In the face of the global energy challenge and progressing global climate change, renewable energy systems and components, such as fuel cells and electrolyzers, which close the energetic oxygen and ...carbon cycles, have become a technology development priority. The electrochemical oxygen reduction reaction (ORR) and the direct electrochemical carbon dioxide reduction reaction (CO2RR) are important electrocatalytic processes that proceed at gas diffusion electrodes of hydrogen fuel cells and CO2 electrolyzers, respectively. However, their low catalytic activity (voltage efficiency), limited long‐term stability, and moderate product selectivity (related to their Faradaic efficiency) have remained challenges. To address these, suitable catalysts are required. This review addresses the current state of research on Pt‐based and Cu‐based nanoalloy electrocatalysts for ORR and CO2RR, respectively, and critically compares and contrasts key performance parameters such as activity, selectivity, and durability. In particular, Pt nanoparticles alloyed with transition metals, post‐transition metals and lanthanides, are discussed, as well as the material characterization and their performance for the ORR. Then, bimetallic Cu nanoalloy catalysts are reviewed and organized according to their main reaction product generated by the second metal. This review concludes with a perspective on nanoalloy catalysts for the ORR and the CO2RR, and proposes future research directions.
Recent progress on nanoalloy catalysts for multielectron reduction reactions such as oxygen reduction reaction and carbon dioxide reduction reaction is discussed. For the first reaction, Pt‐based alloy nanocatalysts are considered, while for the latter, Cu‐based alloy nanocatalysts are discussed. Advantages with respect to single‐element catalysts are reported. Finally, a perspective is given, highlighting future challenges.
Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of ...low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.
Octahedrally shaped Pt–Ni alloy nanoparticles on carbon supports have demonstrated unprecedented electrocatalytic activity for the oxygen reduction reaction (ORR), sparking interest as catalysts for ...low-temperature fuel cell cathodes. However, deterioration of the octahedral shape that gives the catalyst its superior activity currently prohibits the use of shaped catalysts in fuel cell devices, while the structural dynamics of the overall catalyst degradation are largely unknown. We investigate the time-resolved degradation pathways of such a Pt–Ni alloy catalyst supported on carbon during cycling and startup/shutdown conditions using an in situ STEM electrochemical liquid cell, which allows us to track changes happening over seconds. Thereby we can precisely correlate the applied electrochemical potential with the microstructural response of the catalyst. We observe changes of the nanocatalysts’ structure, monitor particle motion and coalescence at potentials that corrode carbon, and investigate the dissolution and redeposition processes of the nanocatalyst under working conditions. Carbon support motion, particle motion, and particle coalescence were observed as the main microstructural responses to potential cycling and holds in regimes where carbon corrosion happens. Catalyst motion happened more severely during high potential holds and sudden potential changes than during cyclic potential sweeps, despite carbon corrosion happening during both, as suggested by ex situ DEMS results. During an extremely high potential excursion, the shaped nanoparticles became mobile on the carbon support and agglomerated facet-to-facet within 10 seconds. These experiments suggest that startup/shutdown potential treatments may cause catalyst coarsening on a much shorter time scale than full collapse of the carbon support. Additionally, the varying degrees of attachment of particles on the carbon support indicates that there is a distribution of interaction strengths, which in the future should be optimized for shaped particles. We further track the dissolution of Ni nanoparticles and determine the dissolution rate as a function of time for an individual nanoparticle – which occurs over the course of a few potential cycles for each particle. This study provides new visual understanding of the fundamental structural dynamics of nanocatalysts during fuel cell operation and highlights the need for better catalyst-support anchoring and morphology for allowing these highly active shaped catalysts to become useful in PEM fuel cell applications.
Shape-controlled octahedral Pt–Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (oxygen reduction reaction, ORR), which makes them fuel-cell ...cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nano-octahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situ analytical techniques with in situ transmission electron microscopy (TEM), in situ X-ray diffraction (XRD), and in situ electrochemical Fourier transformed infrared (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ∼25 times higher ORR activity than for commercial Pt/C (2.7 A mgPt –1 at 0.9 VRHE) was reproducibly observed upon annealing at 300 °C. The electrochemical stability, however, peaked out at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (EDX) in combination with in situ electrochemical CO stripping/FTIR data revealed subtle, but important, differences in the formation and chemical nature of Pt-rich and Ni-rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situ electrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni-rich facets. The insights from our study are quite generally valid and aid in developing suitable post-synthesis thermal treatments for other alloy nanocatalysts as well.
Direct ethanol fuel cells are attractive power sources based on a biorenewable, high energy‐density fuel. Their efficiency is limited by the lack of active anode materials which catalyze the breaking ...of the C−C bond coupled to the 12‐electron oxidation to CO2. We report shape‐controlled PtNiRh octahedral ethanol oxidation electrocatalysts with excellent activity and previously unachieved low onset potentials as low as 0.1 V vs. RHE, while being highly selective to complete oxidation to CO2. Our comprehensive characterization and in situ electrochemical ATR studies suggest that the formation of a ternary surface site ensemble around the octahedral Pt3Ni1Rhx nanoparticles plays a crucial mechanistic role for this behavior.
It's the shape, that matters: The efficient and complete electrooxidation of ethanol at low overpotentials has been a prime research target for a long time. The unique combination of a ternary Pt‐Ni‐Rh ensemble on defined octahedral {111} facets now shows promising activity coupled with good selectivity in this transformation.
Thanks to their remarkably high activity toward oxygen reduction reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high ...activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt–Ni octahedral nanoparticles with high activities up to 1.14 A mgPt –1 combined with improved performance and shape stability compared to previous bimetallic Pt–Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and atomic degradation mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphological particle shape and consequently their performance. Rh surface-doped octahedral Pt–Ni particles were prepared at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30 000 potential cycles, while undoped bimetallic reference nanoparticles show a complete loss in octahedral shape already after 8000 cycles in the same potential window. Detailed atomic insight in these observations is obtained from aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) analysis. Our analysis shows that it is the migration of Pt surface atoms and not, as commonly thought, the dissolution of Ni that constitutes the primary origin of the octahedral shape loss for Pt–Ni nanoparticles. Using small amounts of Rh we were able to suppress the migration rate of platinum atoms and consequently suppress the octahedral shape loss of Pt–Ni nanoparticles.
Shape controlled octahedral Pt-Ni alloy nanoparticles are promising oxygen reduction reaction (ORR) electrocatalysts for cathodes of low temperature Polymer Electrolyte Membrane fuel cells. Organic ...surfactants are used in order to control and tune particle composition, size, shape, and the distribution on the support material. Such methods request intense post-synthesis cleaning, or annealing procedures in order to remove the ligands, demanding for simpler cleaning and activation procedures. Here, we explore the effect of an acetic acid treatment of as-prepared Pt-Ni particles, applied prior to annealing. The resulting nanoparticles underwent an electrochemical surface characterization and were investigated in terms of their ORR activities and electrochemical long-term stability. After acid treatment the particles exhibit a Pt-rich surface, which changed slightly during annealing at 300°C but drastically to a more homogeneous alloy after annealing at 500°C due to Ni surface segregation. Besides changes in the (sub-)surface Pt and Ni composition, the octahedral shape did not survive the 500°C treatment. An improved ORR activity was obtained after annealing at 300°C. Our insights into effects and benefits of the described post-synthesis treatments aid our general understanding, but also may help improve the practical design of suitable treatment protocols of this class of ORR catalyst.
Octahedrally shaped Pt-Ni alloy nanoparticles on carbon supports have demonstrated unprecedented electrocatalytic activity for the oxygen reduction reaction (ORR), sparking interest as catalysts for ...low-temperature fuel cell cathodes. However, deterioration of the octahedral shape that gives the catalyst its superior activity currently prohibits the use of shaped catalysts in fuel cell devices, while the structural dynamics of the overall catalyst degradation are largely unknown. We investigate the time-resolved degradation pathways of such a Pt-Ni alloy catalyst supported on carbon during cycling and startup/shutdown conditions using an
in situ
STEM electrochemical liquid cell, which allows us to track changes happening over seconds. Thereby we can precisely correlate the applied electrochemical potential with the microstructural response of the catalyst. We observe changes of the nanocatalysts' structure, monitor particle motion and coalescence at potentials that corrode carbon, and investigate the dissolution and redeposition processes of the nanocatalyst under working conditions. Carbon support motion, particle motion, and particle coalescence were observed as the main microstructural responses to potential cycling and holds in regimes where carbon corrosion happens. Catalyst motion happened more severely during high potential holds and sudden potential changes than during cyclic potential sweeps, despite carbon corrosion happening during both, as suggested by
ex situ
DEMS results. During an extremely high potential excursion, the shaped nanoparticles became mobile on the carbon support and agglomerated facet-to-facet within 10 seconds. These experiments suggest that startup/shutdown potential treatments may cause catalyst coarsening on a much shorter time scale than full collapse of the carbon support. Additionally, the varying degrees of attachment of particles on the carbon support indicates that there is a distribution of interaction strengths, which in the future should be optimized for shaped particles. We further track the dissolution of Ni nanoparticles and determine the dissolution rate as a function of time for an individual nanoparticle - which occurs over the course of a few potential cycles for each particle. This study provides new visual understanding of the fundamental structural dynamics of nanocatalysts during fuel cell operation and highlights the need for better catalyst-support anchoring and morphology for allowing these highly active shaped catalysts to become useful in PEM fuel cell applications.
In situ
nanoscale imaging of the electrochemical activation and degradation of carbon-supported octahedral Pt-Ni nanocatalysts in real time.
We present a voltammetric, spectroscopic, and atomic-scale microscopic study of how initial interfacial contact with high- and low-pH electrolytes affects the surface voltammetry, near-surface ...composition, CO binding, and electrocatalytic oxygen reduction reaction (ORR) of dealloyed Pt–Ni alloy nanoparticles deployed in fuel cells. The first contact of the catalyst with the electrolyte is critical for the evolution of the catalytically active surface structure, yet still insufficiently understood. Counter to chemical intuition, we find that voltammetric activation protocols in both pH 1 and pH 13 electrolytes result in similarly Ni-depleted surfaces with similar near-surface Ni/Pt ratios to a 2.5 nm depth, yet vastly different ORR reactivities. On the basis of our combined voltammetric, scanning transmission electron microscopy with the spectroscopic mapping by energy dispersive X-ray (STEM-EDX) microscopic and X-ray photoelectron spectroscopy (XPS) analysis, we conclude that oxygen-saturated alkaline electrolytes causes a strong surface segregation of the more oxophilic Ni component toward the particles surface, however in distinctly different ways depending on the pretreatment pH. Data suggest a controlling role of the initial thickness of the Ni-depleted Pt shell for the catalysis-driven segregation process. We analyze and discuss how such subtle differences in initial surface composition can unfold such dramatic subsequent variations in ORR activity as a function of pH. Our findings have practical bearing for the design of active Pt bimetallic ORR catalysts for alkaline exchange membrane fuel cells. If the non-noble oxophilic Pt alloy component is insoluble in the alkaline electrolyte, our results call for an imperative acid-pretreatment to avoid surface blocking by oxygen-induced segregation. If the non-noble oxophilic Pt alloy component is soluble in an alkaline electrolyte, acid or alkaline, even nonpretreated Pt alloy catalyst may be employed.
Alloying Pt with highly oxophilic transition metals such as Rh, Ni, or Sn has been a promising strategy to modify the electrocatalytic surface properties of Pt in order to supply active ...oxygen‐containing species for ethanol electrooxidation. A new, highly active, ternary single‐phased fcc PtRhNi/C nanoparticle electrocatalyst for the electrocatalytic oxidation of ethanol (EOR) is reported and its morphology (XRD and TEM), composition (inductively coupled plasma optical emission spectroscopy), and electrochemical activity are discussed in comparison with the state‐of‐art PtRhSn/C electrocatalyst. The EOR activity of the PtRhNi/C material outperformed the benchmark PtRhSn/C material in acidic and alkaline media, showing high stability, especially in alkaline media. The higher intrinsic EOR activity of the Ni‐containing electrocatalyst lends support to the notion that surface NiOx is an excellent oxygenate‐supplying catalyst component for the oxidation of ethanol.
A perfect match? Platinum, rhodium, and nickel are combined to create a novel electrocatalyst with activity towards ethanol oxidation in alkaline and acidic media, supporting the notion that surface NiOx is an excellent oxygenate‐supplying catalyst component for the complete oxidation of ethanol (see picture).
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