Pt@Metal-Organic Framework (ZIF-8) Thin Films Obtained at a Liquid/Liquid Interface as Anode Electrocatalysts for Methanol Fuel Cells: Different Approaches in the Synthesis

Smart membranes, nanodevices, chemical sensors, and catalytic coatings are some of the applications that make the metal-organic framework (MOF) thin films very important. Encapsulation of nanoparticles in the porous structure of MOFs can lead to the formation of effective catalysts with new unique properties and wide range of applications that may not be obtained by MOFs individually. Three main strategies, ship-in-a-bottle, bottle-around-the-ship, and in situ synthesis including the simultaneous formation of the two components, were applied for the synthesis of Pt(0)@zeolitic imidazolate framework-8 (ZIF-8) thin films at the toluene/water interface. The effects of platinum precursor transfer directions toward the interface on the properties of the films were investigated by using the [PtCl₂(cod)] (where cod = cis,cis-1,5-cyclooctadiene) complex soluble in toluene as the upper phase and K₂PtCl₄ soluble in water as the lower phase. The six obtained films with different morphologies were applied as electrocatalysts for the methanol oxidation reaction. Considerable current density, mass activity, catalyst stability, activation energy, exchange current density, maximum power, and long-term poisoning rate are some of the advantages of the Pt(0)@ZIF-8 catalysts synthesized using the in situ strategy and K₂PtCl₄ as the platinum precursor. Furthermore, we report the formation of Pt@ZIF-8 nanorods at the interfaces without using any stabilizer or template. Our results suggest that the in situ strategy at the liquid/liquid interface is one of the best procedures for the synthesis of Pt(0)@ZIF-8 thin films as a suitable anode electrocatalyst for methanol fuel cells.

Colloidal synthesis of nanoparticles: from bimetallic to high entropy alloys

At the nanoscale, the synthesis of a random alloy (i.e. without phase segregation, whatever the composition) by chemical synthesis remains a difficult task, even for simple binary type systems. In this context, a unique approach based on the colloidal route is proposed enabling the synthesis of face-centred cubic and monodisperse bimetallic, trimetallic, tetrametallic and pentametallic nanoparticles with diameters around 5 nm as solid solutions. The Fe-Co-Ni-Pt-Ru alloy (and its subsets) is considered a challenging task as each element has fairly different physico-chemical properties. Particles are prepared by temperature-assisted co-reduction of metal acetylacetonate precursors in the presence of surfactants. It is highlighted how the correlation between precursors' degradation temperatures and reduction potential values of the metal cations is the driving force to achieve a homogeneous distribution of all elements within the nanoparticles.

Ordered PtFeIr Intermetallic Nanowires Prepared through a Silica-Protection Strategy for the Oxygen Reduction Reaction

Developing efficient and stable Pt-based oxygen reduction reaction (ORR) catalysts is a way to promote the large-scale application of fuel cells. Pt-based alloy nanowires are promising ORR catalysts, but their application is hampered by activity loss caused by structural destruction during long-term cycling. Herein, the preparation of ordered PtFeIr intermetallic nanowire catalysts with an average diameter of 2.6 nm and face-centered tetragonal structure (fct-PtFeIr/C) is reported. A silica-protected strategy prevents the deformation of PtFeIr nanowires during the phase transition at high temperature. The as-prepared fct-PtFeIr/C exhibited superior mass activity for ORR (2.03 A mg_Pt ^-1 ) than disordered PtFeIr nanowires with face-centered cubic structure (1.11 A mg_Pt ^-1 ) and commercial Pt/C (0.21 A mg_Pt ^-1 ). Importantly, the structure and electrochemical performance of fct-PtFeIr/C were maintained after stability tests, showing the advantages of the ordered structure.

Platinum-Tin/Tin Oxide/CNT Catalysts for High-Performance Electrocatalytic Ethanol Oxidation

Ethanol is a promising liquid clean energy source in the energy conversion field. However, the self-poisoning caused by the strongly adsorbed reaction intermediates (typically, CO) is a critical problem in ethanol oxidation reaction. To address this issue, we proposed a joint use of two strategies, alloying of Pt with other metals and building Pt/metal-oxide interfaces, to achieve high-performance electrocatalytic ethanol oxidation. For this, a well-designed synthetic route combining wet impregnation with a two-step thermal treatment process was established to construct PtSn/SnO_x interfaces on carbon nanotubes. Using this route, the alloying of Pt-Sn and formation of PtSn-SnO_x interfaces can simultaneously be achieved, and the coverage of SnO_x thin films on PtSn alloy nanoparticles can be facilely tuned by the strong interaction between Pt and SnO_x . The results revealed that the partial coverage of SnO_x species not only retained the active sites, but also enhanced the CO anti-poisoning ability of the catalyst. Consequently, the H-PtSn/SnO_x /CNT-2 catalyst with an optimized PtSn-SnO_x interface showed significantly improved performances toward the ethanol oxidation reaction (825 mA mg_Pt ^-1 ). This study provides deep insights into the structure-performance relationship of PtSn/metal oxide composite catalysts, which would be helpful for the future design and fabrication of high-performance Pt-based ethanol oxidation reaction catalysts.

Mastering the surface strain of platinum catalysts for efficient electrocatalysis

Platinum (Pt) has found wide use as an electrocatalyst for sustainable energy conversion systems^1-3. The activity of Pt is controlled by its electronic structure (typically, the d-band centre), which depends sensitively on lattice strain^4,5. This dependence can be exploited for catalyst design^4,6-8, and the use of core-shell structures and elastic substrates has resulted in strain-engineered Pt catalysts with drastically improved electrocatalytic performances^7,9-13. However, it is challenging to map in detail the strain-activity correlations in Pt-catalysed conversions, which can involve a number of distinct processes, and to identify the optimal strain modification for specific reactions. Here we show that when ultrathin Pt shells are deposited on palladium-based nanocubes, expansion and shrinkage of the nanocubes through phosphorization and dephosphorization induces strain in the Pt(100) lattice that can be adjusted from -5.1 per cent to 5.9 per cent. We use this strain control to tune the electrocatalytic activity of the Pt shells over a wide range, finding that the strain-activity correlation for the methanol oxidation reaction and hydrogen evolution reaction follows an M-shaped curve and a volcano-shaped curve, respectively. We anticipate that our approach can be used to screen out lattice strain that will optimize the performance of Pt catalysts-and potentially other metal catalysts-for a wide range of reactions.

Pt3Sn nanoparticles enriched with SnO2/Pt3Sn interfaces for highly efficient alcohol electrooxidation

Pt₃Sn nanoparticles (NPs) enriched with Pt₃Sn/ultra-small SnO₂ interfaces (Pt₃Sn@u-SnO₂/NG) were synthesized through a thermal treatment of Pt₂Sn/NG in a H₂ atmosphere, followed by annealing under H₂ and air conditions. The unique structure of Pt₃Sn NPs enriched with Pt₃Sn/SnO₂ interfaces was observed on the Pt₃Sn@u-SnO₂/NG catalyst based on HRTEM. The optimized Pt₃Sn@u-SnO₂/NG catalyst achieves high catalytic activity with an ethanol oxidation reaction (EOR) activity of 366 mA mg_Pt ^-1 and a methanol oxidation reaction (MOR) activity of 503 mA mg_Pt ^-1 at the potential of 0.7 V, which are eight-fold and five-fold higher than those for the commercial Pt/C catalyst (44 and 99 mA mg_Pt ^-1, respectively). The Pt₃Sn@u-SnO₂/NG catalyst is found to be 3 times more stable and have higher CO tolerance than Pt/C. The outstanding performance of the Pt₃Sn@u-SnO₂/NG catalyst should be ascribed to the synergetic effect induced by the unique structure of Pt₃Sn NPs enriched with Pt₃Sn/SnO₂ interfaces. The synergetic effect between Pt₃Sn NPs and ultra-small SnO₂ increases the performance for alcohol oxidation because the Sn in both Pt₃Sn and SnO₂ favors the removal of CO_ads on the nearby Pt by providing OH_ads species at low potentials. The present work suggests that the Pt₃Sn@u-SnO₂ is indeed a unique kind of efficient electrocatalyst for alcohol electrooxidation.

A Robust PtNi Nanoframe/N-Doped Graphene Aerogel Electrocatalyst with Both High Activity and Stability

Insufficient catalytic activity and stability and high cost are the barriers for Pt-based electrocatalysts in wide practical applications. Herein, a hierarchically porous PtNi nanoframe/N-doped graphene aerogel (PtNiNF-NGA) electrocatalyst with outstanding performance toward methanol oxidation reaction (MOR) in acid electrolyte has been developed via facile tert-butanol-assisted structure reconfiguration. The ensemble of high-alloying-degree-modulated electronic structure and correspondingly the optimum MOR reaction pathway, the structure superiorities of hierarchical porosity, thin edges, Pt-rich corners, and the anchoring effect of the NGA, endow the PtNiNF-NGA with both prominent electrocatalytic activity and stability. The mass and specific activity (1647 mA mg_Pt ^-1 , 3.8 mA cm^-2 ) of the PtNiNF-NGA are 5.8 and 7.8 times higher than those of commercial Pt/C. It exhibits exceptional stability under a 5-hour chronoamperometry test and 2200-cycle cyclic voltammetry scanning.

One-pot synthesis of three-dimensional Pt nanodendrites with enhanced methanol oxidation reaction and oxygen reduction reaction activities

The high cost of Pt-based catalysts impedes the practical large-scale commercial application of fuel cells. Controlling the morphology of Pt nanostructure is one of the most promising approaches to promote the electrocatalytical activity and stability toward methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). Herein, we report a facile method for the synthesis of Pt nanodendrites consisting a dense array of dendrites using K₂PtCl₄ as metal resource, L-ascorbic acid as reducing agent and hexadecyltrimethylammonium chloride as capping agent in aqueous solution at 96 °C for 30 min. Owing to the novel structure, the obtained Pt nanodendritic electrocatalyst exhibites superior MOR catalytic performance and particularly long-term stability both in acid and alkaline conditions. the Pt nanodendrites also exhibited a higher half wave potential of 0.86 V and lower tafel slope of 51.69 mV dec^-1 when compared with 20% Pt/C (0.83 V, 93.55 mV dec^-1) in acid conditions toward ORR. This work provides a promising way for the rational design of efficient and robust catalysts for sustainable energy conversion.

High-efficiency methanol oxidation electrocatalysts realized by ultrathin PtRuM-O (M = Ni, Fe, Co) nanosheets

A general method for controlling the synthesis of a class of ultrathin PtRuM-O (M = Ni, Fe, Co) NSs is reported for the first time. By optimizing the metal ratio, the Pt₇RuNi₂-O NS catalyst is found to have the highest electrocatalytic activity (mass activity, 3.57 A mg_Pt^-1) for the MOR among PtRuM-O NSs and PtRu-O NSs, which is 10.5 times higher than that of commercial Pt/C (0.34 A mg_Pt^-1). And the Pt₇RuNi₂-O NSs also have better stability and CO anti-poisoning properties in the prepared materials. In addition, the ultrathin Pt₇RuNi₂-O NS catalyst also shows the highest performance among reported Pt-based catalysts for the MOR in acidic medium.

Unusual Effect of Trace Water on the Structure and Activity of Nix Co1-x Electrocatalysts for the Methanol Oxidation Reaction

Highly active Ni-based catalysts have attracted much attention but are still facing challenges owing to the immature synthetic method. Herein, polyhedral Ni_x Co_1-x alloy was prepared by a facile modified polyol method in which a trace amount of water could halve the particle size of the alloy. The Ni/Co ratios in Ni_x Co_1-x alloy strictly depended on the used amount of water owing to the different solubilities of the precursors. Among them, the Ni_0.6 Co_0.4 nanoparticles obtained with 70 μL of deionized water exhibited the best performance in the methanol oxidation reaction with a peak current density of 116 mA cm^-2 in the presence of 1 m NaOH+0.5 m CH₃ OH solution, which is higher than those of Ni_0.7 Co_0.3 (80 mA cm^-2 ) and Ni_0.5 Co_0.5 (33 mA cm^-2 ). The excellent performance of Ni_0.6 Co_0.4 is attributed to the unique structure with appropriate Ni/Co ratio, which elongates the C-O bond in methanol and lowers the reaction free energy according to DFT calculations.

Bifunctional PtCu electrocatalysts for the N2 reduction reaction under ambient conditions and methanol oxidation

PtCu nanoalloys were employed as bifunctional electrocatalysts in both the N₂ reduction and methanol oxidation, in which the electrocatalytic activity and stability is composition dependent and highly improved compared to their counterpart.