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 IrO x 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 IrO x particle needs more attention, but there is mounting evidence for ligand hole effects in defect-rich IrO x shells that generate preactive oxygen centers.
Catalysts by Platonic design Strasser, Peter
Science (American Association for the Advancement of Science),
07/2015, Letnik:
349, Številka:
6246
Journal Article
Recenzirano
Sophisticated shape-controlled design is yielding ever more active nanocatalysts
Also see Report by
Zhang
et al.
Around 360 BCE, in his work
Timaeus
, the Greek philosopher Plato elaborated on the ...four elements as the basic components of our cosmos: earth, water, air, and fire. He argued that each element consists of small, highly symmetric corpuscles—the cube for earth, the tetrahedron for fire, the icosahedron for water, and the octahedron for air. The faces of the latter three corpuscles consist of equilateral triangles, which—according to Plato—allows air, water, and fire to interconvert. Plato would likely be thrilled to learn that, as recently confirmed by Huang
et al.
(
1
), nanoscale Pt-Ni octahedra are the catalytically most active known material for converting air (molecular oxygen) into water and fire (thermal energy). On page 412 of this issue, Zhang
et al.
(
2
) show that octahedral and cubic hollow shells of just a few atomic Pt layers are also versatile catalysts, with the octahedral shells particularly active for oxygen reduction. Such tiny metallic octahedra may one day become the building blocks of electrodes for electrochemical energy conversion.
NiFe‐based (oxy)hydroxides are highly active catalysts for the oxygen evolution reaction in alkaline electrolyte solutions. These catalysts can be synthesized in different ways leading to ...nanomaterials and thin films with distinct morphologies, stoichiometries and long‐range order. Notably, their structure evolves under oxygen evolution operating conditions with respect to the as‐synthesized state. Therefore, many researchers have dedicated their efforts on the identification of the catalytic active sites employing in operando experimental methods and theoretical calculations. These investigations are pivotal to rationally design materials with outstanding performances that will constitute the anodes of practical commercial alkaline electrolyzers. The family of NiFe‐based oxyhydroxide catalysts reported in recent years is addressed and the actual state of the research with special focus on the understanding of the oxygen‐evolution‐reaction active sites and phase is described. Finally, an overview on the proposed oxygen‐evolution‐reaction mechanisms occurring on NiFe‐based oxyhydroxide electrocatalysts is provided.
NiFe‐based oxyhydroxides are among the most active electrocatalysts for oxygen evolution in alkaline electrolytes. Structural transformations are observed after applying a positive electrode potential leading to a catalytically active phase that is different with regard to the resting state. Recent advances in revealing the catalytically active phase under oxygen evolution operating conditions are presented.
Hydrogen peroxide (H2O2) has a wide range of important applications in various fields including chemical industry, environmental remediation, and sustainable energy conversion/storage. Nevertheless, ...the stark disconnect between today's huge market demand and the historical unsustainability of the currently-used industrial anthraquinone-based production process is promoting extensive research on the development of efficient, energy-saving and sustainable methods for H2O2 production. Among several sustainable strategies, H2O2 production via electrochemical and photochemical routes has shown particular appeal, because only water, O2, and solar energy/electricity are involved during the whole process. In the past few years, considerable efforts have been devoted to the development of advanced electrocatalysts and photocatalysts for efficient and scalable H2O2 production with high efficiency and stability. In this review, we compare and contrast the two distinct yet inherently closely linked catalytic processes, before we detail recent advances in the design, preparation, and applications of different H2O2 catalyst systems from the viewpoint of electrochemical and photochemical approaches. We close with a balanced perspective on remaining future scientific and technical challenges and opportunities.
A comparative investigation was performed to examine the intrinsic catalytic activity and durability of carbon supported Ru, Ir, and Pt nanoparticles and corresponding bulk materials for the ...electrocatalytic oxygen evolution reaction (OER). The electrochemical surface characteristics of nanoparticles and bulk materials were studied by surface-sensitive cyclic voltammetry. Although basically similar voltammetric features were observed for nanoparticles and bulk materials of each metal, some differences were uncovered highlighting the changes in oxidation chemistry. On the basis of the electrochemical results, we demonstrated that Ru nanoparticles show lower passivation potentials compared to bulk Ru material. Ir nanoparticles completely lost their voltammetric metallic features during the voltage cycling, in contrast to the corresponding bulk material. Finally, Pt nanoparticles show an increased oxophilic nature compared to bulk Pt. With regard to the OER performance, the most pronounced effects of nanoscaling were identified for Ru and Pt catalysts. In particular, the Ru nanoparticles suffered from strong corrosion at applied OER potentials and were therefore unable to sustain the OER. The Pt nanoparticles exhibited a lower OER activity from the beginning on and were completely deactivated during the applied OER stability protocol, in contrast to the corresponding bulk Pt catalyst. We highlight that the OER activity and durability were comparable for Ir nanoparticles and bulk materials. Thus, Ir nanoparticles provide a high potential as nanoscaled OER catalyst.
We report a synthetic electrochemical strategy to deliberately modify the catalytic reactivity of Pt bimetallic surfaces. The strategy consists of voltammetric surface dealloying of the non-noble ...constituent from Pt-poor bimetallic precursor compounds. We exemplify this method by dealloying carbon-supported Pt25Cu75 alloy nanoparticle precursors and testing the resulting active catalyst phase for the oxygen reduction reaction (ORR). We show that dealloyed Pt−Cu electrocatalysts exhibit an extraordinary increase in intrinsic reactivity of 4−6 times as compared to pure Pt electrocatalysts. Our results indicate that electrochemical treatment of the alloy precursors selectively dissolves Cu near the particle surface. The partially dealloyed particles constitute the active catalyst phase. While Cu is retained in the core of the particles after dealloying, the essentially pure Pt surface suggests a core−shell structure of the active catalyst. Geometric effects, such as exposure of more active crystallographic facets or a more favorable Pt−Pt surface interatomic distance are proposed to play a key role in the enhancement mechanism. This work suggests that the selective electrochemical dissolution (dealloying) of non-noble components from noble metal bimetallics can serve as a general strategy toward tuning surface electrocatalytic properties.
Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging ...when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential <480 mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pH 13 in seawater‐mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near‐neutral conditions (borate buffer at pH 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pH 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using in situ electrochemical mass spectrometry.
Salty splitting: NiFe layered double hydroxide operates as oxygen evolution reaction (OER)‐selective electrocatalyst in seawater‐mimicking electrolyte within an overpotential range (<480 mV) at technologically‐targeted current densities of 10 mA cm−2. Suitable operating conditions are further identified for seawater electrocatalysis that will aid in the future design of selective seawater electrolyzers and seawater splitting catalysts.
•The buffer capacity of the electrolyte determines the activity and selectivity of Cu.•Our observations are attributed to a different local pH in different electrolytes.•H2 and CH4 formation rate is ...improved when the local pH close to neutral.•Ethylene's formation rate shows Tafel relation independently of local pH.
In the present study we demonstrate that the activity and selectivity of copper during the CO2 electrochemical reduction can be tuned by changing the concentration of the bicarbonate electrolyte. Comparing the absolute formation rate and Faradaic selectivity of H2, CH4, CO, and C2H4 as a function of the applied electrode potential, we show that variations in the bulk buffer capacities of the electrolyte have substantial impact on absolute product formation rates and relative faradic selectivity. We find that high concentrations of bicarbonate improve the overall Faradaic CO2 electroreduction activity, largely due to higher absolute formation rates of H2 and CH4. In lower-concentrated bicarbonate electrolytes with their lower overall activity, the selectivity toward ethylene was drastically enhanced. Following earlier theoretical work, we hypothesize the pH near the copper electrode interface to largely account for the observed effects: diluted KHCO3 solutions allow for more alkaline local pH values during CO2 electroreduction. Our study highlights the controlling role of the interfacial pH on the product distribution during CO2 reduction over a wide electrode potential range.
Development of efficient electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) remain key issues for the commercialization of fuel cells and metal–air ...batteries. In this study, A CoFe2O4/graphene nanohybrid is facilely synthesized via a two-step process and applied as an electrocatalyst for the ORR and the OER. The as-prepared CoFe2O4/graphene nanohybrid demonstrates excellent catalytic activity for the ORR. At the same mass loading, the Tafel slope of CoFe2O4/graphene electrocatalyst for the ORR is comparable to that of the commercial Pt/C (20 wt% Pt on Vulcan XC-72, Johnson Matthey). The ORR on CoFe2O4/graphene mainly favours a direct 4e− reaction pathway. The CoFe2O4/graphene nanohybrid also affords high catalytic activity for the OER. The chronoamperometric tests show that CoFe2O4/graphene catalyst exhibits excellent stability for both the ORR and the OER, outperforming the commercial Pt/C. The high electrocatalytic activity and durability of CoFe2O4/graphene nanohybrid are attributed to the strong coupling between CoFe2O4 nanoparticles and graphene.
•A CoFe2O4/graphene nanohybrid is facilely synthesized.•CoFe2O4/graphene nanohybrid is an efficient bi-functional electrocatalyst for the ORR and the OER.•CoFe2O4/graphene nanohybrid could be used as a potential catalyst for metal–air batteries.
PDF
