Patent Description:
A typical RFB includes a redox flow cell that has a first or positive electrode and a second or negative electrode separated by an ion-conducting separator, such as an ion-exchange membrane. A first or positive fluid electrolyte (sometimes referred to as the posolyte) is delivered to the positive electrode and a second or negative fluid electrolyte (sometimes referred to as the negolyte) is delivered to the negative electrode to drive reversible redox reactions. Upon charging, the electrical energy supplied causes an electrochemical reduction reaction in one electrolyte and an electrochemical oxidation reaction in the other electrolyte. The separator prevents the electrolytes from freely and rapidly mixing but permits selected ions to pass through to balance the redox reactions. Upon discharge, the chemical energy contained in the liquid electrolytes is released in the reverse reactions and electrical energy can be drawn from the electrodes. RFBs are distinguished from other electrochemical energy storage devices by, inter alia, the use of externally-supplied, fluid electrolyte solutions that include reactants that participate in reversible electrochemical reactions.

A prior art redox flow battery system having the features of the preamble to claim <NUM> is disclosed in <CIT>.

From one aspect, the present invention provides a redox flow battery system according to claim <NUM>.

From another aspect, the present invention provides a method for recovering water byproducts in a redox flow battery system according to claim <NUM>.

<FIG> schematically shows portions of an example system <NUM> that includes a redox flow battery <NUM> ("RFB <NUM>") for selectively storing and discharging electrical energy. As an example, the RFB <NUM> can be used to convert electrical energy generated in a renewable energy system to chemical energy that is stored until a later time when there is greater demand, at which time the RFB <NUM> can be used to convert the chemical energy back into electrical energy. The flow battery <NUM> can supply the electric energy to an electric grid, for example.

<FIG> illustrates a non-limiting example of the RFB <NUM>. The RFB <NUM> includes a first electrolyte <NUM> that has at least one electrochemically active species <NUM> that functions in a redox pair with regard to a second electrolyte <NUM> that has at least one electrochemically active species <NUM>. As will be appreciated, the terminology "first" and "second" is to differentiate that there are two architecturally distinct electrolytes. It is to be further understood that terms "first" and "second" are interchangeable in the embodiments herein in that the first electrolyte <NUM> could alternatively be termed as the second electrolyte, and vice versa, or even the same electrolyte, but housed in the opposing tank and reactor volume.

The electrolytes <NUM>, <NUM> are aqueous liquid solutions. For example, the electrochemically active species <NUM>, <NUM> can be based on aqueous solutions of vanadium species. The electrochemically active species <NUM>, <NUM> can include ions of elements that have multiple, reversible oxidation states in a selected liquid solution, such as but not limited to, aqueous solutions or dilute aqueous acids, such as <NUM>-<NUM> sulfuric acid. In some examples, the multiple oxidation states are non-zero oxidation states, such as for transition metals including but not limited to vanadium, iron, manganese, chromium, zinc, molybdenum and combinations thereof, and other elements including but not limited to sulfur, cerium, lead, tin, titanium, germanium and combinations thereof. In some examples, the multiple oxidation states can include the zero oxidation state if the element is readily soluble in the selected liquid solution in the zero oxidation state. Such elements can include the halogens, such as bromine, chlorine, and combinations thereof. The electrochemically active species <NUM>, <NUM> could also be organic molecules or macromolecules that contain groups that undergo electrochemically reversible reactions, such as quinones or nitrogen-containing organics such as quinoxalines or pyrazines. In embodiments, the electrolytes <NUM> and <NUM> are aqueous solutions that include one or more of the electrochemically active species <NUM>, <NUM>. The first electrolyte <NUM> (e.g., the positive electrolyte) and the second electrolyte <NUM> (e.g., the negative electrolyte) are contained in a supply/storage system <NUM> that includes first and second vessels <NUM>, <NUM>.

In one example based on aqueous vanadium electrolyte chemistry with equimolar electrolytes <NUM>, <NUM>, the electrolytes together have an average oxidation state of +<NUM> based upon the use of V<NUM>+/V<NUM>+ and V<NUM>+/V<NUM>+ (which can also be denoted as V(ii)/V(iii) and V(iv)/V(v), although the charge of the vanadium species with oxidation states of <NUM> and <NUM> are not necessarily +<NUM> and +<NUM>) as the electrochemically active species <NUM>, <NUM>. For example, if the electrolyte solution is aqueous sulfuric acid, then the V(iv)/V(v) species of the first electrolyte <NUM> will be present as VO<NUM>+ and VO<NUM>+ and the V(ii)/V(iii) species of the second electrolyte will be present as and V<NUM>+ and V<NUM>+ ions.

The electrolytes <NUM>, <NUM> are circulated by pumps <NUM> to at least one redox flow cell <NUM> of the flow battery <NUM> through respective feed lines <NUM>, and are returned from the cell <NUM> to the vessels <NUM>, <NUM> via return lines <NUM>. As can be appreciated, additional pumps <NUM> can be used if needed, as well as valves (not shown) at the inlets/outlets of the components of the RFB <NUM> to control flow. In this example, the feed lines <NUM> and the return lines <NUM> connect the vessels <NUM>, <NUM> in respective loops L1, L2 with first and second electrodes <NUM>, <NUM>. Multiple cells <NUM> can be provided as a stack within the loops L1, L2.

The cell or cells <NUM> each include the first electrode <NUM>, the second electrode <NUM> spaced apart from the first electrode <NUM>, and an electrolyte separator layer <NUM> arranged between the first electrode <NUM> and the second electrode <NUM>. For example, the electrodes <NUM>, <NUM> are porous electrically-conductive structures, such as carbon paper or felt. The electrodes <NUM>, <NUM> may also contain additional materials which are catalytically-active, for example a metal oxide. In general, the cell or cells <NUM> can include bipolar plates, manifolds and the like for delivering the electrolytes <NUM>, <NUM> through flow field channels to the electrodes <NUM>, <NUM>. It is to be understood, however, that other configurations can be used. For example, the cell or cells <NUM> can alternatively be configured for flow-through operation where the fluid electrolytes <NUM>, <NUM> are pumped directly into the electrodes <NUM>, <NUM> without the use of flow field channels.

The electrolyte separator layer <NUM> can be, but is not limited to, an ionic-exchange membrane, a micro-porous polymer membrane or an electrically insulating microporous matrix of a material, such as silicon carbide (SiC), that prevents the fluid electrolytes <NUM>, <NUM> from freely and rapidly mixing but permits selected ions to pass through to complete the redox reactions while electrically isolating the electrodes <NUM>, <NUM>. In this regard, the loops L1, L2 are isolated from each other during normal operation, such as charge, discharge and shutdown states.

The fluid electrolytes <NUM>, <NUM> are delivered to, and circulate through, the cell or cells <NUM> during an active charge/discharge mode to either convert electrical energy into chemical energy or, in the reverse reaction, convert chemical energy into electrical energy that is discharged. The electrical energy is transmitted to and from the cell or cells <NUM> through an electric circuit <NUM> that is electrically coupled with the electrodes <NUM>, <NUM>.

As known, the electrochemical window of operation for aqueous electrolytes in RFBs is small and is limited by the hydrogen evolution reaction and oxygen evolution reaction. Although RFBs are generally designed to avoid these reaction regimes, the competing balance of operational factors may result in low rates of hydrogen and/or oxygen evolution. Over time with repeated charge/discharge cycles, hydrogen and/or oxygen evolution can lead to electrolyte concentration imbalance, species precipitation, and, therefore, loss of storage capacity. Measures can be taken to detect such conditions with a State-of-Charge cell and implement remedial actions. Such actions, however, may not account for the loss of the hydrogen or oxygen, which may typically be vented from the system. In this regard, as shown in <FIG>, the disclosed system <NUM> includes an electrochemical recovery cell <NUM> that is operable to recover evolved hydrogen and/or oxygen gas and incorporate it back into the electrolyte, thereby reducing electrolyte loss.

As shown in <FIG>, the system <NUM> includes a gas vent passage <NUM> that is connected with the redox flow battery <NUM> and the electrochemical recovery cell <NUM>. The gas vent passage <NUM> receives water byproduct (i.e., hydrogen, oxygen, or both) that evolves from side reactions of one or both of the aqueous electrolytes <NUM>, <NUM>. The system <NUM> further includes a bypass passage <NUM> that is connected with the redox flow battery <NUM> and the electrochemical recovery cell <NUM>. The bypass passage <NUM> receives one of the aqueous electrolytes <NUM>, <NUM>.

The electrochemical recovery cell <NUM> includes a first half-cell 56a that is connected to the gas vent passage <NUM> and a second half-cell 56b that is connected to the bypass passage <NUM>. The first half-cell 56a includes a first electrode 58a, and the second half-cell 56b includes a second electrode 58b. A separator <NUM> is located between the first half-cell 56a and the second half-cell 56b.

Through the gas vent passage <NUM> the electrochemical recovery cell <NUM> receives the water byproduct (gas) as a reactant into the first half-cell 56a, and through the bypass passage <NUM> the electrochemical recovery cell <NUM> receives one of the electrolytes <NUM>, <NUM> as a reactant into the second half-cell 56b. The electrolyte <NUM> or <NUM>, and thus the configuration of which of the electrolytes <NUM>, <NUM> the bypass line <NUM> is connected to, depends on which of hydrogen or oxygen is being recovered.

The reactants spontaneously react (i.e., without being driven by an electrical input) in the electrochemical recovery cell <NUM>, resulting in the generation of water in the second half-cell 56b or the first half-cell 56a that is then fed back into the redox flow battery <NUM>. The loss of the hydrogen or oxygen from the system <NUM> is thereby reduced by reacting the hydrogen or oxygen to produce water and incorporating the water back into the flow battery <NUM>. As an example, for hydrogen recovery, the first (positive) electrolyte <NUM> is used as the reactant in the second half-cell 56b, and for oxygen recovery the second (negative) electrolyte <NUM> is used as the reactant in the second half-cell 56b. Example applicable reactions and approximate voltage potentials are shown below based on an all-vanadium chemistry.

Hydrogen Recovery Reactions:     (first half cell) H<NUM> → <NUM>+ + 2e-; V ≈ <NUM> V vs. RHE (second half cell) 2VO<NUM>+ + <NUM>++ 2e-→ 2VO<NUM>+ + <NUM><NUM>O; V ≈ <NUM> V vs. RHE Overall reaction H<NUM> + <NUM>+ + 2VO<NUM>+ → 2VO<NUM>+ + <NUM><NUM>O; OCV ≈ <NUM> V.

Oxygen Recovery Reactions:     (first half cell) <NUM>/2O<NUM> + <NUM>+ + 2e- → H<NUM>O; V ≈ <NUM> V vs. RHE (second half-cell) 2V<NUM>+ → 2V<NUM>+ + 2e-; V ≈ - <NUM> V vs. RHE Overall reaction: -2V<NUM>+ + <NUM>/2O<NUM> + <NUM>+ → <NUM> V<NUM>+ + H<NUM>O; OCV ≈ <NUM> V.

The electrodes 58a, 58b of the electrochemical recovery cell <NUM> are selected in accordance with the reactants that participate in the reactions in the cell <NUM>. For example, in accordance with the claims, the first electrode 58a has a metal catalyst or a phthalocyanine catalyst material that is capable of catalyzing hydrogen or oxygen, and the second electrode 58b includes a catalyst material that is capable of catalyzing the species in electrolyte <NUM> or <NUM>. The catalyst material of the second electrode 58b excludes any metal catalysts and is carbon paper.

In one example, the first electrode 58a is a metal catalyst and is selected to promote the desired reaction, either the hydrogen oxidation reaction (HOR) or the oxygen reduction reaction (ORR). The catalyst material of the first electrode 58a may be selected based on a balance of performance factors. One of the factors may be the pH of the electrolyte <NUM> or <NUM>. For example, the electrolyte <NUM> or <NUM> may cross over the separator <NUM>, and the first electrode 58a and its catalyst material may thus be exposed to the electrolyte <NUM> or <NUM>. If the catalyst material of the first electrode 58a is not resistant to chemical attack by the electrolyte <NUM> or <NUM>, the catalyst material may degrade over time. In this regard, for acidic electrolytes <NUM> or <NUM>, platinum mono-catalyst may be used for the hydrogen reaction or platinum alloy catalysts for the oxygen reaction. Other platinum group metals or alloys may also be used. In an alternate example, the first electrode 58a uses a rhodium sulfide catalyst, RhxSy, which is a mixture of Rh<NUM>S<NUM> and Rh<NUM>S<NUM>, and is known to promote the HOR and also be tolerant to contamination by species that may cross over from the second half-cell 56b, such as acid or acid ions. For alkaline electrolytes <NUM> or <NUM>, non-platinum group metals, such as but not limited to iron, cobalt, and nickel catalysts, may be used for the hydrogen reaction or silver phthalocyanine or iron phthalocyanine catalysts for the oxygen reaction.

The electrochemical recovery cell <NUM> can also be further adapted for its function for hydrogen or oxygen recovery. For instance, since no electrical input is required, nor is generating electrical output of concern, the electrodes 58a and 58b can be shorted to each other. Furthermore, again since electrical performance is not of concern as in a electrochemical cell for generating or storing electric current, a relatively thick separator <NUM> can be used to reduce electrolyte cross-over. For instance, the separator <NUM> is an ion-exchange membrane (IEM), which may be a perfluorinated IEM, such as perfluorosulfonic-acid (PFSA). Examples of PFSA can include NAFION® or GORE SELECT®. Partially fluorinated IEMs may also be used, such as those based on poly(ethylene-co-tetrafluorethylene) (ETFE) or PVDF. Hydrocarbon IEMs, with suitable oxidative stability may also be used, such as sulfonated poly(aryl ether ketone), sulfonated poly(aryl ether sulfone), sulfonated poly(imide) These IEMs may be either cation-exchange membranes (CEMs) or anion-exchange membranes (AEMs), and the choice will depend on the chemistry of the electrolyte and other performance factors. Further examples of the separator <NUM> may include solid-state ion conductors, especially proton conductors, such as alkaline-earth cerates and zirconate based perovskite materials such as acceptor doped SrCeO<NUM>, BaCeO<NUM> and BaZrO<NUM>. The key performance factor for the separator for this application is the Selectivity, which is herein defined as the ratio of the permeability of the desired charge carrier (i.e., protons and/or hydroxide ions in this case) and the other ions in the electrolyte, which one does not want to transport through the separator. Using a vanadium cation, V4, as an example one can define a dimensionless Selectivity as: <MAT>.

The variables are defined as follows: SV4 is the dimensionless selectivity of protons over vanadyl ions VO<NUM>+, κ is the conductivity in S/m, R is the universal gas constant in J mol-<NUM>·K-<NUM>, T is the absolute temperature in K, F is the Faraday constant in C·mol-<NUM>, PV4 is the permeability of VO<NUM>+ in m<NUM>·s-<NUM>, and CV4 is the concentration of VO<NUM>+ in the solution adjacent to the membrane. The separator should have a dimensionless selectivity of more than <NUM>, and preferably be > <NUM>. The separator should preferably be significantly thicker than those used in flow battery cells, since the ohmic loss of the cell is not a concern. The separator has a thickness of at least <NUM> micrometers, and preferably be in the range of <NUM> to <NUM> micrometers.

<FIG> illustrates another example system <NUM>, which as will be described below is configured for hydrogen recovery. In this disclosure, like reference numerals designate like elements where appropriate and reference numerals with the addition of one-hundred or multiples thereof designate modified elements that are understood to incorporate the same features and benefits of the corresponding elements. In this example, each of the vessels <NUM> and <NUM> include a respective headspace <NUM>. The electrolytes <NUM>, <NUM> do not completely fill the vessels <NUM>, <NUM>. The headspace <NUM> is the region or volume of the vessel <NUM>, <NUM> above the level of the electrolyte <NUM>, <NUM>. The gas vent passage <NUM> is connected with the headspaces <NUM> of the vessels <NUM>, <NUM>.

An inert gas source <NUM> is connected with at least one of the headspaces <NUM>. In this example, the inert gas source <NUM> is connected to the headspace <NUM> of the vessel <NUM> (of the positive electrolyte <NUM>). For instance, the inert gas source <NUM> provides pressurized inert gas, such as but not limited to nitrogen or argon. The inert gas source <NUM> maintains a positive pressure in the headspaces <NUM> and gas vent passage <NUM>, to reduce infiltration of air into the system from the surrounding environment.

The bypass passage <NUM> in this example is connected to the feed line <NUM> of the positive electrolyte <NUM>. Here, the bypass passage <NUM> connects to the feed line <NUM> at a location downstream of the pump <NUM>. The connection may include a valve to control flow of the electrolyte <NUM> into the bypass passage. The flow may additionally or alternatively be controlled by the selected size of the cross-sectional flow area of the bypass passage <NUM>. For example, the cross-section is substantially smaller than that of the feed line <NUM> such that only a relatively small amount of bleed flow exits to the bypass passage <NUM>.

In this example, the system <NUM> also includes a pressure relief valve <NUM> located downstream of the electrochemical recovery cell <NUM> and upstream of the vessel <NUM>. The pressure relief valve <NUM> is operable to release gas from the gas vent passage <NUM> when the pressure in the gas vent passage <NUM> exceeds a threshold pressure of the pressure relief valve <NUM>.

In this example, during operation of the redox flow cell <NUM>, side reactions in the negative electrolyte <NUM> may evolve hydrogen gas. The hydrogen gas migrates to the headspace <NUM> of the vessel <NUM> and is captured by the gas vent passage <NUM> connected to the headspace <NUM>. Although the inert gas source <NUM> maintains a positive pressure in the gas vent passage <NUM> and headspaces <NUM>, there is not a continuous flow of the inert gas through the gas vent passage <NUM>. Rather, as hydrogen evolves, the pressure in the gas vent passage <NUM> and headspace <NUM> increases. Once the pressure increases beyond the threshold pressure of the pressure relief valve <NUM>, the pressure relief valve <NUM> releases gas until the pressure reduces below the threshold. The release causes a transient flow in the gas vent passage <NUM> and headspace <NUM> such that the inert gas and hydrogen pass through the first half-cell 56a of the electrochemical recovery cell <NUM>. The flow of the hydrogen through the electrochemical recovery cell <NUM> thus depends on pressure-release of the pressure relief valve <NUM>.

At the same time, the positive electrolyte <NUM> is either flowing or present in the second half-cell 56b. The hydrogen and electrolyte <NUM> participate in the reactions as described above to thereby produce water in the second half-cell 56b. The bypass passage <NUM> returns to the vessel <NUM> and the water is thus incorporated into the electrolyte <NUM>. In this manner, rather than a loss of hydrogen, the hydrogen is recovered into the system <NUM>. The inert gas and any unreacted hydrogen may be vented from the pressure-relief valve <NUM>. Alternatively, rather than venting, the released gas may be conserved by discharging into the bypass line <NUM> (e.g., using an eductor) prior to entry into the vessel <NUM> or directly into the vessel <NUM>. The electrochemical recovery cell <NUM> can be simply shorted electrically, or the voltage can be controlled to a desired value using a potentiostat <NUM>.

The control and response methodology of the electrochemical recovery cell <NUM> may use a fixed potential approach. In a fixed potential approach, the voltage applied between the electrodes 58a and 58b is held constant at a value below the cell voltage defined by the intended reaction (e.g. <NUM> V for the hydrogen recovery reaction paired with VO<NUM>+/VO<NUM>+). This constant voltage must be applied by electrically connecting the electrodes <NUM>, <NUM> with a fixed-voltage device capable of accepting the current generated by the recovery cell.

As will be appreciated, the system <NUM> can alternatively be modified for oxygen recovery due to oxygen evolution from the positive electrolyte <NUM>. For instance, for oxygen recovery, the bypass line <NUM> is connected off of the feed line <NUM> from the vessel <NUM> that contains the negative electrolyte <NUM> and the bypass line <NUM> returns to the vessel <NUM>. In either case, for hydrogen or oxygen recovery, the resulting water is returned to the opposite electrolyte <NUM>, <NUM> from which the recovered hydrogen or oxygen evolved. However, water is readily transported through the separator <NUM> of the redox flow battery <NUM> and thereby redistributed between the electrolytes <NUM>, <NUM>. Preferably, the pressure of the gas in the electrochemical recovery cell <NUM> should be higher than the pressure of the liquid to help promote the recovery of the liquid water. The system may optionally include a water trap, or liquid-gas separator device <NUM>, between the electrochemical recovery cell <NUM> and the pressure-relief valve <NUM> to help enhance the recovery of the water that may be contained in the gas exiting the cell.

In the example in <FIG>, the gas vent passage <NUM> connects the headspaces <NUM> of both vessels <NUM>, <NUM>. The gas vent passage <NUM> thereby captures evolved hydrogen (or oxygen) from both electrolytes <NUM>, <NUM>, even though only one of hydrogen or oxygen is recovered in that configuration. <FIG> illustrates a further example system <NUM> in which the gas vent passage <NUM> connects to only one of the headspaces <NUM>. Practically, hydrogen evolution primarily occurs in the negative electrolyte <NUM> due to overpotentials in the negative electrode <NUM>. In this regard, the gas vent passage <NUM> is connected only to the headspace <NUM> of the vessel <NUM> of the negative electrolyte <NUM>. Likewise, the inert gas source <NUM> is connected to the vessel <NUM> to maintain the positive pressure in the headspace <NUM> of the vessel <NUM> and the gas vent passage <NUM>. A separate inert gas source and venting system may be used for the headspace <NUM> of the vessel <NUM>, to serve as a cover gas to limit undesired side reactions of the electrolyte <NUM> in the vessel <NUM>. Alternatively, for oxygen recovery, oxygen evolution primarily occurs in the positive electrolyte <NUM> due to overpotentials in the positive electrode <NUM> and the gas vent passage <NUM> would be connected only to the headspace <NUM> of the vessel <NUM>.

<FIG> illustrates another example system <NUM> that is configured for both hydrogen and oxygen recovery. In this example, the system <NUM> includes two gas vent passages 352a, 352b, two electrochemical recovery cells <NUM>, and two bypass passages 354a, 354b. The gas vent passage 352a is connected to the headspace of the vessel <NUM> and one of the electrochemical recovery cells <NUM>, and the gas vent passage 352b is connected to the headspace <NUM> of the vessel <NUM> and the other of the electrochemical recovery cells <NUM>. The bypass passage 354a is connected to the feed line <NUM> from the vessel <NUM> of the positive electrolyte <NUM>, and the bypass passage 354b is connected to the feed line <NUM> from the vessel <NUM> of the negative electrolyte <NUM>.

For hydrogen evolution in the negative electrolyte <NUM>, the hydrogen is captured in the gas vent passage 352a and reacted in the electrochemical recovery cell <NUM> with the positive electrolyte <NUM> from the bypass passage 354a. For oxygen evolution in the positive electrolyte <NUM>, the oxygen is separately captured in the gas vent passage 352b and reacted in the other electrochemical recovery cell <NUM> with the negative electrolyte <NUM> from the bypass passage 354b. In this manner, both hydrogen and oxygen are recovered.

It is to be appreciated that the description above also contemplates a method for recovering water byproducts, such as hydrogen or oxygen, in a redox flow battery system. Such a method may include operating the redox flow battery <NUM>, where the electrolytes <NUM>, <NUM> generate water byproduct from side reactions. The water byproducts are captured in the gas vent passage <NUM>, <NUM>, <NUM>, 352a, 352b that is connected with the redox flow battery <NUM>. The water byproducts are then recovered by passing the water byproducts through the electrochemical recovery cell <NUM>, which reacts the water byproducts to produce water that is incorporated back into one of the electrolytes <NUM>, <NUM>.

Claim 1:
A redox flow battery system comprising:
a redox flow battery (<NUM>) including
a redox flow cell (<NUM>), and
a supply/storage system (<NUM>) external of the redox flow cell (<NUM>), the supply/storage system (<NUM>) including first and second electrolytes (<NUM>, <NUM>) for circulation through the redox flow cell (<NUM>), the first and second electrolytes (<NUM>, <NUM>) being aqueous liquid electrolytes having electrochemically active species (<NUM>, <NUM>) with multiple, reversible oxidation states;
a gas vent passage (<NUM>; <NUM>; <NUM>; 352a) connected with the redox flow battery (<NUM>) to receive water byproduct that evolves from side reaction of the first electrolyte;
a bypass passage (<NUM>; <NUM>; 354a) connected with the supply/storage system (<NUM>) to receive the second electrolyte; and
an electrochemical recovery cell (<NUM>) including a first half-cell (56a) connected to the gas vent passage (<NUM>; <NUM>; <NUM>; 352a) to receive as a reactant the water byproduct and a second half-cell (56b) connected to the bypass passage (<NUM>; <NUM>; 354a) to receive as a reactant the second electrolyte; characterised in that
the first half-cell (56a) includes a first electrode (58a) that has a metal catalyst or a phthalocyanine catalyst and the second half-cell (56b) includes a second electrode (58b) that is carbon paper and excludes any metal catalyst; and in that
the metal catalyst is selected from the group consisting of platinum group metals, nickel, iron, cobalt, and combinations thereof, or includes rhodium sulfide, RhxSy or the phthalocyanine catalyst is selected from the group consisting of silver phthalocyanine, iron phthalocyanine, and combinations thereof.