J. Phys. Chem. C 2007, 111, 18423-18430
18423
Development of Cathode Architectures Customized for H2/O2 Metal-Cation-Free Alkaline Membrane Fuel Cells Christelle Tamain,†,‡ Simon D. Poynton,† Robert C. T. Slade,† Bryony Carroll,† and John R. Varcoe*,† Department of Chemistry, The UniVersity of Surrey, Guildford GU2 7XH, United Kingdom ReceiVed: August 22, 2007; In Final Form: September 24, 2007
Aqueous-electrolyte-free (metal-cation-free) alkaline membrane fuel cells represent a promising new class of low-temperature Pt-free fuel cell. A current hypothesis is that mass transport of (stoichiometric) reactant water to the cathode catalyst reaction sites is the principal origin of the limited power output (water is not a direct reactant in proton-exchange membrane fuel cells (PEMFCs) and only required to keep the protonexchange membrane hydrated for sufficient conductivity); electrode architectures specifically optimized for use in H2/O2 solid polymer electrolyte alkaline fuel cells (SPE-AFC) were previously identified as a research priority. This study directly addresses this challenge and shows that with the correct choice of cathode components significant improvements in power performance can be obtained; 125 mW cm-2 was obtained in a H2/O2 SPE-AFC when a cathode fabricated from Toray carbon paper and Pt/C catalyst (20% mass Pt on Vulcan XC-72R carbon support) was used with a 79 µm thick anion-exchange membrane in hydroxide anion form (cf. 94 mW cm-2 when the same membrane was used with prefabricated Pt-based commercial carbon cloth electrodes that contained 4 mg cm-2 metal loadings and poly(tetrafluoroethylene), PTFE, binder). Importantly, the cathode fabrication methodology reported will allow the easy comparison of the performance of different cathode catalysts, including Pt/C and cheaper carbon-supported non-noble-metal-containing catalysts of different formulations (e.g., different carbon supports and metal particle sizes). A final significant finding was that Pt-free Vulcan XC-72R-only cathodes can produce between 25% and 36% of the power obtained when Pt/C catalysts were used in SPE-AFCs (this is not the case with PEMFCs where carbon is electrokinetically inactive for the oxygen reduction reaction at the cathode); this insight highlights the necessity of recording the background currents, arising from the carbon supports, when testing different catalyst formulations in alkaline media. A recommendation is presented for a standardized test protocol for evaluating these inherently CO2-tolerant fuel cells.
1. Introduction Despite over a century of study and decades of intensive research, the cost of fuel cells still inhibits commercialization. Alkaline fuel cells (AFC) appear to be the most promising technology on a cost basis. Metal-cation-free (aqueous-electrolytefree) H2/O2 solid polymer electrolyte alkaline fuel cells (SPEAFC), containing alkaline anion-exchange membranes (AAEM), represent a technologically important new class of low-temperature fuel cell. The key reasons for the increasing interest in such fuel cells are as follows.1 (1) Catalyst electrokinetics are improved in alkaline, as opposed to acidic, conditions, especially for the oxygen reduction reaction (ORR) at the cathode; cheaper metal catalysts, such as Ni-based anodes2 and Ag-based cathodes, perform well.3-6 The “acid-stability criterion” precludes the use of such non-noble metals in proton-exchange membrane fuel cells (PEMFC),7 though there is increasing research into using non-noble metals to produce active nitrogenmodified carbon catalysts for oxygen reduction in such acidic systems.8 (2) Use of cheaper partially fluorinated and nonfluorinated membranes is feasible. (3) Metal-cation-free SPE* To whom correspondence should be addressed. Phone: +44 1483 686838. Fax: +44 1483 686851. E-mail:
[email protected]. † The University of Surrey. ‡ Current address: L’Ecole Nationale Supe ´ rieure de Chimie de Paris, 11, rue Pierre et Marie Curie, 75231 Paris Cedex 05, France.
AFCs are intrinsically immune to the presence of CO2 (a component of the cathode air supplies): AAEMs that were deliberately exchanged to the CO32- anion form performed better than the OH- analogues and decarbonated (self-purged) when operated in H2/air and methanol/air fuel cells;9 this is generally not the case with traditional aqueous KOH electrolyte AFCs.10 (4) Easily stamped and thin (low resistance) metal bipolar plates can be used with minimal corrosion-derived problems; the expense of bipolar plates is often overlooked compared to the catalysts and electrolyte membranes in fuel cells. Corrosion of metal bipolar plates in PEMFCs is serious without further treatment (coating); graphite bipolar plates are too thick (heavy) and cost too much (gas flow fields have to be manually machined) for most fuel cell applications.11 (5) Contrary to prior wisdom, AAEMs exhibit acceptable ion conductivities for use in fuel cells (>0.03 S cm-1 at 30 °C when fully hydrated).12 AAEMs can also be used in alcohol/air SPE-AFCs because of their tolerance to CO2 and low permeability to alcohols.13,14 Recent reports suggest that a Ni-Fe-Co-based anode catalyst (HYPERMEC, ACTA Nanotech SpA) can fully electro-oxidize ethanol to product CO2;15 full 12 e- oxidation of ethanol is generally not observed in proton-exchange membrane (PEM) based direct ethanol fuel cells (acetic acid and highly toxic acetaldehyde are the predominant products).16 Hence, (bio)-
10.1021/jp076740c CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2007
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TABLE 1: AAEM and Cathode Composition for the MEAs Fabricated in This Study intermediate layerc MEA Af B C D Ef F G Hf If J
a
AAEM
GDL
Solvay Solvay Solvay Solvay Solvay Solvay Solvay Solvay Solvay Surrey
CC CC CC CC CP CP CP CP CP CP
b
binder
PTFEe poly(VBC) ) 0.32
catalyst layerc
carbon
XR-72e XR-72R ) 0.82
catalyst
d
E: Pt ) 0.5: XC-72 ) 2.0 J: Pt ) 0.53: XC-72R ) 2.1 J: Pt ) 0.56: XC-72R ) 2.2 J: Pt ) 0.55: XC-72R ) 2.2 E: Pt ) 0.57: XC-72 ) 2.3 J: Pt ) 0.55: XC-72R ) 2.2 J: Pt ) 0.57: XC-72R ) 2.3 J: Pt ) 0.49: XC-72R ) 2.0 XC-72R ) 2.0 only J: Pt ) 0.55: XC-72R ) 2.2
PTFEe
poly(VBC)
x × × × × × × × × ×
0.73 0.72 0.75 0.42 0.76 0.75 0.78 0.71 0.72 0.75
additional notes identical to anode E-Tek C/PTFE electrode total poly(VBC) ) 0.74 repeat 1 repeat 2 repeat 3 control experiment thinner AAEM
a All anodes consisted of the same alkaline interface polymer treated prefabricated electrodes. b Non-wet-proofed GDLs: CC ) E-Tek type-A carbon cloth and CP ) Toray TGPH-090 carbon paper. c All numbers refer to geometric loadings (mg cmgeo-2). d All catalysts were Pt/C(20%mass): Suppliers: E ) E-Tek (C1-20 HP Pt) and J ) Johnson Matthey (HISPEC3000). e Loadings are unknown: proprietary information. f Additional EIS experiments conducted with these MEAs.
ethanol (higher energy density and lower toxicity compared to methanol)17 is attracting considerable attention for use in SPEAFCs. However, AAEM-based H2/O2 SPE-AFCs currently perform poorly when compared to their acidic PEMFC analogues. A previous combined dc/ac electrochemical study was conducted with the aim of elucidating the causes of the low power outputs.3 The main findings from this prior investigation are summarized below. (1) Unlike in PEMFCS, water is stoichiometric reactant for the ORR at the cathode: H2O(g/l) + 1/2O2(g) + 2e- f 2OH-(AAEM). This has profound implications for the operation of AAEMs in fuel cells; the main source of performance loss was hypothesized to be restricted mass transport of water to the cathode reaction sites. (2) Even with the use of high relative humidity (RH ) 100%) gas supplies, the main source of water to the cathode was the back transport of the water electro-generated at the anode (H2(g) + 2OH-(AAEM) f 2H2O(l/g) + 2e-); the commercial prefabricated cathodes that were used contained PTFE catalyst binder, and it was suspected that this restricted the transport of the water supplied in the humidified oxygen supply to the reaction sites. Enhanced back transport of water from the anode, and therefore superior performances, was observed with thinner AAEMs. (3) Current commercial electrodes for polymer electrolyte fuel cells are optimized for use with proton-exchange membranes (PEMs) and not for AAEMs. The urgent requirement for development of electrodes that are specifically designed for use in AAEM-based SPE-AFCs was highlighted. (4) Ag/C(60% mass, 4 mgAg cm-2) performed as well as Pt/ C(20% mass, 0.5 mgPt cm-2) when used as a cathode catalyst. The Ag/C catalyst performed poorly for the hydrogen oxidation reaction (HOR) at the anode of a SPE-AFC and for the ORR at the cathode of a PEMFC. This current article is concerned with development of cathodes for use in metal-cation-free SPE-AFCs. Studies into the optimization of the anode electrodes (and electrodes for alcohol-based SPE-AFCs) are ongoing and will be reported in due course. The study reported below leads to important (previously unknown) physical insights into the behavior of differing cathode architectures and catalysts in such systems. A principal objective was to investigate the effect of the elimination of PTFE from the cathode formulations on the fuel cell performance; this also allows verification of the above hypothesis.3
2. Experimental Methods Morgane ADP100-2 (Solvay S.A., Belgium) was the primary AAEM used in this study; this cross-linked and partially fluorinated quaternary-ammonium-type anion-exchange membrane (fully hydrated thickness of 153 ( 2 µm) was supplied in the Cl- anion form and has a maximum use temperature of 60 °C and an ion-exchange capacity (IEC) of 1.3 mmol(Cl-) gdry-1. Further in-house evaluated properties of this AAEM are presented in Table 1 in ref 3. A thinner AAEM was also synthesized (as described previously)18 with an IEC of 1.36 mmol(OH-) gdry-1 and a fully hydrated thickness of 79 ( 2 µm. Both of the AAEMs were exchanged to the OH- form immediately prior to any testing by immersion in a large excess of aqueous KOH (1 mol dm-3) at room temperature for 1 h with two changes of solution to ensure complete exchange and subsequent washing in water to remove the excess KOH. Details of the components used to fabricate the alkalineMEAs in this study are summarized in Table 1. Prefabricated electrodes (as used in all of the anodes and the cathode of MEA A) were purchased from E-Tek division of BASF (Somerset, NJ) and contained Pt/C catalyst (20% mass, 0.5 mgPt cmgeo-2, Pt particle sizes in the range 2-5 nm (some agglomeration was observed in several regions) supported on Vulcan XC-72 as determined by TEM analysis), type-A carbon cloth gas diffusion layer (GDL) and PTFE (undisclosed proprietary loadings) as catalyst layer binding material. The prefabricated anodes used in all of the MEAs were spray coated with poly(vinylbenzyl chloride), labeled poly(VBC) from here on, using ethyl acetate as a solvent, to a loading of 0.75 ( 0.03 mgpoly(VBC) cmgeo-2 and then treated with N,N,N′,N′-tetramethylhexane-1,6-diamine (TMHDA) and aqueous KOH (1 mol dm-3) to form an alkaline ionomer interface polymer as previously described.19 All other cathodes were fabricated in house by painting a catalyst ink onto non-wet-proofed GDLs, either type-A carbon cloth (E-Tek) or TGPH-90 carbon paper (Toray), and dried in air. The catalyst inks were formed by stirring a mixture of the catalyst powder in question either supplied by E-Tek (C1-20 HP Pt) or Johnson Matthey (HISPEC3000), both of which contained Pt/C(20% mass), and poly(VBC) in ethyl acetate solvent for several hours. MEA D was synthesized with an intermediate carbon-only layer, which was prepared in the same manner, between the GDL and the catalyst layer (CL). MEA C was produced by painting the catalyst ink onto a commercially available preformed XC-72/PTFE coated (Pt-free) carbon cloth electrode (E-Tek). The cathodes for MEAs A-J were treated with TMHDA and KOH(aq) as for the prefabricated anodes.
Cathodes for Alkaline Membrane H2/O2 Fuel Cells
Figure 1. Beginning of life galvanostatic steady-state fuel cell performance curves (recorded from OCV f high i) at 50 ( 1 °C for MEA A (B), MEA B (9), MEA C (2), and MEA D ((). The additional MEA data (1) was for the Pt/C MEA evaluated previously,3 which differs from MEA A only in containing a lower loading of poly(VBC) (0.27 ( 0.02 mgpoly(VBC) cmgeo-2). Hydrogen (anode) and oxygen (cathode) gases were supplied at 2 dm3 min-1 with dew-point temperatures ) 51 ( 2 °C and no back pressurization.
Details on the experimental techniques used in this study, such as fuel cell testing and electrochemical impedance spectroscopy, are as described previously.3 Beginning of life steadystate galvanostatic cell voltage (Vcell/V) and cell power density (Pcell/mW cm-2) versus current density (i/mA cm-2) plots were recorded with fuel cell temperatures of 50 ( 1 °C and the dew points of the gases, supplied at 2 dm3 min-1, set to 51 ( 2 °C. Fuel cell tests were either conducted by recording data points starting at open-circuit voltage (OCV, i ) 0 mA cm-2) and moving to high current densities (denoted V) or recording data points returning from low cell voltage to OCV (v). 3. Results and Discussion Cathodes Based on Carbon Cloth GDLs. Figure 1 presents the fuel cell performance curves (V scan) of MEAs A-D that were all prepared using carbon cloth GDLs. The principal parameters of relevance are presented in Table 2. MEA A which contained both prefabricated anodes and cathodes produced a power density of 55 mW cm-2; this matches numerous previous experiments (51 ( 5 mW cm-2, n ) 5)3,18 with similar MEAs that varied only in the amount of poly(VBC) polymer used in
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18425 the electrode treatment (see Figure 1 for a selected example). It was observed that MEAs B-D performed poorly compared to MEA A. It was anticipated that eliminating PTFE in the cathodes would allow more of the water contained in the cathode gas supply to reach the catalyst sites and therefore lead to increased performances. However, the principal cause of the very low power for MEA B was that the catalyst powder passed through the carbon cloth (catalyst was located on both sides of and inside of the pores of the cloth) when applied as an ink. This is not unexpected as type-A carbon cloth is highly porous (>70% porosity) and of low tortuosity, being made from interwoven carbon fibers (requiring no polymer binder).20,21 This phenomenon also occurred to a lesser extent for MEA C, which contained the intermediate carbon layer; however, the carbon in this intermediate layer was observed to pass through the carbon cloth as above. Despite little catalyst passing through the preformed C/PTFE-coated carbon cloth electrodes used in MEA D, performance was still low; the PTFE content of this electrode formulation is obviously undesirable as explained in section 1. Cathodes Based on Carbon Paper GDLs. As using carbon cloth GDLs proved unsuccessful, the next logical step was to use Toray carbon paper (another “standard” fuel cell grade GDL).21 This carbon paper material is also based on carbon fibers but in a nonwoven state (so a binder is present, see Figure 2); this material is less porous and more tortuous that the carbon cloth. Importantly, the rougher surface of the carbon cloth leads to reduced water coverage on the material compared to the carbon paper;20 hence, in this respect use of carbon paper appears desirable where maximum water retention at the cathode is required. It was observed when preparing MEAs E-I that very little of the catalyst power passed through the carbon paper GDL and the majority of the catalyst was located on the side of the GDL in contact with the AAEM. Figure 3 presents the performances of the MEAs that contain carbon paper GDLs and Pt/C(20% mass) from two different suppliers: E-Tek (Vulcan XC-72 used as the carbon support) and Johnson Matthey (JM, supplied by Alpha Aesar, Vulcan XC-72R carbon support). Figure 4 shows the TEM micrographs of Pt/C catalysts in the commercial electrodes used in this study: the Pt particle sizes for the E-Tek and JM catalysts are 3.05 ( 0.81 (n ) 120) and 3.11 ( 0.83 nm (n ) 120), respectively. Both carbon black supports have specific surface areas 250 m2 g-1, but XC-72 is a denser pelletted version, which contains additional silicate binders to allow granulation and easier handling. MEA E containing the E-Tek-catalyzed carbon paper electrode did not perform as well as MEA A. However, MEA F containing the JM catalyst produced a significant increased in power output compared to when the prefabricated cathode was used. A maximum power density of 90 mW cm-1 was obtained (a 50% increase over MEA A). This shows an unexpected (and remarkable) sensitivity to the source of the Pt/C catalysts of similar particle sizes. The inset to Figure 3 (expanded low-current region) suggests that the electrokinetics of the different cathodes were similar (the OCVs were also all similar for all three MEAS, 1.06 ( 0.01 V); corroborating evidence for this is presented in the EIS data in the next section (see Figure 8 later). Therefore, the prime suspect for the difference in performance between the two catalysts is mass transport, possibly from the presence of the silicate binder in the XC-72 (denser carbon support material). This is confirmed by the impedance spectroscopy measurements described later. Performances were consistently superior for the performance plots returning from high current densities to OCV (v), which
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TABLE 2: Selected Extracted Electrochemical Data Ppeak/mW cmgeo-2
OCV/V MEA A B C D E F G H I J
a
dc Tafel slope/mV dec-1 d
b
V
b
v
Vb
vb
r/Ω cmgeo2 c
1.073 1.080 1.012 1.043 1.057 1.050 1.072 1.057 0.883 1.067
1.082
55 25 34 32 35 80 75 61 22 104
60
1.7
-93 ( 4
25 ( 2
47 90 88 76 21 125
1.7
-97 ( 4
40 ( 10
1.5 2.0
-92 ( 4 -85 ( 5
44 ( 5 50 ( 6
1.075 1.076 1.084 1.076 0.898 1.076
Rpl/mΩe
a Refer to Table 1. b V ) galvanostatic performance curve recorded from OCV to high current densities. v ) from high current densities to OCV. Recorded at high current densities (from EIS data when collected). d From Figure 8. e The width of the high-frequency semicircular feature in EIS data (see Figure 7).
c
Figure 2. SEM micrographs of the carbon cloth (left) and carbon paper (right) GDLs used in this study.
were always recorded after the plots beginning at OCV and going to high current densities (V) and after an initial high current discharge; this arises from the alkaline ionomer components becoming fully hydrated at high current densities from the increased back transport of the water (that is electrode generated at the anode) through the AAEM.3 This in effect acts as a “conditioning step” and means water transport to the cathode is facilitated on the return v polarization scan; the more uniform and stable hydration levels also explain the smoother profiles of the plots for that scan. To check repeatability of the improved performance, three JM-catalyst-containing MEAs were produced and tested (Figure 5, with both V and v scans). Despite the inevitable performance variability, this MEA configuration consistently outperformed MEA A (our prefabricated cathode benchmark MEA). Peak power densities of 84.8 ( 7.6 mW cm-2 (n ) 3) compared to 60 mW cm-2 for MEA A were observed with the v polarization scans. As a control experiment, a test was conducted with a Pt-free cathode containing only the Vulcan XC-72R carbon black support (MEA I in Figure 5); strict precautions were taken to ensure no Pt contamination. A peak power of 22 ( 1 mW cm-2 was observed, which constitutes 25% of the power produced by the best performing JM-based MEA (MEA F); this is a significant background interference and highlights the importance of taking into consideration the currents generated by the support materials when evaluating different carbon-supported catalysts in alkaline conditions. It is evident from the inset to Figure 5 that the electrokinetics are poorer on the Pt-free carbon catalyst compared to the Pt/C as expected. Carbon, in the absence of impurities, is known to be active for the ORR reaction in alkali22,23 via the 2e- reduction of oxygen (O2 + H2O + 2e- f HO2- + OH-; EQ ) -0.08 V vs SHE) rather than the desired 4e--ORR (O2 + 2H2O + 4e- f 4OH-; EQ ) +0.40 V vs SHE).24 A substantial amount of 2e--ORR and therefore hydroperoxide anion formation on the carbon catalyst supports under alkaline conditions was, therefore, anticipated.
Figure 3. Beginning of life galvanostatic steady-state Vcell vs i and Pcell vs i plots at 50 ( 1 °C for MEA A (B), MEA E (9), and MEA F (2). Test conditions unchanged from Figure 2. Filled symbols are data recorded from OCV f high i (V), and open symbols are high i to OCV (v). (Inset) Magnification of the low i region.
Formation of peroxide byproducts is a serious concern in acidic PEMFCs; such species cause serious PEM degradation, especially on reaction with (always present) trace amounts of metalcation contaminants forming reactive peroxy radicals (Fenton’s reaction mechanism).25 The effect of high hydroperoxide concentrations on the AAEMs is, however, currently unknown (although there is less chance of metal-cation contaminants being
Cathodes for Alkaline Membrane H2/O2 Fuel Cells
Figure 4. TEM micrographs of the catalysts from Johnson Matthey (left) and E-Tek (right).
Figure 5. Beginning of life galvanostatic steady-state Vcell vs i and Pcell vs i plots at 50 ( 1 °C for MEA A (B), the arithmetic mean responses for MEA F, G, and H for both types of polarization scan (V and v) where the error bars indicate one sample standard deviation (9), and MEA I (2). Error bars are not shown in the inset for clarity.
present in the AAEM as cations are selectively rejected by the positively charged -N+R3 groups in membrane).26 The OCV of 0.88 V obtained with this Pt-free-cathode MEA (Table 2) reveals, however, that a mix of 2e--ORR and 4e-ORR must be occurring: an OCV of >0.75 V cannot be obtained (thermodynamically) if 2e--ORR is the exclusively operating mechanism, while an OCV between 1.0 and 1.1 V is expected for a purely 4e--ORR mechanism (see Table 2 and the data presented in ref 3). The Vulcan XC-72R was used as received and not washed, chemically treated, or purified in any way. A factor for the mix of ORR mechanisms could also be
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18427 the absence of spectator ions: Kucernak and Jiang previously discussed and contrasted the differences observed between the mechanisms of electrochemical processes, such as the HOR, in solid-state electrochemical cells (microelectrode pressed against a PEM in the absence of aqueous anionssthe only anions being the SO3- groups attached to the PEM backbone directly analogous to the conditions in PEMFCs) and in classical electrochemical cells containing aqueous electrolytes (with the presence of H2SO4, with specifically adsorbing bisulfate (HSO4-) anions, or HClO4, with nonspecifically adsorbing perchlorate (ClO4-) anions).27 No unattached cations should be present in well-prepared alkaline MEAs for SPE-AFCs (KOH(aq) free). This hints at the tantalizing prospect that with the right treatment/ preparation, carbon-only (metal-free) cathodes that operate the 4e--ORR and produce acceptable current densities could be developed for use in the cathodes of SPE-AFCs. Development of a solid-state (aqueous-electrolyte-free) electrochemical cell (three electrodessthe microelectrode-based working electrode is contacted only with the AAEM and water) for further evaluation of the carbon supports in conditions analogous to those found in SPE-AFCs is required to confirm this new hypothesis. A previous observation was that use of thinner AAEMs with the same electrodes leads to improved power outputs (due to increased back transport of water from the anode to the cathode);3 to confirm this, an experiment was conducted where identical electrodes to those used for MEA F were used with an AAEM of one-half the thickness (similar IEC and ionic resistance, see section 2). The result was dramatic, and a power output of 125 mW cm-2 was obtained with MEA J compared to a maximum of 90 mW cm-2 for the thicker Solvay AAEM (Figure 6). The same thin 79 µm AAEM yielded 94 mW cm-2 with Pt-based (4 mg cmgeo-2 metal catalyst loaded) prefabricated electrodes that contained PTFE binder;18 again, the superiority of the Toray-Pt/C(JM) combination cathode is demonstrated. In Situ Electrochemical Impedance Spectroscopy (EIS) Studies. Selected EIS spectra are presented in Figure 7 and analyzed as described previously.3 The principle features observed are summarized. (1) The internal resistances (ir) of the fuel cells are measured from the high-frequency x-axis intercepts. (2) The potential-independent high-frequency depressed semicircular feature gives an estimate of the ionic resistance of the ionomer in the porous electrodes (i.e., this feature is larger in magnitude when the electrodes have not been subjected to an alkaline ionomer treatment). The width of this feature is designated Rp1 and reported in Table 2 where relevant. (3) The potential-dependent medium-frequency semicircular feature represents the classic parallel RctCdl circuit where Rct is the charge-transfer resistance and Cdl is the double-layer capacitance. As the anodes and membranes used for all of the MEAs tested with EIS are the same, any significant changes to this feature between MEAs is related to changes in cathode electrokinetic properties. This feature is difficult to analyze at medium to high current densities due to the substantial overlap (similar RC time constants) with the high-f feature above. The ac equivalent to the dc Tafel plots can be constructed from this data (see Figure 9 later for an example). (4) The potentialdependent low-frequency feature is typical of a diffusion-derived (long time constant) phenomena and hypothesized to relate to the mass-transport limitations of water to the cathode reaction sites. The width of this feature is designated Rp2, and the values at different current densities are plotted in Figure 10 later. The spectra for all four MEAs evaluated using EIS (MEA A, E, H, and I) show a similar response when under 10 mA
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Figure 7. EIS spectra of MEA A, E, H, and I under galvanostatic discharge at (a) i ) 10 and (b and c) 70 mA cm-2. The high-f, mediumf, and low-f frequency features referred to in the main text are highlighted on the spectra of MEA H (low-i) and MEA E (high-i). Note the high-f and medium-f data have merged in the high-i EIS spectra. Figure 6. Beginning of life galvanostatic steady-state Vcell vs i and Pcell vs i plots at 50 ( 1 °C for MEA A (B), MEA J containing the thinner AAEM (9), and MEA F (2).
cm-2 galvanostatic discharge; at this low current density masstransport-derived phenomena (low-f features) are not manifest. The smaller Rct value observed at i ) 10 mA cm-2 of the XC72R-only electrode was anticipated, as it was operating under a higher overpotential (Vcell ) 0.66 V for MEA I compared with 0.88 V for MEA H). The Rct values at this low current density are all similar for MEA A, E, and H, indicating the electrode kinetics on each of the different cathodes were similar (confirming observations above). The low-f features are significantly larger for MEA E and I compared to MEA A and H in the EIS spectra recorded at i ) 70 mA cm-2. The internal resistances recorded at each current density were used to plot pseudo-dc Tafel plots for MEA A, E, H, and I (i.e., ir-corrected-Vcell/V vs log(i/mA cm-2), plots that have not been mass-transport corrected (Figure 8)). The corresponding Tafel slopes are presented in Table 2. It is apparent that the electrokinetics of all of the Pt/C-containing cathodes are the same (overlapping plots at low current densities), while the plot for the Pt-free cathode is shifted to considerably lower ircorrected potentials due to a combination of thermodynamic (reduced EQ) and kinetic effects (reduced exchange current density). This data confirms the observations discussed above. The equivalent ac Tafel plot is presented in Figure 9 and acts
Figure 8. Tafel plots (d.c. no mass-transport correction applied) for MEA A (B), MEA H (9), MEA E (2), and MEA I (1) where the ir corrections were made using EIS data at each current density and with each MEA.
as further corroboratory evidence; the Tafel slopes were similar to the dc slopes (within experimental error). To obtain further proof that mass transport of water was the primary factor limiting power outputs, the values of Rp2 were plotted at each current density (Figure 10). Fitting these low-f features is difficult; the subtly changing non-steady-state condi-
Cathodes for Alkaline Membrane H2/O2 Fuel Cells
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18429 sized electrodes were operated as the anodes. In each case the MEA performed poorly when compared to the bench mark MEA A. Optimization clearly has to be specific to the anodes. Finally, a MEA comprising of two E-Tek prefabricated electrodes deliberately damaged (patches of catalyst were removed exposing the carbon cloth GDL) was tested to see if this aided water transport to the cathode. Power performance increased from 55 to 67 mW cm-2 (removal of catalyst led to a 22% increase in power!), giving a final piece of evidence that mass-transport limitations dominate over (outweigh) electrokinetic-derived performance losses. Concluding Remarks
Figure 9. Tafel plots (a.c. no mass-transport correction applied) for MEA A (B), MEA H (9), MEA E (2), and MEA I (1). From EIS data.
Figure 10. Values of the fitted resistances of the low-f EIS feature (see Figure 7) for MEA A (B), MEA H (9), and MEA E (2).
tions in the fuel cell during the low-frequency sweeps lead to relatively noisy data. Only the first part of the low-f semicircles are developed (going to lower frequencies ( 160 mA cm-2 (within experimental precision). Figure 7 indicated that MEA E gave larger low-f features than MEA A at i ) 70 mA cm-2 but that this is dramatically reversed at i ) 80 mA cm-2, indicating that these mass-transport limitations are very sensitive to small changes in current density (unlike for O2 diffusion limitations in PEMFC cathodes due to liquid water formation and electrode flooding);28 again, this provides evidence that water, rather than O2, is the species that is at the core of the mass-transport-derived performance losses in SPE-AFCs. Two further brief experiments were conducted. The MEAs were tested in “reverse formation” where the in-house synthe-
All of the data presented above corroborate the hypothesis that diffusional (and/or convectional) mass transport of stoichiometric water to the cathode reaction sites limit power production in intrinsically CO2-tolerant alkaline-anion-exchange membrane (AAEM) containing H2/O2 metal-cation-free solid polymer electrolyte alkaline fuel cells (SPE-AFCs). Cathodes formulated using Pt/C (20% mass, supported on Vulcan XC72R carbon blacksfree of silicate binder) on Toray carbon paper gas diffusion layers yielded an increase in power of between 27% and 50% over use of PTFE-containing prefabricated cathodes (by improving the characteristics of the water mass transport). The electrode fabrication procedure was deliberately simple and creates the ability to evaluate different cathode catalysts in these solid alkaline fuel cells (including profitable Pt-free catalysts). The Vulcan XC-72R carbon black support produces at least 25% of the power of the Pt/C electrodes, and the need to evaluate this background interference is emphasized when evaluating any form of alkaline fuel cell. The tantalizing prospect that suitably treated carbon materials could be used in metal-free cathodes in aqueous-electrolyte-free SPE-AFCs is noted. The research priority must be to target further development of electrodes, including the anodes, for SPE-AFCs; performances are not currently limited by the intrinsic properties of the AAEMs (though thinner, mechanically strong, membranes are desirable as with PEMFCs). As the field of SPE-AFCs is new and immature and as a result of the observations in this study, the following recommendations are made to enable standardized and reproducible testing of the components of AAEM-based SPE-AFC membrane electrode assemblies (MEA): (1) fuel cell tests should be evaluated at 50 °C (to ensure minimal thermal degradation of current generation AAEMs, which have maximum operating temperatures of ca. 60 °C) with fully hydrated and superstoichiometric gas supplies (to minimize mass-transport effects); (2) a 1-2 h high-current discharge step should initially be conducted followed by a quick series of galvanostatic current steps from open-circuit voltage to high-current densities (this acts as a conditioning step for the MEAs allowing full hydration of the alkaline iomomer MEA components); (3) the actual steady-state galvanostatic cell voltage and power density versus current density plots should then be recorded returning from high to low current densities (to ensure smooth and consistent responses) with in situ impedance spectroscopic measurements conducted at each current density (to monitor internal resistances, charge-transfer overpotentials, and mass-transport-derived overpotentials). Acknowledgment. The work was funded by the EPSRC via the University of Surrey Doctoral Training Account. Solvay S.A. (Belgium) and Johnson Matthey (United Kingdom) are thanked for the supply of the anion-exchange membrane and Vulcan XC-72R carbon black, respectively. Dr. Yanling Chen (Micro-
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