Article pubs.acs.org/accounts
Free Electrons to Molecular Bonds and Back: Closing the Energetic Oxygen Reduction (ORR)−Oxygen Evolution (OER) Cycle Using Core−Shell Nanoelectrocatalysts Published as part of the Accounts of Chemical Research special issue “Nanoelectrochemistry”. Peter Strasser* The Electrochemical Energy, Catalysis and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division, Technical University Berlin, Strasse des 17. Juni 124, 10623 Berlin, Germany CONSPECTUS: Nanomaterial science and electrocatalytic science have entered a successful “nanoelectrochemical” symbiosis, in which novel nanomaterials offer new frontiers for studies on electrocatalytic charge transfer, while electrocatalytic processes give meaning and often practical importance to novel nanomaterial concepts. Examples of this fruitful symbiosis are dealloyed core−shell nanoparticle electrocatalysts, which often exhibit enhanced kinetic charge transfer rates at greatly improved atom-efficiency. As such, they represent ideal electrocatalyst architectures for the acidic oxygen reduction reaction to water (ORR) and the acidic oxygen evolution reaction from water (OER) that require scarce Pt- and Ir-based catalysts. Together, these two reactions constitute the “O-cycle”, a key elemental process loop in the field of electrochemical energy interconversion between electricity (free electrons) and molecular bonds (H2O/O2), realized in the combination of water electrolyzers and hydrogen/oxygen fuel cells. In this Account, we describe our recent efforts to design, synthesize, understand, and test noble metal-poor dealloyed Pt and Ir core−shell nanoparticles for deployment in acidic polymer electrolyte membrane (PEM) electrolyzers and PEM fuel cells. Spherical dealloyed Pt core−shell particles, derived from PtNi3 precursor alloys, showed favorable ORR activity. More detailed size−activity correlation studies further revealed that the 6−8 nm diameter range is a most desirable initial particle size range in order to maximize the particle Ni content after ORR testing and to preserve performance stability. Similarly, dealloyed and oxidized IrOx core−shell particles derived from Ni-rich Ir−Ni precursor particles proved highly efficient oxygen evolution reaction (OER) catalysts in acidic conditions. In addition to the noble metal savings in the particle cores, the Pt core−shell particles are believed to benefit in terms of their mass-based electrochemical kinetics from surface lattice strain effects that tune the adsorption energies and barriers of elementary steps. The molecular mechanism of the kinetic benefit of the dealloyed IrOx particle needs more attention, but there is mounting evidence for ligand hole effects in defect-rich IrOx shells that generate preactive oxygen centers.
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INTRODUCTION Ever since the advent of the prefix “nano” in the electrochemical science literature, endeavors to understand and harness “nanoelectrochemistry” have ensued. Electrode structures on length scales of less than one-hundredth of the diameter of a human hair have promised new material properties and new electrochemical applications. The increase in the surface-to-volume ratio of nanoscale matter has been intriguing to surface scientists, as it often generates new phenomena related to catalytic charge-transfer at electrode surfaces and across electrochemical interfaces. This is why “nanoelectrocatalysis” has evolved into such an important field of nanoelectrochemistry. Quite naturally, early work in nanoelectrochemistry was not referred to as such. Carbon-supported platinum particles for use in fuel cell cathodes, investigated in depth by Kunz,1,2 represent some of the earliest examples of nanoscale electrodes. At the same time, Grätzel’s,3,4 Henglein’s,5 and Bard’s6 ground© 2016 American Chemical Society
breaking research on photogenerated colloidal redox catalysts explored examples of heterogeneous interfacial charge transfer at the surface of nanometer-sized metal and semiconductor particles. While the particles were humbly referred to as “microelectrodes”,7 and even though they were not yet supported on a current collector, these works are some of the earliest examples of nanoelectrochemistry. Meanwhile, advances in materials characterization had made possible direct observation of nanometer-sized metallic catalyst particles.8 In parallel to these bottom-up approaches during the 1970s and 1980s, top-down methods to fabricate and utilize nanometersized monometallic electrodes were developed toward the end of that decade.9 Finally, a language transition in the electrochemical materials literature from “colloidal”10−12 to “nano”13−15 commenced in the early 1990s and was completed Received: July 4, 2016 Published: October 24, 2016 2658
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Figure 1. O-cycle in electrochemical energy conversion comprises the oxygen reduction (green arrows, left side) and oxygen evolution (blue arrows, right side) reactions, which are, in turn, coupled to fuel-generation- and fuel-combustion-based H, C, or N cycles (black arrows). Free electrons from generative electricity sources are converted into molecular bonds and back. Reproduced with permission from ref 38. Copyright 2016 Elsevier.
nitrogen-based chemicals has recently attracted much attention for the production of chemicals from excess electricity (“Powerto-X”). Whether purely electrocatalytic “power-to-chemicals” schemes will ultimately prove competitive to conventional gasphase thermal catalytic processes remains to be seen, yet the abundant availability of solar electrolytic hydrogen will, in any scenario, remain critical for the design of future solar refineries.31,32 For decades, the electrocatalytic oxygen cycle, defined by the oxygen reduction reaction33,34 (ORR) and the oxygen evolution reaction35 (OER), has attracted attention. Sixty years ago, John Bockris published the first paper on the electrochemical mechanism of the oxygen electroevolution on Pt in acid electrolytes.35 A decade later, Henri Beer developed dimensionally stable anodes,36 still today our industrial benchmark and the world’s economically most successful designed electrode other hand, gained tremendous attention during the space race era of the mid-1960s, first in alkaline and later in acid conditions.33,37 The attention given to the oxygen electrode can be explained (i) by its notoriously low catalytic rates on virtually all known electrode materials combined with the chemical instability of the majority of known electrode materials at oxygen redox potentials and (ii) based on its critical importance as the only viable, since scalable, proton- and electron-supplying and -consuming redox process for large scale electrolysis and fuel cell systems. In this Account, we describe our recent efforts to design and understand novel nanostructured metal/metal oxide core−shell particles for use as electrocatalysts for the oxygen evolution and oxygen reduction reactions in acidic environments of proton exchange membrane (PEM) fuel cells, PEM electrolyzers, or photoelectrochemical (PEC) devices. Our goal was the development of “nanoelectrocatalysts” offering improved voltage and noble metal atom efficiency, which may one day aid in closing the electrochemical O-cycle of energy conversion and storage.
in the mid-2000s. By then, nanoelectrochemistry also addressed nanoelectrodeposition16 and nanopatterning17 of surfaces using scanning probe microscopy.18 A number of excellent reviews cover various aspects of nanoelectrochemistry14,15,19,20 and more specifically of nanoparticle electrochemistry.21−25 In recent years, the notion of nanoelectrochemistry has been shaped by a successful symbiosis between the nanomaterials sciences and the electrochemical sciences. New sophisticated chemical synthesis techniques21,23,26−30 of nanosized and nanostructured particles, rods, plates, fibers, or precisely shape-controlled aggregates have been developed over the past decade. As a result of this, the materials sciences offer ample new playground for innovative research on charge transfer at nanoscale designer interfaces. On the other hand, the electrocatalytic sciences offer meaning to the materials sciences: nanomaterials endow fundamental and practical functional significance as electrodes owing to their ability to tune activity, stability, or efficiency of charge transfer processes. Early on, (photo)electrochemical charge transfer on nanoscale colloidal particles was studied and discussed in terms of bond making and bond breaking of molecular fuels for the purpose of electrochemical energy storage and conversion.3,4 At the time, particular focus was given to the light-driven electrochemical two-electron production of hydrogen from water. Some 35 years later, nanoelectrocatalysis continues to be perceived as part of a future solution to our global energy challenge. This is because electrolytic cells hold the promise to convert and store the free electrons generated by photovoltaics or wind turbines in the form of molecular chemical bonds of “solar fuels”, such as hydrogen, ammonia, methanol, or hydrocarbons (Figure 1). Galvanic cells, operated separately or coupled with the electrolysis cell, will then convert the solar fuels back to usable electricity. Hence, these two electrochemical types of devices define closed chemical energy loops involving the elements H, O, N, and C. In fact, it was recently argued that mastering the catalysis of a toolbox of a mere 10 critical chemical reactions that comprise these very four elements at a large scale can become the basis of a sustainable future energy system that would build on the interconversion between electricity and chemical energy carriers.31 There is consensus that making these chemical interconversions energy and atom efficient will require innovative nanoscale and nanostructured electrocatalyst materials. What is more, electrolytic one-way valorization pathways of lower-value carbon or
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DEALLOYED METAL/METAL OXIDE CORE−SHELL NANOCATALYSTS Bimetallic alloys consisting of a more-noble and a less-noble metal constituent undergo selective dissolution of the less noble component in oxidative acidic or electrochemical environments.39 Depending on the initial molar fraction of the less noble metal, the selective dissolution can be limited to a few 2659
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Figure 2. (a) Synthetic pathways to core−shell nanoparticles and (b,c) voltammetric profiles during the electrochemical preparation of (b) dealloyed Pt−Cu core−shell nanocatalysts and (c) dealloyed and oxidized Ir−Ni−IrOx core−shell nanocatalyst. Reproduced with permission from refs 52, 57, and 60. Copyrights 2013 American Chemical Society, 2012 The Electrochemical Society, and 2014 Royal Society of Chemistry.
Figure 3. High resolution HAADF-STEM images of dealloyed PtNi (a), PtNi3 (b), and PtNi5 NPs (c), and EELS line compositional profiles (d−f) across the NPs, respectively. Reproduced with permission from ref 47. Copyright 2014 Springer.
between surface diffusion rate of the nobler component and the dissolution rate of the less noble component.44 Recent work in our group explored the electrochemical dealloying of nanometer-sized alloy particles as compared to bulk alloys.38,45 We found that the evolution of nanoporosity is sensitively dependent on the initial alloy particle size.46−51 Below a critical size, no porosity was observed after electrochemical leaching, most likely due to the fast surface diffusion of noble metal surface atoms with reduced coordinative saturation. Instead, the dissolution was confined to the surface region (few layers) of the nanoparticle resulting
atomic layers in the surface region; it may also proceed deeper into the alloy bulk and is then referred to as dealloying.40 Dealloying is a millennia-old craft used in Pre-Columbian Mesoamerica to modify the surface appearance of macroscopic Au−Cu alloy objects; centuries later, brass dealloying posed serious technical challenges to corrosion engineers. Early corrosion research revealed that bulk dealloying generates nanoporous metal structures,41,42 like nanoporous Au (NPG), affording high internal surface area, which was harnessed for decades in gas phase catalysis.43 More recent corrosion research uncovered the mechanism of dealloying as a competition 2660
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Figure 4. ORR activity and stability analysis of (a−c) air-dealloyed PtNi3 catalyst and (d−f) N2-dealloyed PtNi3 catalyst: (a, d) HAADF-STEM images of the catalysts after dealloying but before stability test, (b, e) ORR activity and stability after 10 000 cycles between 0.6 and 1.0 V, and (c, f) size−composition−porosity information before and after stability test (dash lines represent the average Ni content). Reproduced with permission from ref 47. Copyright 2014 Springer.
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in a nonporous core−shell structure. Figure 2a illustrates the preparation of core−shell particles using electrochemical leaching in subporosity size regimes and puts it in direct comparison to other popular synthesis pathways toward core− shell architectures.52 Leaching ensures a homogeneous dissolution over the particle surface, while the experimental parameters, such as dealloying potential, acid, time, or gas environment offer control over the thickness of the noble metal-rich shell.50,53 Alternative synthetic pathways to core− shell nanoparticles, for example, segregation or seeded deposition techniques, show individual pros and cons. Thermal segregation of a metal monolayer, for instance, can be achieved in a simple dry annealing process,54 yet control of the shell thickness and the nature of the segregating metal are often limited compared to wet-chemical strategies. Figure 2b,c displays experimental voltammetric profiles at the start and after completion of the electrochemical dealloying of PtCu355−59 and IrNi360−62 alloy nanoparticles, respectively. Clearly visible in Figure 2b are the Cu dissolution waves (“1 and “2”) that rapidly disappear once a stable multilayer Pt shell has formed on a Pt−Cu alloy core. While the profiles in Figure 2b were applied with an upper turning potential of +1.0 VRHE to maintain a largely metallic Pt shell on a PtCu core (PtCu@Pt), the voltammograms of Figure 2c extend to +1.5 VRHE in order to dealloy the Ni atoms and concomitantly generate an amorphous IrOx particle shell on a Ni-rich IrNi alloy core referred to as IrNi@IrOx. Ample conclusive evidence for the formation of actual core− shell nanoparticle morphologies was provided by X-ray spectroscopic experiments60,63 and electron microscopic techniques.46,49,50,60,64
DEALLOYED PT ALLOY CORE−SHELL NANOCATALYSTS FOR THE ELECTROREDUCTION OF MOLECULAR OXYGEN
Morphology, Composition, and Structure of Surface and Bulk
Over the past decade, a variety of dealloyed Pt core−shell nanoparticles based on Cu, Co, or Ni were prepared and characterized with respect to their suitability as ORR electrocatalysts.38,47−49,65,66 Dealloyed particles larger than a critical diameter displayed a nanoporous morphology, while subcritically sized nanoparticles evolved solid core morphologies. Figure 3 depicts an analysis of a family of dealloyed PtNix core−shell particles with varying initial Ni content. The shell thickness did not follow the initial Ni content in any simple way. At first, more initial Ni caused a thicker Pt shell (0.8 nm) for the PtNi3 precursor, but then resulted in a thinner Pt shell of below 0.6 nm. The exact value of an optimal Pt shell thickness to achieve high activity and sufficient stability has remained elusive. It is believed that the best compromise consists of 2−5 layers of Pt on top of a Pt-poor core.67,68 Similarly, the residual Ni content of the dealloyed particles did not exactly follow the initial Ni content, exhibiting a nonmonotonic pattern, where the dealloyed PtNi3 particles (Figure 3b,e) showed the largest residual Ni content. However, a clear correlation was found between the residual total Ni content and the electrocatalytic ORR activity of the core−shell particles. An ORR activity optimum near the initial Pt/M = 1:3 alloy stoichiometry was repeatedly reported for fcc alloy of Cu, Co, and Ni. The origin of this observation is unclear, but is likely related to the specifics of the dealloying mechanism of that composition, balancing the non-noble metal content in the core and the shell thickness. Emphasis in studies on dealloyed Pt core−shell particles was placed on the structure of the outermost Pt shell layers. Yet, 2661
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Figure 5. Size-selected dealloyed PtNi3 catalyst particles after activation and after long-term aging. (A−D) TEM images of the 5.3 nm PtNi3 (A, B) and the 7.8 nm PtNi3 catalyst (C, D) after activation and after aging. (E, F) Size- (E) and compositional (F) evolutions of differently sized PtNi3 catalysts. (G, H) HAADF-STEM image and EELS elemental mapping of the aged 7.8 nm PtNi3 catalyst (G) and the aged 25 nm PtNi3 catalyst (H). Reproduced with permission from ref 46. Copyright 2016 Wiley.
some of our past work on dealloyed PtCo69 and PtNi50 systems showed that the dealloying process reproducibly led to the spontaneous evolution of nonmonotonic compositional patterns across the particles, which may affect the resulting surface catalysis very much. This is evident from Figure 3e,f where a nonmonotonic distribution of Ni across the particles gave rise to a Ni concentration maximum about 2 nm below the surface with the center of the particle showing Ni depletion. This patterns runs counter to the popular intuitive picture of a monotonic Ni gradient across the leached particles. Obviously, the molecular processes inside the nanoparticle during the dealloying process are much more complex than a simple diffusion process driven by surface dissolution. We have proposed an inverse Kirkendall effect to explain this phenomenon. Its detailed impact on the ORR activity deserves further attention.
inhomogeneities, or electronic ligand effect in cases where nonnoble atoms are in atomic proximity to surface Pt atoms, may affect observed ORR activity trends. Catalytic, Compositional, and Morphological Stability
The stability of dealloyed Pt core−shell nanoparticles is of importance for their deployment and use as electrocatalysts in fuel cell cathodes. The issue of stability of core−shell particles raises a variety of interesting scientific questions. There is the issue of the impact of nanoporosity on stability. To assess this, we compared the ORR performance of two dealloyed Pt−Ni catalysts prepared from an identical PtNi3 precursor. One catalyst was porous (Figure 4a−c), while the other showed nonporous solid core−shell particles (Figure 4d−f). The porous particle ensemble (Figure 4c) showed the transition from porous to solid core−shell morphology at a diameter of 13 nm, while porous particles were essentially absent over the entire 5−35 nm particle diameter range of the solid core−shell sample (Figure 4f). Catalytic ORR tests revealed that the solid core−shell particles showed a slightly higher beginning-of-life activity, but a clearly improved end-of-life activity after thousands of voltammetric cycles. This was evident from the Pt mass-based and surface area-based ORR activity values. Figures 4c,f provides further insight into the molecular origin of the performance differences. For the porous sample, the average Ni content dropped from 43 atom % to about 30 atom %, while the critical porosity diameter plummeted to below 10 nm. The porous particles retained no more than 20 atom % Ni, due to corrosion. The solid core−shell sample presented a different picture. While some porosity was observed for particles above 14 nm after stability cycling, their Ni content
Catalytic ORR Activity
Numerous theoretical and experimental studies have associated the enhanced ORR activity of Pt core−shell catalysts with geometric lattice structure effects, in particular to compressive strain in Pt overlayers caused by compressed Pt bulk alloy lattices underneath. The more Ni is retained in the subsurface, the smaller its lattice parameter, and the larger the ideal compressive strain in the Pt overlayer. Compressively strained Pt layers are known to weaken the adsorption of reactive oxygen intermediates, which is conducive for enhanced kinetic ORR rates. This simple chain of hypotheses has proved surprisingly consistent with a large majority of reported ORR reactivity trends of Pt core−shell particles. Clearly, other factors such as strain relaxation, particle size, facet distribution, surface 2662
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Figure 6. (a, b) STEM-EDX line profiles of Ir (red) and Ni (green) of two dealloyed nanoparticles with initial composition IrNi3.3 (D-IrNi3.3). The red arrows are directions of the EDX line profiles. (c) Ir 4f core level spectrum of dealloyed and surface oxidized (DO-IrNi3.3) nanoparticles at photoelectron kinetic energy of 210, 550, and 1200 eV. Dash lines are metallic Ir. Reproduced with permission from ref 60. Copyright 2014 Royal Society of Chemistry.
the particle size distribution to 10−15 nm. Elemental line scan analysis performed on the as-prepared alloy particles revealed homogeneous single phase nanomaterials. Subsequent electrochemical dealloying and concomitant surface oxidation transformed the metallic alloy into surface Ni-depleted particles with an amorphous IrOx shell. Figure 6a shows a representative elemental analysis across a dealloyed and oxidized IrNi3.3 nanoparticle of about 5 nm. Evidently, the particle was composed of an Ir shell region located on top of a nonporous Ni-rich core. Hence the particles appear to be of the solid core−shell type. The depth profile photoemission analysis in Figure 6c reveals that the surface Ir as well as large portions of the Ir located in deeper subsurface regions of the particle are in an oxidized state, with metallic Ir0 being detectable only at the deepest probe depth, consistent with a somewhat metallic Ir core. Larger particles (Figure 6b) exhibit a clear shell core contrast in dark field mode (inset) suggesting the formation of hollow IrOx nanoparticles. EDX line scans confirm the severe Ni loss from the core of the dealloyed “hollow” particles. The observed size-dependent morphology is qualitatively quite consistent with data and conclusions reached for Pt-based dealloyed nanoparticles, lending support to universal mechanistic principles leading to the evolution of nanoporosity during electrochemical dealloying.
remained high. A majority of the particles that remained core− shell after stability cycling retained more than 50 atom % Ni. This translates into compressed bulk lattices, strained Pt layers, and more favorable ORR kinetics. To better pinpoint the optimal particle size of dealloyed Pt− Ni solid core−shell catalysts, we tracked particle size distribution and corresponding average molar Ni content of a variety of PtNi3 samples with varying diameter (Figure 5a−d). Size control was achieved by defined amounts of reducing agent. Particle size distribution analysis (Figure 5e) revealed that dealloyed PtNi core−shell particles reduce their size initially and then maintain their morphology over thousands of cycles. The Ni metal content showed the least drop in the 6−8 nm size range (Figure 5f). STEM mappings confirmed that solid core−shell particles below 10 nm preserved their morphology during the stability tests (Figure 5g,h), which is why this size range is considered the most favorable for the design of stable and active fuel cell cathodes. A recent report on the performance of size-controlled dealloyed PtNi3 nanoparticles in automotive fuel cell assemblies confirmed their unprecedented stability under realistic test conditions and thus provides additional support to our structure−stability hypothesis derived from the fundamental studies described above.64
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Ir CORE−SHELL NANOCATALYSTS FOR THE ELECTROCHEMICAL EVOLUTION OF MOLECULAR OXYGEN
Support Effects and Buried Interfaces of Dealloyed IrOx Nanoparticles
The data in the previous section were obtained for carbonsupported dealloyed IrNix core−shell particles. On carbon supports, the IrNix precursor alloy particles responded readily to anodic potential cycles with Ni loss and formation of surface enriched IrOx. Recently, we compared and contrasted the formation of dealloyed IrNix core−shell particles on carbon to
Morphology, Composition, and Depth Profile
To explore the viability of dealloyed core−shell concepts for atom-efficient Ir-based OER nanoparticle catalysts, spherical IrNix precursor alloys were prepared using a solvothermal hot injection method.60,61,70 Size-directing capping ligands limited 2663
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Figure 7. (a) HAADF-STEM image of a dealloyed IrNix@IrOx core−shell nanoparticle catalyst synthesized from a meso-ATO-supported IrNi3.3 particle alloy. (b) Elemental mapping of panel a (red, Ir; blue, Ni: green, O; cyan, Sn). Depth-resolved Ir 4f (and Ni 3p) XPS spectra of the (c) meso-ATO-supported IrNi3.3 precursor and (d) the IrNix@IrOx core−shell catalyst after dealloying and oxidation (photoelectron kinetic energies are given; dashed vertical lines are metallic Ir). Reproduced with permission from ref 61. Copyright 2015 Wiley.
Figure 8. (a) Oxygen evolution reaction (OER) activities of IrNix@IrOx core−shell catalysts in comparison to Ir. (b) Ir mass-based activities from panel a at 250 mV overpotential. (c) Oxygen evolution reaction (OER) activities of mesoporous ATO supported IrNix@IrOx core−shell nanoparticle catalysts and IrOx supported on carbon and on commercial ATO. (d) Ir mass-based activity comparison from panels a and c at 280 mV overpotential. Reproduced with permission from refs 60 and 61. Copyrights 2014 Royal Society of Chemistry and 2015 Wiley.
core−shell particles, interfacing to the lower-contrast ATO support discernible on the bottom right of Figure 7a,b. Compositional depth profiles before (Figure 7c) and after dealloying (Figure 7d) revealed, to our surprise, that a low valent, largely metallic Ir state persisted after potential cycling protocols (cf. Figure 2); that is in stark contrast to previous experiments, where the same cycling protocols fully transformed the carbon-supported IrNi3.3 precursor alloy particles into oxidized core−shell IrOx. Only at the smallest probe depths (210 eV kinetic energy), some higher-valent Ir species, presumably Ir3+ or Ir4+, were detected. We convinced ourselves that this was neither due to poor electric conductivity nor
that on high surface area, mesoporous antimony-doped tin oxide (meso-ATO) supports.60,61,71,72 Oxides are considered the catalyst supports of choice for electrochemical water splitting anodes due to their improved corrosion stability compared to carbons. However, the electrochemistry of dealloyed IrOx core−shell particles supported on oxides, in particular the role of the buried interface between active metal oxide and support oxide has been poorly studied. Figure 7a depicts the dark field micrograph of a dealloyed IrNix@IrOx core−shell catalyst obtained from potential cycling of an IrNi3.3 alloy nanoparticle precursor. The catalytically active noble metal oxide displayed a mix of solid and hollow 2664
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Figure 9. Illustration of the hole doping effect of substitution of Ir4+ with Ni2+ in a rutile lattice: (a) IrO6 octahedra in rutile lattice; (b) Ni2+ incorporation results in O 2p holes;74,75 (c) removal of Ni2+ results in more holes. Holes in O 2p orbitals are electrophilic O1−.
electrochemical dealloying.62,73 Our analysis provided evidence that the incorporation of Ni2+ in IrOx lattices prevented crystallization and long-range order. This allowed to prepare Xray-amorphous IrNiOx catalyst films at annealing temperatures, at which pure IrO2 films invariably crystallized in their rutile, catalytically quite inactive structure. The amorphous mixed oxide films thus stabilized significant fractions of labile, easily reducible surface hydroxyl species. Now, as soon as these IrNiOx films were electrochemically polarized at OER electrode potentials in acidic electrolytes, the Ni ions started to leached out, creating a roughened IrOx surface. However, in addition to the sheer enhanced roughness effect, spectroscopic analysis proved that the Ni leaching process drastically altered the active catalyst surface termination toward an even larger population of redox-active surface hydroxyls. As a result of this, the strongly hydroxylated, surface Ni-depleted IrNixOx films exhibited previously unachieved OER specific and Ir mass based catalytic activities and record low Tafel slopes. The study of the electronic structure of the Ni-depleted IrNiOx revealed another remarkable feature: Ni2+ lattice doping gave rise to an unusual 529 eV X-ray absorption feature in the O K-edge spectrum of the IrNiOx films both before and after OER catalysis. In pure crystalline IrO2 films, this excitation was absent. DFT calculations confirmed that this excitation was associated with an additional “oxygen hole” state, when Ni2+ substituted Ir4+ in an octahedral rutile coordination environment.62 While the spectroscopic fingerprints of oxygen hole states were established, their chemical and electronic nature and relation to OER activity remained elusive. Shortly after, an Xray absorption study by Pfeifer et al. of amorphous, hydrous IrOx catalysts provided the first plausible interpretation of similar 529 eV near edge excitation features observed in defectrich amorphous IrOx.74,75 A new hypothesis put forward in that work centered on the formation of holes that are not localized at Ir centers but instead populate O 2p orbitals. These localized holes generated electrophilic O1− species, reflecting a rather covalent bonding character in these defective 5d oxides. This new view may constitute a substantial advance in our understanding of preactive states in OER oxides. Inspired by the interpretation of the data of amorphous IrOx,74,75 we propose that Ni2+ doping results in (formally) two O 2p holes per Ni2+, as illustrated in Figure 9a,b. Hole-doping by Ni, however, can only be effective if the oxygen atoms maintain (at least on the nanoscale) their octahedral rutile-type lattice scaffold. Once the geometric structure of the IrNiOx transforms into other (such as NiO rock salt) type structures, the hole doping effect of Ni ceases; this prediction was confirmed by a
inhomogeneous particle distribution that the IrNi3.3 failed to transform into oxidized Ir particles. As expected, the Ni content dropped during potential dealloying, yet vanished completely even in shallow near-surface regions. These experiments led us to hypothesize the possibility that electronic effects at the buried solid−solid interface between oxide support and IrNiOx core−shell particle play a more important role in the surface electrochemistry of oxide-supported particles than previously assumed. Similar metal−support interactions are well-known in the surface science of heterogeneous gas phase catalytic processes, yet they are much less documented and discussed in electrochemical environments. More work on such metal support effects at buried interfaces is ongoing in our laboratory. Catalytic OER Performance Evaluation
Figure 8 provides a comparative overview of the OER activity of carbon-supported and oxide-supported IrNiOx core−shell catalysts and between meso-porous ATO supports and commercial ATO supports.60,61 The dealloyed core−shell particles based on the IrNi∼3 stoichiometry outperformed others on a Ir-mass basis. Core−shell particles derived from the IrNi3.3 precursor were 3.5× more active than oxidized Ir controls but 10× more active than a previously reported crystalline rutile IrO2 (r-IrO2) catalyst (Figure 8a,b). As Figure 8c shows, the ATO-supported IrNix@IrOx core−shell nanoparticles greatly outperform the electrochemically oxidized IrOx/C particle catalysts, yet fall short in Ir mass-based performance compared to the carbon-supported IrNix@IrOx core−shell particles (Figure 8d). We attribute this to the incomplete Ni dealloying and Ir oxidation. So, even though the ATO-supported IrNix@IrOx core−shell catalyst exhibits a 3× higher performance compared to the IrOx/C or IrOx/Comm catalysts, for ATO particles, this performance could be further boosted to a factor of 7× (at 280 mV overpotential, see Figure 8d), if an IrOx shell structure had fully evolved on ATO. The performance factors reflect enhanced atom-efficient use of the noble metals and translate directly in catalyst cost reduction thanks to reduced electrode Ir-mass loadings in electrolyzer anodes. A Mechanistic Hypothesis for the Enhanced OER Activity in IrNixOx
To gain insight in the origin of the enhanced OER activity of dealloyed IrNix@IrOx core−shell nanoparticles, we explored extended dealloyed IrNiOx model film catalysts in parallel to dealloyed nanoparticles. Specifically, we analyzed the correlations between catalytic activity, geometric structure, and the electronic structure of a 50 nm IrNiOx thin film before and after 2665
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Catalysis Society and the “Ertl Prize” awarded by the Ertl Center for Catalysis.
direct comparison of the local geometric structure and the corresponding XANES data.62 Once exposed to anodic potential or an acidic electrolyte, leaching of Ni2+ introduces (formally) two additional oxygen holes per leached Ni. Electrophilic O1− species may play the role of “pre-active” centers, which become catalytically active targets for nucleophilic attacks of molecular water or terminal hydroxides under anodic OER electrode potentials. These nucleophilic attacks form O−O bonds, ultimately closing in the catalytic oxygen evolution cycle. More work to clarify these fundamental issues related to acid oxygen evolution on metal-depleted IrOx core−shell catalysts is currently ongoing.
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ACKNOWLEDGMENTS The author acknowledges support by DFG (STR 596/3-1) and the German BMBF (03SF0527A LOPLAKAT). I thank Dr. Travis Jones and Dr. Hong-Nhan Nong for helpful discussions. Dedicated to Daniel, Alexander, and Beatriz.
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CONCLUSIONS AND OUTLOOK Closing the energetic O-cycle will remain a critical scientific challenge for future renewable electricity-based energy systems. This is because the oxygen electrode is an attractive protonand electron-coupled redox half-cell reaction involving abundant reactants like oxygen and water, yet it displays very sluggish electrochemical charge transfer rates. For the last 60 years,35 generations of researchers have explored the electrocatalysis of the O-cycle. And there is a chance that even our youngest generation of scientists, born as recently as October 21, 2014, will continue to struggle for a complete molecular understanding and optimization of catalyst materials for the oxygen electrocatalysis by the time they retire in another 60 years. But thanks to past and coming decades of electrocatalysis research, the author is hopeful that mastering the energetic O-cycle will contribute to the transition of our energy systems from fossil to renewable ones. By the time these youngest scientists can read this Account out loud, the world’s share of renewable electricity may have risen to a double-digit value, in some European countries perhaps approaching as much as 40%; by the time these young scientists will comprehend the scientific content of this Account, these numbers may have doubled. To make this transition a reality, a growing integration of renewable electricity using efficient interconversion between electricity and molecular bonds (chemical energy storage) is indispensable. New material concepts resulting from the symbiosis of nanomaterials science and electrocatalytic science will render this possible. Core−shell nanoparticles may be one of them.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest. Biography Peter Strasser is the chaired professor of “Electrocatalysis and Electrochemical Engineering” in the Chemical Engineering Division of the Department of Chemistry at the Technical University Berlin. Prior to his appointment, he was Assistant Professor at the Department of Chemical and Biomolecular Engineering at the University of Houston. Before moving to Houston, Prof. Strasser served as Senior Member of staff at Symyx Technologies, Inc., Santa Clara, USA. In 1999, Prof. Strasser received his doctoral degree in Physical Chemistry and Electrochemistry from the “Fritz-Haber-Institute” of the Max-PlanckSociety, Berlin, Germany, under the direction of Gerhard Ertl. He was awarded the “Otto-Hahn Research Medal” by the Max-Planck Society, the “Otto Roelen” medal for catalysis awarded by the German 2666
DOI: 10.1021/acs.accounts.6b00346 Acc. Chem. Res. 2016, 49, 2658−2668
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