DOI: 10.1002/cphc.201100984

Electrocatalysis Using Porous Nanostructured MaterialsNadine Menzel,* Erik Ortel, Ralph Kraehnert, and Peter Strasser[a]

1. Introduction

Electrochemistry is enjoying growing interest in science andtechnology. In addition to established electrolysis processessuch as the evolution of halogens, for example, chlorine,[1, 2]

galvanic elements such as fuel cells, photoelectrochemical cells(PEC), or batteries have come to the fore in recent years. Inparticular, low-temperature fuelcells, such as polymer electrolytemembrane fuel cells (PEMFC)[3–5]

or direct methanol fuel cells(DMFC),[6–8] enable efficient anddirect electrochemical conver-sion of hydrogen or methanolinto electricity. Hence, they rep-resent key technology for thefuture production of electricityfrom renewable, clean, and sus-tainable resources. The wide-spread practical use of this tech-nology requires the develop-ment of more efficient processesand, in particular, new efficientelectrocatalysts. Moreover, adeeper understanding of the re-action mechanisms and kineticsis also important to improve theelectrocatalytic performance.

The efficiency of an electroca-talytic reaction is expressed as the overpotential, h (sum oflosses of electric potential), that is, the difference between atheoretical potential required for a catalytic reaction and thepotential required in the practical experiment. These lossesresult from different physicochemical phenomena, for example,kinetic reaction overpotentials and ohmic overpotentials.Ohmic overpotentials are a consequence of electric resistanceof the electrolyte, transport limitations of charged reactantspecies, and conductor and contact resistances. Minimizingthese overpotentials is one of the main challenges for electro-chemists in the development of efficient electrocatalysts. How-ever, such optimization requires a fundamental understanding

of all individual steps that contribute to an electrocatalytic pro-cess.

Scheme 1 illustrates an example of a porous electrode andthe sequence of coupled physicochemical steps that constitutean electrocatalytic process. The chemical transformation of re-

actants occurs in the reaction layer near the surface of theelectrode. The catalytic reaction itself proceeds at the interfacebetween (accessible) the electrode surface and electrolyte. Itconsists of adsorption, reaction, and desorption. However,these catalytic steps are often preceded by diffusive transport

The performance of electrochemical reactions depends strong-ly on the morphology and structure of the employed catalyticelectrodes. Nanostructuring of the electrode surface representsa powerful tool to increase the electrochemically active surfacearea of the electrodes. Moreover, it can also facilitate faster dif-fusive mass transport inside three-dimensional electrodes. This

minireview describes recent trends in the development of syn-thesis routes for porous nanostructured electrode materialsand discusses the respective important electrocatalytic applica-tions. The use of structure-directing agents will play a decisiverole in the design and synthesis of improved catalysts.

Scheme 1. Schematic representation of the individual physicochemical steps that constitute an electrocatalyticprocess. The process is illustrated for the example of a porous electrode applied to an oxidation reaction.

[a] N. Menzel, E. Ortel, Dr. R. Kraehnert, Prof. Dr. P. StrasserThe Electrochemical Energy, Catalysis, and Materials Science LaboratoryDepartment of Chemistry, Chemical Engineering DivisionTechnical University BerlinStraße des 17. Juni 124, 10623 Berlin (Germany)Fax: (+ 49) (0)30 314-22261E-mail : nadine.menzel@tu-berlin.de

ChemPhysChem 2012, 13, 1385 – 1394 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1385

of reactants to the electrode interface, chemical reactions oc-curring in the electrolyte, and mass transport from the bulk ofthe electrolyte to the reaction layer. Moreover, the products ofthe electrocatalytic surface reaction are also the subject of sim-ilar reaction and transport steps. The chemical and geometricalconfiguration of the catalytic electrode provides control overall of these individual steps.

Optimized catalytic electrodes show a low intrinsic overpo-tential for the desired reaction path and a high overpotentialfor undesired side reactions. Moreover, they provide high elec-trical conductivity and good mechanical, chemical, and electro-chemical stability. To maximize the number of active sites, theyalso possess a high active surface with good accessibility tothe reactants. The intrinsic catalytic properties are controlledby the choice of the local composition of the electrodes. Incontrast, surface area and transport properties can be opti-mized by nanostructuring of the electrode material.

In general, electrodes consist of a current collector and asubstrate that is coated with the electrocatalyst. This catalyst isformed either completely of the active material or by a catalystsupport onto which active particles are deposited. The shapeof the electrodes can be flat, rough, or 3D porous. Flat orrough electrodes are often made from a solid bulk metal. How-ever, due to their low surface-to-volume ratio they are onlyuseful when highly active and cheap materials are used. Incontrast, porous structures provide a higher specific (active)surface area, but at the expanse of increasing manufacturingeffort. Porous electrodes exist for both bulk and supported cat-alysts. In the latter, pores are introduced into the support ma-terial. The porosity of the catalyst material, that is, its nano-structure, controls not only the active surface area, but alsothe transport of reactants and products. These can exist ineither liquid or gas phases (e.g. gas evolution reactions). Whilesmall pores provide a higher surface area, they can also de-crease the rate of mass transport. The synthesis of porous cata-lysts therefore requires control over the obtained pore size.

Porous electrocatalysts have been synthesized in many dif-ferent ways. Synthesis methods include chemical reduction orelectrochemical deposition from metal salt solutions, deposi-tion by sputtering or precursor evaporation, sol–gel synthesis,powder technology, or spraying and dip-coating methods.However, the degree of porosity and control over the electro-catalyst nanostructure varies significantly between the differentsynthesis methods.

This minireview focuses on synthesis methods that enabledirect control over the catalyst nanostructure through physicalsynthesis parameters or with auxiliary structure-directingagents. Synthesis options for porous catalysts and porous cata-lyst supports are described in Section 2. Moreover, the role ofthe nanostructure in improving the performance of the elec-trocatalysts is discussed for three selected reactions in Sec-tion 3.

Nadine Menzel received her Engineer-

ing Diploma in 2008 at the Technical

University Berlin, Germany, in chemical

engineering. She is currently working

on her doctoral thesis at the same uni-

versity in the group of Prof. P. Strasser.

Her research is focused on the funda-

mental understanding of mechanistic

aspects of the electrochemical chlorine

evolution which also includes synthesis

of electrocatalysts and electrochemical

measurements.

Erik Ortel received his M.Sc. in 2008 at

the Technical University of Applied Sci-

ences Berlin, Germany, in pharmaceuti-

cal and chemical engineering. He is

currently a Ph.D. student at the Techni-

cal University of Berlin in the Junior

Research Group DEPOKATnm of Dr.-Ing.

Ralph Kraehnert. His research is mainly

focused on the synthesis of mesopo-

rous oxide films for catalytic applica-

tions.

Ralph Kraehnert was awarded a

degree as Dr.-Ing. in 2005. His work on

Pt-catalyzed oxidation of ammonia

was carried out at the Institute of Ap-

plied Chemistry Berlin under the guid-

ance of Prof. M. Baerns. Since 2007 he

has led an independent Junior Re-

search Group financed by the NANO-

FUTUR program of BMBF. He develops

methods for a rational design of cata-

lysts and the synthesis of porous

oxides, colloidal nanoparticles and cat-

alysts with controllable nanostructure.

Peter Strasser is a full professor in the

Chemical Engineering Division of the

Department of Chemistry at the Tech-

nical University of Berlin. Prior to his

appointment, he was an Assistant Pro-

fessor at the Department of Chemical

and Biomolecular Engineering at the

University of Houston and prior to that

he served as a senior member of staff

at Symyx Technologies, Inc. In 1999,

Peter Strasser obtained his doctoral

degree in physical chemistry and elec-

trochemistry from the Fritz-Haber Institute of the Max-Planck Soci-

ety, Berlin, under the direction of Professor Gerhard Ertl. He studied

chemistry at Stanford University, USA, the University of Tuebingen,

Germany, and the University of Pisa, Italy, and received his diploma

degree (M.S.) in chemistry in 1995.

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N. Menzel et al.

2. Synthesis of Porous NanostructuredElectrocatalysts

The synthesis of porous nanostructured electrocatalysts can berealized by nanostructuring of either the conductive support(mostly carbon) or the electrocatalytically active material(mostly metals or metal oxides). Possible synthesis routes fornanostructured electrocatalysts described below are 1) nano-casting by using templates, 2) dealloying of alloys, and 3) elec-troassisted deposition or dissolution.

2.1. Nanocasting with Templates

Nanocasting is the process in which a template with a lengthscale of nanometers is surrounded or filled with the material(or a precursor for it) to be cast. The initial template is re-moved thereafter from the stabilized precursor phase, therebya material with open porosity can be obtained. Different tem-plates allow precise control over the synthesized porous struc-ture.[9, 10] In a review from 2010, Walcarius highlighted recenttrends for template-directed porous electrodes in the field ofelectroanalysis.[11] The following part only focuses on the mainnanocasting synthesis strategies that lead to porous templatedelectroactive catalysts and are highlighted by a few examples.

Figure 1 illustrates the nanocasting concept of using mi-celles of amphiphilic polymer molecules as soft templates.Upon deposition, the micelles assemble into well-defined peri-odic mesostructures, such as hexagonal or cubic phases, inwhich the voids between the micelles are filled with the pre-cursor (Figure 1I). Subsequent treatment of the composite re-moves the template (typically by template extraction or heattreatments) and results in mesoporous materials (Figure 1II).[12]

The general concept allows the synthesis of both mesoporouscarbon and metal oxides. The precursor used (organic precur-sors for carbon and metal alkoxides or metal salts for metaloxides) and synthesis conditions permit the synthesis of meso-porous bulk catalysts (Figure 1II ; e.g. Pt,[13] Pt/Ru,[13] RuO2,

[14]

IrO2[15]) or mesoporous catalyst supports with incorporated

active species (Figure 1IIa ; e.g. Pt in carbon,[16] Pt–Pb incarbon/Nb2O5

[17]). The mesoporous material (Figure 1II) result-ing from soft templating can itself be replicated again; thismaterial then acts as a “hard template” (Figure 1III). The hardtemplate, for example, mesoporous silica SBA-15, is infiltratedwith a precursor. After template removal (by HF in the case ofsilica), a negative mesoporous replicate (Figure 1III) is obtained,which resembles the initial structure of the soft templates (Fig-ure 1I).[18] In analogy to the soft-templated route, mesoporouselectrocatalytic active materials (Figure 1III ; Pt–Ru[19]) or meso-porous supports (Figure 1IIIa) can be achieved. Such mesopo-rous supports obtained by the hard-templating pathway (Fig-ure 1IIIa) are, for example, OMCs,[20] which are widespread inthe field of nanostructured electrocatalysts (see Sections 3.1and 3.2). Nevertheless, carbon-free, hard-templated mesopo-rous WO3 was also successfully used as a nanostructured elec-trode catalyst support.[21]

Examples of materials obtained by the nanocasting ap-proach are given in Figure 1 for both the synthesis of bulk cat-

alysts (A, B) and support materials with incorporated activecomponents (C, D). Figure 1 A illustrates a TEM image of a mes-oporous iridium oxide film prepared by the soft-templatedpreparation pathway (Figure 1II). The iridium oxide film wasobtained by dip-coating through deposition of iridium acetateprecursor and PEO-b-PB-b-PEO polymer templates[22] on titani-um electrodes. After heat treatment at 300 8C, the soft-tem-plate decomposed and iridium oxide with a cubic mesostruc-ture with pore sizes of 16 nm and a specific surface area of248 m2 g�1 (Kr-BET) was obtained.[15] The catalytic performanceof templated versus untemplated IrO2 catalyst in the oxygenevolution reaction (OER) is discussed in Section 3.3.

Figure 1 B shows a TEM image of Pt–Pb nanoparticles incor-porated into the pores of a mesoporous niobium oxide–carbon composite support. The synthesis procedure reportedby Orilall et al.[17] is based on the self-assembly of PI-b-PEOblock copolymer templates with NbCl5/Nb(OEt)5 and noble-metal precursors [(dimethylcyclooctadiene)platinumand triphe-nyl(phenylethynyl)lead]. After heat treatments (700 8C underargon and subsequently 875 8C under reductive atmosphere), acrystalline, hexagonally arranged, cylindrical, mesoporousniobium oxide–carbon composite with Pt–Pb nanoparticles in-corporated into the mesopores was obtained. The materialssynthesized in one pot show pores with an average diameterof around 25 nm and specific surface areas (N2-BET) between

Figure 1. An illustration of the nanocasting pathway by soft (I) and hard (II)templates. TEM images of representative electrocatalysts based on mesopo-rous supports and mesoporous materials as “active” phases are presented.A) Poly(ethylene oxide)-b-poly(butadiene)-b-poly(ethylene oxide) (PEO-b-PB-b-PEO) soft-templated IrO2 (reprinted with permission from[15]), B) Pt–Pb in-corporated in poly(isoprene-block-ethylene oxide) (PI-b-PEO) soft-templatedcarbon–Nb2O5 composite (reprinted with permission from ref. [17]), C) meso-porous silica SBA-12hard-templated Pt–Ru (reprinted with permission fromref. [19]), and D) Pt incorporated in ordered mesoporous carbon (OMC; re-printed with permission from ref. [6]).

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137 and 217 m2 g�1. These electrocatalysts show promisingelectrochemical activity toward formic acid oxidation.

The TEM image in Figure 1 C presents a nanostructuredPt–Ru electrocatalyst synthesized through the hard-templatingapproach (Figure 1III), reported by Jiang and Kucernak.[19] First,they synthesized hexagonal mesoporous SBA-12 silica usingnonionic surfactant Brij 76 as the soft template and tetraeth-oxysilane (TEOS) as the silica source. After calcination a meso-porous silica hard-template was obtained. Subsequently,H2PtCl6 and RuCl3 were impregnated into the silica templateand heat-treated at 300 8C. Thereafter, the silica template wasremoved by using a solution of 10 wt % HF at room tempera-ture. The retained nanostructured Pt–Ru material with meso-pores of about 3 nm in diameter demonstrates high activity to-wards the electrochemical oxidation of methanol.

Figure 1 D shows a TEM image of an OMC film with incorpo-rated Pt nanoparticles (synthesis path IIIa). Here, Su et al.[6] firstprepared hexagonal mesoporous silica SBA-15 with the surfac-tant P123 and a SiO2 precursor. The calcined mesoporous SBA-15 hard template was impregnated with benzene (carbonsource) by chemical vapor deposition (CVD). After carboniza-tion at 900 8C the silica hard template was then removed withHF solution. Platinum nanoparticles were then deposited onthe prepared mesoporous carbon by the borohydride reduc-tion method. The enhanced activity of this material in electro-chemical methanol oxidation is described in Section 3.2.

In addition to templating routes that start with amphiphilicmolecules, porous anodic aluminum oxide (AAO)[23] hard tem-plates can also be used for the preparation of nanostructuredelectrocatalysts. For example, Zhao et al.[24] used AAO tem-plates for the synthesis of Pt–Ru and Pt nanowire array electro-des on Ti/Si substrate. This catalyst synthesis started with thepreparation of the AAO template by anodization of a thin Alfilm on a conductive Ti/Si substrate in oxalic acid solution at20 V (see also Section 2.3). The resulting AAO template con-tained uniform cylindrical pores that were aligned perpendicu-lar to the substrate surface. The pores were then filled with ametal by electrodeposition from an electrolyte solution con-taining, for example, H2PtCl6 and RuCl3. A platinum plate wasused as a counter electrode and the AAO/Ti/Si structure wasthe working electrode. The as-prepared composites were im-mersed in a solution of NaOH to remove the AAO template.The brush-shaped Pt–Ru nanowires obtained were about30 nm in diameter and 1 mm in length. They were tested formethanol electro-oxidation. Figure 2 A shows a SEM image ofthe surface morphology of the prepared Pt–Ru nanowires. In asimilar way, Liu et al.[25] used AAO hard templates to synthesizemultisegment Pt–Ni nanorods as methanol electro-oxidationcatalysts by sequential electrodeposition of Pt and Ni intonanoporous anodic alumina membranes.

Moreover, Alia et al.[8] used silver nanowires as hard tem-plates to synthesize porous platinum nanotubes for oxygen re-duction and methanol oxidation reactions. The porous plati-num nanotubes, with a thickness of 5 nm, an outer diameterof 60 nm, and a length of 5–20 mm, were synthesized by gal-vanic displacement of silver nanowires, which were formed by

ethylene glycol reduction of silver nitrate. Figure 2 B shows aTEM image of the respective templated electrocatalysts.

In the case of a nanostructured electrocatalyst with regularpore sizes above 50 nm, spherical latex templates (e.g. polymeror silica spheres) can be employed.[26] These spheres are assem-bled into colloidal crystals with highly uniform 3D structuresand are then infiltrated with a carbon or metal oxide precursor.Alternatively, the spheres and the precursor are codeposited.Thereafter, the template is usually removed by calcination(polymer) or HF etching (SiO2) to produce porous catalysts orcarbon supports. Chai et al.[27] synthesized ordered, uniform,porous carbon frameworks with pore sizes in the range of 10to 1000 nm by infiltration of colloidal SiO2 templates withphenol and formaldehyde (carbon source) followed by poly-merization and carbonization of the carbon precursor. Theporous carbons were used as supports for Pt–Ru alloy nano-particles in methanol fuel cell catalysts. Figure 3 A shows a TEM

image of this catalyst. Furthermore, Su et al.[28] synthesizedpolystyrene-templated macroporous silica as a hard templateto deposit carbon on the surface of this inverse silica opal. Thecarbon deposition was carried out by using a CVD methodwith benzene vapor as the carbon precursor. Then, carbon/silica composite was immersed in HF solution to remove thesilica. The macroporous carbon was used as a support for plati-num nanoparticles and showed strong performance improve-ments in the methanol oxidation reaction due to nanostructur-ing of the catalyst support (see Section 3.2). Figure 3 B shows a

Figure 2. Electron microscopy images of hard-templated electrocatalysts.A) AAO-templated Pt–Ru nanowires (reprinted with permission fromref. [24]), B) Ag nanowire templated Pt nanotubes (reprinted with permissionfrom ref. [8]).

Figure 3. TEM images of A) Pt–Ru incorporated in macroporous carbon (re-printed with permission from ref. [27]) and B) hollow carbon spheres with in-corporated Pt (reprinted with permission from ref. [28]).

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N. Menzel et al.

TEM image of the hollow carbon spheres with a diameter ofaround 200 nm with incorporated platinum nanoparticles.

It is clear that, in contrast to the soft-templated synthesisroute, the hard-templating approach demands more prepara-tion steps. However, hard templates are often more stableagainst the precursor system and a precise negative replica ofthe template can be created more easily than with soft tem-plates. In general, nanocasting of templates is one of the mostpromising synthesis strategies to precisely control the porousstructure of nanostructured electrocatalysts.

2.2. Nanoporous Materials Made by Dealloying

Another important synthetic process to obtain 3D highlyporous electrocatalytic electrodes is chemical dealloying[29]

(CD). In this synthesis single-phase, multicomponent bulkalloys are formed by melting. From this alloy, the less-noblemetal is selectively dissolved by an etching process inacids,[30, 31] alkalis,[32, 33] or electrochemically.[34, 35] The removal ofthe less-noble metal can result in mesopores being formed inthe remaining metal. The synthesis of mesoporous electrocata-lysts by chemical or electrochemical dealloying has, for exam-ple, been reported for Au from an Ag–Au alloy,[36–41] Pt fromCu–Pt[34, 42] or Si–Pt,[42] Cu from Mn–Cu,[35, 43] and Al–Cu[32] andAg from Al–Ag.[31, 33] Control over the size of the mesoporesduring electrochemical or chemical dealloying results fromvarying the starting alloy composition,[36] the electrochemicalpotential driving dissolution,[36] the electrolyte temperature,[34]

the dealloying/etching time,[35] or employing thermal annealingafter dealloying.[36] A potential drawback of the syntheticmethod is the fact that complete removal of the alloying com-pound cannot be guaranteed; hence, the second metal mightalso affect the electrochemical performance of the resultingcatalysts.

Typical electron microscopy images of nanoporous goldfilms made by dealloying from different Ag–Au alloys are pre-sented in Figure 4.Figure 4 a depicts a cross-sectional SEM

image of dealloyed Au32 atm %Ag68 atm %, which shows an openporosity in the whole film. The planar view of a dealloyedAu26 atm %Ag74 atm % film (Figure 4 b) indicates ligament spacingsin the order of 10 nm.

The dealloying method described can also serve for the syn-thesis of a support such as porous gold. When compared withtypical carbon supports, the deposition of an atomically thin

layer of platinum on gold can improve the catalyst/substratebinding while providing a high dispersion of the active plati-num phase.[30] Furthermore, it is possible to use the dealloyedporous material as a hard template for the nanostructuring ofplatinum-based electrocatalysts formed by epitaxial casting.[44]

An epitaxial skin of a second material is coated on a mold thatis subsequently dissolved away. In this case platinum is grownepitaxially onto the nanoporous gold by an electroless platingprocess described by Ding et al.[30] This is a simple strategy tosynthesize free-standing noble-metal membranes with a hier-archical bicontinuous porous architecture.

With dealloying it is possible to synthesize porous 3D cata-lysts in a simpler and faster way than with the templatingtechniques described in Section 2.1. However, the synthesis ofordered nanopore structures by dealloying has not been re-ported so far.

2.3. Electrochemical Deposition and Dissolution

For some electrode compositions, material-specific syntheticroutes for nanostructures exist. This includes, in the case of ti-tania, the electrochemical dissolution of titanium to create TiO2

nanorods; metals such as platinum can also be nanostructuredby electrochemical deposition in the presence of structure-di-recting agents.

Schmuki et al. synthesized highly ordered, self-organizedTiO2 nanotubes by electrochemical anodization of titani-um.[45–47] This synthetic route represents a simple electrochemi-cal oxidation reaction of a metallic titanium substrate underspecific conditions, such as a sufficient anodic voltage andacidic fluoride (0.05–0.5 m) containing electrolytes. However,oxide nanotubes with well-defined lengths and diameters andvertical alignment relative to the substrate are obtained. Thedefined nanostructure results from varying the pH of the elec-trolyte[48, 49] (influences the thickness of the tube walls), thevoltage[50, 51] (diameter of the tubes), or anodization time[47, 51]

(tube length). Figure 5 shows one example of TiO2 tubes ob-

tained by electrochemical anodization of a titanium foil in0.135 m HF in ethylene glycol electrolyte at 30 V for 5 h. Afteranodization the sample was annealed in air for 1 h at 450 8C.Macak et al.[45] demonstrated the influence of TiO2 nanotubesas supports for Pt/Ru nanoparticles in the methanol oxidationreaction.

Electrochemical and hydrothermal methods can be com-bined to synthesize porous platinum electrodes. In the hydro-

Figure 4. SEM images of nanoporous Au made by selective dissolution of Agfrom an Ag–Au alloy. a) Cross-section of dealloyed Au32 atm %Ag68 atm %.b) Planar view of dealloyed Au26 atm %Ag74 atm %. (reprinted with permissionfrom ref. [36]).

Figure 5. Electron microscopy images of TiO2 nanotubes made by anodiza-tion of Ti. a) Plan view and b) cross-section of the TiO2 nanotubes (reprintedwith permission from ref. [46]).

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thermal-assisted seed growth method,[52] platinum particles arefirst deposited electrochemically on titanium substrates with-out structure-directing agents. Thereafter, porous platinumstructures were reported to grow by hydrothermal treatmentof these substrate for 10 h at 100 8C using autoclaves contain-ing ethylene glycol, a platinum salt (H2PtCl6), and HCl, for ex-ample.[53]

Nanostructured (mesoporous) electrodes of metals such asPt[54, 55] and Au[56, 57] can also be synthesized by electrochemicaldeposition through electroreduction of a metal salt in the pres-ence of hard or soft templates, such as silica colloids,[56, 57] poly-styrene (PS) particles,[57] or micelles of octaethylene glycolmonohexadecyl ether (C16EO8)[54, 55] (see Section 2.1). Moreover,ruthenium oxide[58] was synthesized in a similar way; however,in this case, the structure-directing agent was polystyrene.Methanol oxidation on porous platinum catalysts was studiedby Garcia et al.[55]

In many syntheses it is necessary to use a templatingmethod before the electrochemical deposition of the electro-catalytic active material can be carried out. Therefore, numer-ous preparation steps are needed; however, precise control ofthe pore diameters and layer thickness is possible. In the caseof anodization, only a 1D structure could be synthesized. Ad-vantages of this method are the precisely tunable tube lengthsand diameters and also direct electrical connectivity betweenthe substrate and support or catalytically active material dueto direct growth of the tubes from the substrate.

3. Applications as Electrocatalysts

A complete overview of all reported applications of nanostruc-tured electrocatalysts is beyond the scope of this minireview.However, we focus on three selected electrochemical reactions,namely, oxygen reduction, methanol oxidation, and oxygenevolution, to illustrate the advantages of the nanostructuredelectrodes. Two of these examples employ supported catalysts,namely, the oxygen reduction reaction (Section 3.1) and meth-anol oxidation (Section 3.2), whereas the oxygen evolution re-action uses the unsupported catalyst IrO2 (Section 3.3).

3.1. Oxygen Reduction Reaction on Carbon-SupportedPlatinum

One of the most serious challenges in the development ofproton-exchange membrane fuel cells (PEMFC) is the oxygenreduction reaction (ORR). ORR is a complex electrode reactiondue to it irreversibility and the very slow kinetics. Two possiblereaction pathways for the ORR exist : the direct 4-electron re-duction pathway [Eq. (1):

O2 þ 4 Hþ þ 4 e� ! 2 H2O ð1Þ

and the two-electron reduction pathway with hydrogen perox-ide as an intermediate [Eqs. (2) and (3)]:

O2 þ 2 Hþ þ 2 e� ! H2O2 ð2Þ

H2O2 þ 2 Hþ þ 2 e� ! 2 H2O ð3Þ

Platinum-based materials are the most commonly used cata-lysts for ORR,[4, 5, 59] often in the form of unsupported large crys-talline platinum black particles.[60]

A decrease in the platinum loading was achieved by the in-troduction of carbon black as a catalyst support (commercialPt/C).[61, 62] Due to the high surface area of carbon black, betterspatial distribution and utilization of the active platinum parti-cles were achieved. However, the ORR activity of a catalyst alsodepends significantly on the size of the platinum particles andthe spatial distribution of these particles over the support.[63]

Despite the high surface area of carbon black, two main disad-vantages exist : Due to its dense structure carbon black sup-ports show significant mass-transfer limitations. Moreover, theyalso undergo thermal degradation at high fuel-cell workingtemperatures.[64, 65]

A significant improvement in the ORR performance can beachieved by using nanostructured carbon supports, as sum-marized in a review by Antolini.[62] In particular, OMC[62, 66–70] at-tracted great attention as a catalytic support material for ORRdue to their controllable pore sizes and high surface areas (seeSection 2.1). Higher spatial distribution of the active nanoparti-cles and more facile diffusion of reactants and products due tothe mesoporous structure led to an increase in the catalyticperformance. Figure 6 demonstrates the activity increase of

ORR on OMC supports. The OMC support was synthesized byhard templating with SBA-15 mesoporous silica. Sucrose wasused as the carbon source. At an identical Pt loading and com-parable Pt particle size, the OMC-supported Pt catalyst clearlyshows enhanced ORR activity compared with the standardPt/C catalyst. At a potential of about 0.55 V versus SCE [�0.31 V vs. a reversible hydrogen electrode (RHE)] the current

Figure 6. Potential–current density plots for oxygen reduction on Pt cata-lysts in 0.5 m H2SO4 at room temperature on gas diffusion electrodes madeof Pt/OMC with Pt particle sizes of 40 (! ) and 4 nm (~), and commercialPt/C with a Pt particle size of 2 nm (*) at loadings of 0.6 mg per cm2 of Pt.Potentials are given versus a saturated calomel electrode (SCE; reprintedwith permission from ref. [67]).

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density (per cm2 of electrode) of a Pt/OMC catalyst is twice ashigh as the activity of a commercial Pt/C electrocatalyst.

Furthermore, Yamada et al.[68] reported that OMC facilitatedbetter spatial distribution of small Pt particles (�5 nm) andpromoted improved electrolyte transportation; this resulted ina 2.5 higher limiting current density at a potential of 0.4 Vversus RHE. Hayashi et al.[69] and Vengatesan et al.[70] also con-firmed such an improvement of the catalytic performance dueto better spatial distribution of the platinum nanoparticles inthe ordered mesoporous structure and better mass transportof the reactants and products in and at the electrocatalyst. Fur-ther information on the use of unsupported porous catalystsin fuel cells can be found in a recent review by Antolini andPerez.[71]

3.2. Methanol Oxidation on Carbon-Supported Platinum

DMFCs are promising highly efficient and clean energy sourcesfor applications in electric vehicles and portable devices. Theoxidation of methanol is an important reaction for the designof DMFCs. The total oxidation process of the methanol oxida-tion can be written as shown in Equation (4):

CH3OHþ H2O! CO2 þ 6 Hþ þ 6 e� ð4Þ

This total oxidation process can be divided in the three con-ventional reactions for a platinum catalyst shown in Equa-tions (5), (6), and (7):.[72]

Ptþ CH3OH! Pt-COþ 4 Hþ þ 4 e� ð5Þ

Ptþ H2O! Pt-OHþ Hþ þ e� ð6Þ

Pt-COþ Pt-OH! 2 Ptþ CO2 þ Hþ þ e� ð7Þ

A typical cyclic voltammogram for the oxidation of methanolon platinum is shown in Figure 7.

The cyclic voltammetry profiles show that, at a potential ofaround 0.2 V versus SCE, methanol molecules adsorb at theplatinum surface [Eq. (5)] . Water is oxidized at a potential ofabout 0.4–0.45 V vs. SCE [Eq. (6)] . If both species are adsorbedon the surface, then the formation of CO2 can occur [Eq. (7)] ,as indicated in the cyclic voltammogram by the increase in thepotential in the forward scan at this point (see Figure 7). Dueto formation of Pt�OH or Pt�O,the current density reaches amaximum value and decreasethereafter (�0.6–0.7 V vs. SCE).MeOH decomposition and COremoval do not proceed effi-ciently on Pt�O surfaces. In thebackward scan, CO desorptionstarts at a potential of about0.7 V vs. SCE and reaches a maxi-mum current density at around0.4 V vs. SCE. The onset potentialand current density at a selectedpotential are important for the

comparison of different catalysts. Lower onset potentials corre-spond to better catalysts.

Much effort has been made to enhance the performance ofmethanol oxidation. One method was the improvement ofelectrocatalytic activity and minimization of CO poisoning dueto the use of Pt–Ru[19, 27] alloys or other metals or metalalloys.[73–75] Furthermore, great work has been made to improveaccess to large areas of the catalytic material, as well as facili-tate electron transfer and mass transport due to structuringand well-developed porosity of the support.

The standard catalytic activities of platinum catalysts oncarbon black (Pt/C) were determined by differentgroups.[6, 8, 76, 77] Results given in Table 1 show that specific cur-rent densities with relation to the electrochemical surface area(ECSA) are between 0.1 and 0.4 mA per cm2 of Pt at 0.7 Vversus RHE for standard Pt/C catalysts. The onset potential isaround 0.6 V versus RHE.

For catalysts with an OMC support, the onset potential isslightly lower (�0.1 V) and the specific current density isaround 0.4 mA per cm of Pt, which is higher than the valuesfor a commercial catalyst (Table 1). Su et al.[6, 28] demonstratedthat catalysts with ordered meso- and macroporous carbon

Figure 7. Typical cyclic voltammograms for the electro-oxidation of metha-nol at platinum catalysts in 0.5 m H2SO4 + 1 m CH3OH at room temperature.Pt/XC-72 is a commercial catalyst and Pt/CMK-5-xxx-60 are synthesized or-dered porous carbons, in which the last number denotes the graphitizationtime and the second number is the graphitization temperature. Potentialsare given versus SCE (reprinted with permission from ref. [16]).

Table 1. Catalytic activity of methanol oxidation for Pt (20 wt %) on OMC supports and commercial carbon atroom temperature.

Synthesis procedure of Pt20 wt %/C and Pt20 wt %/OMC Onset poten-tial [V] vs. RHE

j [mA per cm2 of Pt]at 0.7 V vs. RHE

cMeOH

[mol L�1]Ref.

commercial Pt/C (E-TEK) �0.5 �0.15 1 [28]

commercial Pt/C (E-TEK) �0.6 �0.1 1 [6]

hard templating with silica opal, CVD for 2 h at1000 8C

�0.45 �0.4 1 [28]

hard templating with SBA-15, CVD for 1 h at 900 8C �0.55 �0.3 1 [6]

hard templating with SBA-15, CVD for 60 min at500 8C and graphitization at 850 8C

�0.55 �0.63 1 [16]

hard templating with SBA-15, wet impregnation,functionalization in 2 m HNO3 for 0.5 h

�0.6 0.5 [7, 20]

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Porous Nanostructured Materials

supports (pore size: �3.5[6] and �70 nm,[28] respectively) had ahigher electrocatalytic activity towards methanol oxidationthan standard catalysts. This results from better mass transportof the reactants and products due to higher spatial distributionof platinum particles on the mesoporous material. Lei et al.[16]

also showed that the electrocatalytic activity of Pt supportedon OMC (pore size �3–4.7 nm) was higher than that of stan-dard catalysts. Moreover, Calvillo and co-workers reported thatfunctionalization with HNO3 to create oxygen groups at thesurface of OMC improved the performance of methanol oxida-tion due to better spatial distribution and anchoring of theplatinum particles on the support.[7, 20]

This short summary of OMCs as one example of a porousnanostructured catalyst support for methanol oxidation showsthat the characteristics of the carbon support for platinum cat-alysts have great impact on electrochemical properties inmethanol oxidation. The OMCs offer a larger accessible surfacearea than that of standard carbon blacks; this results in betterspatial distribution of the platinum particles on the supportand enhances catalytic activity. Furthermore, the area of theelectrode/electrolyte interface increases, which enhancescharge and mass transport through the porous Pt/OMC cata-lyst and decreases the diffusion distance of electrolyte ions.

3.3. OER on Unsupported IrO2

OER is the anodic half-cell reaction of the water-splitting pro-cess. The efficient oxidation of water is a great challenge in thedevelopment of viable, cost-effective, durable technologies forthe conversation of light into storable fuels. The electrochemi-cal water-splitting reaction is divided into two half-cell reac-tions. The hydrogen evolution reaction (HER) proceeds at thecathode and the OER at the anode. The oxidation process isshown in Equation (8):

2 H2O! O2 þ 4 Hþ þ 4 e� ð8Þ

OER is a very complex process with several surface-adsorbedintermediates and the mechanism of OER on these differentelectrodes is still a matter of discussion. More detailed informa-tion about the water-splitting reaction and it half-cell reactionscan be found in a recent review by Dau et al.[78]

Among numerous catalytic materials, such as binary and ter-nary ruthenium metal alloys,[79, 80] ruthenium oxides, or rutheni-um mixed metal oxides,[81–84] iridium oxide is regarded as oneof the most stable and active catalysts for the OER.[15, 85–89]

Recent studies on the synthesis of colloidal iridium oxide nano-particles[85, 87, 90] reported an increase in the electrochemicallyactive surface area and a reducing OER overpotential. Iridiumparticles were deposited on the electrode support. The over-potential for the OER, determined at a current density of0.5 A cm�2, was around 0.25 V versus RHE, as reported by Naka-gawa et al.[87] However, synthetic method did not provideaccess to controlled pore structure and crystallinity of thelayers. Recently, we highlighted a soft-templated synthesis ofnanocrystalline mesoporous IrO2 catalyst layers. The mesopo-rous IrO2 catalyst layers were synthesized by soft-templating

methods with polymer micelles, as described in Section 2.1.Soft templating created mesopores of about 16 nm in diame-ter, as shown in Figure 1 A.

Figure 8 A provides information about the active surfacearea of the film, which was determined electrochemically in apotential window between 0.4 and 1.4 V versus RHE.[15] The

templated mesoporous (Figure 8 A, a) film exhibits a signifi-cantly larger active surface area than the untemplated IrO2

sample (Figure 8 A, b). Figure 8 B shows cyclic voltammogramsof the OER recorded on IrO2 films with and without templatedmesoporosity. These voltammograms were recorded underquasi-steady-state voltammetric responses (scan rate 6 mV s�1).Both catalysts are active for OER; however, the templated mes-oporous catalyst clearly shows a higher catalytic activity pergeometric surface area of the planar electrode (see Figure 8 B).Therefore, the improved performance of the templated meso-

Figure 8. A) Cyclic voltammograms of a) templated and b) untemplated IrO2

films in a potential window between 0.4 and 1.4 V versus RHE and a scanrate of 50 mV s�1. B) Cyclic voltammograms for the OER in a potentialwindow between 1 and 1.6 V versus RHE recorded with a scan rate of6 mV s�1 on a) templated and b) untemplated IrO2 films. The current density,j, is normalized to the geometric surface area of the working electrode (re-printed with permission from ref. [15]). Insets show SEM images of a) tem-plated and b) untemplated IrO2 films calcined at 450 8C.

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N. Menzel et al.

porous catalyst in Figure 8 B originates from an increase in theaccessible surface area.

4. Summary and Outlook

Nanostructured conductive porous materials are attractive can-didates as supports or active components of catalytic electro-des. The porous nanostructure significantly increases the cata-lytically active surface area and accelerates mass transport;hence, porous electrodes are particularly beneficial for gas re-actions in gas diffusion electrodes. Beyond the examples dis-cussed herein, porous electrodes are also suited for gas reac-tions such as hydrogen evolution and reduction, formic acidformation,[91] CO and glucose oxidation, or the chlorine evolu-tion reaction. In most cases, the intrinsic catalytic activity is notinfluenced by the structuring of the electrode support or thecatalytic species.

Depending on the detailed nature of the reacting molecules,mass transport and diffusion rates, optimum pore diameters,wall thicknesses, and catalyst or support morphologies maydiffer significantly. Hence, individual optimization of the elec-trode material is required for each application. We have em-phasized three different synthetic routes that are applicable forthe synthesis of well-ordered, porous, nanostructured elec-trode materials. Close collaboration between material scientistsand electrochemists will promote new promising develop-ments in the field of porous nanostructured materials as highlyactive electrocatalysts.

Acknowledgements

We thank Tobias Reier, Mehtap Oezaslan, and Fr�d�ric Hasch� forassistance and aid in obtaining information about the topic. Inaddition, we thank the German Cluster of Excellence in Catalysis(UNICAT) funded by the German National Science Foundation(DFG) managed by the Technical University Berlin for financialsupport. E.O. and R.K. acknowledge generous funding from BMBFwithin the frame of the Nanofutur program (FKZ 03X5517A).

Keywords: carbon · electrochemistry · mesoporous materials ·nanostructures · oxygen

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Received: December 9, 2011

Published online on February 14, 2012

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