Recent advances in electrocatalysts toward the oxygen reduction reaction: the case of PtNi octahedra

Terrace-Rich Ultrathin PtCu Surface on Earth-Abundant Metal for Oxygen Reduction Reaction

The activity and stability of the platinum electrode toward the oxygen reduction reaction are size-dependent. Although small nanoparticles have high Pt utilization, the undercoordinated Pt sites on their surface are assumed to have too strong oxygen binding strength, thus often leading to compromised activity and surface instability. Herein, we report an extended nanostructured PtCu ultrathin surface to reduce the number of low-coordination sites without sacrificing the electrochemical active surface area (ECSA). The surface shows (111)-oriented characteristics, as proven by electrochemical probe reactions and spectroscopies. The PtCu surface brings over an order of magnitude increase in specific activity relative to commercial Pt/C and nearly 4-fold enhancement in ECSA compared to traditional thin films. Moreover, due to the weak absorption of air impurities (e.g., SO2, NO, CO) on highly coordinated sites, the catalyst displays enhanced contaminant tolerance compared with nanoparticulate Pt/C. This work promises a broad screening of extended nanostructured surface catalysts for electrochemical conversions.

Theory of Anisotropic Metal Nanostructures

A significant challenge in the development of functional materials is understanding the growth and transformations of anisotropic colloidal metal nanocrystals. Theory and simulations can aid in the development and understanding of anisotropic nanocrystal syntheses. The focus of this review is on how results from first-principles calculations and classical techniques, such as Monte Carlo and molecular dynamics simulations, have been integrated into multiscale theoretical predictions useful in understanding shape-selective nanocrystal syntheses. Also, examples are discussed in which machine learning has been useful in this field. There are many areas at the frontier in condensed matter theory and simulation that are or could be beneficial in this area and these prospects for future progress are discussed.

Controlled Synthesis of Carbon-Supported Pt-Based Electrocatalysts for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells are playing an increasing role in postpandemic economic recovery and climate action plans. However, their performance, cost, and durability are significantly related to Pt-based electrocatalysts, hampering their large-scale commercial application. Hence, considerable efforts have been devoted to improving the activity and durability of Pt-based electrocatalysts by controlled synthesis in recent years as an effective method for decreasing Pt use, and consequently, the cost. Therefore, this review article focuses on the synthesis processes of carbon-supported Pt-based electrocatalysts, which significantly affect the nanoparticle size, shape, and dispersion on supports and thus the activity and durability of the prepared electrocatalysts. The reviewed processes include (i) the functionalization of a commercial carbon support for enhanced catalyst–support interaction and additional catalytic effects, (ii) the methods for loading Pt-based electrocatalysts onto a carbon support that impact the manufacturing costs of electrocatalysts, (iii) the preparation of spherical and nonspherical Pt-based electrocatalysts (polyhedrons, nanocages, nanoframes, one- and two-dimensional nanostructures), and (iv) the postsynthesis treatments of supported electrocatalysts. The influences of the supports, key experimental parameters, and postsynthesis treatments on Pt-based electrocatalysts are scrutinized in detail. Future research directions are outlined, including (i) the full exploitation of the potential functionalization of commercial carbon supports, (ii) scaled-up one-pot synthesis of carbon-supported Pt-based electrocatalysts, and (iii) simplification of postsynthesis treatments. One-pot synthesis in aqueous instead of organic reaction systems and the minimal use of organic ligands are preferred to simplify the synthesis and postsynthesis treatment processes and to promote the mass production of commercial carbon-supported Pt-based electrocatalysts.

Graphical Abstract

This review focuses on the synthesis process of Pt-based electrocatalysts/C to develop aqueous one-pot synthesis at large-scale production for PEMFC stack application.

Deposition of Pt Nanoparticles by Ascorbic Acid on Composite Electrospun Polyacrylonitrile-Based Carbon Nanofiber for HT-PEM Fuel Cell Cathodes

The efficient use of renewable energy sources requires development of new electrocatalytic materials for electrochemical energy storage systems, particularly fuel cells. To increase durability of high temperature polymer electrolyte fuel cell (HT-PEMFC), Pt/carbon black based catalysts should be replaced by more durable ones, for example Pt/carbon nanofibers (CNF). Here, we report for the first time the quantitative ascorbic acid assisted deposition of Pt onto electrospun polyacrylonitrile-based CNF composite materials. The effect of their subsequent post-treatment at various temperatures (250 and 500 °C) and media (vacuum or argon-hydrogen mixture) on the Pt/C catalyst morphology is investigated. All obtained samples are thoroughly studied by high resolution electron microscopy, and Pt electrochemically active specific surface area was evaluated by cyclic voltammetry.

One-pot production of a sea urchin-like alloy electrocatalyst for the oxygen electro-reduction reaction

Designing a cost-effective catalyst with high performance towards the oxygen electro-oxidation reaction (ORR) is of great interest for the development of green energy storage and conversion technologies. We report herein a facile self-assembly strategy in a mild reducing environment to realize an urchin-like NiPt bimetallic alloy with the domination of the (111) facets as an efficient ORR electrocatalyst. In the rotating-disk electrode test, the as-obtained NiPt nanourchins (NUCs)/C catalyst demonstrates an increase in both onset potential (0.96 V_RHE) and half-wave potential (0.92 V_RHE) and a direct four-electron ORR pathway with enhanced reaction kinetics. Additionally, the as-made NiPt NUCs/C electrocatalyst also shows impressive ORR catalytic stability compared to a commercial Pt NPs/C catalyst after an accelerated durability test with 15.29% degradation in mass activity, which is 3.04-times lower than 46.48% of the Pt NPs/C catalyst. The great ORR performance of the as-made catalyst is due to its unique urchin-like morphology with the dominant (111) facets and the synergistic and electronic effects of alloying Ni and Pt. This study not only provides a robust ORR electrocatalyst, but also opens a facile but effective route for fabricating 3D Pt-based binary and ternary alloy catalysts.

High Oxygen Reduction Activity of Pt-Ni Alloy Catalyst for Proton Exchange Membrane Fuel Cells

In order to fill the research gap of high metal loading of high performance PtNi alloy catalysts, a PtNi/C alloy nano-catalyst with metal loading more than 50 wt.% and core-shell like structure was prepared by ethylene glycol reduction, high temperature annealing, and acid pickling. The electrochemical test results showed that the prepared PtNi alloy catalyst had excellent electrochemical activity: the electrochemical surface area (ECSA) was 63.8 m2·gPt−1, and the mass activity (MA) was 0.574 A·mgPt−1, which is 2.73 times greater than those of the Pt/C JM (Johnson Matthey) catalyst. The durability of the PtNi/C catalyst was further investigated. After 30 K cycles of accelerated durability test, the ECSA and MA of the PtNi/C alloy catalyst decreased by 10.2% and 31.2%, respectively. The PtNi/C alloy catalyst prepared in this study has excellent catalytic activity and overcomes the problem of insufficient durability of traditional alloy catalysts and has the potential for large-scale commercial application.

Surface unsaturated WOx activating PtNi alloy nanowires for oxygen reduction reaction

PtNi alloy nanoparticles display promising catalytic activity for oxygen reduction reaction (ORR), while the Ostwald ripening of particles and the dissolution/migration of surface atoms greatly affect its stability thus restricting the application. Herein, the WO_x-surface modified PtNi alloy nanowires (WO_x-PtNi NWs) exhibiting enhanced ORR catalytic property is reported, which has high aspect ratio with the diameter of only 2 ∼ 3 nm. It is found that the WO_x-PtNi NWs shows a volcano relationship between the ORR activity and the content of WO_x. The WO_x-(0.25)-PtNi NWs has the best performance among all the synthesized catalysts. Its mass activity (0.85 A mg^-1_Pt) is reduced by only 23.89% after 30k cycles durability test, which is much more stable than that of PtNi NWs (0.33 A mg^-1_Pt, 45.94%) and Pt/C (0.14 A mg^-1_Pt, 57.79%). Hence this work achieves an effective regulation of the ORR activity for PtNi alloy NWs by the synergistic effect of WO_x on Pt.

Highly dispersed Pt nanoclusters supported on zeolite-templated carbon for the oxygen reduction reaction

The formation of highly dispersed Pt nanoclusters supported on zeolite-templated carbon (PtNC/ZTC) by a facile electrochemical method as an electrocatalyst for the oxygen reduction reaction (ORR) is reported. The uniform micropores of ZTC serve as nanocages to stabilize the PtNCs with a sharp size distribution of 0.8–1.5 nm. The resultant PtNC/ZTC exhibits excellent catalytic activity for the ORR due to the small size of the Pt clusters and high accessibility of the active sites through the abundant micropores in ZTC.

Electrochemically synthesized highly dispersed Pt nanoclusters (PtNCs) stabilized by the nanocages of zeolite-templated carbon (ZTC) exhibit excellent electrocatalytic performance toward the oxygen reduction reaction.

Catalytic Nanoframes and Beyond

The ever-increasing need for the production and expenditure of sustainable energy is a result of the astonishing rate of consumption of fossil fuels and the accompanying environmental problems. Emphasis is being directed to the generation of sustainable energy by the fuel cell and water splitting technologies. Accordingly, the development of highly efficient electrocatalysts has attracted significant interest, as the fuel cell and water splitting technologies are critically dependent on their performance. Among numerous catalyst designs under investigation, nanoframe catalysts have an intrinsically large surface area per volume and a tunable composition, which impacts the number of catalytically active sites and their intrinsic catalytic activity, respectively. Nevertheless, the structural integrity of the nanoframe during electrochemical operation is an ongoing concern. Some significant advances in the field of nanoframe catalysts have been recently accomplished, specifically geared to resolving the catalytic stability concerns and significantly boosting the intrinsic catalytic activity of the active sites. Herein, general synthetic concepts of nanoframe structures and their structure-dependent catalytic performance are summarized, along with recent notable advances in this field. A discussion on the remaining challenges and future directions, addressing the limitations of nanoframe catalysts, are also provided.

A facile approach to high-performance trifunctional electrocatalysts by substrate-enhanced electroless deposition of Pt/NiO/Ni on carbon nanotubes

Trifunctional electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are crucial for many electrochemical devices. Herein, novel trifunctional electrocatalysts of a hierarchically-structured Pt/NiO/Ni/CNT with ultrasmall Pt nanoparticles (∼2 nm) were synthesized via substrate-enhanced electroless deposition. The resulting catalysts exhibited a higher ORR activity (half-wave potential at 0.942 V) than that of the benchmark commercial Pt/C (20 wt%) and a similar OER activity (overpotential of 350 mV at 10 mA cm-2) to that of RuO2 in alkaline media. Moreover, the newly-developed Pt/NiO/Ni/CNT catalysts also showed a high mass activity superior to Pt/C towards the HER in both acid and alkaline electrolytes. The Pt/NiO/Ni/CNT catalysts, when used for overall water splitting, self-powered by two Zn-air batteries with the Pt/NiO/Ni/CNT air electrodes connected into series, displayed superb performance with 1.61 V at 10 mA cm-2. This work represents a breakthrough in the development of facile approaches to trifunctional catalysts from low-cost Earth-abundant materials for efficient energy conversion and storage.

New Carbon Nanofiber Composite Materials Containing Lanthanides and Transition Metals Based on Electrospun Polyacrylonitrile for High Temperature Polymer Electrolyte Membrane Fuel Cell Cathodes

Electrospinning of polyacrylonitrile/DMF dopes containing salts of nickel, cobalt, zirconium, cerium, gadolinium, and samarium, makes it possible to obtain precursor nanofiber mats which can be subsequently converted into carbon nanofiber (CNF) composites by pyrolysis at 1000-1200 °C. Inorganic additives were found to be uniformly distributed in CNFs. Metal states were investigated by transmission electron microscopy and X-ray photoelectron spectroscopy (XPS). According to XPS in CNF/Zr/Ni/Gd composites pyrolyzed at 1000 °C, nickel exists as Ni0 and as Ni2+, gadolinium as Gd3+, and zirconium as Zr4+. If CNF/Zr/Ni/Gd is pyrolyzed at 1200 °C, nickel exists only as Ni0. For CNF/Sm/Co composite, samarium is in Sm3+ form when cobalt is not found on a surface. For CNF/Zr/Ni/Ce composite, cerium exists both as Ce4+ and as Ce3+. Composite CNF mats were platinized and tested as cathodes in high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC). Such approach allows to introduce Pt-M and Pt-MOx into CNF, which are more durable compared to carbon black under HT-PEMFC operation. For CNF/Zr/Ni/Gd composite cathode, higher performance in the HT-PEMFC at I >1.2 A cm-2 is achieved due to elimination of mass transfer losses in gas-diffusion electrode compared to commercial Celtec®P1000.

Bimetallic PtAu electrocatalysts for the oxygen reduction reaction: challenges and opportunities

Highly active, durable oxygen reduction reaction (ORR) electrocatalysts have an essential role in promoting the continuous operation of advanced energy technologies such as fuel cells and metal-air batteries. Considering the scarce reserve of Pt and its unsatisfactory overall performance, there is an urgent demand for the development of new generation ORR electrocatalysts that are substantially better than the state-of-the-art supported Pt-based nanocatalysts, such as Pt/C. Among various nanostructures, bimetallic PtAu represents one unique alloy system where highly contradictory performance has been reported. While it is generally accepted that Au may contribute to stabilizing Pt, its role in modulating the intrinsic activity of Pt remains unclear. This perspective will discuss critical structural issues that affect the intrinsic ORR activities of bimetallic PtAu, with an eye on elucidating the origin of seemingly inconsistent experimental results from the literature. As a relatively new class of electrodes, we will also highlight the performance of dealloyed nanoporous gold (NPG) based electrocatalysts, which allow a unique combination of structural properties highly desired for this important reaction. Finally, we will put forward the challenges and opportunities for the incorporation of these advanced electrocatalysts into membrane electrode assemblies (MEA) for actual fuel cells.

Recent advances in pristine tri-metallic metal-organic frameworks toward the oxygen evolution reaction

Pristine metal-organic frameworks (MOFs) have received much attention in recent years due to their high specific surface areas, large porosity, excellent pore size distributions, flexible structure, and remarkable catalytic properties. The design of functional MOFs that can function as efficient HER and OER catalysts is significant in solving the energy crisis but remains a big challenge. Tri-metallic metal-organic frameworks show a good application prospect in water oxidation. In this review, we are going to focus on the latest progress and future trends in the development of pristine trimetallic MOFs with respect to the OER. The synergistic effect between multi-metal active sites is effective at improving the intrinsic activity of MOFs toward the OER. By summarizing the synthesis method of tri-metallic MOFs and observing their performance toward the oxygen evolution reaction, we hope that this review will trigger new developments in coordination chemistry, electrochemistry, nanomaterials and energy materials.

Integration mesoporous surface and hollow cavity into PtPdRh nano-octahedra for enhanced oxygen reduction electrocatalysis

Design and synthesis of Pt-based nanocrystals with controlled structural diversity and complexity can potentially bring about multifunctional properties. In this work, we present a facile two-step strategy for the construction of the PtPdRh mesoporous octahedral nanocages (PtPdRh MONCs). This unique nanoarchitectonics rationally integrates multiple advantages (i.e. the octahedral shape, hollow cavity and mesoporous surface) into one catalyst, which facilitates the efficient utilization of noble metal atoms at both of the interior and exterior surfaces. As expected, the resultant PtPdRh MONCs could effectively catalyze the oxygen reduction reaction (ORR) under acidic conditions. The demonstrated ORR activity and catalytic durability are superior to the commercial Pt/C catalyst. The present study would provide a general guidance for architectural and compositional engineering of noble metal nanocrystals with desired functionalities and properties.

Reactive nanotemplates for synthesis of highly efficient electrocatalysts: beyond simple morphology transfer

Efficient electrocatalysts for energy conversion in general, and fuel cell operation and water electrolysis in particular, are pivotal for carbon-free hydrogen production. While the requirements of successful electrocatalysts include a high number density of catalytically active sites, high surface-to-volume ratio, inherently high catalytic activity, and robustness of the catalyst surface structure under harsh operating conditions, it is extremely difficult to synthesize nanocatalysts that could possess all these structural characteristics. Nanotemplate-mediated synthesis, namely, the coating or filling of a template with a desired material phase followed by the removal of the template, has captured the interest of researchers because of the ease of creating hollow-structured nanocatalysts with a high surface to volume ratio. Recent studies, however, have revealed that nanotemplates could be more than just passive supports because they greatly affect catalytic performance by creating an unusual synergy between the substrate and catalyst and by providing dopants to the actual catalyst phase owing to their reactive nature. In this review, we discuss the most notable recent advances in the nanotemplate-based synthesis of electrocatalysts as well as the unusual effects of nanotemplates on the performance of nanocatalysts. We also provide an outlook for this fledgling field so that future research efforts could be focused on the development of practically useful electrocatalysts that could shape the future of energy technologies.

Dissolution of Platinum Single Crystals in Acidic Medium

Platinum single crystal basal planes consisting of Pt(111), Pt(100), Pt(110) and reference polycrystalline platinum Pt(poly) were subjected to various potentiodynamic and potentiostatic electrochemical treatments in 0.1 M HClO4 . Using the scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC-ICP-MS) the transient dissolution was detected on-line. Clear trends in dissolution onset potentials and quantities emerged which can be related to the differences in the crystal plane surface structure energies and coordination. Pt(111) is observed to have a higher dissolution onset potential while the generalized trend in dissolution rates and quantities was found to be Pt(110)>P(100)≈Pt(poly)>Pt(111).