢loss⁢⁢(C)⁢/⁢faraday⁢⁢constant⁢⁢(C⁢⁢mol-1)=mol.⁢of⁢⁢electronMol.⁢e-×0.5⁢⁢mol⁢⁢MnO2⁢⁢formation=mol.⁢MnO2⁢⁢produced

In general terms, cycles 1 and 2 may not necessarily be consecutive cycles.

Precipitate removal is based on operation of the system below 0.1V until the charge measured (which is associated to Ti(III) production) is equal to the capacity loss calculated above.

Example 1

A 5 cm2cell, using graphite felt with thickness of 4.6 mm as its liquid electrode, standard hydrogen electrode with Pt loading of 0.4 mg/cm2and 30% PTFE as gas half-cell, and Nafion 117 as proton exchange membrane was tested initially following the conditions below:1. Electrolyte with 1M Mn and 1M Ti in 5M H2SO4was used.2. Electrolyte was supplied at 50 ml/min throughout the whole experiment.3. Hydrogen gas (99.99% purity) was supplied at the rate of 100 ml/min.

The protocol was used to carry out the following experiments:1. The cell was galvanostatic charged and discharged at 100 mA/cm2for 10 cycles where its performance evaluation indexes (Energy efficiency (EE), Voltage efficiency (VE) and Coulombic efficiency (CE)) was calculated (shown inFIG.4(a)).2. The cell was charged at constant voltage of 1.8V until current density dropped to 10 mA/cm2(shown inFIG.4(b)).3. A discharge cycle was attempted at 100 mA/cm2, however the cell immediately reached cut off voltage (0.65) which indicates that all the Mn3+active species have precipitated by producing MnO2(Mn4+).4. To regenerate the electrolyte and remove the precipitate, Ti4+was reduced to Ti3+. In order to achieve this electrochemical reaction, cell was potentiostaticly discharged at constant potential of 0.1V, until the current density dropped to 10 mA/cm2(shown inFIG.4(c)).5. After regenerating the electrolyte and removing the precipitate, similar testing to step1was carried out and results are reported (shown inFIG.4(d)).