WO2022023770A1

WO2022023770A1 - Fuel cell

Info

Publication number: WO2022023770A1
Application number: PCT/GB2021/051980
Authority: WO
Inventors: Javier Rubio GARCIA; Anthony Kucernak; Andres Parra PUERTO
Original assignee: Imperial College Innovations Limited
Priority date: 2020-07-31
Filing date: 2021-07-30
Publication date: 2022-02-03
Also published as: GB202011937D0

Abstract

Provided herein is a method of operating a fuel cell. The method comprises providing a catholyte comprising permanganate (VII) or manganate (VI) or manganese (IV) species to a cathode compartment of the fuel cell. The method further comprises providing an Mn2+- formation promotor to the catholyte. The method further comprises discharging the fuel cell.

Description

FUEL CELL

Technical field

The present disclosure relates to fuel cells. The disclosure relates more particularly, but not necessarily exclusively, to methods of operating fuel cells to improve performance and supress deposits thereon.

Background

A fuel cell is an electrochemical apparatus used for power delivery by means of a chemical redox reaction involving a fuel and an oxidant. Usually the oxidant is oxygen, but other suitable oxidants are also possible. Polymer electrolyte fuel cells are fuel cells utilising proton or hydroxide conducting membranes as electrolyte. These fuel cells typically require platinum group metals (e.g. Pt, Ru) as catalysts for the oxidation and reduction half-reactions occurring within the fuel cell. Manganese-based oxidants have been contemplated for use in fuel cells and batteries due to high oxidation states for certain manganese species. A benefit of these oxidants is that they do not require platinum group metals to catalyse their reaction. However, in practice, by-products of redox reactions in manganese-based systems can quickly build up on electrode surfaces and inhibit performance. As these intermediates are at a higher oxidation state than the preferred product species (Mn(ll)), production of these intermediate species also represent a loss of energy that the system can produce. Thus production of intermediate insoluble materials can (a) foul the electrode and flow channels and prevent good operation of the system by creating blockages; and (b) reduce the achievable energy output for the system.

US5549991 A depicts an aluminium permanganate battery which operates with an alkaline electrolyte and achieves high energy density. However, as described in that patent, the permanganate is reduced to MnO₂. Thus, only 3/5 (60%) of the maximum charge of the permanganate oxidiser is realised compared to reduction to Mn(ll) and a precipitate build-up is expected. Summary

According to a first aspect of the present application, there is provided a method of operating a fuel cell, comprising: providing a catholyte comprising permanganate (VII) or manganate (VI) or manganese (IV) species to a cathode compartment of the fuel cell, providing an Mn²⁺-formation promotor to the catholyte, and discharging the fuel cell.

According to a second aspect of the present application, there is provided a fuel cell, comprising: a cathode, in a cathode compartment comprising a permanganate (VII) or manganate (VI) or manganese (IV) catholyte; wherein:

(a) the catholyte further comprises a Mn²⁺-formation promotor; and/or

(b) the catholyte further comprises a precursor to the Mn²⁺-formation promotor ; and/or

(c) the fuel cell further comprises a supply fluidly connected to the cathode compartment for delivering fluid thereto, the supply comprising an Mn²⁺- formation promotor and/or a precursor to that Mn²⁺-formation promotor .

According to a third aspect of the present application, there is provided a catholyte for use in a fuel cell, comprising: a permanganate (VII) or manganate (VI) or manganese (IV) species; and an Mn²⁺-formation promotor and/or an precursor to the Mn²⁺-formation promotor.

According to a fourth aspect of the present application, there Is provided a use of a catholyte in a fuel cell, wherein the catholyte comprises: a permanganate (VII) or manganate (VI) or manganese (IV) species; and an Mn²⁺-formation promotor .

According to a fifth aspect of the present application, there is provided a kit of parts comprising a catholyte and an anolyte, wherein: the catholyte comprises: a permanganate (VII) or manganate (VI) or manganese (IV) species; and an Mn²⁺ formation promotor; and the anolyte comprises: (a) selected from a carboxylic acid, an alcohol or a carbohydrate (such as a sugar) or a chemical hydride, for example wherein the anolyte comprises formic acid, methanol, ethanol, isopropanol, glucose, ammonia, hydrazine, borohydride, glycerol or glucose;

(b) hydrogen (such as a bimolecular hydrogen-rich anolyte, such as hydrogen gas, reformate or hydrogen derived from biological or other sources); or

(c) a metal selected from zinc, aluminium, calcium, iron and silicon.

Definitions

In accordance with standard terminology in the field of fuel cells, the term "anode” refers to an electrode in the half cell that contains the oxidation reaction (i.e. the anode compartment) and "cathode" refers to an electrode in the half cell that contains the reduction reaction (i.e. the cathode compartment).

The terms “anolyte" and “catholyte" are used to denote the fluid in contact with the anode and cathode respectively.

A fuel cell as described herein comprises an electrochemical cell for the conversion of chemical energy into electricity. A fuel cell comprises an anode compartment comprising an anode and an anode fluid (i.e, a gas or liquid) and the cathode compartment comprising the cathode and the catholyte liquid comprising said permanganate (VII) or manganate (VI) or manganese (IV) species. A selective membrane or porous separator may be provided between the two compartments, configured to exchange ions (e.g. protons) between the two compartments.

The compartments of electrolyte fluid (catholyte and anolyte) may be charged separately with electrolytes that are able to undergo reduction-oxidation reactions. The redox reactions cause a net flow of electrons between the compartments, thus generating an electrical current.

As used herein, the expression " Mn²⁺-formation promotor” denotes a chemical species that is able to inhibit formation and precipitation of intermediary MnO₂ (or other insoluble manganese compounds) during reduction of manganese species and to allow reduction of high oxidation state manganese species to Mn²⁺ Insoluble manganese oxide precipitate species, such as MnO₂. MnO, M₂O₃ and Mn₃O₄, which would otherwise be deposited during discharge of the fuel cell in the absence of said promotor, are supressed (as described below).

Manganese electrochemistry involves a variety of species, as follows (Source: Bard, A. J.; Parsons, R.; Jordan, J., Standard Potentials in Aqueous Solution., International Union of Pure and Applied Chemistry, 1985):

+7 +6 +5 +4 +3 +2 0

With reference to permanganate (VII) reduction, the electrochemical reaction of MnO₄ ^_to Mn²⁺ involves 5 electrons, typically occurring through Mn³⁺ or MnO₂ as an intermediate. Under normal conditions in acid, MnO₂ typically forms as a major by-product. MnO₂ Is insoluble in aqueous media and will tend to precipitate. This precipitate can cover the electrode surface, and/or clog the liquid flow channels used for readant distribution blocking the access of manganese electrolyte species and therefore reducing performance over time. Similar Issues occur with MnO, M₂O₃ and Mn₃O₄ oxides. MnO₂ is also lost from further reaction meaning that some of the oxidising capacity of the MnO₄ is lost (l.e. we recover only three of the possible five electrons). This reduces the performance of the system, increases the weight of reactant material required, and necessitates cleanlng or replacement of the electrodes. The Mn²⁺-formation promotor is able to supress this, preserving performance and enabling complete redution of MnO₄- to Mn²⁺. Likewise, Mn (VI) species can be reduced to Mn²⁺ with suppression of MnO₂ and the other oxides. Similarly, Mn(IV) species may be mobilised and allow reduction to Mn²⁺ and the other oxides.

As used herein, the term “oxophilicity” refers to the tendency of certain chemical compounds to form oxides by abstraction of oxygen. Oxophilicity is suitably measured as a function of Lewis acidity as measures utilising methods such as Gutmann-Beckett method (Polymer, 37 (1996), pp. 4629-4631, Monatsh. Chem., 106 (1975), pp. 1235-1257) According to Inorg. Chem. 2016, 55, 9461 -9470 (the entire contents of which are hereby incorporated by reference), Manganese oxophiiicity is 0.4 and promotors with oxophilicities above this would be suitable.

As used herein, the "precursor" to the Mn²⁺-formation promotor denotes a chemical species that is able to undergo an electrochemical or chemical reaction to generate the Mn²⁺- formation promotor. For example, the precursor may be reduced to generate the Mn²⁺- formation promotor.

The term "consists essentially of as applied to a designated component Is used herein to denote that one or more specific further components can be present, as long as those further components do not materially affect the essential characteristics of the designated component.

As applied to a cathode, for example, (and particularly with reference to a cathode consisting essentially of carbon, e.g. not comprising platinum and/or ruthenium, or not comprising metal) It will be appreciated that the essential characteristics of that cathode is to permit electrochemical reactions to take place. Platinum, ruthenium and other metals are often used to catalyse electrochemical reactions in fuel cells and, in this context, the term "wherein the cathode consists essentially of carbon" may be understood to mean that the cathode does not Include components (such as the aforementioned metals) which enable catalytic behaviour for such electrochemical reactions.

Suitably, the term "consists essentially of carbon" may be interpreted such that the designated component primarily comprises carbon (i.e. there is a majority of carbon). Suitably, such a designated component comprises greater than or equal to about 85% of carbon, more suitably greater than or equal to about 90%, more suitably greater than or equal to about 95%, most suitably greater than or equal to about 99% of carbon.

Detailed description

According to a first aspect of the present application, there is provided a method of operating a fuel cell, comprising: providing a catholyte comprising permanganate (VII) or manganate (VI) or manganese (IV) species to a cathode compartment of the fuel cell, providing an Mn²⁺-formation promotor to the catholyte, and discharging the fuel cell. It has surprisingly been found that performance of such fuel cells can be improved, and buildup of MnO₂ and/or other oxides on electrode surfaces during discharge of the fuel cell can be suppressed, by provision of a Mn²⁺-formation promotor in the catholyte comprising permanganate (VII) or manganate (VI) or manganese (IV) species. This preserves performance (e.g. keeping current relatively constant over time) as compared with a fuel cell which does not include such a promotor. Moreover, reduction of the Mn species in the presence of the promotor is able to generate manganese species having a lower oxidation state, giving rise to greater electron generation and thereby greater power generation overall.

Furthermore, such a system enables the effective use of permanganate (VII) or manganate (VI) or manganese (IV) catholytes in fuel cells, whereas previously these may have been overlooked. Such catholytes are useful for a variety of reasons. For example, permanganate and manganate species have high diffusivity and are thereby able to engage in fast redox reaction kinetics. The thermodynamics of permanganate (VII) or manganate (VI) or manganese (IV) reduction are superior than oxygen reduction, leading to a higher theoretical cell potential. The fuel cells of the present application may permit higher voltages, energy densities and/or power as compared with existing systems under comparable conditions.

Furthermore, the fuel cell does not depend on oxygen (e.g. oxygen gas) catholytes, permitting application of the fuel cell in areas where little or poor oxygen is available (e.g. undersea, space, high altitude, in mines, etc.). Moreover, the system can avoid the use of compressed gas tanks (in some implementations).

Yet further, fuel cells of the present application may adopt thinner ion conducting membranes separating the anode and cathode compartments.

The concentration of permanganate (VII) or manganate (VI) or manganese (IV) species may be at least about 10 mM, such as at least about 0.05M, preferably at least about 0.1 M. The concentration between about 0.1 M to 7.3M is preferred.

The permanganate (VII) or manganate (VI) or manganese (IV) species may comprise any suitable counter-ion, such as potassium, sodium, magnesium, calcium, or lithium. Preferably the permanganate (VII) or manganate (VI) or manganese (IV) species may be sodium or potassium permanganate (VII), sodium or potassium manganate (VI) or sodium or potassium manganate (IV). In some Implementations, the catholyte Is an aqueous catholyte.

The catholyte may be a permanganate (VII) catholyte. In the context of a permanganate system, the reaction in the cathode compartment may be as follows:

It will therefore be appreciated that a proton source may be required for fuel cells employing permanganate (VII) or manganate (VI) catholyte species. Accordingly, the catholyte may comprise a proton source (e.g. an acid, such as a Brensted acid).

Acidic catholytes are well known In the art and any standard acidle catholyte may be used In accordance with the present disclosure. Suitable acids Include sulphuric acid, which may be an aqueous solution of concentrated sulphuric acid, methanesulfbnlc acid (MSA) or trifluoromethaneaulfonte acid (TFSA), phosphoric, pyrophosphate, hydrochloric, nitric, perchloric, periodic, boric or mixtures thereof. Preferably, the proton source to sulphuric acid.

The pH of the catholyte may be about -2 to 2, preferably -1 to 1.

The acid may be provided at an amount relative to the permangenate (VII) or manganate (VI) or manganese (IV) species In the cathotyta, such as a stoichiometric excess of acid relative to the permanganate (VII) or manganate (VI) or manganese (IV) species. The Ideal ratio of acid depends on the composition and reaction occurring on the anode of the ftiel cell. For example, the acid may be present at a molar concentration of about 1 :1 to 6:1 acid to permanganate (VII) or manganate (VI) or manganese (IV). The Ideal ratio depends on the Mn species used and the reaction occurring on the ftiel cell anode. For example, the 1:1 to 6:1 ration to suitable where the anode reactant to hydrogen or methanol, but for other Implementations (such as the permanganate example using hydrogen or methanol as a fuel) a motor concentration may be about 2:1 to 6:1 permanganate (VII) or manganate (VI) or manganese (IV). The catholyte may, for example, comprise about 0.3 M permanganate (VII) and about 1 M H₂SO₄. Where the anolyte comprises a metal (such as zinc, aluminium, calclum, Iron and silicon), the ratio may be greater than 1:1 to 6:1 acid to permanganate (VII) or manganate (VI) or manganese (IV), preferably greater than about 7:1 , preferably In the range about 8:1 to 10:1. The electrolyte may comprise zinc species, aluminium species, calcium species, iron species, silicon species or a combination thereof, preferably zinc species, aluminium species, or a combination thereof.

The Mn²⁺-formation promoter may have an oxophilicity greater than 0.4, such as about between about 0.6 and 1.0, preferably wherein the species has an oxophilicity of about 1.0.

The Mn²⁺ promotor may comprise a metal, semi-metal or non-metallic species. The metal species may comprise titanium, iron, tin, niobium, tungsten, aluminium or silicon. Other species may be Phosphorus. Preferably, the metal is titanium.

The Mn²⁺-formation promotor may comprise Ti⁴⁺, Al³⁺ or Si⁴⁺, P⁵⁺, W⁶⁺, Nb⁶⁺ and/or combinations thereof; preferably Ti⁴⁺ .

The promotor may be present as an oxide or oxy-anion, for instance pyrophosphate (P₂O₇ ^4-), perchlorate, acetate or oxalate. The promotor may alternatively be any of the species listed on page 9463 of Inorg. Chem. 2016, 55, 9461-9470 with a value above 0.4 (Mn), the entire contents of which are hereby incorporated by reference for the limited purpose of these species. Suitable species comprise carbon, boron, titanium, lanthanum, neodynium, cerium, ytterbium, aluminium, silicon, scandium, vanadium, yttrium, zirconium, niobium, tantalum, tungsten, magnesium, barium, sulphur, phosphorus. Preferred species comprise oxo complexes or coordinating carbon groups and which are soluble (e.g. in water), for example, carbon, boron, titanium, cerium, vanadium, yttrium, tungsten, magnesium, barium, sulphur and phosphorus.

The Mn²⁺-formation promotor may comprise sulphate (SO4² ). The Mn²⁺-formation promotor may be Ti(SO₄)₂.

The method may further comprise providing an anolyte selected from a carboxylic acid, an alcohol or a carbohydrate (such as a sugar). The anolyte may comprise formic acid, alcohols (such as methanol, ethanol, isopropanoi), glucose, ammonia, hydrazine, borohydride, glycerol or glucose.

The method may further comprise providing an anolyte in an anode compartment of the fuel cell, the anolyte comprising hydrogen (such as a bimolecular hydrogen-rich anolyte, such as hydrogen gas, reformate or hydrogen derived from biological or other sources). Hydrogen may be stored externally to the anode compartment, in a container, which may be a pressurised gas source vessel. The hydrogen gas may be supplied to the anode compartment by one or more conduits.

The method may further comprise electrochemically or chemically generating said Mn²⁺- formation promotor from a precursor. The precursor may comprise Ti³⁺, Fe²⁺, Sn²⁺, Al²⁺ or Si⁴⁺, preferably Ti³⁺. The precursor may comprise Ti₂(SO₄)₃ . The precursor may comprise SO₄ ²

Electrochemically generation of the Mn²⁺-formation promotor may occur in an electrochemical cell separate from, and fluidly connected to, said cathode compartment. Alternatively or additionally, electrochemically generating may occur in the cathode compartment of the fuel cell. In this way, the precursor species is converted to the Mn²⁺- formation promotor species in sitir, that is, in the same compartment as the catholyte. Such a configuration provides a compact and efficient design which requires no further modification to existing systems, save to Include a catholyte comprising the precursor species. In situ set up also means that the method can be controlled so as to produce an adequate amount of Mn²⁺-formation promotor depending on the extent of precipitate build up in the catholyte compartment.

Electrochemically generating may be conducted at or below a voltage sufficient to oxidise the precursor to the Mn²⁺-formation promotor. Electrochemically generating may be above a voltage at which reduction of the precursor occurs.

Electrochemically generating may be at or below about -0.3 V, optionally below about -0.4 V; optionally below about -0.5 V; optionally below about -0,6 V.

Electrochemically generating may be above about -0.7 V, optionally above about -0.6 V; optionally above about -0.5 V; optionally above about -0.4 V, optionally said electrochemically generating is conducted at about -0.3 V.

Examples of suitable electrodes (for the anode or cathode) are well known in the art. Catalysed porous carbon electrodes are suitable for use in the present disclosure, fry example catalysed carbon paper, cloth, felt or composite. The carbon may be graphitic, amorphous, or have glassy structure. The anode may be a catalysed electrode and the cathode may be a non-catalysed electrode.

The cathode may comprise carbon. In some instances, the cathode may comprise carbon or be coated with a catalyst comprising gold, platinum, palladium, Iridium, ruthenium, rhenium, rhodium or osmium. In some instances, the cathode comprises or is coated with platinum and/or ruthenium. In some Instances, the cathode consists essentially of carbon. It has been unexpectedly found that carbon- based electrodes are sufficient for the electrochemical reactions of the present application, thus representing a simpler, more straightforward and cheaper fuel cell than prior art systems. Nonetheless, the metals described herein may be useful to catalyse the electrochemical reactions herein and hence use of electrodes comprising such metals may nonetheless be beneficial.

The catalyst used in the anode may be of noble metals such as for example platinum, gold, palladium, iridium, ruthenium, rhenium, rhodium, osmium or combinations thereof, including alloys for example a platinum/ruthenium alloy or binary catalyst such as PtCo, PtNi, PtMo etc. or ternary catalyst PtRuMo, PtRuSn, PtRuW etc. or chalcogenides/oxides as RuSe, Pt- MoOx etc. The catalyst may be a carbon-based catalyst, such as a catalyst described in Liang, J; Zheng, Y; Vasileff, A; Qiao, S (2018) ‘Carbon-Based Electrochemical Oxygen Reduction and Hydrogen Evolution Catalysts', ISBN: 9783527811458. Some binary/ternary or other than pure precious metal catalysts can be more tolerant to probable catalytic poisoning as results of catholyte species crossover.

A selective membrane may be provided between the two compartments, configured to exchange ions between the two compartments. The membrane may be a membrane capable of selectively passing protons (hydrogen ions), which means that the membrane may be a proton exchange membrane or a membrane which is permeable to protons. The membrane may be one which is substantially impermeable to manganese cations. Proton exchange membranes are well known in the art, for example, the Nation™ ion exchange membrane produced by Chemours.

(a) the catholyte further comprises an Mn²⁺-formation promotor; and/or (b) the catholyte further comprises an Mn²⁺-formation promotor precursor; and/or

(c) the fuel cell further comprises a supply fluidly connected to the cathode compartment for delivering fluid thereto, the supply comprising an Mn²⁺- formation promotor .

The supply may comprise an electrochemical cell separate from said cathode compartment. Conduits may enable supply of fluid between the cathode compartment and the electrochemical cell.

The supply may be configured to mix the Mn²⁺-formation promotor with the catholyte.

Features described above in the context of the first aspect apply equally, mutatis mutandis, to the second aspect, and vice versa.

According to a third aspect of the present application, there is provided a catholyte for use in a fuel cell, comprising: a permanganate (VII) or manganate (VI) or manganese (IV) species; and an Mn²⁺ promotor or an Mn²⁺-fbrmation promotor.

Features described above in the context of the first and second aspects apply equally, mutatis mutandis, to the third aspect, and vice versa ,

According to a fourth aspect of the present application, there is provided a use of a catholyte in a fuel cell, wherein the catholyte comprises: a permanganate (VII) or manganate (VI) or manganese (IV) species; and an Mn²⁺-formation promotor or an Mn²⁺-formation promotor precursor.

Features described above in the context of the first, second and third aspects apply equally, mutatis mutandis, to the fourth aspect, and vice versa.

According to a fifth aspect of the present application, there is provided a kit of parts comprising a catholyte and an anofyte, wherein: the catholyte comprises: a permanganate (VII) or manganate (VI) or manganese (IV) species; and a Mn²⁺-formation promotor or an Mn²⁺-formation promotor precursor; and the anolyte comprises: (a) selected from a carboxylic acid, an alcohol, a carbohydrate (such as a sugar) or a chemical hydride, for example wherein the anolyte comprises formic acid, methanol, ethanol, isopropanol, glucose, hydrazine, borohydride, glycerol or glucose;

(b) hydrogen (such as a bimolecular hydrogen-rich anolyte, such as hydrogen gas, reformate or hydrogen derived from biological or other sources) or

(c) a metal selected from zinc, aluminium, calcium, iron and silicon

Features described above in the context of the first, second, third and fourth aspects apply equally, mutatis mutandis , to the fifth aspect, and vice versa.

Brief description of the figures

The application will now be further described, by way of example only, with reference to the accompanying figures, in which:

Fig. 1 is a series of graphs showing current against time for (a) the reduction of a 0.3 M KMnO₄ solution on a rotating disk electrode, and (b), and (c) exhaustive electrolysis of different anolyte and catholyte compositions in a fuel cell system.

Fig. 2 is a series of polarisation and power curves for fuel cells employing methanol as the anolyte.

Fig. 3 is a polarisation and power curve for a fuel cell employing hydrogen or hydrogen rich gas as the anolyte.

Fig. 4 shows a series of polarisation and power curves for fuel cells employing various anolytes.

Fig. 5 shows a the time variation of a fuel cell at constant polarisation using methanol as a fuel in the presence and absence of the Mn²⁺-formation promotor Examples

General

Single fuel cell assemblies were as follows: 4.6 mm graphite felt (SGL group, Germany, Sigracell) and Nation 212 (Fuel cell Store, 50.8 pm) were used as cathode and membrane respectively. As anode 4 mg cm^-2 PtRu on carbon paper (Fuel cell Store) was used for the DMFC single cell setup and 0.4 mg cm^-2 Pt on Toray carbon paper with microporous layer (Johnson Matthey) for the H₂ PEM single cell setup.

For direct methanol fuel cell tests, the same anode was used, Nation 115 (Fuel Cell Store, 127 pm) and 0,4 mg cm^-2 Pt on Toray carbon paper with microporous layer (Johnson Matthey) membrane and cathode electrode were used, respectively.

For the H₂ fuel cell tests, the same electrode was used for anode and cathode, 0.4 mg cm^Pt on Toray carbon paper with microporous layer (Johnson Matthey), and N212 (Fuel cell Store, 50.8 pm thick) as membrane.

Cell were assembled using Tygaflor glass-reinforced FIFE uncompressible gaskets, to achieve a compression around of the electrodes by 25% at 4 N m of torque. The area for all the electrodes was 5 cm², using the Scribner Associates single cell fixtures.

Fuel cell testing was done using an RFB test station (857, Scribner Associates) for the liquid- liquid experiments and the fuel cell station (850e, Scribner Associates) for the gas-liquid experiments.

For methanol fuel cell tests, the Anolyte was prepared by mixing methanol In ultrapure water (Millipore Milli-Q water purification system; <18.2 ΜΩ cm) to obtain the different concentrations used in the experiments. The same preparation was done for the other feedstocks (formic acid, ethanol, isopropanol and glucose).

For H₂ fuel cell tests, pure H₂ (99.999%, BIP Plus Air Products) and Reformate H₂ (78.6% H₂, 18.5% CO₂, 2.9% CH₄, and 20 ppm CO, Air Products) were used.

Rotating disk electrode (RDE; Pine Instruments) experiments a three-electrode cell setup was used, with separate compartments for each electrode. An RDE with a glassy carbon of 5 mm was used as working electrode and SCE and Pt as reference and counter electrode, respectively. The electrolyte was purged with N₂ prior any experiment. All the RDE experiments were done using an Autolab PGSTAT32M (Metrohm).

Electrochemical generation of a Mn²⁺-formation promoter from an Mn²⁺ promotor precursor

Ti(SO₄)₂ was synthesized electrochemically from a 0.5 M solution of Ti₂(SO₄)₃ (20wt% in 1- 4wt% sulfuric acid, Alfa Aesar) in 1.5 M H₂SO₄, to finally obtain a mother solution of 1 M

Ti(SO₄)₂ in 1 M H₂SO₄. The catholyte used in the fuel cell tests was created by dissolving the proper amount of KMnO₄ (97%, Sigma-Aldrich) to get the desired concentration (e.g. 5 mM, 0.1 M and 0.3 M), mixed equimolar with the as-synthesised Ti(SO₄ )₂, in 1M H₂SO₄ utilising water from a Millipore Milli-Q water purification system (<18.2 ΜΩ cm).

Fig. 1(a) shows the results of chronoamperometry studies conducted at 1.2 V vs. RHE in a three electrode cell setup, with a glassy carbon as working electrode in a rotating disk configuration and an 1 M H₂SO₄ electrolyte comprising 0.3 M KMnO₄ with (upper line) and without (dashed line) the Mn²⁺-formation promotor 0.3 M Ti(SO₄)₂

The Mn²⁺-formation promotor, TI(IV), enabled a clean electrode surface with measured constant current whereas, in the absence of Ti(IV) additive, MnO₂ coated the electrode surface and current dropped dramatically.

Figs. 1(b) and (c) were conducted at 0.6 V in a fuel cell using (b) MeOH as limiting reactant (Cathode, 250 ml_: 0.3 M KMnO₄ + 0.3 Ti (IV) in 1M H₂SO₄; Anode, 50 mL: 1 M MeOH in 1 M H₂SO₄) and (c) KMnO₄ as limiting reactant (Cathode, 25 mL: 0.1 M KMnO₄ + 0.1 M Ti (IV) in 1 M H₂SO₄; Anode, 100 mL: 1 M MeOH), the insets show the charge available, charge collected and the excess of the counter electrolyte respectively.

Polarisation and power

Fuel cells were assembled as above, using methanol or hydrogen as anolyte:

Methanol fuel cell

Anode: Cathode:

Hydrogen fuel cell

Anode:

Cathode:

Polarisation and power curves are shown in Figs. 2 and 3, for methanol and hydrogen respectively.

A ca. 5-200x increase in current density was observed regardless of the fuel utilised when Ti additives were included (Fig. 2(a), 1 M MeOH + KMnO₄ without as compared to with Ti⁴⁺). Fig. 2(a) (methanol) shows a methanol (anode) air (cathode) cell for comparison. Fig. 2(b) shows the effect of varying MeOH concentration. All measurements taken with a catholyte comprising 0.1 M Ti(IV) in 1 M H₂SO₄.

Fig. 3 shows the performance comparison at 25 °C of a standard H₂ PEMFC using Air and KMnO₄, demonstrating a significant increase in the performance, using 100 mL of 0.3 M KMnO₄ + 0.3 M Ti(IV) in 1.45 M H₂SO₄.

Similar experiments as shown with MeOH were conducted with a variety of other anolytes - polarization and power curves for these are shown in Fig. 4.

Achieved Eocv (the achievable open circuit voltage; representing the potential achieved from the system when no current is flowing) and the percentage of Eocv to the theoretical standard potential are tabulated below. Hydrogen-oxygen and methanol-oxygen (direct-methanol fuel cell, “DMFC") results are shown for Comparison.

The majority of the measurements with permanganate catholytes were taken with a catholyte comprising 100 mL: 0.1 M KMnO₄ + 0.1 M Ti(IV) in 1 M H₂SO₄ . For the high performance FC using H₂ as fuel, the catholyte composition was: 0,3 M KMnO₄ + 0.3 M Ti(iV) in 1.45 M H₂SO₄ using 100 mL as well.

The data shown in the table above are examples of standard DMFC using Air and O₂ at room temperature (between 25 to 30 °C) compared with the KMnO₄ technology, observing a big increase in power density using KMnO₄ instead of Air or even pure O₂. The fuel in all the cases is methanol (MeOH) at different concentrations going from 1 M to 8 M. However, for standard DMFC using liquid feed, the membrane required needs to be thicker, as the crossover of MeOH at higher concentrations is increased, resulting in a decrease the cell voltage due to the mix potential effect, as in the cathode side Pt is used as catalyst. In the case of using KMnO₄, as the cathode do not require any catalyst the mix potential effect is removed and we have the possibility to use higher concentrations (Fig. 2) of MeOH and thinner membranes, keeping a good performance in the single cell. As shown in the table, our working voltage is approximately 3 times higher compared with the other systems.

As mentioned before, as KMnO₄ does not need a catalyst, we can use a carbon graphite felt as electrode, decreasing the total Pt loading of the cell. Even using MeOH in alkaline solution (shown in the examples [4] and [5]), which improves the kinetics of the MeOH oxidation but increase the cost of the fuel, the performance at room temperature is much lower compared with the KMnO₄ , which can use MeOH in water solution achieving much higher performance.

Efficiency

The efficiency using H₂ as fuel at room temperature (25 °C) Is close to the obtained in standard H₂ fuel cells at high temperature (between 60 to 90 °C). However, the operational voltage at a current density of 0.5 A cm^-2 is doubled, from around 0.6 V in a standard H₂ fuel cells to 1.2 V for the KMnO₄ fuel cell which improves the power density of the system significantly. We have Included a comparison of the standard H₂ fuel cell setup using Air at the same temperature conditions. A part of the increase in performances, the use of this oxidant opens the possibility of using H₂ of different purities and sources. Using H₂ reformate as fuel in the standard H₂ - Air fuel cell, the performance was decreased dramatically, however, using KMnO₄ only shows some fluctuations in the voltages in the polarization curves due to adsorption of the impurities of the H₂ reformate, with further recovery due to their fully oxidation to CO₂. Further, the disclosure comprises examples according to the following clauses:

Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described aspects.

Claims

1. A method of operating a fuel cell, comprising: providing a catholyte comprising permanganate (VII) or manganate (VI) or manganese (IV) species to a cathode compartment of the fuel cell, providing an Mn²⁺-formation promotor to the catholyte, and discharging the fuel cell.

2. The method according to claim 1 , wherein the catholyte is a permanganate (VII) catholyte.

3, The method according to any preceding claim, wherein the Mn²⁺-formation promotor has an oxophilicity greater than 0.4, such as about between about 0.6 and 1.0, preferably about 1.0.

4. The method according to any preceding claim, wherein the Mn²⁺-formation premotor comprises a metal species, optionally wherein the metal species comprises titanium, carbon, boron, vanadium, or silicon, or mixtures thereof, preferably wherein the metal is titanium.

5. The method according to any preceding claim, wherein the Mn^-formation premotor comprises titanium.

6. The method according to any preceding claim, wherein the Mn²⁺-formation premotor comprises Ti⁴⁺, Al³⁺ or Si⁴⁺, or mixtures thereof, preferably Ti⁴⁺, optionally wherein the Mn²⁺ premotor is Ti(SO₄ )₂.

7. The method according to any preceding claim, wherein the Mn²⁺-formation promotor is Ti(SO₄ )₂.

8. The method according to any preceding claim, wherein the Mn²⁺-formation promotor comprises SO₄ ^2-, pyrophosphate (P₂O₇ ^4-), perchlorate, acetate or oxalate.

9. The method according to any preceding claim, further comprising providing an anolyte selected from a carboxylic acid, an alcohol, a carbohydrate (such as a sugar) or a chemical hydride.

10. The method according to any preceding claim , wherein the anolyte comprises formic acid, alcohols (such as methanol, ethanol or isopropanol), glucose, hydrazine, ammonia, borohydride, glycerol or glucose.

11. The method according to any one of claims 1 to 8, further comprising providing an anolyte in an anode compartment of the fuel cell, the anolyte comprising hydrogen (such as a bimolecular hydrogen-rich anolyte, such as hydrogen gas, reformate or hydrogen derived from biological or other sources).

12. The method according to any preceding claim, wherein the electrolyte further comprises zinc species, aluminium species, calcium species, iron species, silicon species or a combination thereof, preferably zinc species, aluminium species, or a combination thereof.

13. The method according to any preceding claim, wherein catholyte comprises a pH from -2 to 2, preferably about -1 to 1.

14. The method according to any preceding claim, wherein the cathode comprises carbon, optionally wherein the cathode is coated, optionally wherein the cathode comprises or is coated with metal, optionally wherein the cathode comprises or is coated with gold, platinum, palladium, iridium, ruthenium, rhenium, rhodium or osmium; optionally wherein the cathode comprises or is coated with platinum and/or ruthenium.

15. A fuel cell, comprising: a cathode, in a cathode compartment comprising a permanganate (VII) or manganate (VI) or manganese (IV) catholyte; wherein:

(a) the catholyte further comprises an Mn²⁺-formation promotor; and/or

(b) the catholyte further comprises an Mn²⁺-formation promotor precursor; and/or

(c) the fuel cell further comprises a supply fluidly connected to the cathode compartment for delivering fluid thereto, the supply comprising an Mn²⁺-formation promotor and/or an Mn²⁺-formation promotor precursor.

16. The fuel cell according to claim 15, wherein said supply comprises an electrochemical cell separate from said cathode compartment.

17. A catholyte for use in a fuel cell, comprising: a permanganate (VII) or manganate (VI) or manganese (IV) species; and an Mn²⁺-formation promotor or an Mn²⁺-formation promotor precursor.

18. Use of a catholyte in a fuel cell, wherein the catholyte comprises: a permanganate (VII) or manganate (VI) or manganese (IV) species; and an Mn²⁺-formation promotor or an Mn²⁺-formation promotor precursor.

19. A kit of parts comprising a catholyte and an anolyte, wherein: the catholyte comprises: a permanganate (VII) or manganate (VI) or manganese (IV) species; and an Mn²⁺-formation promotor or an Mn²⁺-formation promotor precursor; and the anolyte comprises:

(a) selected from a carboxylic acid, an alcohol, a carbohydrate (such as a sugar) or a chemical hydride, for example wherein the anolyte comprises formic acid, methanol, ethanol, isopropanol, glucose, ammonia, hydrazine, borohydride, glycerol or glucose;

(c) a metal selected from zinc, aluminium, calcium, iron and silicon.

20. A method, fuel cell, catholyte, use or kit of parts substantially as described herein with reference to the accompanying description and/or figures.