Carbon dioxide, as a greenhouse gas, has a critical impact on global climate change. Hence, reducing its emissions and/or transforming it into value-added chemicals is of paramount importance to ...society. Such a goal can be achieved through the development of electrochemical catalytic processes, which showed initial successes in synthesizing CO and cosynthesis of H2 (syngas) from CO2. However, to further advance the technology toward more complex hydrocarbons (possibly fuels) and more complex organic molecules to be used in industrial chemical synthesis of rubber/resins and plastics or even fine chemical synthesis of pharmaceuticals, catalytic structures have to be designed beyond simple materials. One approach consists of executing the required steps linking CO and small alcohols and the desired products through chemical reactions catalyzed by enzymes or microorganisms, hence combining them with the electrocatalysts used for the initial steps of the CO2 reduction. Here, we discuss the electrocatalysts, enzymes, and bacteria to be used for those cascade reactions. Inorganic catalysts can be directly utilized for initial steps of CO2 reduction resulting in formate, carbon monoxide, and methanol, which then can be further reduced by employing enzymatic catalysts such as carbonic anhydrase and formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase. Formate, carbon monoxide, and methanol can also be converted to acetyl-CoA and pyruvate, which are critical intermediates in the production of various chemicals of critical importance. Furthermore, if a chemolithoautotrophic bacteria is employed, the former can be used as a feedstock for biomass. A special emphasis is here given to the compatibility of the different classes of electrocatalysts used in the cascade: heterogeneous electrocatalysts (metals and metal oxides/carbides/nitrites, atomically dispersed transition metal–nitrogen-carbon, etc.), enzymatic catalysts (individually purified enzymes or multienzymatic complexes), bacterial cells (pure or mixed cultures of different classes, planktonic, or biofilm-forming), and their integration in singular bed reactors or multireactors units required for the formation of the products of interest. The electrocatalysts and related cascade reaction pathways are summarized in a descriptive “connectivity map” identifying the most important reaction pathways.
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.
The electrical performance of a proton exchange membrane fuel cell is limited by the slow oxygen reduction reaction (ORR) kinetics. Catalytic improvements for the ORR have been obtained on alloyed ...PtM/C or M-rich-core@Pt-rich-shell/C catalysts (where M is an early or late transition metal) in comparison to pure Pt/C, due to a combination of strain and ligand effects. However, the effect of the fine nanostructure of the nanomaterials on the ORR kinetics remains underinvestigated. Here, nanometer-sized PtNi/C electrocatalysts with low Ni content (∼15 atom %) but different nanostructures and different densities of grain boundary were synthesized: solid, hollow, or “sea sponge” PtNi/C nanoalloys, and solid Ni-core@Pt-shell/C nanoparticles. These nanostructures were characterized by transmission and scanning transmission electron microscopy, X-ray energy dispersive spectroscopy, synchrotron wide-angle X-ray scattering (WAXS), atomic absorption spectroscopy, and electrochemical techniques. Their electrocatalytic activities for the ORR were determined and structure–activity relationships established. The results showed the following: (i) The compression of the Pt lattice by ca. 15 atom % Ni provides mild ORR activity enhancement in comparison to pure Pt/C. (ii) Highly defective PtNi/C nanostructures feature up to 9.3-fold enhancement of the ORR specific activity over a commercial Pt/C material with similar crystallite size. (iii) The enhancement of the ORR kinetics can be ascribed to the presence of structural defects, as shown by two independent parameters: the microstrain determined from WAXS and the average COads electrooxidation potential (μ1 CO) determined from COads stripping measurements. This work indicates that, at fixed Ni content, ORR activity can be tuned by nanostructuring and suggests that targeting structural disorder is a promising approach to improve the electrocatalytic properties of mono- or bimetallic nanocatalysts.
Due to their interesting electrocatalytic properties for the oxygen reduction reaction (ORR), hollow Pt‐alloy nanoparticles (NPs) supported on high‐surface‐area carbon attract growing interest. ...However, the suitable synthesis methods and associated mechanisms of formation, the reasons for their enhanced specific activity for the ORR, and the nature of adequate alloying elements and carbon supports for this type of nanocatalysts remain open questions. This Review aims at shedding light on these topics with a special emphasis on hollow PtNi NPs supported onto Vulcan C (PtNi/C). We first show how hollow Pt‐alloy/C NPs can be synthesized by a mechanism involving galvanic replacement and the nanoscale Kirkendall effect. Nickel, cobalt, copper, zinc, and iron (Ni, Co, Cu, Zn, and Fe, respectively) were tested for the formation of Pt‐alloy/C hollow nanostructures. Our results indicate that metals with standard potential −0.4<E<0.4 V (vs. the normal hydrogen electrode) and propensity to spontaneously form metal borides in the presence of sodium borohydride are adequate sacrificial templates. As they lead to smaller hollow Pt‐alloy/C NPs, mesoporous carbon supports are also best suited for this type of synthesis. A comparison of the electrocatalytic activity towards the ORR or the electrooxidation of a COads monolayer, methanol or ethanol of hollow and solid Pt‐alloy/C NPs underlines the pivotal role of the structural disorder of the metal lattice, and is supported by ab initio calculations. As evidenced by accelerated stress tests simulating proton‐exchange membrane fuel cell cathode operating conditions, the beneficial effect of structural disorder is maintained on the long term, thereby bringing promises for the synthesis of highly active and robust ORR electrocatalysts.
Defective is the new chic: Taking advantage of their highly defective structure (disordered surface, high polycrystallinity, contracted lattice, and open porosities), hollow Pt‐alloy/C nanoparticles can be used to achieve a high catalytic enhancement for both the oxygen reduction reaction and oxidation reactions such as CO oxidation or alcohol oxidation, thus opening the path to a generation of electrocatalysts designed around structural defects.
Due to their increased surface area to volume ratio and molecular accessibility, microporous and mesoporous materials are a promising strategy to electrocatalyze the cathodic oxygen reduction ...reaction (ORR), the key reaction in proton-exchange membrane fuel cells (PEMFC). Here, we synthesized and provided atomically resolved pictures of porous hollow PtNi/C nanocatalysts, investigated the elemental distribution of Ni and Pt atoms, measured the Pt lattice contraction, and correlated these observations to their ORR activity. The best porous hollow PtNi/C nanocatalyst achieved 6 and 9-fold enhancement in mass and specific activity for the ORR, respectively over standard solid Pt/C nanocrystallites of the same size. The catalytic enhancement was 4 and 3-fold in mass and specific activity, respectively, over solid PtNi/C nanocrystallites with similar chemical composition, Pt lattice contraction, and crystallite size. Furthermore, 100% of the initial mass activity at E = 0.90 V vs RHE (0.56 A mg–1 Pt) of the best electrocatalyst was retained after an accelerated stress test composed of 30 000 potential cycles between 0.60 and 1.00 V vs RHE (0.1 M HClO4 T = 298 K), therefore meeting the American Department of Energy targets for 2017–2020 both in terms of mass activity and durability (0.44 A mg–1 Pt, mass activity losses < 40%). The better catalytic activity for the ORR of hollow PtNi/C nanocatalysts is ascribed to (i) their opened porosity, (ii) their preferential crystallographic orientation (“ensemble effect”), and (iii) the weakened oxygen binding energy induced by the contracted Pt lattice parameter (“strain effect”).
The catalytic performance of extended and nanometer-sized surfaces strongly depends on the amount and the nature of structural defects that they exhibit. However, whereas the effect of steps or ...adatoms may be unraveled with single crystals (“surface science approach”), implementing reproducibly in a controlled manner structural defects on nanomaterials remains hardly feasible. A case that deserves particular attention is that of bimetallic nanomaterials, which are used to catalyze the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). Point defects (vacancies), planar defects (dislocations and grain boundaries), and bulk defects (voids, pores) are likely to be generated in alloy or core@shell nanomaterials based on Pt and a transition metal due to the high lattice mismatch between the two elements. Here, we report the morphological and structural trajectories of hollow PtNi/C nanoparticles during thermal annealing under vacuum, N2, H2, or air atmosphere by in situ transmission electron microscopy and synchrotron X-ray diffraction. We evidence atmosphere-dependent restructuring kinetics, which enabled us to synthesize a set of catalysts with identical chemical compositions and elemental distributions but different morphologies, crystallite sizes, and lattice strain. By combining the results of Rietveld and pair-distribution function analyses and electrochemical measurements, we demonstrate that the structurally disordered areas located at the interface between individual crystallites are highly active for two reactions of interest for PEMFC devices: the electrochemical COads oxidation and the ORR. These results shed fundamental light on the effect of structural defects on the catalytic performance of bimetallic nanomaterials and should aid in the rational design of more efficient ORR electrocatalysts.
Fuel cell technology is on its verge of deployment as one of the solutions for decarbonization of transportation. It currently uses platinum-based catalysts, being the largest materials cost factor, ...subject to market volatility, limited availability, and unfavorable geopolitical source location. Hence, Earth-abundant elements-based materials, and among those, platinum-free catalysts, could be an ultimate solution. Among several such catalysts, the transition-metal nitrogen-carbon ones have shown adequate activity and promise in durability, the latest being the most vulnerable treat. Recent years have seen initial successes in incorporation of iron-nitrogen-carbon catalysts in fuel cells and their evaluation under automotive relevant conditions. The catalysts can be described as N-doped, graphene-like carbonaceous materials, with transition metal atomically dispersed and associated with the pyridinic nitrogen-containing in-plane or edge defects in graphene. Here, we provide a view on these materials' chemical composition and morphology that provide for the reactivity and stability of transition-metal-containing active sites.
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Our current energy production model strongly relies on fossil fuels, thus being flawed at different levels: (1) the fossil fuels’ transformation generates CO2, the main protagonist of global warming, and (2) their reserves are finite. Thus, shifting toward renewable energies has become a necessity. When focusing on electrification of mobility as one of the main de-carbonization strategies, proton exchange membrane fuel cells (PEMFCs) are an option for medium and heavy-duty vehicles and a suitable replacement in train and ship engines, thanks to their ability to generate power from H2 and O2. The oxygen reduction reaction is currently catalyzed by Pt-based nanomaterials thanks to their exceptional activity and adequate stability, provided by the intrinsic properties of platinum metal (i.e., “close-to-optimal” oxygen intermediate binding and water release). However, platinum is scarce, unevenly distributed, and expensive. This raises the need to investigate other materials, prone to replace it in the upcoming decades and ensure broader introduction of fuel cell technology. While metal-nitrogen-carbon appears as the most advanced of this new platinum group metal-free (PGM-free) electrocatalysts generation, they exhibit lower activity than the Pt-based materials and substantially suffer in durability. Closing this activity gap and providing avenues to reach a sensible stability is a critical challenge for PEMFCs improvement and commercialization.
Assessing the critical features of a complex, multi-site, electrocatalyst. This manuscript discusses (1) iron-nitrogen-carbon electrocatalyst various actives sites (i.e. Fe and/or N-containing moieties, Fe-based nanoparticles) and (2) the guidelines to follow to synthesize a highly active, highly durable, electrocatalyst (i.e. a preferential exposition of the carbon plane versus carbon edges, low micropore volume, low density of structural defects in the carbon basal plane, and an absence of Fe-based nanoparticles).
The kinetic isotopic effect (KIE) of oxygen reduction reaction (ORR) was studied via the investigation of both Koutecky−Levich and Tafel methods on atomically dispersed iron-containing, a.k.a. ...iron−nitrogen−carbon (Fe−N−C) electrocatalyst. This type of catalyst has been under intensive development for use as a platinum-group-metal-free cathode catalyst in polymer electrolyte membrane fuel cells. The KIE value derived from the Tafel method (the slopes of the semilogarithmic representation of the polarization data) is effectively 1, indicating that for this Fe–N–C electrocatalyst, the rate-determining step (RDS), i.e., first electron charge transfer, is independent of the proton/deuteron ratio (H+/D+). This finding suggests that the RDS of the Fe–N–C catalysts is not the main factor limiting its performance. Thus, through careful optimization of structure and morphology resulting in overcoming other limitations, Fe–N–C catalysts could, in principle, successfully compete with Pt-based ORR electrocatalysts. In contrast, the KIE value derived from the Koutecky–Levich method, based on the analysis of linear sweep voltammetry in the diffusion-limited region of polarization response at varied convective conditions (electrode rotating speeds), is approximately 2, thus implying that in mass-transport-controlled region of ORR, the mechanism and, hence, the RDS are H+-dependent. This behavior, combined with the understanding that Fe–N–C display multiple active sites, suggests a more complex and more limited mechanism of ORR for the sites involved in hydrogen peroxide production and further reduction, a.k.a. “parallel” or peroxide pathway. The catalysts exhibiting this hydrogen peroxide production (bifunctional or 2 × 2 e–) pathway will be intermittently inferior in ORR due to the concerted proton/electron charge-transfer process as RDS.
The influence of the texture, structure, and chemistry of different carbon supports on the morphological properties, oxygen reduction reaction (ORR) activity, and stability of porous hollow PtNi ...nanoparticles (NPs) was investigated. The carbon nanomaterials included carbon blacks, carbon nanotubes, graphene nanosheets, and carbon xerogel and featured different specific surface areas, degrees of graphitization, and extent of surface functionalization. The external and inner diameters of the supported porous hollow PtNi/C NPs were found to decrease with an increase in the carbon mesopore surface area. Despite these differences, similar morphological properties and electrocatalytic activities for the ORR were reported. The stability of the synthesized electrocatalysts was assessed by simulating electrochemical potential variations occurring at a proton exchange membrane fuel cell (PEMFC) cathode during startup/shutdown events. Identical location transmission electron microscopy (IL-TEM) and electrochemical methods revealed the occurrence of a carbon-specific degradation mechanism: carbon corrosion into CO2 and particle detachment were noticed on carbon xerogels and graphene nanosheets while, on carbon blacks, surface oxidation prevailed (C → COsurf) and did not result in modified electrical resistance of the catalytic layers, rendering these carbon supports better suited to prepare highly active and stable ORR electrocatalysts.
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