WO2024095010A1

WO2024095010A1 - Proton-conductive solid oxide fuel cell and method of operation

Info

Publication number: WO2024095010A1
Application number: PCT/GB2023/052872
Authority: WO
Inventors: Didem Berceste Beyribey PRICE; Sneh Lata JAIN; Vinay MULGUNDMATH; George Michael CARINS; John Thomas Sirr Irvine
Original assignee: Zem Fuel Systems Ltd
Current assignee: Zem Fuel Systems Ltd
Priority date: 2022-11-03
Filing date: 2023-11-03
Publication date: 2024-05-10
Anticipated expiration: 2025-05-03
Also published as: GB202216387D0; EP4612744A1

Abstract

A method of operating a fuel cell, wherein the fuel cell comprises an anode, a cathode and a proton conducting solid oxide electrolyte arranged between the anode and the cathode, and the method comprises feeding a fuel to the fuel cell, wherein the fuel comprises methanol or ammonia. Also described is a fuel cell comprising an anode, a cathode and a solid oxide electrolyte, wherein the solid oxide electrolyte comprises BaCe0.7Zr0.1Y0.16Zn0.04 (BCZYZ), and wherein the solid oxide electrolyte is arranged between the anode and the cathode; a method of manufacturing said fuel cell and a system comprising said fuel cell.

Description

PROTON-CONDUCTIVE SOLID OXIDE FUEL CELL AND METHOD OF OPERATION

FIELD

This relates to a fuel cell, methods of operating a fuel cell, and a system comprising a fuel cell, particularly a proton conductive solid oxide fuel cell.

BACKGROUND

Shipping is a central mode of the transport industry, and decarbonisation of the sector is required to meet Net Zero targets. Currently, the shipping industry generates about 3% of global CO2 emissions and 17% of atmospheric pollution. Large vessels with a power consumption of more than 130 kW or modified since 2000 must control emissions, including SO_X, NO_X and CO2, in so-called emission control areas, where the vessels approach ports and are close to the land. The targets are to cut at least 50% of GHG emissions from shipping by 2050 and eliminate it as soon as possible before the end of the century.

Emission control could be achieved by implementing clean and alternative fuels and technology, for example, moving from fossil fuels to renewable energy. Fuel cells became an attractive solution for low-carbon energy generation, and green hydrogen is often considered the likely energy carrier. However, hydrogen is problematic for long-distance and heavy-duty applications due to its low energy density.

Various green fuels have been considered for maritime use, including ammonia, liquid hydrogen, synthetic Liquid Natural Gas (LNG), methanol and diesel. Among these, ammonia could be the most promising. Ammonia has a high energy density (4 kWh/kg), hydrogen content (17.7 wt. %) and its use a fuel does not produce CO2 emissions. Also, the costs related to the liquefaction of hydrogen could be avoided because ammonia at room temperature can be easily liquidised at low pressures (~10bar). Additionally, there is already existing infrastructure for transport, storage and distribution of ammonia. Similarly, methanol is considered to be a good hydrogen storage medium since it is liquid at room temperature, and existing infrastructure is in place for methanol transport, storage and distribution.

Solid Oxide Fuel Cells (SOFCs) are high-temperature fuel cells where the electrolyte is made of ion-conductive solid ceramics. Traditional SOFCs conduct oxide ions (O^2-) across a solid electrolyte to the fuel side, where H2 is electrochemically oxidised, generating a current. In state-of-the-art devices, 8% doped yttria-stabilised zirconia (Zro.84Yo.-i6O2.08, YSZ) is used as the electrolyte, Ni/YSZ composite as the anode and LSM/YSZ composite (La_xSri._xMnO3 - Zro.84Yo.i6O1.92) as the cathode

Another type of SOFC is proton conductive (P-SOFC), where hydrogen protons are conducted to the oxygen side. However, their high sintering temperature can cause production and barium evaporation issues. Therefore, there is scepticism about the application of P-SOFC on a large scale, and only a few have managed to produce a cell with a large surface area delivering sizable power.

SUMMARY

Aspects of the present disclosure relate to methods of operating a proton conductive solid oxide fuel cell, a proton conductive solid oxide fuel cell, a system comprising a solid oxide fuel cell and a vehicle comprising a solid oxide fuel cell.

According to a first aspect, there is provided a method of operating a fuel cell, wherein the fuel cell comprises an anode, a cathode and a proton conductive solid oxide electrolyte arranged between the anode and the cathode, and the method comprises: feeding a fuel to the fuel cell, wherein the fuel comprises methanol or ammonia.

Typically, the fuel may comprise or may be supplied as a liquid fuel.

Fuel cells comprising proton conductive solid oxide electrolytes are beneficially suitable for the direct cracking of ammonia and methanol at the fuel cell anode, at lower temperatures compared to traditional oxide conducting solid oxide fuel cells. The lower operating temperatures may also provide for reduced production costs and operating costs. Furthermore, fuel cells comprising proton conductive solid oxide electrolytes do not produce nitrogen oxides as a waste product.

The present disclosure provides a method of operating a fuel cell which may have applications within heavy transport, for example within the shipping industry. Also, in the protonic type of SOFC according to the present disclosure, water is produced at the cathode. This may minimise corrosion issues and the generation of NO_X which can result from electrochemical oxidation when oxide ions diffuse to the anode in classic oxide conducting SOFCs.

The proton conductive solid oxide electrolyte may comprise BaCei-_x.yZr_xY_yO3-6 (BCZY). The proton conductive solid oxide electrolyte may comprise BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ).

The provision of a solid oxide electrolyte comprising BaCei-_x.yZr_xY_yO3-6 (BCZY) advantageously can provide for direct fuelling with, for example, ammonia or methanol. Specifically, BaCei-_x.yZr_xY_yO3-6 (BCZY) provides for high catalytic activity for the NH3 cracking reaction. Furthermore, BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ) which comprises 4% Zinc doped BCZY (BaCei-_x.yZr_xY_yO3-6) may overcome challenges in manufacturing fuel cells comprising proton conductive solid oxide electrolytes because BCZYZ exhibits better sintering properties compared to un-doped BCZY.

The anode may comprise BCZYZ and nickel oxide (NiO). The method may further comprise reducing the NiO prior to feeding the fuel cell with the liquid fuel. Reducing the NiO to nickel metal may provide for the formation of electronically conductive pathways for electrons from the anode during operation of the fuel cell.

The method may comprise cracking ammonia at the anode of the fuel cell into hydrogen and nitrogen. The method may further comprise oxidising the hydrogen at the anode of the fuel cell to form H⁺ ions and electrons.

The method may comprise reacting methanol with water at the anode of the fuel cell to form carbon dioxide, H⁺ ions and electrons.

In use, the H⁺ produced at the anode travel through the electrolyte to the cathode, and the electrons produced at the anode are supplied to an external circuit which is connected to the anode and the cathode. The H⁺ ions combine with oxygen supplied to the cathode and electrons from the external circuit to generate water. The cathode may comprise BCZYZ and LaosSro^CoosFeosCh-s (LSCF). The cathode may provide a catalytically active and electrically conductive layer for the electrochemical combination of protons, electrons and oxygen to generate water, in the form of steam.

The method may comprise arranging the cathode to be exposed to air. The method may comprise supplying air to the cathode of the fuel cell, and reducing oxygen present in air with H⁺ ions and electrons generated at the cathode.

The method may comprise operating the fuel cell at a temperature above approximately 300°C. The fuel cell may be configured to be operated at temperatures between approximately 300°C to 800°C, preferably 650°C to 750°C. The method may comprise heating the fuel cell using a furnace. The method may comprise utilising heat produced from the reactions taking place at the cathode to provide subsidiary heat for the endothermic reactions taking place at the anode. This may contribute to improved fuel cell efficiency.

The method may comprise operating the fuel cell for up to 200 hours, for example up to 150 hours, 160 hours, 170 hours, 180 hours, 190 hours or 200 hours. The method may comprise operating the fuel cell continuously.

The method may comprise switching between fuel sources. For example, the method may comprise feeding a first fuel, e.g. ammonia, to the fuel cell for a selected period of time, then feeding a second fuel, e.g. methanol, to the fuel cell for selected period of time, or vice versa. By such provision, the versatility and utility of the fuel cell may be improved. For example, the fuel cell may be capable of powering a vehicle whilst not being reliant on the availability of a single source of fuel.

The method may comprise operating a plurality of fuel cells, e.g. in series. The method may comprise providing the fuel cell, or fuel cells in a vehicle. The vehicle may comprise a shipping vessel, such as a cargo ship, a passenger ship, a ferry. The vehicle may comprise an aircraft, for example an airplane such as a cargo plane.

The method may comprise using the current generated by the fuel cell to power the vehicle.

According to a second aspect, there is provided a fuel cell comprising an anode, a cathode and a solid oxide electrolyte, wherein the solid oxide electrolyte comprises BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ), and wherein the solid oxide electrolyte is arranged between the anode and the cathode.

The provision of a solid oxide electrolyte comprising BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ) advantageously can provide for direct fuelling of the fuel cell with, for example, ammonia or methanol. Specifically, BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ) provides for high catalytic activity for the NH3 cracking reaction. Furthermore, BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ) which comprises 4% Zinc doped BCZY (BaCei._x. yZr_xY_yO3-₅) may overcome challenges in manufacturing fuel cells comprising such electrolytes because BCZYZ exhibits better sintering properties compared to un-doped BCZY.

The fuel cell may comprise a thin film of BCZYZ electrolyte. For example, the thin film of BCZYZ electrolyte may be 1 pm, 2 pm, 3 pm, 4 pm or 5 pm in thickness. The thickness of the electrolyte may be selected to minimise proton flux at a desired operating temperature. The anode may comprise BCZYZ and nickel oxide (NiO). The anode may comprise a catalytically active porous structure. In use, the NiO is reduced to nickel metal prior to operating the fuel cell, wherein the nickel may provide electronically conductive pathways for electrons from the catalytically active sites. The anode may be configured to provide structural support for other components of the fuel cell. The anode may comprise a tubular configuration. A tubular anode may provide for improved structural integrity and thermal stability of the fuel cell during operation. The tubular anode may provide for an increased active surface area compared to, for example, planar anode arrangements. The anode may comprise 60.2 weight % NiO, 32.4 weight % BCZYZ and 7.4 weight % carbon black.

Alternatively, the anode may comprise a substantially planar configuration.

The cathode may comprise BCZYZ and Lao.sSro^Coo.sFeo.sOs-s (LSCF). The cathode may comprise a plasticiser. The cathode may comprise a pore former, for example, carbon black. The cathode may provide a catalytically active and electrically conductive layer for the electrochemical combination of protons, electrons and oxygen to generate water, in the form of steam.

The fuel cell may further comprise a cathode current collector. The cathode current collector may comprise silver. For example, the cathode current collector may comprise silver wires and silver paste which may be arranged on a surface of the cathode.

The fuel cell may comprise an anode current collector. The anode current collector may comprise nickel wire mesh. The nickel wire mesh may be contacted to a surface of the anode using silver paste.

The fuel cell may comprise a total active surface area which includes the surface area of the anode and the cathode. The active surface area of cathode may be 36 cm².

The fuel cell may further comprise a support assembly. The support assembly may comprise a metal support assembly, for example a nickel support assembly. The support assembly may be arranged to provide mechanical support for the fuel cell. The support assembly may be arranged to form a fuel supply channel and an exhaust gas channel. The support assembly may further comprise an insulator. The insulator may comprise zirconia.

The fuel cell may be configured to be operated at temperatures above approximately 300°C. The fuel cell may be configured to be operated at temperatures between approximately 300°C to 800°C, preferably 650°C to 750°C. According to a third aspect, there is provided a method of manufacturing a fuel cell, wherein the method comprises: providing an anode comprising BCZYZ (BaCeo.yZro.iYo.ieZno.tMOs-c) and nickel oxide (NiO), coating the anode in BCZYZ electrolyte to form an electrolyte coated anode; and further coating the electrolyte-coated anode in a mixture comprising BCZYZ and Lao.8Sro.2Coo.5Feo.503-6 (LSCF) to form a cathode surrounding the electrolyte coated anode.

The method may comprise extruding the anode comprising BCZYZ and nickel oxide (NiO).

The method may comprise extruding the anode into a tubular form. A tubular anode may provide for improved structural integrity and thermal stability of the fuel cell during operation. The tubular anode may provide for an increased active surface area compared to, for example, planar anode arrangements.

The method may comprise dip coating the anode in BCZYZ electrolyte to form the electrolyte-coated anode. The electrolyte may comprise 4 % Zinc doped BCZYZ electrolyte

The method may comprise dip coating the electrolyte-coated anode in the mixture comprising BCZYZ and Lao.sSro^Coo.sFeo.sOs-s (LSCF) to form the cathode surrounding the electrolyte coated anode.

The method of manufacture may comprise heating the anode prior to dip coating the anode in the electrolyte. The heating may comprise bisque firing. The method may comprise heating the anode in a vertical configuration. The method may comprise bisque firing the anode at a temperature of 1300 °C. The method may comprise increasing the temperature of the anode at a rate of 2°C/min up to 1300°C. The method may comprise exposing the anode to 1300°C heat for at least 180 minutes. The method may comprise allowing the anode to cool from 1300°C at a rate of 3°C/min.

The method may comprise drying the electrolyte coated anode after the first dip coating step. The method may comprise drying the electrolyte coated anode in an oven. The method may comprise drying the electrolyte coated anode at least 100 °C, for example 105 °C for approximately one hour.

The method may comprise heating the electrolyte coated anode prior to the further dip coating step. This method step allows the formation of the electrolyte membrane while providing further sintering of the anode. The method may comprise heating the electrolyte coated anode to at least 1500°C. The method may comprise exposing the electrolyte coated anode to 1200°C for approximately 10 minutes and then to 1500°C for approximately 10 hours.

The method may comprise dipping the electrolyte coated anode into a cathode slurry comprising BCZYZ and Lao.sSro^Coo.sFeo.sOs-s (LSCF). The method may comprise subsequently drying the fuel cell. The method may comprise drying the coated anode at least 100 °C, for example 105 °C for approximately one hour.

The method may further comprise heating the fuel cell after coating with the cathode slurry. The method may comprise heating the fuel cell to at least 1150 °C. The method may comprise heating the fuel cell at 1150 °C for approximately 2 hours. This heating step may allow for the formation of a sintered porous cathode.

The method may comprise selecting a desired fuel cell size, for example a desired length and cutting the fuel cell to this length.

The method may comprise forming current collectors at the anode and cathode. For example, the current collectors may comprise end collars. The end collars may comprise at least one of nickel wire, nickel sheeting, silver wires and silver paste.

The method may comprise forming a support assembly for the fuel cell. The support assembly may comprise a metal support assembly, for example a nickel support assembly. The support assembly may be arranged to provide mechanical support for the fuel cell. The support assembly may be arranged to form a fuel supply channel and an exhaust gas channel. The support assembly may further comprise an insulator. The insulator may comprise zirconia.

According to a fourth aspect, there is provided a system comprising a fuel cell according to the second aspect.

The system may further comprise a fuel reservoir comprising fuel, e.g. liquid fuel, and at least one conduit arranged for the supply of the fuel, e.g. liquid fuel, from the fuel reservoir to the fuel cell. The system may further comprise a second reservoir configured to hold or store a second fuel; and at least one second conduit arranged for the supply of the second fuel to the fuel cell. The system may comprise a plurality of fuel reservoirs. For example, the system may comprise a fuel reservoir containing ammonia and a fuel reservoir containing methanol. The system may comprise a flow control arrangement operatively associated with the fuel reservoir. The flow control arrangement may provide for the selection of the flow rate of fuel to be supplied to the fuel cell.

The system may comprise a water supply. The system comprise at least one conduit arranged for the supply of water to the fuel cell.

The system may comprise an air supply system. The air supply system may comprise, for example, a fan arrangement configured to direct air to the fuel cell.

The system may further comprise a furnace. The furnace may be arranged to heat the fuel cell to a desired operating temperature. The furnace may be configured to reach temperatures above approximately 300°C. The furnace may be configured to be reach temperatures between approximately 300°C to 800°C, preferably 650°C to 750°C. The furnace may comprise a temperature controller, wherein the temperature of the furnace may be accurately controlled.

The system may comprise an exhaust system. The exhaust system may comprise at least conduit arranged to provide a flow of reaction products (for example, water, nitrogen, hydrogen, air, and carbon dioxide) from the fuel cell to a location external to the fuel cell.

The system may further comprises a vehicle arranged to be powered by the fuel cell. The vehicle may comprise a shipping vessel, such as a cargo ship, a passenger ship, a ferry.

According to a fifth aspect of the disclosure, there is provided a vehicle comprising a fuel cell according to the second aspect or a system according to the fourth aspect, wherein the fuel cell is arranged to provide power to the vehicle.

The vehicle may comprise a shipping vessel, such as a cargo ship, a passenger ship, or a ferry. The vehicle may comprise an aircraft, for example an airplane such as a cargo plane.

For the purposes of the present disclosure, it should be understood that the features defined above or described below may be utilised, either alone or in combination with any other defined feature, in any other aspect, embodiment, or example or to form a further aspect, embodiment or example of the disclosure.

BRIEF DESCRIPTION OF FIGURES

These and other aspects will now be described by way of example with reference to the accompanying drawings, of which:

Figure 1 shows a schematic of a proton conductive solid oxide fuel cell; Figure 2 shows a schematic of a fuel cell based on a proton conducting solid oxide electrolyte being fuelled with ammonia;

Figure 3 shows a schematic of a fuel cell based on a proton conducting solid oxide electrolyte being fuelled with methanol/water;

Figure 4 shows a schematic of a system according to an embodiment;

Figure 5 shows a schematic of a vehicle according to another embodiment;

Figures 6a to 6d shows SEM micrographs of the fuel cells’ structure after reduction;

Figures 7a to 7j show the electrochemical performance of a fuel cell according to an embodiment when fuelled with ammonia or a mixture of hydrogen and nitrogen;

Figures 8a to 8d show the development of Equivalent Circuit for impedance analysis at OCV and fuel cell operation with ammonia;

Figures 9a and 9b show Arrhenius plots of ohmic and polarization resistance for the fuel cell operation with ammonia;

Figures 10a to 10e show the electrochemical performance of the fuel cell according to the present disclosure at various flow rates of ammonia;

Figures 11a and 11b show the performance of the fuel cell operation with ammonia over an operating time of 170 hours;

Figure 12 shows the electrochemical performance of the fuel cell operating with ammonia at various operation periods; and

Figure 13 shows plots of stack voltage and cell power against current at different temperatures for a fuel cell according to an embodiment operating with methanol.

DETAILED DESCRIPTION

The cells of the present disclosure are fuel cells based on a proton-conducting solid oxide electrolyte. The cells can be described as a Solid Oxide Fuel Cells (SOFC). This means that the main materials used in the construction of the cell are solid metal oxides. Technologies based on solid oxides are generally intended for use at high temperature, as it is only at high temperatures that the novel attributes of these oxides begin to work; ionic conductivity for example. The general architecture of a fuel cell 10 according to a first embodiment can be seen in Figure 1. A fuel cell is composed of three main components; an anode electrode 14, a cathode electrode 16 and an ionically-conductive but electronically-insulating electrolyte 12. The electrodes 14, 16 are generally porous to gases, and the electrolyte 12 provides a gas-tight seal between the two electrodes 14, 16. This assembly 10 is often referred to as a ‘cell’. External to these components, there is an electrical circuit with a load which sinks electrical power from the fuel cell.

Each of the layers shown in Figure 1 are made of different materials, and these materials perform different functions within the cell. The electrolyte 12 of the cell is made of BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ), which is an oxide that is proton conducting at temperatures above approximately 300°C. The membrane is designed to be as thin as possible to minimise resistance to proton flux at the operating temperature.

The anode 14 has two functions. Firstly, the anode 14 serves the purpose of providing a high surface area, catalytically active porous structure on which some of the electrochemical (and chemical) reactions occur and separation of protons and electrons. The anode 14 is composed BCZYZ and nickel oxide (NiO). The BCZYZ helps to transport protons from catalytically active sites. The NiO is reduced to nickel metal prior to operating the cell, and the metal provides electronically conductive pathways for the electrons from the catalytically active sites. The second function of the anode 14 is to provide mechanical support for the rest of the cell. The anode 14 is thick in cross section, but highly porous. During manufacture of the fuel cell 10, the anode support 14 is the first part of the cell 10 to be produced, as an extruded green (unfired) tube.

The cathode 16 is made up of a mixture of BCZYZ and Lao.sSro^Coo.sFeo.sOs-s (LSCF). Similar to the anode support 14 in function, the cathode 16 provides a high surface area catalytically active and electrically conductive porous layer for the electrochemical combination of protons, electrons and oxygen to generate water (steam).

On each of the anode 14 and cathode 16, a current collector 18 is provided. In the case of the cathode 16, the current collector 18 the form of silver wires and silver paste that is applied to the surface of the cathode electrode 16. In the case of the anode support 14, the current collector 18 takes the form of a woven nickel wire mesh contacted to the porous anode support structure 14 applying silver paste.

Figure 2 shows simplified schematic of a tubular fuel cell 10 having a proton conducting electrolyte being fuelled with ammonia. Ammonia is supplied to the anode 14 via an inlet 20. At the anode 14, ammonia is cracked into hydrogen and nitrogen, of which the hydrogen is oxidised into form H⁺ ions and electrons. The nitrogen and excess hydrogen leaves the fuel cell via an exhaust outlet 24. Ammonia was entirely decomposed to H2 and N2 thanks to the catalytic activity of the Ni/BCZYZ composite anode 14.

The electrons are conducted to an external circuit 30 (shown in Figure 3) where they do work in the load 32. The protons are transported through the electrolyte 12 to the cathode 16 where they combine with oxygen and electrons from the external circuit to generate water (steam). Steam is exhausted from the cathode via an exhaust outlet 26.

Figure 3 shows a simplified schematic of a fuel cell 10 based on a proton conducting electrolyte being fuelled with methanol/water. At the anode 14, methanol reacts with water to form carbon dioxide, H⁺ ions (protons) and electrons, and the latter are conducted to the external circuit 30 where they do work in the load 32. The protons transport through the electrolyte membrane 12 to the cathode 16, where they combine with oxygen and electrons from the external circuit 30 to generate water (in the form of steam). Carbon dioxide gas is exhausted from the anode 14, and steam is exhausted from the cathode 16.

Figure 4 shows schematic of a system 100 according to an embodiment comprising fuel cell 110. The system 100 comprises a fuel reservoir 120. The fuel reservoir can contain ammonia or methanol. The fuel reservoir 120 can comprise multiple reservoirs 120a, 120b containing different fuels to allow for switching between fuel sources if desired. The system 100 comprises piping 122 for delivery of fuel to the fuel cell 110 which is located with a furnace 130. The furnace 130 is provided to heat the fuel cell 110 to a required operating temperature.

The system 100 also comprises a water supply 123 for the supply of water to the anode when the fuel cell is being fuelled by methanol. The system is also provided with an air supply 128 for the provision of air to the cathode. The air supply is fan driven to increase the air flow to the cathode of the fuel cell 110, thereby providing for improved conversion rates of oxygen at the cathode.

An exhaust arrangement 126 is also provided to convey exhaust gases from the fuel cell 110 during operation to a location external to the fuel cell 110.

Figure 5 shows a vehicle 200 comprising fuel cell system 100. The vehicle can be a ship, for example a cargo ship. EXAMPLES

EXAMPLE 1 - DIRECT AMMONIA FUEL CELL

Cell preparation

Evaluated in this work were tubular cells were fabricated by extrusion of NiO/BaCeo.7Zro.iYo.i6Zno.o403-6 (NiO/BCZYZ) tubular anode and dip-coating of BCZYZ electrolyte followed by Lao.sSro^Coo.sFeo.sOs-o/BaCeo.yZro.iYo.ieZno.o^s-o (LSCF/BCZYZ) cathode. The process parameters were optimised to achieve thin and uniform functional layers

Cell Assembly

100pm Inconel 625, 1 mm nickel wire and 1 mm nickel sheet were formed into collars and brazed onto both ends of the cell using a CuO/Ag braze material. The assembly was then heated to 1030°C at 10 °C/min, allowed to dwell for 12 minutes and cooled to room temperature at 10 °C/min. This burned off binders and solvent vehicles and melted the braze material, ensuring a hermetic seal between the collars and the cell.

One open and one closed end cap were formed of Inconel 625 and nickel and were brazed to each end of the cell using proprietary braze materials and methods. The open cap was brazed to an alumina tube for support and interface with the test infrastructure.

Ferro 903B AG LTCC conductor paste was applied to the cathode to ensure optimal current collection. A 2mm silver (99.9%) wire was formed into the cathode current bus, and this was affixed to the electrode by wrapping 1 mm silver wire at regular intervals. Additional conductor paste was then applied to the wires to enhance the contact between the silver wires and the conductor.

Electrochemical Test/ Data collection

Electrochemical tests were carried out using Biologic (VSP-300) and EZ lab software. The cell was characterised for about seven days, including reduction, running under OCV conditions, electrochemical characterisation (l-V and EIS), and about 140h of stability at a constant voltage and 133 ml/min of pure NH3.

Before electrochemical characterisation, the anode was reduced in 100 ml/min of H2 and 100 ml/min of N2 mixture at 650 °C overnight. Current-voltage (l-V) and electrochemical impedance spectroscopy (EIS) were used to characterise the cell's performance at 25 °C intervals between 650 °C and 750 °C after reduction. The gas flow rate used for the analysis was 133 ml/min of anhydrous ammonia, or a synthetic mixture of hydrogen/nitrogen was used to replicate the equivalent molar value of gas, (H2(200 ml/min) I N2(66 ml/min)) assuming total decomposition of ammonia. The cathode side was open to the air atmosphere of the furnace. Additionally, 500 ml/min of air was delivered to the furnace cavity to ensure faster air exchange with the environment. For the stability test, the cell was discharged at constant potential (-0.25V relative to OCV).

The EIS was taken in the 20000 Hz - 0.1 Hz frequency range with the excitation voltage amplitude of 20 mV. The free Matlab-based DRTtool by Wan et al.⁶¹ was used to construct DRT. Before conducting analysis, the quality of impedance spectra was investigated with Kramers-Kroning (K.K.) transforms.^62-64 As the real and imaginary parts are connected through Kramers-Kroning transformations, only the imaginary part of impedance could be considered. Good quality data should not show a more significant residual than 0.5%.

Complex non-linear least-squares (CNLS) approximation was used for EIS analysis with the equivalent circuit, consisting of elements connected in series or parallel that correspond to individual electrode processes. The impedance spectra were fitted to the equivalent circuit using Z-view software.

Using an Agilent micro GC, the anode exhaust gas was analysed at 100- second intervals for Hydrogen, Nitrogen and residual ammonia. Using Soprane software, the chromatograms were analysed to give percent gas composition. Maximum power was determined by connecting a series of low resistance, high power resistors as load across the cell.

XRD and SEM characterisation

XRD analysis was carried out using a PANalytical X-ray diffractometer with CuKal radiation (A = 1.54056A) in the range 20 = 10-90. Scanning electron microscopy was carried out using an FEI Scios to image fractured cross-sections of the tubular cell.

The cell described as “after reduction” in Figure 6, was reduced with pure H2 and characterised by l-V and EIS over one day in pure ammonia fuel. The cell was from the same manufactured batch as the cell whose performance was evaluated.

Results and discussion

XRD Analysis

Crystallographic information on the electrode materials was determined by X-ray diffractometry. BaCeo.7Zro.1Yo.i6Zno.04O3- s (BCZYZ) possessed single phase of orthorhombic symmetry (Pbnm space group) in accord with prior studies. For LSCF, single perovskite phase was observed with R-3m space group. LSCF with the Co/Fe ratio of 50/50 provided a pure phase in rhombohedral symmetry which was more stable than in the orthorhombic.

Characterisation of cells structure

A tubular cell was produced by extrusion of NiO/ BCZYZ anode support and multistep co-firing process with dip-coated BCZYZ electrolyte and LSCF/ BCZYZ cathode. Figure 6 (a) shows the BSE/SEM analysis of the cell’s microstructure after reduction with H2. The SEM analysis of the cell’s crosssection shows dense ~40 pm BCZYZ electrolyte (312), porous -1100 pm Ni/BCZYZ anode support (314) and -5 pm LSCF/BCZYZ (300) cathode 316.

The image in Figure 6 (a) was obtained with a Back Scattering Electron probe (BSE) and was used for porosity measurement of the anode with Imaged software, thanks to the high contrast between the resin, ceramic and metal. Two phases visible on the anode 314 side are Ni (304) with a darker colour, and a brighter phase, BCZYZ (302). The anode 314 shows around 15% total porosity, made of large pores formed between the particles and smaller pores at the BCZYZ/Ni interface and in the Ni itself, most likely due to NiO reduction, Figure 6 (c). Figure 6 (d) shows the LSCF/BCZYZ cathode. LSCF is denoted by 306, BCZYZ by 302. The electrode 316 was sintered as the composite to assure a good interface with BCZYZ electrolyte; Figure 6 (b) shows a dense layer of BCZYZ electrolyte. Both electrodes have good adhesion with BCZYZ electrolyte; no cracking or delamination is present in the cell’s structure.

Electrochemical characteristic

The cell’s performance tested at 133 ml/min of NH3 at 650 - 750 °C is shown in Figure 7. Figure 7a shows the l-V curve of cell operating at 650-750 °C with 133 ml/min of NH₃. Figures 7(b) - (c) EIS and DRT at 650-750 °C at OCV conditions. Figures 7(d) - (e) EIS and DRT at 650-750 °C at 10A in fuel cell mode. Figure 7(f) shows l-V curve of cell operating at 650 °C with H2/N2 mixture and NH3. Figures 7(g) - (h) show corresponding EIS and DRT at OCV conditions. Figures 7(i) - (j) show corresponding EIS and DRT at 10A in fuel cell mode.

The generated power increased significantly with rising temperature, and up to 7.7W was generated at 750 °C from a single cell with 36 cm² of surface area when tested in the 0-10A range. Due to the limitation of power load, the peak power was determined by connecting a series of low resistance, high power resistors as the load for the cell and was equal to 8.5W, reaching 0.236 W/cm² maximum power density.

When applying the changing current, the slope of the line seems to decrease, especially visible for the lower furnace temperature, which could be related to cell temperature increasing through Joule heating - according to the temperature measurement inside a cell. The slope change is lower at higher temperatures and even increases at 750 °C. Behaviour could be related to increased concentration losses, leading to competition between temperature and concentration factors as well as changing Joule heating with higher electrolyte conductivity at higher temperature.

The OCV was between 1.08 and 1.01 ; decreasing with rising temperature, which agrees with the thermodynamics of H2 containing gas rather than the direct oxidation of NH3. For the direct conversion of ammonia, the theoretical OCV value increases with the rising temperature and is much higher than for H2.³² Nevertheless, the high value of OCV indicates good gas-tightness of the cell, sufficiently dense electrolyte and robust structure.

Considering a high operating temperature and excellent catalytical properties of Ni/BCZY based materials for ammonia decomposition, it was expected that ammonia would be almost entirely cracked to H2 and N2 on the anode; and rather than direct oxidation of ammonia, the hydrogen will be used in the electrochemical reaction.

Figure 7(f) compares the current-voltage and impedance characteristics of the cell’s performance at 133 ml/min flow of NH3 and the mixture of H2/ N2 (200/66 ml/min), which have the same flow and stochiometry assuming complete ammonia decomposition. The same shape of the l-V curves suggests similar electrode kinetics in both cases, further confirmed by impedance data showing no difference in the shape of the EIS or DRT curve for the whole frequency range (Figure 7 (g-j)). The slightly lower performance of ammonia could be explained by the lower temperature inside the cell measured by the thermocouple (the 650 °C reported in the plot is furnace temperature). A thermocouple was placed in the top part of the cell; the measured temperature was 661 °C for H2/N2 mixture and 653 °C for NH3 fuel while at maximum current (10A) 694 °C and 679 °C, respectively. At OCV condition (Figure 7 (g)), the P2 & P3 process increases at a higher rate than P1 and P4 for ammonia fuel, which could be due to the higher activation energy of those as indicated in Figure 9 (a). Respectively, the higher increase of P1 and P2 in comparison to P3 and P4 when current was applied (Figure 7 (h)) could be related to the change of their activation energy at fuel cell operation, Figure 9 (b). Higher temperature inside a cell than in the furnace for OCV conditions may be due to small leaks in the cell; thus, burning of H2 and heating the cell from outside; we do not expect that O2 would diffuse into the cell through the eventual leaks due to slight overpressure (8 mbar) on the fuel side. The higher temperature at the fuel cell operation comes from the exothermic reaction of H2 oxidation and Joule heating. The smaller increase in temperature for the ammonia fed of the cell is due to the endothermic reaction of ammonia cracking, partially compensating for the exothermic fuel cell processes.

The ammonia cracking was further confirmed by outlet gas analysis at 750 °C with gas chromatography, where no ammonia in the gas stream was detected.

Figure 7 (b) - (e) shows EIS and DRT analysis at 650-750 °C in NH3 fuel, and (g) - (j) compares impedance analysis of NH3 and H2/N2 mixture at 650 °C; measured at OCV conditions and under 10A current in fuel cell mode. The Nyquist plot of impedance spectra illustrates ohmic resistance and polarisation of processes on anode and cathode. A fuel cell’s polarisation usually is affected by the diffusion of gases, their conversion on the electrode, transport of and charge transfer reaction. Those frequency-dependent polarisation processes in the upper part of the Nyquist plot above the x-axis illustrate processes from anode and cathode on the fuel cell spectra. The ohmic resistance R_s, which mainly comes from the resistance of wires, electrolyte and contact between current collector and electrode, is indicated by the intersection point on the Nyquist point with the x-axis at high frequency. In most cases, an impedance spectrum is affected by inductance from testing equipment, which changes the position of the intersection point; thus, it must be extracted (or modelled) for better estimation of ohmic resistance and high-frequency process.

Not many processes could be distinguished from EIS spectra at OCV (Figure 7(b)) and fuel cell operation (Figure 7 (d)). In the temperature range 650-700 °C, responses entirely merged, while, for 700-750 °C, two arcs could be separated; however, they also could be a sum of many processes at close frequencies. The equivalent circuit could be constructed using complex non-linear least squares (CNLS) fits to characterise and separate individual processes. However, the fitting becomes challenging without previous identification of some of their parameters, as the method requires a pre-defined model with a specific number and type of elements. Often, the equivalent circuit elements could be misinterpreted, and even a substantially different system could respond similarly. Alternatively, the number, size, and frequency of those responses could be predetermined with DRT constructed from impedance spectra; the DRT gives a much higher resolution than Nyquist of Bode plots and could be used even as the sole representation. For the OCV conditions at the investigated temperature range (Figure 7 (c)), the DRT shows at least four processes, with a dominant contribution at middle to low frequency; similarly, DRT at 10A (Figure 7 (e)) shows up to four processes; but, with the size and frequency changed under applied anodic polarisation.

Apart from the number and approximation of characteristic frequency of circuit elements, DRT gives the possibility to find their resistance, as the area under the peak corresponds to the ohmic resistance of the dynamic process this was achieved by fitting Gaussian peaks into DRT (Figure 8 (a&c)). The fitting of Gaussian peaks was made with Matlab based free program, a peakfit. Figure 8 shows the Development of Equivalent Circuit for impedance analysis at OCV and fuel cell operation with 133 ml/min NH3. Figure 8 (a) is a Gaussian fit to DRT of impedance data at 650°C at OCV & Figure 8 (b) shows Equivalent Circuit with four processes. Figure 8(c) shows Gaussian fit to DRT of impedance data at 650°C at 10A and Figure 8 (d) shows Equivalent Circuit with five processes.

The cell’s polarisation and ohmic resistance, obtained from Gaussian fitting, were plotted on the Arrhenius dependence for OCV and fuel cell operation, shown in Figure 9, where Figure 9 shows Arrhenius plots of ohmic and polarization resistance at 650- 750 °C (a) at OCV conditions and (b) at 10A fuel cell mode R_s, the ohmic resistance is dominant and is responsible for the highest voltage drop in the cell’s operation. The activation energy slightly decreased as the cell voltage was depressed under load. The ohmic resistance dropped under applied current, which could be related to the increased temperature inside a cell during the operation, but other factors than the temperature could influence it. The decrease in ohmic resistance could be related to an increase in proton carrier concentration due to a higher rate of H2O production on the cathode side, thus increasing the concentration of hydroxyl ions in the vicinity of the electrolyte/cathode interface. The high-frequency P1 process, which is most likely charge-transfer-related increasing at an applied current, also its activation energy (Ea) increasing from 0.75 eV to 2 eV. The P2 behaves almost the opposite way; its activation energy at OCV condition from 2.5 eV decreases to 1.1 eV. The P3 is the main contribution to the polarisation of the cell at OCV; its activation energy changes significantly under applied voltage, from very high at OCV to almost no temperature dependence at applied current; this may indicate some dissociative-adsorption processes coupled with gas diffusion, which becomes a dominant factor at higher overvoltage. Also, the low Ea of P4 suggests the process coupled with a gas diffusion. One more process, P5, becomes visible at 10A (Figure 7 (e)), which also becomes visible in a test we conducted at lower NH3 flow, Figure 10, suggesting a anode’s concentration loss. The linearity of the plots confirms the correctness of the Gaussian estimates, also validated by equivalent circuit fitting Figure 8 (b&d), which gives very close results. With the number and polarisation resistance of elements known, the equivalent circuit could be constructed consisting of ohmic resistance (Rs) and four semicircles; the inductance (3.7x1 O'⁷) was fitted to the impedance at 650 °C and extracted from the data. The semicircles were modelled by resistance, and constant phase elements (CPE) connected parallel (R/CPE). R represents the polarisation process’s resistance, while CPE represents its capacitive behaviour and is modelled by two parameters. With the R and CPE known, the value of capacitance and characteristic frequency could be calculated, Table 1. Due to the diminishing contribution of polarisation at a higher temperature, the fitting was done at 650 - 700 °C, which offers a better estimation.

Char. Freq./ Ea/ eV Capacitance/

_K. Hz At 650 - 750 F

No. Description _{AT 65Q O}Q OQ _{AT 65Q O}Q

OCV/10A OCV/10A OCV/10A

The ohmic resistance of the

Rs cell - 0.44/ 0.36

P1 Cathode 1310/ 7584 0.78/ 2 0.08/ 0.004 P2 Anode 35/ 256 2.5/ 1.1 0.47/ 0.15

P3 Anode 3.6/ 29 1.77/ 0.04 5.6/ 2.8

P4 Cathode 0.18/ 1.9 0.49/ -0.01 460/ 59

Table 1 : The overview of processes in P-SOFC tubular cells indicated at 133 ml/min flow of NH3.

Although a more detailed analysis needs to be done to precisely assign circuit elements to specific processes or electrode, we can still observe their behaviour and determine their fundamental parameters. In the usual approach, their dependency on various conditions is measured, e.g. how processes change with gas composition on one of the electrodes, applied voltage or when working at various temperatures. Theoretically, the processes could be identified by the value of their characteristic frequencies, capacitance and activation energy. However, one should keep in mind that the processes on porous electrodes and interface highly depend on the cell’s structure and characteristics, and direct comparison between the same type of system but different cells is not straightforward; also, obtaining such data for P-SOFC is relatively rare, as most of the work has been done on Ni/YSZ system.

Figure 10 shows the electrochemical performance of the tubular cell at the various flow of NH3. The l-V measured at lower ammonia flow, 66 ml/min, has lower OCV than one measured at 133 ml/min and shows the fuel starvation at -0.75 V in fuel cell mode; however, the kinetics seems to improve as the slope of the curve is slightly lower, Figure 10 (a).

The EIS at OCV conditions demonstrates that ohmic and polarisation resistance decreases at lower ammonia flow, Figure 10 (b). The DRT gives insight into separate processes and shows that P2, P3, and P5 are affected by the fuel flow, Figure 10 (c); which allows allocating those responses to the anode. The P5 likely shows a concentration loss on the anode; it increases at higher anodic overpotential and becomes dominant at the fuel starvation point Figure 10(d).

The decrease in total resistance is mainly due to the decrease in ohmic resistance R_s, and in resistance of P2 and P3 processes. One possible explanation is the higher temperature inside a cell due to the smaller concentration of ammonia and the extent of endothermic cracking reaction. However, the measured temperature inside the cell was only around 6 °C higher for the test with 66 ml/min of ammonia, and it can not explain the large change in ohmic and polarisation resistance. The P2 and P3, which decreased under the lower flow of ammonia, could be related to mass transfer at the anode side.

Four steps could be considered for NH3 utilisation in the P-SOFC system on the anode, including gas diffusion, ammonia adsorption on the electrode’s surface, ammonia decomposition to N2 and H2 and splitting of H2 to produce electrons and protons, which are transported through the electrolyte to the cathode. Knowing the high activity of Ni for H2 splitting, H2 from ammonia cracking would absorb and dissociate on Ni to monoatomic hydrogen, which would be transferred to BCZYZ, e.g. through the “spillover” process. It has previously been observed that the high-frequency process decreases at applied potential, contrary to shown results where the high frequency arc seems to increase. Also, in the tubular cell of the present disclosure, the processes originating from the anode seems to be higher than contribution from cathode.

P1 and P4 are not much affected by the fuel flow and are believed to come from the oxygen side. P4 seems to be slightly affected at 0.1V applied overvoltage and moved to a lower frequency. At higher overpotential, the process can not be recognised as it most likely merged with P5. The P1 process increases at higher voltage, similar to the previous observations. In the LSCF based cathode in P-SOFC, the oxygen gas will absorb and dissociate on the surface. The oxygen species would diffuse to the interface with the electrolyte, where at TPB would form water with the proton conducted from the electrolyte. A similar value of frequency and capacitance for the LSCF electrode allows us to assign the P1 process to interfacial resistance at the cathode, which includes water formation in charge transfer between oxygen species and protons. While P4 is most likely related to concentration losses due to low activation energy.

Cell operation and stability

Figure 11 shows the performance of the cell during operation for a total time of about 170 h. Figure 11 (a) shows temperature of the furnace and cell; and voltage signal of exhaust flow; Figure 11 (b) shows voltage of the cell and generated current. During operation, the cell was run at OCV conditions, characterised by l-V and impedance methods and discharged at a constant voltage. The temperature of the furnace and inside the cell plus the signal measured by the mass flow controller at the anode exhaust were registered for the whole operation time, Figure 11 (a). The thermocouple was placed inside the cell in the top part to measure the cell’s temperature. The mass flow controller generated a voltage signal equivalent to the exhaust mass flow, assuming pure H2. As the composition of gases changes during the operation (ratio of H2 to N2), it is not possible to measure an exact flow with this method; however, for the certain operation conditions, assuming a constant H2/N2 ratio, it gives a good indication of its relative change. The cell stability test was conducted at a constant potential (-0.25V relative to OCV) in 4 cycles under 133 ml/min flow of pure NH3, Figure 11 (b). Between the discharging cycles, the cell was kept at OCV conditions or characterised by l-V and EIS characteristics (indicated in the figure by large voltage changes).

At the start of the test, the cell was heated up to 650 °C and reduced in 100 ml/min of H2 and 100 ml N2. During cell reduction, a constant signal was measured by the mass flow controller at the exhaust, which increased after about an hour from the start of reduction and achieved a maximum after another hour. During the first hour of reduction, it is likely that all H2 was utilised for NiO reduction, and the signal increased afterk^ started to break through. Afterwards, the exhaust gas flow signal started to decrease slightly, which could be related to the development of small leaks during the initial period. The temperature inside the cell increased during reduction to a maximum of 668 °C, likely due to the exothermic reaction and decreased to 663 °C afterwards and stabilised. Next, the cell was switched to 200 ml/min of H2 and 66 ml/min of N2 and electrochemically characterised and compared with 133 ml/min flow of NH3 as shown in Figure 7 (f-j).

Afterwards, the cell was held on the first stability run at 650 °C, fuelled by 133 ml/min of NH3 and atmospheric air on the cathode for ~20h. The cell showed the highest degradation during this period, up to 12% of the current loss. The temperature inside the cell decreased while the exhaust gas flow signal slightly increased, following the changes in the generated current, which indicates changes in the cell temperature due to the Joule heating and exothermic reaction of oxidation, where the increase of gas exhaust signal indicates loss of fuel utilisation.

Next, the temperature was increased to 750 °C, the cell was electrochemically characterised, and another stability run (~20 h) was made at - 0.25V relative to OCV, where the 7.5 % current degradation was registered, mainly during the first 10h. As previously mentioned, a signal measured by the mass flow controller at the anode exhaust and temperature inside the cell followed trends in generated current. Before the third stability cycle, the gas chromatograph (GC) was connected to the exhaust flow from the cell prior to the mass flow sensor, which is indicated by the drop of the gas flow signal measured at the exhaust. Also, the third stability run was made for ~20 h at -0.25V relative to OCV at 750 °C with 133 ml/min of NH3. The degradation rate decreased, and about 2.5% of current was lost during this run. The behaviour of the exhaust gas flow signal was different than in the previous discharging cycles, as it slightly decreased with the current loss. This could be related to the increase in leakage rate, which affects generated current due to lower concentration of the fuel. In the next longer discharging cycle for a continuous 75h of operation, there are some changes in the gas flow and the current seems to follow its pattern. Nevertheless, the cell during the last stability cycle at 750 °C showed no current loss, in contrast to the previous cycles. After discharging, the cell was characterised at various temperatures by l-V and EIS.

The cell’s performance was characterised by l-V and EIS methods on various days of its operation, Figure 12 (a). Figure 12 shows the electrochemical performance of the tubular cell at various operation periods at 750°C - Figure 12(a) is an l-V curve of cell operating in fuel cell mode; Figures 12(b)-(c) show corresponding EIS and DRT at OCV conditions. Figures 12(d)-(e) show corresponding EIS and DRT at 10A in fuel cell mode. The measurement of the pristine cell was made after about 20 hours of the gas reduction; they indicate the highest OCV and best electrode kinetics. After 67h of operation, which includes gas reduction, and two stability runs (20h at 650 °C and 20h at 750 °C), the cell performance decreased due to the slightly lower OCV and increase of both ohmic and polarisation resistance. According to the previous assignment of the polarisation processes, both anode and cathode are affected. The next characterisation point is after 165h of the cell’s operation, which is after the fourth stability cycle, Figure 12. The polarisation resistance seems to increase further for all mentioned processes; however, the ohmic resistance decreases; thus, the cell shows very similar performance as at the previous points of analysis. EXAMPLE 2 - DIRECT METHANOL FUEL CELL

Cell testing

Cell testing was carried out in a Single Cell Test Stand (SCTS). The test stand is comprised of:

1. A furnace for heating the cell

2. A gas control system for metering gases supplied to the stack

3. A DC load (or DC supply for electrolysis) system for sinking of electrical power from the stack

4. A data acquisition system for logging of experimental environmental and stack performance parameters

5. A cell support and interface system

6. A mechanical and balance of plant system for supporting the test

7. A HPLC pump for delivering accurate liquid flow to the cell.

Cell assembly

The test in this example comprises a single cell having a composition as outlined in table 2. The cell takes the form of an extruded green tube.

Table 2~ Composition of extruded green tubes

The extruded green tube underwent multiple processes after extrusion in order generate single cell assemblies.

Experimental

The required methanol/water mixture was calculated stoichiometrically to be a 1:1 ratio.

The amounts of each were weighed then mixed and transferred into a carboy connected to the Agilent HPLC pump.

Once the assembly and methanol/water preparation processing steps were completed, the cell was ready for reduction of the anode. This step is required for reduction of the NiO content in the anode support to nickel metal. This nickel metal then provides an electrically conductive pathway between the anode current collector wires and the electrocatalytically active sites of the anode.

To achieve reduction of the NiO, the cell was heated to 650°C and hydrogen containing gas was flowed through the anode. The sequence of operations was as follows:

1. The furnace was heated at 10°C/min to 650°C

2. Once the cells reached a steady temperature, 0.05 SLPM Nitrogen gas and 0.05SLPM Hydrogen gas was flowed to the cell through the porous steam generator

Once reduction was deemed complete through monitoring of the OCV, it was noted that there appeared to be a sense wire issue. The cell was cooled to room temperature, the issue resolved, and the cell heated back to 650°C at 5°C /min still under reducing conditions.

The Methanol/water mix was pumped slowly to prime the pipework to ensure as little time as possible where the cell was running with no flow. The gas supply to the anode was switched off and the gas-supplying valve closed to prevent liquid flowing the wrong way down the pipework and damaging the mass flow controllers. The Methanol/water mix was then set to 0.05ml/min. 0.5 SLPM of air was then flowed to the cathode side of the cell (into the furnace open volume) to ensure adequate oxygen was available.

When the Methanol/water switch over was complete, a series of IV tests at different furnace temperatures were run. In each case, the current was ramped at 2A/min up until just past peak power, then 2A/min down by manually adjusting a load bank connected across the cell. The 650°C furnace setting was run first, followed in sequence by 675°C, 700°C, 725°C and 750°C.

Figure 13 shows the results of the IV tests. The maximum peak power was seen at the highest temperature test of 750°C, whereas the lowest peak power was seen in the 675°C test. This suggests that there was a degradation event between the 650°C and the 675°C IV tests as there is a significant drop in performance between the two. 650°C post-test plots were performed after all other IV testing to determine if there was a difference between the cell at the start of test and post testing.

Table 3 lists the peak cell power developed for each of the furnace setpoint temperatures, plus the current, cell voltage and current density at peak power. This example shows the ability of the fuel cell according to the present disclosure to use Methanol as a fuel source when operating as a fuel cell. Using higher temperatures appears to provide greater current density and power. Table 3 - Peak power and current at peak power for each of the furnace temperature setpoint IV tests

Claims

CLAIMS:

1. A method of operating a fuel cell, wherein the fuel cell comprises an anode, a cathode and a proton conductive solid oxide electrolyte arranged between the anode and the cathode, and the method comprises: feeding a fuel to the fuel cell, wherein the fuel comprises methanol or ammonia.

2. The method of claim 1 , wherein the fuel comprises or is supplied as a liquid fuel.

3. The method of claim 1 or 2, wherein the proton conductive solid oxide electrolyte comprises BaCei-x-yZr_xYyO3-6 (BCZY), optionally BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ).

4. The method of any preceding claim, wherein the anode comprises BCZYZ and nickel oxide (NiO).

5. The method of claim 4, further comprising reducing the NiO prior to fuelling with the liquid fuel.

6. The method of any preceding claim comprising cracking ammonia at the anode of the fuel cell into hydrogen and nitrogen; or comprising reacting methanol with water at the anode of the fuel cell to form carbon dioxide, H⁺ ions and electrons.

7. The method of any preceding claim, wherein the cathode comprises BCZYZ and Lao.8Sro.2Coo.5Feo.503-6 (LSCF).

8. The method of any preceding claim comprising arranging the cathode to be exposed to air.

9. The method of any preceding claim comprising operating the fuel cell at a temperature above 300°C.

10. The method of any preceding claims comprising utilising heat produced from the reactions taking place at the cathode to provide subsidiary heat for the endothermic reactions taking place at the anode.

11. The method of any preceding claim comprising operating the fuel cell for up to 170 hours.

12. The method of any preceding claim, comprising feeding a first fuel to the fuel cell for a first period of time, and feeding a second fuel to the fuel cell for a second period of time.

13. The method of any preceding claim comprising providing the fuel cell in a vehicle.

14. A fuel cell comprising an anode, a cathode and a solid oxide electrolyte, wherein the solid oxide electrolyte comprises BaCeo.7Zro.iYo.i6Zno.o403-6 (BCZYZ), and wherein the solid oxide electrolyte is arranged between the anode and the cathode.

15. The fuel cell of claim 14, wherein the solid oxide electrolyte is a thin film electrolyte.

16. The fuel cell of claim 14 or 15, wherein the anode comprises BCZYZ and nickel oxide (NiO); and/or the cathode comprises BCZYZ and Lao.sSro^Coo.sFeo.sOs-s (LSCF).

17. The fuel cell of any of claims 14 to 16, wherein the anode has a tubular configuration.

18. A method of manufacturing a fuel cell according to claims 14 to 17, comprising providing an anode comprising BCZYZ and nickel oxide (NiO), coating the anode in BCZYZ electrolyte to form an electrolyte coated anode; and further coating the electrolyte coated anode in a mixture comprising BCZYZ and Lao.8Sro.2Coo.5Feo.503-6 (LSCF) to form a cathode surrounding the electrolyte coated anode.

19. The method of claim 18, further comprising extruding the anode comprising BCZYZ and nickel oxide (NiO), and/or wherein the coating and further coating comprise dip coating.

20. A system comprising: a fuel cell according to any of claims 14 to 18; a first reservoir configured to hold or store a first fuel; and at least one first conduit arranged for the supply of the first fuel to the fuel cell.

21. The system of claim 20, wherein the system further comprises: a second reservoir configured to hold or store a second fuel; and at least one second conduit arranged for the supply of the second fuel to the fuel cell.

22. A system according to claim 20 or claim 21 , wherein the first fuel is provided as a liquid fuel and/or wherein the second fuel is provided as a liquid fuel.

23. The system of any one of claims 20 to 22, wherein the system further comprises a vehicle arranged to be powered by the fuel cell.

24. A vehicle comprising a fuel cell according to any one of claims 14 to 18 or a system according to any one of claims 20 to 23.

25. The vehicle of claim 24, wherein the vehicle is a ship.