GB2518680A

GB2518680A - Water removal in a fuel cell

Info

Publication number: GB2518680A
Application number: GB1317270.5A
Authority: GB
Inventors: Jignesh Karsan Devshi Patel; Pratap Rama; Paul Leonard Adcock
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2013-09-30
Filing date: 2013-09-30
Publication date: 2015-04-01
Also published as: WO2015044683A1; GB201317270D0

Abstract

An electrochemical fuel cell assembly 1 includes a fuel cell having a fuel delivery inlet 3 and a fuel delivery outlet 4. The fuel cell stack 2 has a membrane-electrode assembly 5 and a fluid flow path coupled between the fuel delivery inlet and the fuel delivery outlet for delivery of fuel to the membrane-electrode assembly. A fuel delivery conduit 6 is coupled to the fuel delivery inlet for delivery of fluid fuel to the fuel cell. A flow control device 11in the fluid delivery conduit, which may be an ejector, is configured to provide a pulse modulated inlet pressure at the fuel delivery inlet. The pulse modulation may be provided as a function of fuel cell operating conditions. The amplitude and frequency of the pulse modulation may be selected to create sufficient flow disturbance to encourage disturbance and dislodgement of water collecting in the fluid flow path. The control functions, which are used to regulate the pulse modulated fuel pressure, may be humidity of the fuel cell, cell voltage, resistance or impedance, fuel cell electrical load, fuel concentration, water levels or temperature.

Description

WATER REMOVAL IN A FUEL CELL

The present invention relates to fuel cells and in particular, though not exclusively, to proton-exchange membrane type fuel cells in which hydrogen is supplied to an anode side of the fuel cell, oxygen is supplied to a cathode side of the fuel cell and water by-product is produced at and removed from the cathode side of the fuel cell.

Such fuel cells comprise a proton exchange membrane (PEM) sandwiched between two porous electrodes, together comprising a membrane-electrode assembly (MEA). The MEA itself is conventionally sandwiched between: (i) a cathode diffusion structure having a first face adjacent to the cathode face of the MEA and (ii) an anode diffusion structure having a first face adjacent the anode face of the MEA. The second face of the anode diffusion structure contacts an anode fluid flow field plate for current collection and for distributing hydrogen to the second face of the anode diffusion structure. The second face of the cathode diffusion structure contacts a cathode fluid flow field plate for current collection, for distributing oxygen to the second face of the cathode diffusion structure, and for extracting water reaction product from the MEA. The anode and cathode fluid flow field plates conventionally each comprise a rigid, electrically conductive material having fluid flow channels in the surface adjacent the respective diffusion structure defining flow paths for delivery of the reactant gases (e.g. hydrogen and oxygen) and removal of the exhaust gases (e.g. unused oxygen and water vapour).

An important consideration in the operation of such fuel cells is the management of water and inert gases such as nitrogen within the MEA and within the flow fields delivering fluids to the MEA. Membranes used in fuel cell manufacture typically allow small quantities of water and nitrogen to pass through the membrane from the cathode side to the anode side. While it is important that the MEA remains suitably hydrated during use, failure to control the MEA humidification and the level of inert gas concentration on the anode sides of the fuel cells in the stack can result in dilution of the fuel and/or blockage of the flow paths and hence poor electrical cell performance and/or premature cell failure.

The present invention is particularly related to the management of non-fuel fluids and solids in the fuel delivery (anode) flow paths in a fuel cell or fuel cell stack. Non-fuel fluids for removal may particularly include water, although removal of inert gases such as nitrogen, other gaseous contaminants, particulates and debris that may otherwise build up in the fuel delivery flow paths within the fuel cell may also be relevant.

One approach to management of the anode flow paths is to periodically purge the anode flow paths with a purge gas, such as nitrogen. This can be effective in flushing out contaminants from the anode flow paths, but has potential disadvantages in disrupting electrical output of the fuel cell and requiring a local source of nitrogen purge gas.

Another approach is to periodically purge the anode with a higher than normal flow of the fuel gas, e.g. hydrogen. This has a potential disadvantage of being wasteful of fuel.

Such a periodic purge of the anode may be effected by opening a purge valve in an anode exhaust line. This reduces the pressure at the anode outlet, thereby temporarily increasing flow which entrains water and other contaminants out of the fuel cell.

It is an object of the present invention to provide an alternative way to purge anode flow paths of non-fuel fluid and/or solid contaminants.

According to one aspect, the present invention provides an electrochemical fuel cell assembly comprising: a fuel cell having a fuel delivery inlet and a fuel delivery outlet, the fuel cell having a membrane-electrode assembly and a fluid flow path coupled between the fuel delivery inlet and the fuel delivery outlet for delivery of fuel to the membrane-electrode assembly; a fuel delivery conduit coupled to the fuel delivery inlet for delivery of fluid fuel to the fuel cell; a flow control device in the fluid delivery conduit configured to provide a pulse modulated inlet pressure at the fuel delivery inlet.

The flow control device may be configured to provide said pulse modulated inlet pressure as a function of fuel cell operating conditions. The flow control device may be configured to deliver the pulse modulated inlet pressure as a series of pressure pulses each including a positive and/or negative excursion from a baseline pressure. Each pressure pulse may comprise a low pressure pulse followed by a high pressure pulse, relative to a baseline pressure. The frequency of pressure pulses may be determined as a function of fuel cell operating conditions. The depth of each low pressure pulse below baseline pressure and I or the height of each high pressure pulse above baseline pressure may be determined as a function of fuel cell operating conditions. The flow control device may be configured to vary the amplitude and I or frequency of the pulse modulation of the inlet pressure to the fuel cell as a function of one or more of: humidity in a fuel circuit of the fuel cell assembly; cell voltage, resistance or impedance; fuel ceO electrical load; fuel concentration in a fuel circuit of the fuel cell assembly; water levels in a fuel circuit of the fuel cell assembly; temperature in a fuel circuit of the fuel cell assembly. The flow control device may be configured to vary the amplitude and / or frequency of the pulse modulation of the inlet pressure to the fuel cell as a function of electrochemical impedance spectroscopy (EIS) analysis of the fuel cell performance.

The baseline pressure may be varied according to fuel cell power demand. The electrochemical fuel cell assembly may include an electrochemical impedance spectroscopy analysis system configured to apply an AC current or voltage modulation to at least one cell output, the flow control device being configured to vary the pulse modulation of the inlet pressure as a function of measured impedance of one or more cells at one or more frequencies.

According to another aspect, the present invention provides a method of operating an electrochemical fuel cell assembly comprising: providing a fuel cell having a fuel delivery inlet and a fuel delivery outlet, a membrane-electrode assembly and a fluid flow path coupled between the fuel delivery inlet and the fuel delivery outlet for delivery of fuel to the membrane-electrode assembly; delivering fuel to the fuel cell via a fuel delivery conduit coupled to the fuel delivery inlet; controlling a flow control device in the fluid delivery conduit to provide a pulse modulated inlet pressure at the fuel delivery inlet.

The pulse modulated inlet pressure may be controlled as a function of fuel cell operating conditions.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a fuel cell assembly incorporating a pressure control device in a fuel delivery inlet in a system having a fuel recirculation circuit; Figure 2 shows schematic diagrams illustrating an alternative ejector-based pressure control device and an alternative pump-based pressure control device in a fuel recirculation circuit; Figure 3a is a schematic diagram of a fuel cell assembly incorporating a pressure control device in a fuel delivery inlet in a system without a fuel recirculation circuit; Figure 3b is a schematic diagram of a fuel cell assembly similar to figure 3a but incorporating a fixed orifice ejector: and Figure 4 shows a pulsing scheme suitable for use with the fuel cell assemblies of figures Ito 3.

With reference to figure 1, an electrochemical fuel cell apparatus I comprises a fuel cell stack 2 having a fuel delivery inlet 3 and a fuel delivery outlet 4. The fuel cell stack 2 is constructed from a plurality of fuel cells 5 in stack formation according to well known principles. Each cell 5 has a membrane-electrode assembly (MEA), an anode fluid flow path for delivering fluid fuel to the anode side of the MEA, and a cathode fluid flow path for delivering fluid oxidant to the cathode side of the MEA. The cathode flow path and cathode fluid delivery infrastructure for delivering oxidant fluid (such as air) to the MEA and exhausting reaction by-product (such as water) from the MEA may be configured in entirely conventional manner and are not shown or described here.

The fuel delivery inlet 3 is coupled to a fuel delivery conduit 6 which is fed from a fuel supply 7 such as the hydrogen supply shown. The fuel delivery conduit thereby delivers fluid fuel to the fuel cell stack 2 where it is distributed to each cell within the stack using appropriate manifolds (not shown). The fuel delivery outlet 4 is coupled to a recirculation conduit 8 which is configured to return unused fuel back to the fuel delivery conduit 6 as will be described in more detail shortly. The recirculation conduit 8 also includes a bleed valve 9 which is coupled to allow a flow of fluid to enter a bleed line 10. The bleed valve 9 may be of any suitable type to control fluid to be bled from the fuel delivery outlet and recirculation conduit to the bleed line between zero and a maximum flow, such as an on-off type valve or, as shown in figure 1, a variable orifice bleed valve which can be used to continuously vary the flow rate therethrough between a minimum and maximum value.

The recirculation conduit B extends to an ejector 11 in the fuel delivery conduit 6. The ejector 11 is a variable orifice ejector. The ejector 11 has a high pressure fuel line inlet M which may be designated as the "motive" inlet; a discharge outlet D configured to discharge fuel and recirculating fluids to the fuel delivery inlet 3 of the fuel cell stack; and a suction inlet S configured to provide a low pressure suction force to the recirculation conduit 8.

A suitable variable flow ejector has been described in "Hydrogen Recirculation Ejector", University of Delaware Fuel cell Research Lab (http://www.rne.udel.edu/research grouDs/prasad/research/eiector.htmfl.

The recirculation conduit 8 preferably also includes components for the removal of water and/or water vapour from the recirculating fuel. These components may include, for example, a first water separator or condenser 12 for extracting water vapour and/or liquid water; a heat exchanger 13 for extracting heat from the recirculation fluid; and a second water separator or condenser 14 for extracting water vapour and/or liquid water.

The water outlets or drains of the water separators 12, 14 may be provided with automatic drains or solenoid valves 15, 16.

The apparatus 1 may further include one or more sensors of various types. In a preferred arrangement as shown in figure 1, a pressure sensor 21 is connected to the fuel delivery conduit downstream of the discharge outlet D of the ejector 11. In a preferred arrangement as shown in figure 1, a fuel concentration sensor 22 is connected to the fuel delivery conduit downstream of the discharge outlet of the ejector 11. The fuel concentration sensor 22 is, in a preferred arrangement a full range (0 -100%) hydrogen concentration sensor. The apparatus may also include a temperature sensor 27 and a humidity sensor 26 connected to the fuel delivery conduit downstream of the discharge outlet D of the ejector 11.

The apparatus 1 further includes a bleed controller 20 coupled to the variable orifice bleed valve 9 for controlling a bleed rate of the bleed valve. The bleed controller 20 is provided with one or more inputs, one of which may include a fuel concentration sensor output 22a from the fuel concentration sensor 22.

In one arrangement as shown in figure 1, the fuel cell stack 2 also includes a plurality of cell voltage monitoring outputs 23 which provide voltage levels from one or more individual cells 5 within the stack 2. Cell voltages can be measured for every cell in the stack or possibly for selected cells only. The bleed controller 20 may also be provided with an input corresponding to the cell voltage monitoring output 23a.

The apparatus 1 includes a fuel controller 25 coupled to the ejector 11 for controlling the ejector variable orifice and thereby determining fuel pressure at the discharge outlet D. The fuel controller 25 may be coupled to receive as input the pressure sensor output 21a and the cell voltage monitoring outputs 23a.

The bleed line 10 may lead to a vent for disposal of fuel to atmosphere, or may lead to a suitable fuel recovery apparatus.

In the fuel cell apparatus I as shown in figure 1, the fuel delivery conduit 6, the recirculation conduit 8, and the fuel flow paths within the fuel cell stack 2 may be defined as a fuel circuit. Fuel is recirculated using the ejector 11. The bleed valve 9 and the bleed line 10 are used to remove excess water and I or inert or contaminant gases from the fuel circuit which would otherwise build up within the fuel circuit by releasing a controlled amount of fuel as a carrier gas. The bleed valve 9 and the bleed line 10 may also be effective in purging particulates and debris that may otherwise build up in the fuel circuit.

The ejector 11 is preferably configured as a variable geometry ejector which is capable of actively controlling the anode inlet pressure. The ejector is controlled by the fuel controller 25 to modulate the anode inlet pressure such that it delivers periodic pressure pulses in the baseline anode inlet pressure within the fuel delivery conduit 6.

With reference to figure 4, exemplary pressure pulses are shown for the anode inlet pressure in fuel delivery conduit 6. The objective is to control the ejector discharge outlet 0 to modulate the baseline (normal) anode inlet pressure with a series of pressure pulses, preferably having a sham transient increase in pressure that serves to create a disturbance or disruption within the anode flow paths in the fuel cell stack 2.

In a first example shown in figure 4a, a series of simple square waves 40 are used, having a pulse width w and pulse height h, with the rising edges 40a providing a pressure pulse from the baseline pressure 41. In another example shown in figure 4b, a series of inverted square waves 42 are used, having a pulse width wand pulse height h, with the rising edges 42a providing a pressure pulse as the pressure returns to the baseline pressure 41.

In another preferred example shown in figure 4c, it is recognised that a larger pressure pulse can be provided by using a combined waveform 44 having a negative pulse 44a of width Wi and height h1, a positive pulse 44b of width w2 and height h2, both relative to baseline 41, which are combined to create a larger rising edge 44c of height h1 + h2.

The pulses 44 first reduce the anode inlet pressure by amplitude h1 from the baseline pressure 41 for the period of time w1, and then rapidly increase pressure to an amplitude h2 above the baseline pressure 41 for a period of time w2 before returning pressure to the baseline 41.

The amplitudes of the pulses 40, 42, 44a, 44b may be any suitable amplitude appropriate to creating the necessary flow disturbance that is within the capability of the variable geometry ejector 11, preferably while maintaining fuel flow, i.e. the amplitude modulation does not actually result in fuel shut off, and preferably is of sufficiently short duration that it does not result in sufficient starvation of fuel to the fuel cells that there is significant reduction in power output. Preferably, the rising edge 40a, 42a, 44c is as steep as possible for maximum percussive effect on the flow channels 5 in the fuel cell stack 2. The pulse widths w1 and w2 need not be equal. The pulse heights h1 and h2 need not be equal. Successive pulses in the series can be separated by a delay interval d, which can be varied. Successive pulses in the series need not be identical and could have varying profiles.

Other features of the waveform profile may be adjusted. For example, figure 4d illustrates a sawtooth waveform 46 having a sharp rising edge 46a and a more gradual return 46b to the baseline flow. Figure 4e illustrates an alternative sawtooth waveform 48 having a combined negative pulse 48a and positive pulse 48b from the baseline 41, together creating a sharp rising edge 48c.

The baseline pressure 41 may be generally varied (over a much longer timescale than the pressure pulses 40, 42, 44a, 44b) according to fuel cell demand, e.g. power demand on the fuel cell stack and thus fuel demand of the cells in the stack. The power demand may be determined according to known fuel cell control techniques, e.g. monitoring load voltage and current.

Although the examples of figure 4 are particularly described with reference to modulating a series of sharp rising edges 40a, 42a, 44c, 46a, 48c onto the baseline anode pressure 41, the modulation may alternatively or additionally focus on creating a series of sharp falling pressure edges, e.g. by inversion of the waveforms 44, 46, 48, which may also provide sufficient disruption to the anode flows to disturb, dislodge and then entrain non-fuel liquids and particulates into the anode fluid flows and exhaust them from the fuel cell.

In a general aspect, the ejector 11 is illustrative of a pressure control device in the fuel delivery conduit which is configured to provide pulse modulation of the anode inlet pressure at the fuel delivery inlet. The pressure control device thereby generates a succession of short pressure pulses to disturb water in the fuel cell stack and the baseline fuel flow entrains this water out of the stack via the fuel delivery outlet 4. The extracted water or other non-fuel liquid or particulates can be removed from the recirculation conduit 8 using the liquid separators 12, 14.

The operation of the ejector 11 to create the desired pressure pulses is controlled by the fuel controller 25. This may operate in concert with the bleed controller 20 which controls purging of the fuel delivery outlet 4 to the bleed line 10. When a purge is initiated through bleed valve 9 and bleed line 10, the pressure at the fuel delivery outlet 4 is reduced which further increases flow rate through the anode and helps entrain water and other non-fuel liquids, gases or particulates out of the fuel cell stack 2.

Contaminants which are ejected from the fuel cell stack by the pulse modulated hydrogen flow may be either removed from the recirculation line 8 by water separators 12, 14, or may be removed via the bleed line 10.

The characteristics of the pressure pulses and/or the frequency of the pressure pulses may be varied dependent on various operating parameters of the fuel cell stack. The amplitude (e.g. h orb, + h2) of the pulse can be adapted to how much water needs to be removed. The frequency of the pulses can depend on current being drawn from the fuel cell stack 2 and how long the load has been applied on the fuel cell stack 2 which determines how much water is produced on the anode side.

Thus, in a general aspect, the anode pressure modulation under the control of fuel controller 25 may be controlled as a function of fuel cell operating conditions, and these may be based on one or more direct measurements from the fuel cell stack. The measurements may include any one or more of (i) humidity (e.g. relative humidity) in the fuel circuit, e.g. as measured by relative humidity sensor 26 in the fuel delivery conduit 6, or water levels in the fuel circuit; (ii) cell voltage, or cell voltages in the fuel cell stack, as measured by the cell voltage monitor 23, e.g. cell voltage balance, cell voltage profile of the fuel cell stack, including one or more of: individual cell voltages, mean cell voltage, variance in cell voltages, minimum cell voltage in the stack, maximum cell voltage in the stack, rates of change of each parameter, worst performing cell parameters; (iii) load on, or current drawn from, the fuel cell or fuel cell stack, e.g. instantaneous load or load profile over time, such as how long load has been applied; (iv) fuel concentration in the fuel circuit, e.g. in the discharge flow from the ejector 11 or suction flow to the ejector, e.g. as measured by the hydrogen concentration sensor 22 in the fuel delivery conduit 6; (v) temperature in the fuel circuit, e.g. as measured by the temperature sensor 27 in the fuel delivery conduit 6; (vii) cell resistance or cell impedance, e.g. resistance or impedance of one or more cells in the stack, groups or subsets of cells, or all cells in the stack; (vi) electrochemical impedance spectrascopy (EIS) analysis of the fuel cell or fuel cell stack.

The measurements for control parameters such as humidity, fuel concentration and temperature may be taken at any suitable location in the fuel circuit, such as between the discharge outlet D and the fuel delivery inlet 3 or in the recirculation conduit upstream of the suction inlet S. Measurements may be taken downstream of any water separators 12, 14, condensers or heat exchangers 13 to avoid or reduce measurement inaccuracies caused by high moisture levels. Alternatively, humidity I water content could be monitored at the fuel delivery outlet to determine the rate of water dislodgement during pressure pulsing.

Another measurement which may be used as a control parameter for either or both fuel controller 25 and bleed controller 20 could be liquid removal rate which can be measured at the outlets 28 of the water separators I condensers 12, 14.

Some of these measurements may require additional sensors, or repositioned sensors, to those shown in the configuration of figure 1. In a general aspect, there is flexibility in sensor location. For example, the temperature and pressure sensors 27. 21 could be positioned in the recirculation conduit 8, preferably between the suction inlet S and any water separator or condenser or heat exchanger devices 12, 13, 14 (i.e. downstream of the devices 12, 13, 14).

The measurements of cell voltage can be made using the cell voltage monitoring outputs 23. The measurements of cell resistance can be made using the cell electrical connections 23 otherwise used for cell voltage monitoring. Measurements of cell impedance can be made using the technique of electrochemical impedance spectroscopy (EIS) analysis. In this technique, a small AC perturbation I modulation in voltage or current is imposed on the fuel cell stack, for example via the electrical load on the stack. The corresponding AC current or voltage response of the cells in the stack is measured to determine the resistive, capacitive andlor inductive behaviour of the cell or cells at that particular AC frequency. Physical and chemical processes in the cell or cells under test have different characteristic time constants and thus these can be analysed using different frequency AC perturbations / modulations.

By way of example, the impedance analysis can determine whether the level of flooding or humidity of a cell or group of cells is increasing or decreasing and the anode pressure modulation (e.g. amplitude and/or frequency of pulses) can be modified directly in response to the detected increase or decrease.

The electrical measurements of cell voltage, cell resistance or cell impedance can be made on an individual cell basis for one or more cells, or on one or more groups I subsets of cells, or on the whole stack of cells together.

Many variations may be made to the apparatus as shown in figure 1 and as described in relation thereto.

The ejector 11 may be replaced with any form of fuel regulating device which is capable of modulating the pressure in the fuel delivery conduit 6 such as a pump, injector or a multi-stage ejector. For example, figure 2 shows comparative schematic arrangements for an ejector-based system (figure 2a) and a pump-based system (figure 2b). Figure 2a shows the arrangement of a fuel source 7 coupled to a pressure control device 30 which feeds the motive inlet of an ejector 31, fuel cell or fuel cell stack 2 and water separator 12 in a recirculation conduit 8. The ejector 31 is preferably a variable orifice ejector configured to create the required pulse modulated inlet pressure to the fuel cell stack 2.

However, if the pressure controller 30 is adapted to create pressure modulation, the ejector 31 may be a fixed orifice ejector. The relative positions of the pressure controller and the ejector 31 could be reversed, i.e. with the ejector 31 being upstream of the pressure controller 30. By contrast, figure 2b shows an arrangement of a fuel source 7 coupled to a pressure control device 33 which directly feeds the fuel cell or fuel cell stack 2, with a water separator 12 and recirculation pump 34 in a recirculation line 8.

The inlet pressure modulation function as described above may also be applied to a fuel cell stack system without fuel recirculation.

Figure 3a illustrates an alternative exemplary architecture for the fuel cell apparatus 50.

Corresponding features of the apparatus use the same reference numerals as in figure 1 and perform corresponding functions unless otherwise indicated.

In figure 3a, the apparatus 50 has a fuel delivery conduit 6 which is coupled to a fuel source 7 via a flow / pressure regulator 51. The flow / pressure regulator 51 may be controlled by fuel controller 25. The fuel cell stack includes a fuel delivery outlet 4 which is directly coupled via a variable orifice bleed valve 9 to a bleed line 10. In this arrangement, no fuel recirculation is provided. The modulation of inlet pressure is effected as a function of one or more of the control parameters I operating conditions of the fuel cell stack 2 as previously discussed. In the case of the figure 3a arrangement, however, the relevant sensors are disposed between the fuel delivery outlet 4 and the bleed valve 9, as shown by the positions of hydrogen concentration sensor 22, humidity sensor 26, pressure sensor 21 and temperature sensor 27.

If a fuel concentration sensor 22 is used to provide fuel concentration measurements, these can be affected by changes in humidity and it may be preferable to include a water separator or condenser 12 upstream of the sensor 22. Similarly, it may also be beneficial to use a humidity sensor 26, and pressure sensor 21 and a temperature sensor 27 to provide measurements for correcting or adjusting fuel concentration measurements.

In the arrangement of figure 3a, the fuel controller 25 may also operate in concert with the bleed controller 20 which controls purging of the fuel delivery outlet 4 to the bleed line 10. It may be appropriate to implement pressure modulation only during periods of purge or during periods of high flow purge to ensure adequate flow rate through the anode to entrain water and other non-fuel liquids, gases or particulates out of the fuel cell stack 2. Contaminants which are ejected from the fuel cell stack by the pulse modulated hydrogen flow may be either removed via the water separator 12 outlet 28 or may be removed via the bleed line 10.

Figure 3b shows a variation in which a fixed orifice ejector 52 is used in combination with the pressure / flow regulator 51. The fixed orifice ejector may have the suction inlet coupled to the water separator 12. The position of the fixed orifice ejector 52 may be downstream of the pressure I flow regulator as shown, or it may be upstream of the pressure I flow regulator 51 as shown in dashed outline at 52'.

It will be understood that although the various embodiments illustrated show plural types of sensors including fuel (hydrogen) concentration sensors, pressure sensors, humidity sensors and temperature sensors as well as cell voltage monitoring, some sensors may be omitted if they are not required for the purposes of any fuel controller inputs.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims

CLAIMS1. An electrochemical fuel cell assembly comprising: a fuel cell having a fuel delivery inlet and a fuel delivery outlet, the fuel cell having a membrane-electrode assembly and a fluid flow path coupled between the fuel delivery inlet and the fuel delivery outlet for delivery of fuel to the membrane-electrode assembly; a fuel delivery conduit coupled to the fuel delivery inlet for delivery of fluid fuel to the fuel cell; a flow control device in the fluid delivery conduit configured to provide a pulse modulated inlet pressure at the fuel delivery inlet.
2. The electrochemical fuel cell assembly of claim 1 in which the flow control device is configured to provide said pulse modulated inlet pressure as a function of fuel cell operating conditions.
3. The electrochemical fuel cell assembly of claim 1 in which the flow control device is configured to deliver the pulse modulated inlet pressure as a series of pressure pulses each including a positive and/or negative excursion from a baseline pressure.
4. The electrochemical fuel cell assembly of claim 3 in which each pressure pulse comprises a low pressure pulse followed by a high pressure pulse, relative to a baseline pressure.
5. The electrochemical fuel cell assembly of claim 3 in which the frequency of pressure pulses is determined as a function of fuel cell operating conditions.
6. The electrochemical fuel cell assembly of claim 3 in which the depth of each low pressure pulse below baseline pressure and I or the height of each high pressure pulse above baseline pressure are determined as a function of fuel cell operating conditions.
7. The electrochemical fuel cell assembly of claim 1 in which the flow control device is configured to vary the amplitude and for frequency of the pulse modulation of the inlet pressure to the fuel cell as a function of one or more of: humidity in a fuel circuit of the fuel cell assembly; cell voltage, resistance or impedance; fuel cell electrical load; fuel concentration in a fuel circuit of the fuel cell assembly; water levels in a fuel circuit of the fuel cell assembly; temperature in a fuel circuit of the fuel cell assembly.
8. The electrochemical fuel cell assembly of claim 1 in which the flow control device is configured to vary the amplitude and I or frequency of the pulse modulation of the inlet pressure to the fuel cell as a function of electrochemical impedance spectroscopy (EIS) analysis of the fuel cell performance.
9. The electrochemical fuel cell assembly of claim 3 in which the baseline pressure is varied according to fuel cell power demand.
10. The electrochemical fuel cell assembly of claim I further including an electrochemical impedance spectroscopy analysis system configured to apply an AC current or voltage modulation to at least one cell output, the flow control device being configured to vary the pulse modulation of the inlet pressure as a function of measured impedance of one or more cells at one or more frequencies.
11. A method of operating an electrochemical fuel cell assembly comprising: providing a fuel cell having a fuel delivery inlet and a fuel delivery outlet, a membrane-electrode assembly and a fluid flow path coupled between the fuel delivery inlet and the fuel delivery outlet for delivery of fuel to the membrane-electrode assembly; delivering fuel to the fuel cell via a fuel delivery conduit coupled to the fuel delivery inlet; controlling a flow control device in the fluid delivery conduit to provide a pulse modulated inlet pressure at the fuel delivery inlet.
12. The method of claim 11 further including controlling the pulse modulated inlet pressure as a function of fuel cell operating conditions.