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Annu. Rev. Mater. Res. 2003. 33:55779doi: 10.1146/annurev.matsci.33.022802.090953
Copyright c 2003 by Annual Reviews. All rights reserved
NEW ELECTROCATALYSTS BY COMBINATORIALMETHODS
Eugene S. Smotkin and Robert R. Daz-MoralesDepartment of Chemistry, University of Puerto Rico at Rio Piedras, San Juan,
Puerto Rico 00931; email: [email protected]; [email protected]
Key Words fuel cells, array screening, heuristic rules
s Abstract Combinatorial methods provide a means for accelerating the discoveryof fuel cell catalysts. The first example of parallel fuel cell catalysts screening was anindirect method that used fluorescent chemosensors to detect changes in pH in proxim-ity to electrocatalyst spots. Serial direct electrochemical methods have been developedthat use voltammetry, chronoamperometry, and scanning electrochemical microscopy.An array fuel cell screens catalysts simultaneously, using high-performance fuel cellcomponents. Heuristic models based on mechanistic and spectroscopic studies pro-vide guidance for library development, and detailed studies of discovered catalysts
can help to refine these models. The remaining challenges are the development ofhigh throughput synthetic methods that can enable the use of discovery level and fo-cus level screening. Until these synthetic methods are developed, a greater emphasisshould be placed on smaller libraries with design of experiment strategies leveragedwith informatics and data mining.
INTRODUCTION
Combinatorial Discovery of Heterogeneous CatalystsTraditional catalyst discovery methods are labor intensive and involve time-con-
suming trial and error procedures. Combinatorial synthetic methods are being de-
veloped to process large libraries that, if combined with high throughput screening
integrated with data mining and predictive modeling, are expected to accelerate
the catalyst discovery process. Over 50 years ago, Mittasch first applied combi-
natorial approaches to the discovery of heterogeneous catalysis, using arrays of
reactors to screen large libraries of catalysts for the synthesis of NH3 from H2 and
N2 (1). Twenty years later, Hanak applied combinatorial methods to optimize the
compositions of superconductors (2).
Two levels of resolution typify combinatorial catalysts screening subsystems
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558 SMOTKIN DIAZ-MORALES
parallel or serial. Ideally, the focus screen should provide more detailed response
functions such as kinetics, selectivity, and lifetime studies. The focus subsystem
is intended to generate strong leads for further development and provide guidance
for scale-up studies.Commonly used methods in heterogeneous catalyst screening are optical and
mass spectrometric (MS) because of their broad applicability and dynamics.
Holzwarth et al. have demonstrated IR thermography as a heterogeneous, catalyst-
screening tool by taking into account different emissivities of catalyst clusters (4).
IR thermography is a fast parallel method ideally suited for down-selection of cat-
alyst candidates at the discovery level for evaluation at the focus level. Scanning
MS has emerged as a versatile method of analysis. Two basic concepts for the ap-
plication of scanning MS include transient modes (5, 6) and continuously operated
reactor arrays (7). Resonant multiphoton ionization (REMPI) give more direct in-formation but in a serial fashion. Nayar et al. developed a very rapid serial MS tech-
nique, laser-activated membrane introduction mass spectrometry (LAMIMS) (3).
LAMIMS is a discovery subsystem compromise, e.g., a very rapid serial technique
capable of complete product analysis while screening under realistic conditions.
Reddington et al. pioneered combinatorial discovery level screening for elec-
trocatalysts. Their first application was direct methanol fuel cell electrocatalysis.
The optical screening method was based on the detection of the pH drop in prox-
imity to array spots catalyzing the methanol oxidation half reaction (8). Liu &
Smotkin developed an array fuel cell for parallel focus and/or discovery levelscreening (9). The array fuel cell has the advantage of parallel catalyst evaluation
under actual fuel cell conditions at controlled temperatures, catalyst loadings, and
fuel stoichiometric ratios. Serial electrochemical methods have been developed by
Sullivan et al. (10), Jiang & Chu (11), Warren et al. (12), and Pope & Buttry (13).
Shah & Hillier have developed scanning probe methods of analysis based on the
scanning electrochemical microscope (14).
The development of high throughput screening methods requires concomitant
development of high throughput catalyst preparative methods. Senkans statement,
. . . real heterogeneous catalysts are extended solids with multiple discontinuitiesin structure and composition allowing very limited systematic variation in these
properties with overall composition, (15) is appropriate for fuel cell electro-
catalysis, where catalyst preparative methods are kinetically controlled and yield
metastable, mixed-phase materials. Thus the parameter space extends beyond com-
positional (16) and includes preparative details such as temperature, addition rates
of reagents, headspace composition, choice of reducing and oxidizing agents and
even geometric aspects of reaction vessels. The variation of these noncomposi-
tional parameters can generate hundreds of different catalysts for a unique nominal
composition.
Th Si l C ll F l C ll
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COMBINATORIAL ELECTROCATALYSTS 559
generate electricity. It was another 120 years before the NASA space program
demonstrated the first practical application in space flight. Approximately 160
years after Groves observation, fuel cells have still not entered the consumer
market. The transition from the space program, where highly purified H2 andO2 areused, to terrestrial applications using commodity fuels and air, awaits the discovery
of better catalysts that can accommodate the impurities included in H2 derived
from hydrocarbon-based fuels (reformate). The most studied catalyst poison is
CO, a contaminant in reformate fuel and an intermediate in direct methanol fuel
cells (DMFCs). Other concerns are sulfur-containing and nitrogenous (particularly
from autothermal reforming) catalyst poisons. Aricoetal.haveprovidedathorough
review of fuel cell catalysis (17).
Gottesfeld & Zawodzinski provide an excellent review of polymer electrolyte
membrane (PEM) fuel cell systems and catalysis (18). Figure 1 schematizes a fuelcell based on NafionTM 117 (eq wt. 1100, thickness .007 inch). The catalytic layer
and the gas diffusion layers (GDLs) are 10 and 150 m thick, respectively. The
heart of the fuel cellthe PEM sandwiched between the electrocatalytic layers
is the membrane electrode assembly (MEA). Another practice is to catalyze the
GDLs, which are then pressed against PEM (i.e., GDL-integrated MEA) dur-
ing fuel cell assembly. The single cell is completed by current-collecting plates
with machined flow fields for delivery of fuel and oxidants. Reformate fuel cell
catalysts are typically supported on carbon (1 m), whereas DMFC catalysts
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560 SMOTKIN DIAZ-MORALES
are unsupported. In most cases, the catalysts, incorporated into alcoholic inks that
include solubilized polymer electrolyte, are decal-transferred to the polymer (19)
or applied to the GDLs. The finalized catalyst environment has no mobile anions.
Issues such as contact of catalyst nanoparticles with the polymer electrolyte andcontrolled hydrophobicity (to avoid cathode flooding) are inextricably linked to
the catalyst preparation method. The complex environment of fuel cell catalysts
as well as the electrode chemistries must be considered when designing screening
methods.
Reformate Fuel Cells
Significant losses arise in reformate fuel cells because of poisoning by carbon
monoxide. The effects of CO on fuel cell performance were first described by
Gottesfeld & Pafford, who described the effects of CO levels ranging between
5 and 100 ppm (20). The reforming of liquid fuels yields hydrogen with high
levels of CO. Methanol reformate is typically 25% CO2 and 1% CO. Although CO
can be removed by the combination of a water-gas-shift reactor and a preferential
oxidation unit to below a ppm, CO can be generated at the electrode surface
by insitu reduction of CO2 (the reverse water-gas-shift reaction). Combinatorial
methods are also being applied to water-gas-shift catalysts (3). PtRu is somewhat
resistant to CO poisoning but cannot tolerate transient spikes (as high as 0.1% CO)
common to fuel processors. A recent evaluation of CO poisoning on Pt suggests
that at ideal operating conditions (
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COMBINATORIAL ELECTROCATALYSTS 561
and electrochemical studies are being used to develop a comprehensive model that
considers d-band vacancies, geometric factors, and the chemisorption of oxygen
species (25). XAS showed that alloying in carbon-supported Pt/Cr, Pt/Co, and
Pt/Ni contracts the Pt-Pt bond distances. This would enhance a dual site mecha-nism (Equation 3). In addition, the alloying inhibits chemisorption of oxygenated
species, which are responsible for formation of the passivating layer. Toda et al.
studied sputter-deposited Pt-based alloys, including Ni, Co, and Fe, for the ORR
(25a). X-ray photoelectron spectroscopy (XPS) of the sputter-deposited alloys af-
ter electrochemical evaluation showed, without exception, no detectable alloying
components. However, supported alloy catalysts have shown high stabilities and
can retain non-noble alloying elements during 60009000 h of operation in PEM
fuel cells (24). Thus the relevance of sputter-deposited alloys that are not realisti-
cally supported is questionable. Several empirical studies have shown that first-rowtransition metals, including Cr, Co, Ni, and Fe (22, 25, 26), have promoting ef-
fects, although the optimum composition of these alloys remains unknown. In the
ideal, promoting elements are sought that maintain the proper structure and parti-
cle size while inhibiting the adsorption of passivating oxygen-containing species.
Marcovic & Ross (22) describe the anion dependence (ClO4 versus HSO
4 ) of
the performance rankings of Pt3Co, Pt3Ni, and Pt, confirming the importance of
anion adsorption on the ORR rate. They found that the ORR activation energies
for the alloys are similar to that of Pt, which suggests a common serial 4-electron
ORR mechanism. Marcovic & Ross suggest that among Pt ensemble, structural,and electronic effects, the latter is the dominating factor in Pt-OHads energetics.
The careful consideration of these models when designing combinatorial li-
braries, coupled to high throughput fundamental science enabled by in situ high
throughput screening devices, could go a long way toward advancing our under-
standing of ORR kinetics.
Direct Methanol Fuel Cells
DMFCs are based on CH3OH oxidation at anode. Although on a per mass basis
PtRu is more active when supported on carbon, the sluggish kinetics of methanol
oxidation demands high MEA catalyst loadings (4 mg cm2) that can be ac-
commodated only by unsupported catalysts (27). MEA fabrication methods must
be developed to enable the use of the more active supported catalysts. The gas
diffusion layers at the anode side, which were designed for gas fed fuel cells, must
be replaced with current-collecting diffusion layers that are optimized for liquid
feed fuel cells and supported catalyst anode layers.
Unlike H2 fuel cells, where polarization losses are primarily at the cathode,the losses at the DMFC anode and cathode are comparable. The consensus is that
methanol oxidation proceeds by a bifunctional mechanism (28 29) that involves
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CH bond activation
Pt-(CH3OH)ads Pt-(CH3O)ads + H++ e. 6a.
Pt-(CH3O)ads
Pt-(CH2O)ads+
H
++
e
. 6b.Pt-(CH2O)ads Pt-(CHO)ads + H
++ e. 6c.
Pt-(CHO)ads Pt-(CO)ads + H++ e. 6d.
Water adsorption
M+ H2O M-(H2O)ads. 7.
CO oxidation
Pt-(CO)ads +M-(H2O)ads Pt+M+ CO2 + 2H++ 2e. 8.
OverallCH3OH+ H2O CO2 + 6H
++ 6e. 9.
CO free pathways have been proposed but the relevance to steady-state fuel
cell performance is unclear (30, 31). Methanol oxidation kinetics is sluggish be-
cause PtRu does not adequately activate H2O (Equation 7). Crossover methanol
from the anode to the cathode hampers O2 reduction by wasting Pt sites at the
cathode for the direct reaction of methanol with O2 (severely reducing fuel ef-
ficiency), generating a mixed potential that reduces cell voltage, and producing
additional H2O that causes flooding and increases the required O2 stoichiometricratio.
Anode catalysts capable of H2O activation at more negative potentials and
cathode catalysts insensitive to methanol (permitting return of methanol to the
anode) are needed. Cathode catalysts operating at more positive potentials would
permit polarization of the anode to more positive potentials without compromising
operating cell voltages. At more positive anode potentials CH activation (Equation
6) becomes a more kinetically important consideration (32).
Methanol-insensitive cathode catalysts would improve the DMFC fuel effi-
ciency by permitting return of crossover methanol to the anode stream. This wouldexclude Pt, which effectively adsorbs methanol (Equations 6ad). Alonso-Vante
& Tributsch (33) and Alonso-Vante et al. (34) discovered methanol-tolerant non-
noble metal catalysts 15 years ago. These catalysts were originally reported
as Chevrel phases composed of Ru and Se doped with Mo and other elements.
Although chalcogenide-based catalysts have been demonstrated to be methanol
insensitive, their performance is an order of magnitude less than that of Pt. The
development of methanol-insensitive cathode catalysts would be ideally suited for
combinatorial studies.
Heuristic Models for Catalyst Library Development
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COMBINATORIAL ELECTROCATALYSTS 563
that do not make sense chemically. The development of platinum alloy methanol
oxidation catalysts is a good example of this approach. CO, a surface-bound in-
termediate in methanol oxidation, rapidly poisons pure Pt. Adatoms and alloying
elements such as Ru and Sn cause Pt to oxidize methanol at much lower overpo-tentials (3546), and several models have been proposed to explain this effect. The
bifunctional mechanism proposes that water (required in Equation 8) can be bound
more efficiently by oxophilic metals that alloy with or physically contact nanoscale
grains of Pt (4749). A second model involves a ligand effect, in which the d-band
occupancy of Pt is changed by alloying elements, lowering the activation barrier
for CO oxidation (29, 5052). A third model, more recently proposed, claims that
elements such as Ru and Os do not alloy with Pt but provide a mixed-conductor
hydrous oxide phase that improves contact between the Pt surface and the PEM
(53).The above models provide a starting point for library development. Ley et al.
(54) and Liu et al. (55) reported a strategy for ternary catalyst library development
based on the bifunctional mechanism. They hypothesized that Equation 8 demands
similarity between the promoter MO and the PtC bond energy of590 kJ
mol1. A plot of MO binary bond energies in proximity to the PtC bond energy
(Figure 2) suggests a component library that includes Mo, Ru, Os, Sn, and Re.
Figure 2 Ley diagram for bifunctional mechanism requiring concerted breaking of
PtC and MO bonds, where M is Pt or an alloying component. Components along
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564 SMOTKIN DIAZ-MORALES
Figure 3 Consideration of phase equilibria in library design. Two binary phase dia-
grams are combined to generate approximate ternary diagrams. The Ley model selects
nominal compositions near the single-phase region. Actual catalysts are mixed phase
systems. [Reprinted with permission from (55). Copyright 1996 The Electrochemical
Society, Inc.]
Ley et al. further assumed that the alloy phase was the key catalytic phase and thus
avoided compositions that significantly departed from the single-phase regimes.
They constructed approximate ternary phase diagrams from binary diagrams to
generate ternary libraries (Figure 3) and tested this idea by examining ternary
Pt-Ru-Os compositions that lay within the face-centered cubic (fcc) single-phase
region of the ternary alloy phase diagram. They correlated the effectiveness of
alloying elements with metaloxygen bond strengths and rationalized why Ru is a
more effective alloying element than Os. Ley et al. were first to show that ternary
combinations give higher activities than simple binaries in the single-phase region.
Liu et al. corroborated this with rotating disc electrode studies of Nafion-coated,
arc-melted Pt, Pt Ru, and PtRuOs electrodes (56).
COMBINATORIALMETHODS FOR ELECTROCATALYSIS
Single-Cell Versus High-Throughput ScreeningThe traditional method for optimizing fuel cell catalysts is to prepare bulk catalysts
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COMBINATORIAL ELECTROCATALYSTS 565
analysis can be applied to MEA-bound catalysts before and after fuel cell testing.
The success of single-cell testing requires invariant counter electrode performance.
It is convenient to operate the counter electrode as a hydrogen reference electrode in
the galvanostatic mode. In either case, reproducible water management is requiredif single-cell tests are to be comparable. The preparation of catalyst ink for MEA
preparation takes two days. Three days of electrochemical conditioning followed
by least two days of steady-state data acquisition are required to ensure steady-
state performance on uncharted catalyst libraries. The testing of ten catalysts on a
single-cell test stand, in triplicate, would take over half a year. The development
of reliable combinatorial screening methods is an alternative.
Optical Screening Methods
Reddington et al. were the first to demonstrate parallel fuel cell catalyst screen-ing (8). Their optical method is based on the fact that electrochemical half re-
actions consume or generate ions to maintain charge neutrality. Methanol oxida-
tion anode Equations 6 and 8 generate protons, and the ORR consumes protons
(Equations 2 and 4). If a chemosensor (57, 58) (an indicator molecule for the
ion of interest) is present, then its change in absorbance or emission pinpoints
the location, usually a set of individual spots in a catalyst array, where the half
reaction is occurring. Fluorescence is preferred over absorbance as a detection
method because of its greater sensitivity. An advantage of this screening method
is simplicity because of the fact that it is an intrinsically parallel technique andrequires only aqueous indicator solutions and a hand-held UV lamp. Figure 4 a,
illustrates the optical screening method with a small ternary array of Pt-Rh-Os
DMFC anode catalysts supported on a Toray carbon electrode. In this case, quinine
(pKa = 5.5), an acid-sensitive fluorescent indicator, provides a pH map of the ar-
ray. At low overpotential (Figure 4a, center frame), one spot shows the brightest
fluorescence and is ringed around by six less active spots. This region contains the
best methanol oxidation catalysts in the array. The image at the right (Figure 4a)
illustrates that at sufficiently high overpotentials, all the catalyst spots in the array
are active. Figure 4b shows robotic preparation of a 715-member array by delivery
of salts of five elements followed by borohydride reduction. At right is the top
view of the screening cell. Figure 4c demonstrates how a quaternary library can
be scanned and zoomed for refinement of the library. The array-screening cell is
an unfolded version of the quaternary compositional phase diagram. The Empire
State Building appearance results from the removal of redundant points along the
phase boundaries. This method has since been applied to cathodes (8), electrolyz-
ers (59), amperometric sensors (60), photoredox reactions (61), and anodes by
other workers (62). Two major drawbacks are that the method is indirect and does
not allow one to measure current directly, and the synthetic method for preparation
of the catalyst array is not scalable. State-of-the-art catalysts cannot be incorpo-
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566 SMOTKIN DIAZ-MORALES
advantage over focus level electrochemical screening. This may change if scalable
high throughput synthetic methods are developed.
Electrochemical Screening MethodsElectrochemical array screening cells have been independently developed by
Sullivan et al. (10), Jiang & Chu (11), Warren et al. (12), and Liu & Smotkin
(9). The methods of Sullivan and Warren were similar, with square grids of 64
individually addressable microelectrodes fabricated on silicon wafers by litho-
graphic methods. Sullivan et al. studied alkanethiol modified gold electrode arrays
and was able to show good correlation between the rate of electron transfer to solu-
tion phase redox couples and the optical fluorescent response using fluorescein as
an indicator. Warren et al. electrodeposited PtRu on an array of Pt electrodes and
was able to confirm by potential step experiments that PtRu 50:50 was the most
active catalyst for methanol oxidation in 0.5 M H2SO4. Chu & Gilman previously
reported similar findings (63).
Jiang & Chu developed a room-temperature array method that uses a movable
probe with an electrolyte link to a reference and more distant counter electrode
(11). Figure 5 shows the array electrode system consisting of an electronically
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COMBINATORIAL ELECTROCATALYSTS 569
Figure 9 XRD spectra of the four classes of catalysts, before (dotted lines) and afterarray screening (solid lines) samples obtained on the MEA after removal of the GDL
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TABLE 1 XRD-total pattern fitting results
Sample a/ A Displacement/deg 2
Pt (cathode) 3.9160(3) 0.009(5) 3.42313Reetz PtRu 3.8711(7) 0.253(6) 0.56467
Adam PtRu 3.8906(5) 0.352(1) 2.76493
J.M. PtRu 3.8830(8) 0.349(8) 0.61446
loading characteristics as the array prepared catalysts. The reducing strength of
NaBH4, which enables incorporation of metal ions with a wide variety of redox
potentials, is also responsible for the inability of NaBH4 to prepare high surfacearea catalysts. NaBH4 has a sintering effect, precluding preparation of nanostruc-
tured catalysts (A.S. Arico, private communication). Surface areas greater than
30 m2 g1 could not be prepared by borohydride reduction. Attempts to scale up
the method for preparation of bulk catalysts were met with difficulty. In some
cases, even with optimized reaction conditions, the borohydride method yielded
actual bulk compositions that were quite different from the nominal compositions.
In contrast, the Watanabe et al. method (41, 43) yields >80 m2 g1 nanostructured
catalysts. This method involves the oxidation of the sulfite complex of Pt to PtO2,
mixing with RuCl3 to form a mixed Pt/Ru colloid, and reduction with hydrogen togive a highly active PtRu catalyst. Unfortunately, the pH conditions required for
the Watanabe method preclude the inclusion of many other elements (Os, Mo, W,
and others) owing to incompatible Pourbaix boundaries. Other array preparative
methods include electrodeposition (12, 64, 65) and sputter deposition (74, 75). It
remains to be seen whether array preparative methods developed to accommodate
high throughput screening methods are capable of yielding competitive catalysts.
The best methods for catalyst preparation remain to be the highly optimized sin-
gle beaker methods. It is likely that the ideal high throughput preparative method
will be the adaptation of these laborious, but effective, methods to process controlrobotics. Withan et al. have investigated the use of sputter deposition to modify
MEA catalytic layers (75). If sputtering upon catalyzed GDL surfaces proves to
be effective, it opens a wide window for combinatorial studies.
COMBINATORIAL DISCOVERIES OF FUELCELL
ELECTROCATALYSTS
Direct Methanol Fuel Cell AnodesReddington et al. examined combinations of Pt, Ru, Os, Ir, and Rh in quaternary
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Cathodes for PEM Hydrogen/Air Fuel Cells
Recent combinatorial studies employed thin film catalyst libraries made by sputter
deposition of the elements (74). Active Pt-Fe binary and Pt-Co-Cr ternary compo-
sitions were found, as anticipated from earlier work on high surface area catalysts.
Other active catalysts with apparent activities higher than that of Pt-Co-Cr were
also found in the screening experiments, but their compositions have not yet been
disclosed. A representative ternary, Pt50Co25Cr25, was synthesized in scaled-up
form as a carbon-supported catalyst and was tested by rotating disk voltammetry.
A Tafel analysis of these electrodes was informative. Pure Pt/C gave a reduced
Tafel slope at high potentials, whereas the carbon-supported alloy maintained a
slope of 120 mV/decade. The reduced Tafel slope has previously been identified
with the formation of an oxide film on Pt (78).
CLOSING THE LOOP ON DISCOVERYFOCUS TESTING
AND CHARACTERIZATION
Phase Identification and Structural Characterization
Pt-Ru anode catalysts provide a good example of the complexity of phase behav-
ior found with fuel cell catalysts. Ley et al. reported that the lattice parameters
(fcc) of high surface area mixed metal catalysts containing Ru had higher latticeparameters than those observed with arc-melted alloys of the same composition
(54, 55). This results from phasing out some of the smaller Ru atoms from the alloy
phase, leaving a Pt-rich alloy with a correspondingly larger lattice parameter (67).
Figure 11 compares lattice parameters of the fcc phase for high surface area cat-
alysts to those of arc-melted alloys prepared by Ley et al. (67). Gasteiger et al.
reported 10% Ru as optimum at room temperature and about 33% Ru at 60C
on arc-melted alloy surfaces (79). Arico reported 50% Ru as optimal on high
surface area catalysts (although at 130C) (80). Chu & Gilman reported 50%
Ru as optimum on high surface area catalysts at room temperature (63). Theseseemingly controversial reports can be reconciled by examination of the lattice
parameter data. The arc-melted materials are single-phase disordered fcc ther-
modynamic materials with lattice parameter variations that obey Vegards law.
The high surface area catalysts were prepared by kinetically controlled synthetic
methods where uncontrolled phase segregation occurs. All of the high surface
area catalysts of the Ley (and later Gurau) studies were prepared by borohydride
reduction and had only fcc lattice XRD patterns. Gurau confirmed that the high
lattice parameters resulted from incomplete incorporation of the Ru into the fcc
lattice. Thus, if it is the fcc alloy phase that supports the catalytic surface, thena correlation can be drawn between the lattice parameter of the alloy phase of
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COMBINATORIAL ELECTROCATALYSTS 573
Figure 11 Fcc lattice spacings of catalysts and arc-melted alloys. [Reprinted
with permission from (67). Copyright 1998 American Chemical Society]
80%. This qualitatively explains the discrepancy between the optimum for high
surface area catalysts and the optimums reported by Gasteiger for single-phase
alloys. Long and coworkers report that in DMFCs, the active electrocatalyst for
methanol oxidation is not a bimetallic alloy but a mixed phase containing Pt metal
and hydrous ruthenium oxides (RuOxHy) (53). OGrady et al. (81) studied the
structure of Pt-Ru catalysts using X-ray absorption and concluded that in the po-
tential window where methanol oxidation occurs, the metal oxides were reducedto their metallic form. Similarly, in a series of recent work by Russells group
using in situ EXAFS they conclude that for a well-mixed Pt-Ru/C electrocatalyst
the Pt and Ru are both metallic in nature, and exist as a bimetallic alloy (82, 83).
An unsupported PtRuOxcatalyst has been commercialized by E-TEK for DMFC
anodes.
In situ fuel cell spectroscopic and electrochemical studies are ideal for study
of fuel cell catalysts in the most realistic environment. Dinh et al. demonstrated
in situ fuel cell CO stripping voltammetry as a methodology for the characteri-
zation of the catalysts/solid polymer electrolyte interface (84). They were able tocalculate the active area, intrinsic catalytic activity, and, in some cases, the surface
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anions are eliminated. In addition to CO2, methylformate and formaldehyde were
observed as products in the methanol vapor fuel cell. Also, Liu demonstrated that
the catalyst Nafion-coating affects anode kinetics in DMFCs (56). Sanicharane
integrated on MEA-surface FTIR studies with exhaust transmission spectroscopyto differentiate between adsorbed species and exhaust species in proximity to the
surface. Viswananthan et al. demonstrated the first X-ray absorption near edge
structure (XANES) spectroscopy on a high performance reformate-air fuel cell to
study the structure of carbon-supported Pt-Ru anode electrocatalysts (66). The fuel
cell was operated in a normal mode without the use of supplemental electrolytes.
The in situ Pt LIII-edge and Ru K-edge XANES of the fuel cell MEAs showed
metallic characteristics under all operating conditions. It is not unexpected that
hydrogen fuel cell anode catalysts are metallic because the polarization of anode
catalysts is typically less than 100 mV.The important lesson for combinatorial analysis in this case is that phase identi-
fication is not a trivial issue because the best catalysts may consist of several phases
(in addition to the support). Which phases are present depends on the method of
preparation, as well as on the electrochemical history of the catalyst. There clearly
exists a need for high throughput structural characterization techniques that could
complement the fast synthesis/screening methods that have been reported to date
for electrochemical arrays. Ideally, one could obtain phase maps and correlate
them with activity maps as a function of composition and preparative conditions.
Refining Heuristic Models
Recently, concentration-dependent current-voltage curves, together with CD3OH
and CH3OD kinetic data, have shown unambiguously that CH bond activation be-
comes a kinetically comparable effect within the fuel cell relevant potential regime
for two high-performance mixed metal catalysts (PtRu and PtRuOsIr) (32). At po-
tentials above 0.4 V, CH activation becomes the dominant barrier to methanol
oxidation. This result could be of practical importance because anode potentials
between 0.4 and 0.5 V versus a dynamic hydrogen electrode (DHE) will be well
within DMFC operating potentials window when low crossover membranes are
developed. The inclusion of CH bond activation as a possible rate-determining
step also has implications for future catalyst discovery, since it suggests an ex-
pansion of the composition space to include CH activator elements. Figure 12
is a Ley diagram that has been modified to include CH activators. Interestingly,
Figures 2 and 12 suggest that Os can serve both as a CH activator as well as a
water activator, which is consistent with DFT calculations of Kua & Goddard (89).
CONCLUSIONS AND OUTLOOK
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(e.g., hundreds over a year rather than tens of thousands). Focus level screening
should be coupled to high throughput fundamental science. Design of experiment
strategies must leverage acquired data with advanced informatics to increase the
probability of successful discoveries.There is an opportunity for high throughput electrochemical methods to advance
our understanding of which phase or phases are important to the catalyzed reactions
and what role metal components play in those phases. The finding of Reddington
et al. (8) that Ir substantially improves the performance of PtRuOs as a DMFC
anode evoked work by Lei et al. (32) to study CH activation. Serendipitous finding
can force one to re-examine mechanistic models.
The screening bottleneck has been removed. We now require scalable high
throughput synthetic methods yielding bulk catalysts that can be compared head
to head against state-of-the-art catalysts. The black magic associated with kinet-ically controlled catalyst preparative methods must be removed by careful design
of experiment strategies to identify and understand all of the factors involved in
synthetic methods. This suggests the need for precise process control robotics in
the development of high throughput synthetic methods.
ACKNOWLEDGMENTS
This work was supported by grants from theArmy Research Office (grant DAAH04-
94-G-0055), the Army Research Laboratory, Collaborative Technology Alliancein Power and Energy, NuVant Systems Inc., and the University of Puerto Rico at
Rio Piedras.
The Annual Review of Materials Research is online at
http://matsci.annualreviews.org
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Figure 4 (a) An array of 28 Pt-Rh-Os catalyst spots in 6 M aqueous methanol,
pH 6, quinine indicator. (Left) Image in white light. (Center) Fluorescence image at
low overpotential. (Right) Fluorescence image at high overpotential, where methanol
oxidation occurs at every spot in the array. [Reprinted with permission from Science.
Copyright 1998 Am. Assoc. Adv. Sci. (8).] (b) (Left) Robotic delivery of metal salt
solutions to a 715-well pentanary array. A Toray paper carbon sheet is sandwiched
between flexible gaskets (red), which define the wells. (Right) Top view of a screeningcell for 715-member electrode arrays. [Reprinted with permission from (16) Copyright
2003 J h Wil & S ] ( ) A t A ti i id tifi d d
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Figure 7 Component of array fuel cell. (Center) Array flow field, Clockwise fromupper right: array MEA, graphite flow field sensor, counter electrode flow field, as-
sembled cell with multielectrode potentiostat. Provided by NuVant Systems, Inc.,
Chicago, IL.
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Figure 8 (Top) Mass transport corrected IV curves for control experiment array.
(Bottom) IV curves for the ranking of four catalyst, PtRu (Johnson Matthey), PtRu
(reduce by NaBH4), PtRu oxide (E-TEK), and Pt (Johnson Matthey) in DMFC anode
operating region. [Reprinted with permission from (91). Copyright 2002 Elsevier.]
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Figure 10 PtRuOsIr composition map, looking through the PtRuOs ternary
face. Increasing IR concentration is shown in gray scale. Black dots on the PtRu and
PtOs axes shows the binary solubility limits, and the ternary region is approximatelydefined by the area above the dashed line joining these binary limits. Anode catalyst
composition are indicated by open circles, and the region of highest catalytic activity
is shown as a circle containing the best catalyst found by combinatorial screening,
PT44Ru41Os10Ir5 (). [Reprinted with permission from (67) Copyright 1998 American
Chemical Society.]
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