CA2245228A1 - Fuel cell co sensor - Google Patents

Abstract

The CO concentration in the H2 feed stream to a PEM fuel cell stack is monitored by measuring current and/or voltage behavior patterns from a PEM-probe communicating with the reformate feed stream. Pattern recognition software may be used to compare the current and voltage patterns from the PEM-probe to current and voltage telltale outputs determined from a reference cell similar to the PEM-probe and operated under controlled conditions over a wide range of CO concentrations in the H2 fuel stream. A
CO sensor includes the PEM-probe, an electrical discharge circuit for discharging the PEM-probe to monitor the CO concentration, and an electrical purging circuit to intermittently raise the anode potential of the PEM-probe's anode to at least about 0.8 V (RHE) to electrochemically oxidize any CO
adsorbed on the probe's anode catalyst.

Description

FUEL CELL CO SENSOR

The Gol,c .~ of the United States of America has rights in this invention pursuant to Ag~ n~ No. DE-AC02-90CH1043S awarded by the U.S. Department of Energ~.

TECHNICAL FIELD
The present invention relates to a carbon monoxide (CO) sensor, and a method for llloni~uling the CO co.~ tldion in the reçu~ ate fuel stream to a PEM fuel cell for controlling such collcelllldtion.
BACKGROUND OF THE INVENTION
Fuel cells have been proposed as a power source for many applications. So-called PEM (proton eYr1~qr~ge ~I,~ el cells ~a.k.a.
SPE (solid polymer ekctrolyte) fuel cells] potentially have high energy and 15 low weight, and accol-i~ly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a "membrane electrode ass.,lllbly" (a.k.a. MEA) co~lis.llg a thin, proton trqn~mi~sive, solid polymer ~ l~la~ electrolyte having an anode on one of its faces and a cath~xle on the opposite face. The ll~ UlC elc.,~odc-20 assembly is sandwiched l~h.~ll a pair of cle~;l,ic&lly co~ cti~ el~which serve as current collectors for the anode and cqtllode, and contain appro~liate cl~ and/or Op~ therein for di~ ulil~g the filel cell's gaseous reÇ rtqnt~ over the su-r~ces of the rcs~li~ anûde and cathode catalysts. The rhqnnP-l~/opcllings for the rc~ are often rcf~lled to as "flow ch~nn~ls". A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.

PEM fuel cells are typically H2-02 fuel cells wll~rein hydrogen 5 is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., ~2)~ or air (i.e., ~2 a~mixed with N2). The solid polymer n~e,-lb~ s are typically made from ion exchqnge resins such as perfluolo~d sulfonic acid. O~e such resin is NAFIONTM sold by E. I. DuPont deNemeors & Co. S .ch ~cl..b~ es are 10 well known in the art and are desc.ibed in U.S. Patem 5,272,017 and 3,134,697, and in Journal of Power Sources, Volume 29 (1990), pages 367-387, inter alia. The anode and cqthode themselves typically colll~lisc finely divided carbon particles, very finely divided catalytic particles sul pGl~d on the intern~l and external surfaces of the carbon particles and proton 15 conductive resin hlle. ",i ~gled with the catalytic and carbon particles. Onesuch melllblal1e electrode assembly and fuel cell is described in U.S. Patent 5,272,017 issued December 21, 1993 and ~esiEn~l to the acsign~e of the present invention.

The hydrogen used in the fuel cell can be derived from the reformqtion of ~u ~ l or other or~,~ics (e.g., hydrocall,ons).
Unfo.~u~ely, the reformate exiting the lerol~l co-.~s undesirably high conce--l-alio~ of carbon monn~ .o which can quickly poison the catalyst of the fuel cell's anode, and xco~di~l~r must be removed. For example, in the - 25 m~shqnol l~folll~tion process, .-~ m~l and water (as steam) are ideally reacted to generate hyd.oge.l and carbon dioxide according to the reaction:

CH30H +H2O~CO2 + 3H2 This reaction is accomplished heterogeneously within a ch-~ir~l reactor that provides the n~cess~ry thermal energy throughout a catalyst mass and actually yields a ~eful~late gas comprising hydrogen, carbon dioxide, carbon monoxide, and water. One such ,.,fol,ller is described in U.S. Patent No.
4,650,727 to Vand~ll,or~l,. Carbon monoxide (i.e., about 1-3 mole %) is contained in the H2-rich lefo",late/effluent exiting the lefo".l~l, and must be removed or reduced to very low nontoxic (i.e., to the anode) co~ tlalions (i.e., less than about 20 ppm) to avoid poisonirlg of the anode by ads~rption onto the anode catalyst. The unreacted water serves to };.~ iA;ly the fuel gas 10 and prevent drying of the MEA.

It is known that the carbon monoxide, CO, level of the reformate/effluent exiting a m~thqnol reformer can be reduced by l~tili7ing a so-called "shift" reaction In the shift reactor, water (i.e. steam) is injecte~l15 into the m~th~nol reformate/effluent exiting the lefo"ll~l. in the p ~,scnce of a suitable catalyst~ to lower its t~lllpcl~lu,~, and increase the steam to carbon ratio therein. The higher steam to carbon ratio serves to lower the carbon monoxide content of the refoll,lal~ accoldi,lg to the following ideal shift reaction:

CO+H20~C02+H2 Some CO survives the shift reaction and remains in the ,ero",~te. De~ding upon the reformate flow rate and the steam injection rate, the carbon 25 monoxide content of the gas exiting the shift reactor can be as low as 0.5 mole %. Anyresidual...~ olis cGu~e~ d tocarbondioxideandhydrogen in the shift reactor. Hence, shift reactor effll)ent co"~.ises hy~ugell, carbon dioxide, water and some carbon monoxide.

~, , The shift reaction is not enough to reduce the CO content of the reformate enough (i.e., to below about 20 ppm). Therefore, it is nPcescqry to further remove carbon monoxide from the hydrogen-rich l~f~ a~ stream exiting the shift reactor, and prior to supplying it the fuel cell. It is known to 5 further reduce the CO content of H2-rich reformate exiting the shift rcactor by a so-called ~PROX" (i.e., ~f. lelltial oxidation) reaction ~;led in a suitable PROX reactor and can bc either (1) adiabatic (i.e. where the lempfldlule of the catalyst is allowed to rise during ox~ tion of the CO), or (2) isoll,c.lllal (i.e. where the teln~l~tu~ of the catalyst is ~ d 10 subst~nti~lly cons~lll during oxidation of the CO). The PROX reactor comprises a catalyst bcd operated at temperatures which promote the plefelelllial oxidation of the CO by injecting controlled ~mollnt~ of air into the effluent from the shift reactor to consume the CO without con!i~.",ing/oxidi~ing ~ul~l~llial q~ s of the H2. The PROX reaction is 15 as follows:

CO+1/202 ~ CO2 Desirably, the 02 required for the PROX reaction will be about two times the 20 stoichiometric amount required to react the CO in the n f~,lmate. If the amount of O2is substantially less than about two times the stoi~ l ic amount n~e~led, i~ rr~ie~l CO oxidation will occur. On the other hand, if the amount of O2e~ ds about two times the ~ l. ic amount n~e~e(l, excessive consulllylion of H2 results. Co~ul.lyli~n of the H2 raises the - 25 tem~e.alule of the gas, which in turn causes the formation of CO by the reaction of H2 with CO2, known as the reverse gas-shift reaction. Hcnce, careful control of the amount of air injec~ d in the PROX reaction is essential to control the CO content of the ~efo.llldte feed stream to the fuel cell. The - PROX process is des~;libed in a paper entitled ~MEth~nol Fuel ~oce~cin~ for Low Te~ alu-c Fuel Cells" published in the Program and Abstracts of the 1988 Fuel Cell Serninqr, Octobcr 23-26, 1988, Long Beach, California, and in U.S. Patent Vande~l,orgll et al 5,271,916, int~r ,qliq Whether an a~iqbqtic or isothermal PROX reaction, a controlled amount of ~2 (i.e., as air), is mixed with the reformate exiting the shift reactor, and the ~i~-lule passed through a suitable PROX catalyst bed known to those skilled in the art. To control the air input rate, the CO
concentration in the gas exiting either the shift reactor or thc PROX reactor ismeasured, and based thcreon, the ~2 conce~ ation needed for the PROX
reaction adjusted. However, sensilive, real time, CO sensors have not heretofore been available, and accordillgly system response to CO
collcelltlalion variations has becn slow. This is particularly troublesome in dynamic systems where the flow rate, and CO cont~nt, of thc H2-rich reformate vary contin~ cly in response to variadons in the power d~nq-n~lc on the fuel cell system. Since the amount of ~2 (e.g., air) supplied to the PROX reactor must vaIy on a real dme basis in order to accon~odate the varying power cle~nqn~lc on the system, there is a need for a rapid response CO sensor to contim~ cly monitor the CO in the lefollllate stream and thelerl.Jl-l (1) ,.,~;nlqin the proper oxygen-to-carbon monoxide c~ lalion ratio in the PROX reactor, and/or (2) divert the ,~f(J.~le stream away from the fuel cell until the CO content thereof falls within acceptable levels.

The present hl~ provides a sensi~ive CO sensor utili7ing a mini PEM fuel cell as a probe, and a method for real time monitoring of the CO concentration in the refch,llate feed stream to a PEM ~el cell as a means .

to control the operation of the fuel cell system. In accordance with the present invention, the sensor is repeatedly refreshed by ~ ging any CO
thererl~lll to m~int~in the CO sensitivity of the sensor. CO purging may be effected ch~mir~lly or electrocll~nlir~lly as described h~leil~ar~r. The S invention is useful during system start-up to d- t .... ;~-~ when the CO level of the PROX effluent is sufficiently low that such effluent can be dilecL~d to the fuel cell without poisoning the anode catalyst. The invention is particularly useful for the real-time control of the amount of ~2 (i.e., as air) supplied to the PROX reaction in response to the CO concentration in the H2 gas stream 10 exiting the PROX reactor so as to m~ximi7-o CO col~un~tion while minimi7ing H2 co~ul,l~tion in the PROX reactor. The CO concentration in the lefoln~te may be n~asuled at various locations in the le~ol-naLe fuel stream to a fuel cell (e.g., after the l.,fol.l.cr, shift or PROX reactions).

In accordance with a plefe.. ~,d embodiment of the present invention, there is provided a CO sensor colll~)lismg a PEM-probe, and a method of using the PEM-probe to ~ its se~ili~dly and provide real time control of the CO content of thc ~folll.ale fuel stream to a PEM, H2-02 fuel cell stack. The PEM-probe is esse~ lly a mini PEM fuel cell which, 20 like the stack's cells, has an anode and cathode affixed to opposite sides of a proton exchange ll,ell~ ane and a hydrogen flow channel coLlrlvlltillg the anode that receives hydrogen from the hydrogen-feed manifolds supplying the stack. The PEM-probe's anode will pl~fe,lably have a smaller area and a lower catalyst loading (i.e., g/cm2) than the stack's cells for hlcl~scd CO
25 se~ ivily col~al~,d to that of the stack itself. Most p~f~.~bly, the surface area of the PEM-probe's ele.;llod~s will be less than about 10~ that of the stack's electrodes, and the catalyst loading will be about half the catalyst loading in the stack's cells. Moreover, in accordance with the present invention, sensitivity of the PEM-probe is e.-h~n~ecl even further by h~Le,~ ently purging the probe's anode catalyst of any CO that might have become adsorbed thereon while mol~ilc~ g the reformate gas fed to the fuel cell. The frequency of purging is such as to m~int~in the catalyst in a substantially CO-free, or near CO-free, condition where the probe is most 5 effective and responsive in detPcting CO buildup on its catalyst over short intervals. In this regard, the probe is quite effective/responsive during the early stages of co,.~ ion, but less so as the probe becomes more and more col-lA~ ed with CO. CO yulging will preferably be effected by raising the anode potential sufficiently [(i.e., to at least 0.8 V lll~asul.,d 10 against a reversible hydrogen electrode (RHE)] to eleclloch-- ..ir~lly oxidize any CO on the catalyst to CO2 by reaction with the water present in the fuel stream. This may be accomplished by reverse biasing or short ci~cui~ g the PEM-probe, as described hereinafter. Alternatively, the probe may be flushed with ~2 (e.g., air) to ch~mir~lly oxidize the CO.
The l>ref~ d CO sensor includes means for effecting the intermittent electroch~rnir~l yul~hlg of the PEM-probe to remove adsorbed CO. In one embo~im~l t, the CO sensor colll~,lises: a gas-ll~nilcli~g PEM-probe including a proton eYrh~nge ll~n~b.dne having an anode and a c~thocle 20 affLlLed to opposing firse and second surfaces of said llle.,lb,~; a first electrical current collector eng~ging ~e anode; a second electrical current collector çng~gin~ the cathode; an ele~llieal disc~ge- circuit co~eetable between the current collectors, wherein the disch~ge circuit has a first electrical r~ re valued for discharging the PEM-probe at a first rate 25 selected to monitor the degrading output of the PEM-probe inri(lçrt to CO
co"l~",in~;on of the anode; an electrical ~ ~ing circuit connectable l~t~en the current collectors, wherein the ~lghlg circuit has a second ele~.ltical resistance which is less than the first electrical les;~l~nre such that upon discharge of the PEM-probe through the second resistance ~e potential of the anode is raised to at least 0.8 V (RHE) to effect electroch.onlic~l oxidation ofany CO adsorbed on the anode; and an electrical switch in electrical series connection between the current collectors and adapted to i~ y, alternately electrically connect the current collectors to the discharge and theS purging circuits. In this embo~limPnt, the sensor will also preferably include a motorized valve for ~h-ltting off H2 flow to the PEM-probe during the ~u~ g stage. Most preferably, the switch for ~wilchillg between the discl~e and the ~,ul~,hlg circuits will be built into the H2 shut-off valve for simlllt~n~oll~
stopping of the H2 flow to the probe and connPcti~ it to the ~ ;ing circuit 10 during the purging cycle and vice versa during the dischargc cycle.

In another, and most l)lcr~lled embo~imPnt, a CO sensor is provided that comprises: a gas-moniLoli"g PEM-probe including a proton exchange membrane having an anode and a cathode affixed to opposil~g first 15 and second surfaces of said membrane; a first electrical culTent collector eng~ging the anode; a second electrical current collector engaging the c~th~ ~e; an electrical discharge circuit cormectable between the current collectors; the discharge circuit having a first electrical l~,;.;~t~re valued for discharging the PEM-probe at a rate selected to monitor the degrading output 20 of the PEM-probe il-rid~ ll to CO-co.~l~.";.~lion of the anode; an electricalpurging circuit coml~clable b.,t~ll the current collectors and in~k--lin~ a voltage source that illlpOSCS a reverse electrical bias on the PEM-probe sufficient to raise the pol."l~ial of the anode to at least about 0.8 V (RHE) toeffect elecLIoch~ l oxidation of any CO adsorbed on the anode; and an 25 electrical switch in electrical series comlcclion ~h.~n tne current collectors and adapted to i--l- ,.I;~h ~-lly, ~ ",t. Iy COlllleCl the contacts to ~e discharge and ~ ing circuits. This embodiment is seen to permit the quickest and most controllable purging of the anode, without the need to shut off the H2 flow.

In accordance with the process of the present invention the PEM-probe is inte,.~ c.llly purged of any CO buildup on its catalyst.
Between such purgings the current and/or voltage outputs of the probe is/are monitored and colnpal~d to reference standards to det~,line the CO
5 conce,ll,ation in the ,. f~,."ale (e.g., PROX crn"~ ). More specifirqlly, the process invention contemplates:

a. providing a CO sensor including a monitoring PEM-probe co~ lisillg a proton e~rrhq~e IllC,l~ having an anode and a cathode affixed to opposillg first and second surfaces of the ll,. mb,anc wherein the anode colllplise,s a catalyst which is susceptible to poisoning inrident to the adsorption of CO by the catalyst and conceq~nt p,oE;.~ ssi.re degradation of the catalyst from a peak pclro~ ce level in the early stages of CO adsorption to a poor pc.r~JI .. qnr~ level at later stages of such adsol~ion;

b. cont tin~ the anode with a portion of the H2 feed stream to the fuel cell over a plurality of pred~ 1 time intervals;

c. cont: ~tin~ the c~th~ with oxygen;

d. dischalging the PEM-probe during the time intervals;

e. monilolillg the clecl,ical output from the PEM-probe during the disch~gi,l~ to ~,el~,ate an output signal having a behavioral pattern indicative of variations in the CO
concentration in the feed stream;

f. from a lcfele~ce PEM-probe similar to the S monitoring PEM-probe, det~ a plurality of telltale electrical outputs which are correlated to known CO
concentrations in the feed stream;

g. storing the telltale electrical outputs in a readable memory;

h. col,l~alillg the output signal from the monitoring sensor to the telltale electrical outputs from the lefcr~nce PEM-probe to identify a telltale electrical output that is ~ ly similar to the behavioral pattern to det~ ".~il~e the CO
concentra$ion in the feed stream; and i. periodically, ~ ,-"g the catalyst of the CO between the time intervals to ..~ the catalyst at subst~nti~lly its peak ~eLru~ ce level.

Once the CO concentration has been dele ...;~-~1, a det~l~ination can be made as to what adj~stm~nt~ to the system are required. Hence for example, in one scenario, the ~2 injection rate to the PROX reactor may be varied, or in 25 another scenario, the PROX efflu~-nt may be di~ec~d away from the fuel cell stack until its CO content falls within acceFt~le limits (i.e. below ca. 20 PPM) BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better ulldel~lood when considered in the 5 light of the following detailed de~~iplion thereof which is given he~rlel in conjunction with the following dl~Wll~gS of which:

Figure 1 is a schematic illustration of a bipolar, PEM fuel cell stack, and a ~"efelled CO ensor ll~ ,ror in accordance with the present 10 invention;

Figure 2 illustrates an exploded, perspective view of a PEM-probe according to one elllb~li~ of the present invention;

Figure 3 is a sectioned view of the PEM-probe of Figure 2; and Figure 4 sch~rnqtirqlly depicts another embodiment of a CO
sensor in accordance with the present invention.

Briefly, the sensor of the present invention preferably monitors the current through, and voltage across, a CQI.PI~ load co.~ rt~ to the PEM-probe which is connect~d to the h~y~ogen fuel feed manifold to the fuel 25 cell stack for testing the gas therein. A voltage sensing device senses any voltage fllu~t l3tions across the COl.~" lt load over a ~,cd( t~ l time interval and outputs a voltage signal which ~lesel~ts the behavior pattern of the voltage fluctuations over that interval. A current sensing device senses any current fluctuations through the CO~I~u~t load over a predet~ d time ... . , ... .. . . .. ~. , , .~ . .. , .. , ,. ~ ..... .. .

interval and outputs a signal which represents the behavior pattern of the current fluctuations over that interval. A first data processing device serves as a data acquisition unit, and, using conventional technology, sa~ the current and voltage signals, conditions the signals to filter out signal noise, 5 and converts them to digital data streams. A suitable nle.l.ol~ device stores telltale voltage and current outputs which have been correlated to known CO
concentrations at various lelllpelalures and pl~,s~.lles in a gas like that of the reformate. In this regard, the telltale out~uts will have been previously generated empirically from a refer~nce ~ ell which is similar to the PEM-probe 10 and has been discharged in a manner similar to the PEM-probe (e.g., has the same resi~t~n~e thereacross as the load co.~ led to the PEM-probe). The reference cell is operated over a wide range of known CO-coll~nLIaLions in the H2 feed stream to develop a library of telltale current andlor voltage outputs corresponding to dirre~e.ll CO-concellLlaLions. Finally, a second data 15 processing device (e.g., a personal COll~ l) receives the digital data streams, plots a curve of the behavior pattern of the voltage and current fluctuations from the PEM-probe over a given time interval, and coll4,ares those voltage and current behavior ~)&lte. ~s to the telltale voltage and current outputs d( t~ in~d from the l~felelue cell in order to rnatch, or otherwise 20 identify, at least one of the telltale outputs that is subs~ lly similar to the behavior pattern being cu-l~ar~d. ~f~.r~ly, the col~,?alisol1 mPth~ology and telltale outputs are those descIil)ed in cope~ i .g U.S. patent application U.S. Ser. No. 08/807,S59 filed February 28, 1997 in the n~me of M.
Meltzer, and assigned to the ~ccign~-of this invention, which is int~nfle~ to 25 be herein incorporated by lefe~.~ce. ~lternqtively, rather tban plotting the entire behavior pattern and telltale outputs, an abbreviated r~lationsl~ip between the behavior patt~,lllS and the telltale outputs can be used. For exarnple, the starting and ending voltages are d~t~ l for the beE,;i-.-;n~
and end of a predf t~ ;n~ in~n.~t of tirne and the voltage c~ ~,c s over , that i~clelnclll ~csllm~d to vary linearly with time. The leÇ~,.ellce cell's telltale voltages collespollding to known CO-collcc.lllalions in the H2 stream are detelll~ined in the same manner. The slopes of the two curves are then compared. In either event, a substantial match between the PEM-probe's S output pattern and a telltale output from the refe,e~ce cell intlic~l~s the real time CO-concentration in the hydrogen-feed stream which is then used to trigger adjustments to the lefollller, shift and/or PROX reactions to reduce theCO content of the H2 feed stream to the fuel cell stacl~, or to divert the refo~ ate stream away from the fuel cell stack, if n~ essary.
More specifically, Figure 1 depicts a stack 2 of individual fuel cells 4 each comprising a MEA 6 having a proton conductive resin ~ e.~lan 8 sandwiched between an anode 10 on one surface thereof and a cathode 12 on the opposite surface thereof. A cathode flow channel 16 is provided adjacent the cathode 12 for flowing an oxygen-rich gas (i.e., preferably air) by, and into contact with, the cathode 12. Similarly, an anode flow channel 14 is provided adjacent the anode 10 for flowing hydrogen fuel by, and into contact with, the anode 10. The ~ an~ 8 will ple~ably con~l;se a perfluoronated sulfonic acid polymer such as NAFIONTM as is well known in the PEM fuel cell art. Each individual cell 4 is s~ ated from the next cell 4 in the stack by a bipolar plate 18, which is a col~]ucLi~e plate (e.g., metal, carbon, etc.) which S~,~aldteS ~e several cells one from the next while conAllcting electrical current in ele.,hical series directly from one cell to the next. End plates 24 and 26 terminate the stack 2 and define the lesp~cli~re cathode and anode flow Cll~ tlC for the end cells 28 and 30 of the stack 2.
An oxygen-feed manifold 32 s~ s air to the several csthYle flow channels 16. Similarly, an hydrogen-feed manifold 34 supplies hydrogen fuel to the several anode flow çhqnn~!c 14. An hydrogen exhaust manifold 36 collects anode exhaust gas from the several anode flow chsnn~lc 14 for discl~,~ from . CA 0224S228 1998-08-06 the stack. Similarly, a cathode exhaust gas manifold 38 collects exhaust gas from the cathode flow channels 16.

Stack pe.ro,~ ce degrades due to carbon monoxide poisoning S of the anode catalyst. Such poisonil~~ is a potcl.lial problem when there is excess (i.e., more than about 20 PPM) CO in the hydrogen-feed stream which can result from .nerr~ci~"l ...~ ol/hydloc~l,oll ~efc)~ll~ing, shift andlor PROX reactions known to artesans skilled in this art. Accordingly. when the presence of excess CO in the H2 fuel stream is evident, efforts Ir!;lst be made 10 to correct the problem, preferably at its source. To this end, the present invention provides a sensitive, rapid re~onse carbon monoxide sensor (CO-sensor) 40, and method of O~.aLi-lg same, which senses CO concellt~ation in the reÇc",na~e fuel strearn in the manifold 34. CO-sensor 40 includes a probe 41 (heleillarl." PEM-probe) which is naught but a small ~i.e., mini) PEM fuel 15 cell sirnilar to the cells 4 in the stack 2, except for size and possibly catalyst loading. While mon,lolillg the fuel stream in manifold 34, the PEM-probe 41 is discharged in such a manner as to output an electrical signal whose behavioral pattern over time is dependent on the CO con~l~dLion in the lefolmate fuel stream. The output signal behavior pattern is comparcd to 20 certain telltale outputs from a rcr.,lc~ce PEM probe i~kntic~l to the gas-monitoring PEM-probe which have been cOIl~lal~d to known co~ t~atiorls of CO in H2 at various tclmpc~ es and p,essu,cs. Conven~ionql pattern recognition technology is plcf,ll~ for reliably co~l~ing the PEM-probe's 41 output(s) to the telltale output(s) of the lcr~ ce cell. However, less 25 sophictic~ted telltale outputs (e.g., a~io~ e slope of vol~ge degradation curve) may also be used. Monilol~g the el~LIical pe.rol~l ance of the PEM-probe 41 and co~ ing it to the e~ ed pelrulll.ancc under known CO
concentrations conditions provided by the lef~,l.,llce PEM probe provides a direct knowledge of the CO collce~Lration in the ~Çoll~l~ feed stream to the fuel cells 4 comprising the stack 2. From this knowledge, needed corrections can be made to the reformer, shift or PROX reactions to bring the CO
concentration to within acceptable levels. ~lt~ ively, the fuel stream can be diverted away form the fuel cell stack until the CO content thereof is - 5 corrected to within acceptable limits. To control the PROX reaction: (1) the CO concentration is measured at a given ~2 injection rate to the PROX
reactor; (2) the ~2 injection rate is il~ ased and the CO co~ .ation again determined; and (3) if the CO collce~ lion goes down, too little O2is being injected and if it goes up, too much O2is being injected. The process is repeated at dirr~lellt ~2 injection rates until Opli lu~tion is achieved.

Like the cells that comprise the stack 2, the PEM-probe 41 includes an anode 42 and cathode 44 on the opposite surfaces of a proton exchange m~ll,bldlle 50 (see Figures 2 and 3). Conventional conductive diffusion 43 and 45 contact the anode 42 and cathode 44 le~ycc~ively. Such material may comprise carbon paper, fine wire mesh, sintered porous metal (e.g., tit~nillm or niobium). The PEM-probe 41 im~hl~es an anode flow channel 46 in the housing 54 which co----n~ ateS with the hydrogen-feed manifold 34 via applo~lidte flow passages (e.g. inlet 49 and/or conduit 48), as well as the hydrogen eYh~lst manifold 36 via outlet 51 and conduit 53. The cathode 44 is exposed to ambient air via o~ni..g S2 in the PEM-probe's housing 54. Ambient air operation keeps the PEM probe l~alul~. low without external cooling, which ~ ses the CO sensitivity of the PEM-probe. A perforated metal current collector 49 COIl~Cl~ the carbon plaper 45 25 and conducts current to terlTlin~l 47 thereof which exits the housing S4 through slot 55. Preferably, the PEM-probe 41 will have a lower catalyst loading than the stack cells 4 to illc~ase its S~,~Silivily to low CO-co~lcellL~dlions. Most pler. ~dbly, the stack cells 4 will have anodes 10 and cathodes 12 whose surface areas are much greater (e.g., as much as 10 times greater) than the surface areas of the anode 42 and cathode 44 of the PEM-probe 41. This small area, coupled with lower catalyst loadings, provides a PEM-probe with heightened CO-co~ ion sensitivity. By way of example, an H2-O2 PEM fuel cell stack 2 having individual cells 4 with about S 100 in2 of electrode area, can be effectively monitored with a PEM-probe 41 having an electrode area of about 1 in2 to 2 in2, and about one half the catalyst loading (i.e., g/cm2) of the stack cells 4. The co~lui~ 48 and 53 may include valves 57 and 57' for isolating the probe 41 from the H2 manifolds 34 and 36 during purging, if desired. The conduits 48 and 53 may also include air inlet 10 59 and outlet 61 respectively with associated valves 63 and 64 for controlling the flow of purging air through the PEM-probe 41 (i.e., when the air ~-ughlg embodiment is used).

Because of its small size and/or low catalyst loading, the anode 15 catalyst of the PEM-probe poisons at a faster rate then the fuel cell stack it is monitoring. Hence, the degradation rate of the ele~ll ical outputs of the PEM-probe is greater than that of the stack and provides a more dc.-lo~lldlive in~ic~tor of CO concentrations in the H2 fuel stream to the stack. However, the PEM-probe becomes proglcssi~ely less scllsilive to CO co~ .tion 20 variations as it becomes more poisoned. In this regard, the peak ~lroll,.ancelevel of the catalyst is at the point wbere it has sul.s~ i911y no, or very little, CO adsorbed therein, and the poorest pc,Ç .n.~-~ when a ~J1,~ 1 amount of CO is adsorbed on the catalyst. In acco~ance with the present invention, sensilivily of the PEM-probe is m9i"l;~in~1 near its peak p~lr~ s~.~e level by - 25 hllfl~ f nly ~"llghlg the PEM-probe's anode catalyst of any CO that might be adsorbed therein. Preferably, ~Ju~ing will be accomplished by raising the anode polc.llial to a level sllffiriP~t to electroch~ .~;r~lly oxidize the CO toCO2 in the prese,~ce of water. Typically, this requires raising the anode potential to at least 0.8 V as lllca~ d against a lcve.sible h~rogen electrode (RHE), and may be accomplished by (1) periodically short cil~;uiling the PEM-probe as by discha~ging it through a low ~;,i~ ce load, or (2) most preferably, by periodically reverse biasing the PEM-probe by means of a sUpplem~ont~ry voltage source. Alt~ a~ ,ly, CO purging may be err~;~d by S flushing the anode catalyst with oxygen (e.g. air) to çh~ lly oxidize the CO. The CO sensitivity of the PEM-probe may also be illcl~ased by cooling the H2 stream by means of a heat eYch~nge (not shown) hlscit~d in feed line 48 to the PEM-probe 41. Cooling to about 20~C-90~C is useful to condense out excess water which impedes sensitivity of the probe.
The most p~efelled purging terllnique is to reverse bias the PEM-probe, as this t~chni-lue is seen to be most easily controlled and pelrollllable without ~htltting off the flow of gas to the PEM-probe. To this end, a CO sensor 40 (see Figure 1) is provided which includes a PEM-probe 15 41, a voltage source 78 (e.g. a capacitor, or voltaic device such as a battery or connection to one or more cells 4 of the stack 2) in a 1~Ulgil4~ circuit P, a motorized switch 80, and a discharge circuit D. The switch 80 will preferably be coupled to a timer or clock which periodically s~ri~hes the PEM-probe between a discharge mode through load L of the dischalge circuit 20 D, and a reverse biased mode in the l~ulging circuit P. The l~ulgillg circuit P
also includes a small (e.g., about 0.5 ohm) resistor to avoid a high current surge upon switching ~l~cen circuits D and P. More specifi~lly, the PEM-probe 41 is coupled to a constant load L in a disch~ge circuit D (see Fig. 1).
A voltage sensing device 65 (e.g., voll~l~t~l) senses the voltage across the 25 load L while a current sensing device 67 (e.g., ~ t~ I) senses the current flowing in the disch~,c circuit D. The PEM-probe 41 will typically operate with closed circuit voltages of about 0.4~.9 volts and ~;ull~l~ den~ities of about 0.1 to l.0 amps/cm2. The voltage sensing device 65 may be any such device as is well known in the art and is capable of ~U~ a signal 58.

. .

The current sensing device 67, on the other hand, may either be (1) a discrete such device as is well known in the art and is capable of ou~ g a signal 60, or (2) _ay be the voltage sensing device 65 from which current can ~ulo.--~ti~tly be calculated using Ohm's law. The output signals S8 and 60 of 5 voltage sensing device 65 and current sensing device 67 lespe~ ely are inputted into a co~l~enlional high speed analog-to-digital coll~elter 62 (i.e., data ~cq~ ition unit) which conditions the signal to elimin~ noise, and generates digital data streams 64 and 66. A p,ef~ ,d such high speed converter useful with pattern recognition technology a SCU4 data ~cq~ hion 10 system sold by Generic Insllullle.ll~ and Systems Corporation (GenIASTM), as it is capable of reading inputted data, and rnaking all needed c~,...l,ul~tions~ in real time.

In accordance with another embodiment of the invention, the 15 PEM-probe is deprived of H2 and es~enti~lly short circuited or dischd~ged through a relatively low resisl~nce so as to raise the anode potential up to theoxidation potential of the CO (i.e., 0.8 V RHE). To this end, a CO sensor is provided, as shown in Figure 4, which col"~,ises a PEM-probe 82 lilte that described above, a normal discharge circuit 84 dischd~ g through load 86, a 20 short-cil~;uiling purging circuit 88, a motorized switch 90 and a motorized valve 91 for cutting off the H2 during ~ ing The motorized switch 90 and motorized valve 91 will preferably be c~u~,led to a timer or cloc-k which periodically switches tbc PEM-probe l~h..,en (1) a normal CO-mollit~ g discharge mode through load 86 of the disc}~lge circuit 84, and (2) a rapid 25 discharge mode effected by short Cil~;ui~ g the PEM-probe in the ~U~ lg circuit 88. Most p,efe,dbly, the H2 cutoff valve 91 and the switch 90 will be integrated into the same structureJdevice such that H2 cutoff and switching between discharge and ~ lgillg circuits are effected ~imlllt~nloously When using the reverse biasing embodiment, the anode catalyst will preferably col,lpLise platinum black, and the diffusion layer will preferably comprise a porous metal in order to survive the reverse polarity reaction. For the other embo~imPnt~, carbon-~u~,~.led pl~tinllrn catalyst and S a carbon/gldphile diffusion layer may be used.

The current and the voltage of the PEM-probe are preferably both sampled on a regular basis (e.g., every 10 to 100 mi11i~conds) during a specified discharge interval that can vary from about 100 milliseconds to 10 about 10,000 milli~econds. The resulting signals 5~ and 60 are conditioned by the converter 62, and the average voltage and current are plotted over that interval of time. These plots depict the behavior p3~ for the voltage and the current outputs over that time interval. These behavior ~alt~,Lns are inputted as data streams 64 and 66 into the data processor 68 where they are 15 col,lpal~,d to predetermined lefe~ cc current and/or voltage telltale outputsstored in memory 70. Operating conditions of the stack (stack operationals) such as fuel/air stream t~ ,.alu e and ~res~u~e (i.e., taken from sensors not shown) are also illpull~d to the data processor 68 to insure that the proper telltale voltage and/or current are selecte~ from the library 70 for a given 20 voltage/current behavior oul~ulled from the sensor 40. The lefc.encc voltage and current telltale outputs are en,~ilically ~lete....;~.~d before hand at various lel"lxlalu,l~s and pres~ures from a l~f.,,e~ cell which (1) is similar to the PEM-probe 41, (2) is discharged through a co~l~l load having the same value as the constant load L of the CO-sensor 40, and (3) is o~.al~d over a 25 wide range of carbon monoxide conce.,LIdtions in the H2 feed stream. A large library of such telltale outputs is stored in the ~ ol~ 70, and is available forthe co~ ~ison to the voltage and current behavior p~tternc produced by the PEM-probe 41. The voltage behavior pattern and the current behavior pattern of the PEM-probe 41 are co",palcd to each of the many ref;~ ce voltage and current telltale outputs on file in the memory 70 until at least one of the reference current and/or voltage .siy~ es closest to the behavior pattern of the PEM-probe's current is i~entifi~d, and/or one of the rer~,e~ce voltage si~llal~l.es closest to the behavior pattern of the PEM-probe's voltage is 5 identified. Once a "match~ is made ~I~.~cn a referei~ce telltale output and a behavior pattern, the CO-col~c~ alion in the H2 feed stream is ~lct. ~
from which adjustments can be made, as nPecle~l . A perfect match bcll.. e~ n the behavior patterns and the telltale outputs is not nPcessqry. Rather, a suitable match will be found if the telltale output is ~bsl~ lly similar to the 10 behavior pattern with which it is being co~ ,aled. By "subs~ntislly similar"
is meant a degree of similarity that falls within certain pattern r~,cog~ ion tolerances that the stack desi~nPr or operator can include in the pattern recognition software to be described ~,~Çtcr. These tolerances permit a "match" to be made even though the signature and the pattern are not 15 identical.

The data processor 68 includes a common digital co,.~ el with associated read-only memory (ROM3, read-write random access ~m~ly (RAM), electrically prv~;l~llll,able read-only lllellloly (EPROM), lll~llol~ for20 storing a library of pl~,tcte~ d ~.,r~,e~ce current and voltage si~ .es for CUlll~drillg to voltage and current ~dt~ .l s produced by the PEM-probe 41, and input/output sections which interface with the A-D co--~,~r 62 and the PROX control 72 that controls the air injection rate to the PROX reactor by means of control signal 74 to a controllable ~~je. lor 76, or the like. The read-25 only memory (ROM) of the digital comyut~l contains the instructionsnPcessqry to implement the basic input/output instructions. The ele~t,ically pro~ able read-only memory (EPROM) contains the instructions nPcessqry to implement the data processor's own internal control, data manipulation, and cc,.. ~ lion algolil-llls. The processor 68 co.~.,....llic~trs with the A-D converter 62 and the PROX control 72 by means of any applopliate co.. niration l~lwolL. protocol, many of which are known in the art. A standard 486 or ~ntiulll COll~ ~, with 16 meg of RAM, Running Windows~ 3.1 or Windows~9 95, and fitted with an ACB 530 bus 5 control board is ~eql~q~tr for this purpose. A specific program for C~l jillg out the functions of the processor 68 may be accomplished by ~d~.l skill in the art using conventional info,-l,ation ~rocec~ing l~n~qges.

Either the complete voltage and/or current pattern from the 10 PEM-probe 41 may be used, or an abridged pattern (i.e., appro~rimqte slope of degradation curve) characterized by (1) a current and/or voltage reading at the beginning of a discharge cycle and (2) a current and/or voltage reading at the end of a discharge cycle may be used. Preferably, the co",~lete pattern will be used, and can be recognized using co,~ rcially available pattern 15 recognition programs. Pattern recognition programs are known in thc art and have been used for "~l~rous applications such as to (1) identify sea crealules from their acoustic pallcllls, (2) identify body hormonal ch~r~es from sensor measu,el"e"ls, (3) identify the r,~ ul~ point in a tool using vibration pall~l-,s, (4) identify land vehicles from their acoustic and seismic signatures, (S) 20 identify wear patterns in materials from thir~n~ss n~as.~,e~n~, (6) identify intruders in secure areas using ..,ic~o~rave and IR n~aa."el.~s (7) idc~iry automotive intrusion from shock and ~r~u~lic ~~ , and (8) idcl~lir~ faulty power-seat assemblies from acoustic pallc,ns, inter alia. Pl~Lllcd pattern recognition sorlwa,e for the CO conc~,l,L,~lion mol,il~..,lg tc~hnilue of the 25 present invention is ess~nti~lly analog pattern rec~nition suflw~e which, based on current and voltage measul~ ~.,l~ taken over the ~ ir~ time intervals, is capable of e-l a~ing voltage and current behavior p..l~ll~s that can be coll-~ared to refe.el~ce current and voltage telltale outputs within a defined tolerance range. From such co~?alisons, the carbon mono~ide co~,lt-alion in the H2 feed stream to the stack can be de~,~ ed, and based thereon n~cessqry adjustments to the ~fo,lller, shift and/or PROX reactions made. A
pl~relled such pattern recognition software is collu~,c.~;ially available under the name Failure/Wear PredictorTM (FWP~TM colnll~e,cially available from the 5 GenIA~STM, supra. The PWPTM sorl~ale has embedded therein GENMATCHTM software (also sold by GenLASTM), which is a programmable analog pattern recognition program which can simllltqn~ously measure an all,iLtal y nurr;~er of pattern f~ al~lres~ and includes three dirr~,e"l toler~qnres for addressing several features of a pattern rather than just a single feature 10 (e.g., a peak) thereof. That software CO~ lS of a template-mqtrhing process based on a reference si~qhlre (i.e., telltale output) created in advance from a reference cell operated under controlled conditions. It is neither amplitude-sensitive nor time-sensitive in that input signals over wide dynamic ranges (e.g., microvolts to volts taking place over periods from nanoseconds to 15 mimltes) are normqli7~d to just 600 dilllclssionless units in amplitude (Y axis) and 2000 dimensionless units in time (X axis). Following normqli7q~ n of the signals, an qrc~lm~ t~d slope, known as "qngksl-m"l is co.~ ed for each of the 2000 data points of the normqli7~d input data while traversing the signal contour. .Anglesum is proportional to the ~cc~ lllqt~ slope of the 20 curve in such a way that as the curve increases along a positive slope the anglesum increases in n-qgnihlde, and as the curve decleases along a negative slope the anglesum decreases in ma~itl~ . The pattern reCog~itir)n process utilizes the anglesum values, within defined tolerances, as dcfined in the refel~nce ~ignqhlres. In this regard, all lcf~ ~e telltale outputs contain a 25 series of intervals wl~ h~ qngl~s~m values and tolerances are used to characteri_e each interval. These intervals are the disel~ating factors used for signal recognition. If the intervals from the reference "match" (i.e., with consideration of all tolerances) like intervals in the behavior ~ lls from the PEM-probe, a "match" is declared and i~ lir.cation is complete. The program uses two interval types for its recognition process: so-called "key"
and "standard" intervals. The key intervals allow phase adj~ctm~nt of the eferellce telltale outputs to the behavior pdllClllS from the PEM-probe as well as a first pass discrimination by the recognition process. The ~d~d 5 intervals are then used for the remqit~ing recognition process. Key intervals are selected for uniqueness and serve to minimi7~ search/col.l~alison time through the reference telltale output ~?tqhace as well as to phase align the rere,el~ce intervals with thr~ data being i~ntified. Hence, key intervals allow the software to quickly a ~certain wl~ll~.,r the behavior pattern contains the 10 initial characteristics required by the rcfe,~.-ce signature. If the characteristics of the key intervals are found in the PEM-probe pattern, a full comparison is initiqte~ using the remaining slandard intervals. Standard intervals are, by definition, all intervals other than the key intervals. For the PEM-probe's behavior pattern to contain the characteristics of the ~ef,lellce key intervals, it 15 must satisfy two criteria. First, the an~l~sllm values of the ~cf~r~LIce telltale outputs must match corresponding qngl~sllm values in the PEM-probe's pattern, within the same intervals. Second, the sepal~tion (llum~r of data points apart) of the two intervals must be the same as that in the lefel~nce telltale outputs. Hence, it is both the intervals and their separation which 20 d~elll~ilR a match.

The lert~ce output tL,llylate co~isls of a series of signal intervals to which both X and Y tol~anccs are qe~ign~. Each telltale output can be divided into as many as 2000 se~ each of which is ~uuJIded by a 25 signal mq.~imllm and ,..;..i...~.... The behavior of the signal ~t~ segl~ t boundaries is modeled by a mea~ul~,ll,cnt including amplitude change, average rate of amplitude change, and ~ uus rate of amplitude ch. nge.
Tolerances can be acsi~l in three areas, for each segment i.e., so-called "qngl~sllmtolelallce", Ubittol~ lce" andUmqcl~in~tolfl~ce". Bittoklal~ce identifies the number of elçm~-ntc (points) beyond the start and end points of the specified leference interval within which the m~tchine process sea~ches for an anglesum match. For example, consider a reference interval with start and end points at data elem~ntc 65 and 135, r~,spec~ ely~ and a bit tole,ance 5 of 5. The mAtrhing process will then look at ~n~lesl-m values in the signal pattern with start and end points of (60, 130), (61, 131), (62, 132), (63, 133),(64, 134), (65, 135), (66, 136), (67, 137), (68, 138), (69, 139), and (70, 140), when trying to match with the refc.~.~ce interval Anglesl-m. If the bit tolerance = 0, then the angl~sl-m of the co~responding interval in the data is 10 compared directly to the ~ngles~lm of the co.lcs~ol~ding interval in the lefelellce pattern. ~n~les--m tolerance provides an allowance for variation in the ~nglesl~m values being co~ alcd. This tolerance dictates the allowable error in ~ sl~m values bel-.c~ll an interval in a lefe,ellce telltale output anda corresponding interval in the PEM-probe's data set. Consider a leff re.~ce interval with start point at 65 and end point at 135 with an ~n~lPs~m value =
100, bit tolerance = 0, and anglesum tolerance = S. The intervals will match if the ~nglçs-~m for the signal data interval starting at 65 and ending at 135 is within the range of 95 c signal A~gl. ;,l-.., s 105. Masking tolel~ce stipulatesthe number of non-m~t~hin~ intervals that can exist and still provide recognition. For example, consider a lef,le.~ce pattern with 30 intervals and a m~C1~in~ tolerance equal to 5. If the ~ cl of ~efe.~nce intenals found to match corresponding intervals in the signal data set is 2 25 there is a match.
Otherwise, the lefc~ence telltale output does not match the PEM-probe's behavior pattern.
During the m~tchi~g process, the software moves the ref~.e.~ce telltale output segment (the L~llplal~) back and forth along the X-axis wi~in the limits set by the bit tolerance. The soflw~e looks for a match with a'data segment from the PEM-probe's behavior patterns having an ~n~l~s~ ~n the selected upper and lower tole.~ce limits. F.ss~ntiqlly then, the l~ rhing process is as follows: (1) a specified width of voltage andJor currcnt data is extracted from the PEM-probe; (2) this data is norrnqli~Pd to an qnf~l~s~lrn of 600 points, and an element composition of 2000 points; (3) the l~f"e~ce telltale output template is moved across the data set from the PEM-probe; (4) 5 when a match is found with certain key intervals, the template and PEM-probe data sets are locked in phase, and each data set is jittered in phase along the X-axis looking for the qngl~sllrn match; and (S) if the l,~l~r of data segments specified by the mq~kin~ tole,al ce is met t~e PEM-probe data set is considered to match the template. When such a ~~atch is made, the CO-10 cuncclllla~ion in the H2 feed stream is ~letermin~d The processor 68 is pro~ m~d to ~.rOIlll the colllpalisonprocess. That is to say, rligiti7~ voltage and current values from the data acquisition unit 62 are fed to the processor 68 which c~lclll~tPs the behavior 15 patterns thereof as I = f (t) and/or V = f (t) over a pre~et~ ;n~l illcle.llcof time. These behavior patterns are then co~ d to the ~fe.e,lce telltale outputs stored in memory 70. If a behavior pattern and a l.,f~ ce telltale output subst~nti~lly match (as described above), a control signal 78 to the PROX control module 72 is issued to take coll~;live action (i.e., adjust air 20 injection rate).

While the invention has been disclosed in terms of a specifir, embodiment thereof it is not intf~ to be limited thereto, but rather only to the extent set forth ~l."~f~l in the claims which follow.

Claims (11)

1. A carbon monoxide sensor comprising a gas-monitoring PEM-probe including a proton exchange membrane having an anode and a cathode affixed to opposing first and second surfaces of said membrane, a first electrical current collector engaging said anode, a second electrical current collector engaging said cathode, an electrical discharge circuit connectable between said current collectors, said discharge circuit having a first electrical resistance valued for discharging said PEM-probe at a first rate selected to monitor the degrading output of said PEM-probe incident to CO
contamination of said anode, an electrical purging circuit connectable between said current collectors, said purging circuit having a second electrical resistance which is less than said first electrical resistance such that upon discharge of said PEM-probe through said second resistance the potential of said anode is raised to at least 0.8 V (RHE) to effect electrochemical oxidationof any CO adsorbed on said anode, and an electrical switch in electrical series connection between said current collectors and adapted to intermittently, alternately electrically connect said current collectors to said discharge and said purging circuits.

2. A sensor in accordance with claim 1 including a timer operatively connected to said switch for effecting said alternate connecting of said current collectors at predetermined time intervals.

3. A sensor in accordance with claim 2 including a timer operatively connected to said switch for periodically effecting said alternate connecting of said current collectors.

4. A sensor in accordance with claim 1 including a passage for admitting a gas to be monitored to said anode, and a valve operatively associated with said passage for restricting the flow of said gas to said anode when said switch connects said current collectors to said purging circuit.

5. A carbon monoxide sensor comprising a gas-monitoring PEM-probe including a proton exchange membrane having an anode and a cathode affixed to opposing first and second surfaces of said membrane, a first electrical current collector engaging said anode, a second electrical current collector engaging said cathode, an electrical discharge circuit connectable between said current collectors, said discharge circuit having a first electrical resistance valued for discharging said PEM-probe at a rate selected to monitor the degrading output of said PEM-probe incident to CO contamination of said anode, an electrical purging circuit connectable between said current collectors, said purging circuit including a voltage source having a potential capable of raising the potential of said anode to at least about 0.8 V (RHE) to effect electrochemical oxidation of any CO adsorbed on said anode, and an electrical switch in electrical series connection between said current collectors and adapted to intermittently, alternately connect said contacts to said discharge and said purging circuits.

6. A sensor according to claim 5 wherein said voltage source is a charge storage device.

7. A sensor according to claim 6 wherein said charge storage device is a voltaic device.

8. A sensor according to claim 6 wherein said voltage source is a capacitor.

9. A sensor according to claim 5 including a timer operatively connected to said switch for periodically effecting said alternate connecting ofsaid contacts.

10. In a fuel cell system comprising (a) a stack of PEM H2-O2 fuel cells each comprising principally a proton exchange membrane having an anode and a cathode affixed to opposing first and second surfaces thereof, a first flow channel adjacent said anode for flowing hydrogen into contact with said anode, and a second flow channel adjacent said cathode for flowing an oxygen-bearing gas into contact with said cathode, (b) an oxygen-feed manifold supplying oxygen to said cells, and (c) an hydrogen-feed manifold supplying hydrogen to said cells, the improvement comprising: a carbon monoxide sensor communicating with said hydrogen-feed manifold for sensing the concentration of any CO in said manifold, said sensor comprising a gas-monitoring PEM-probe including a proton exchange membrane having an anode and a cathode affixed to opposing first and second surfaces of said membrane such that said anode from said PEM-probe is exposed to said hydrogen from said hydrogen-feed manifold, a first electrical current collector engaging the anode from said PEM-probe, a second electrical current collector engaging the cathode of said PEM-probe, an electrical discharge circuit having a first electrical resistance valued for discharging said PEM-probe at a rate selected to monitor the degrading output of said PEM-probe incident to CO contamination of said anode, an electrical purging circuit adapted to raise the potential of said anode from said PEM-probe sufficiently to effect electrochemical oxidation of any CO adsorbed on said anode, and an electrical switch in electrical series connection between said current collectors and adapted to alternately connect said contacts to said discharge and said purging circuits.

11. A carbon monoxide sensor comprising a gas-monitoring PEM-probe including a proton exchange membrane having an anode and a cathode affixed to opposing first and second surfaces of said membrane, a first electrical current collector engaging said anode, a second electrical current collector engaging said cathode, an electrical discharge circuit connectable between said current collectors, said discharge circuit having a first electrical resistance valued for discharging said PEM-probe at a first rate selected to monitor the degrading output of said PEM-probe incident to CO
contamination of said anode, an electrical purging circuit connectable between said current collectors an adapted to raise the potential of said anode to at least 0.8 V (RHE) to effect electrochemical oxidation of any CO adsorbed on said anode, and an electrical switch in electrical series connection between said current collectors and adapted to intermittently, alternately electrically connect said current col1ectors to said discharge and said purging circuits.