Surface-modulated palladium-nickel icosahedra as high-performance non-platinum oxygen reduction electrocatalysts

Recent Advances of Electrocatalysts and Electrodes for Direct Formic Acid Fuel Cells: from Nano to Meter Scale Challenges

Direct formic acid fuel cells are promising energy devices with advantages of low working temperature and high safety in fuel storage and transport. They have been expected to be a future power source for portable electronic devices. The technology has been developed rapidly to overcome the high cost and low power performance that hinder its practical application, which mainly originated from the slow reaction kinetics of the formic acid oxidation and complex mass transfer within the fuel cell electrodes. Here, we provide a comprehensive review of the progress around this technology, in particular for addressing multiscale challenges from catalytic mechanism understanding at the atomic scale, to catalyst design at the nanoscale, electrode structure at the micro scale and design at the millimeter scale, and finally to device fabrication at the meter scale. The gap between the highly active electrocatalysts and the poor electrode performance in practical devices is highlighted. Finally, perspectives and opportunities are proposed to potentially bridge this gap for further development of this technology.

Decorating Pd-Au Nanodots Around Porous In2O3 Nanocubes for Tolerant H2 Sensing Against Switching Response and H2S Poisoning

With the recently-booming hydrogen (H2) economy by green H2 as the energy carriers and the newly-emerged exhaled diagnosis by human organ-metabolized H2 as a biomarker, H2 sensing is simultaneously required with fast response, low detection limit, and tolerant stability against humidity, switching, and poisoning. Here, reliable H2 sensing has been developed by utilizing indium oxide nanocubes decorated with palladium and gold nanodots (Pd-Au NDs/In2O3 NCBs), which have been synthesized by combined hydrothermal reaction, annealing, and chemical bath deposition. As-prepared Pd-Au NDs/In2O3 NCBs are observed with surface-enriched NDs and nanopores. Beneficially, Pd-Au NDs/In2O3 NCBs show 300 ppb-low detection limit, 5 s-fast response to 500 ppm H2, 75%RH-high humidity tolerance, and 56 days-long stability at 280 °C. Further, Pd-Au NDs/In2O3 NCBs show excellent stability against switching sensing response, and are tolerant to H2S poisoning even being exposed to 10 ppm H2S at 280 °C. Such excellent H2 sensing may be attributed to the synergistic effect of the boosted Pd-Au NDs' spillover effect and interfacial electron transfer, increased adsorption sites over the porous NCBs' surface, and utilized Pd NDs' affinity with H2 and H2S. Practically, Pd-Au NDs/In2O3 NCBs are integrated into the H2 sensing device, which can reliably communicate with a smartphone.

Phosphorus-Doping Enables the Superior Durability of a Palladium Electrocatalyst towards Alkaline Oxygen Reduction Reactions

The sluggish kinetics of oxygen reduction reactions (ORRs) require considerable Pd in the cathode, hindering the widespread of alkaline fuel cells (AFCs). By alloying Pd with transition metals, the oxygen reduction reaction's catalytic properties can be substantially enhanced. Nevertheless, the utilization of Pd-transition metal alloys in fuel cells is significantly constrained by their inadequate long-term durability due to the propensity of transition metals to leach. In this study, a nonmetallic doping strategy was devised and implemented to produce a Pd catalyst doped with P that exhibited exceptional durability towards ORRs. Pd3P0.95 with an average size of 6.41 nm was synthesized by the heat-treatment phosphorization of Pd nanoparticles followed by acid etching. After P-doping, the size of the Pd nanoparticles increased from 5.37 nm to 6.41 nm, and the initial mass activity (MA) of Pd3P0.95/NC reached 0.175 A mgPd-1 at 0.9 V, slightly lower than that of Pd/C. However, after 40,000 cycles of accelerated durability testing, instead of decreasing, the MA of Pd3P0.95/NC increased by 6.3% while the MA loss of Pd/C was 38.3%. The durability was primarily ascribed to the electronic structure effect and the aggregation resistance of the Pd nanoparticles. This research also establishes a foundation for the development of Pd-based ORR catalysts and offers a direction for the future advancement of catalysts designed for practical applications in AFCs.

Multidimensional Electrochemistry Decodes the Operando Mechanism of Hydrogen Oxidation

Being an efficient approach to the utilization of hydrogen energy, the hydrogen oxidation reaction (HOR) is of particular significance in the current carbon-neutrality time. Yet the mechanistic picture of the HOR is still blurred, mostly because the elemental steps of this reaction are rapid and highly entangled, especially when deviating from the thermodynamic equilibrium state. Here we report a strategy for decoding the HOR mechanism under operando conditions. In addition to the wide-potential-range I-V curves obtained using gas diffusion electrodes, we have applied the AC impedance spectroscopy to provide independent and complementary kinetic information. Combining multidimensional data sources has enabled us to fit, in mathematical rigor, the core kinetic parameter set in a 5-D data space. The reaction rate of the three elemental steps (Tafel, Heyrovsky, and Volmer reactions), as a function of the overpotential, can thus be distilled individually. Such an undocumented kinetic picture unravels, in detail, how the HOR is controlled by the elemental steps on polarization. For instance, at low polarization region, the Heyrovsky reaction is relatively slow and can be ignored; but at high polarization region, the Heyrovsky reaction will surpass the Tafel reaction. Additionally, the Volmer reaction has been the fastest within overpotentials of interest. Our findings not only offer a better understanding of the HOR mechanism, but also lay the foundation for the development of improved hydrogen energy utilization systems.

Development of Magnetizable, Nickel-Ferrite-Decorated Carbon Nanocomposites as Hydrogenation Catalyst for Aniline Synthesis

Catalysts with magnetic properties can be easily recovered from the reaction medium without loss by using a magnetic field, which highly improves their applicability. To design such systems, we have successfully combined the magnetic properties of nickel ferrite nanoparticles with the positive properties of carbon-based catalyst supports. Amine-functionalized NiFe2O4 nanoparticles were deposited on the surfaces of nitrogen-doped bamboo-like carbon nanotubes (N-BCNT) and carbon nanolayers (CNL) by using a coprecipitation process. The magnetizable catalyst supports were decorated by Pd nanoparticles, and their catalytic activity was tested through the hydrogenation of nitrobenzene (NB). By using the prepared catalysts, high nitrobenzene conversion (100% for 120 min at 333 K) and a high aniline yield (99%) were achieved. The Pd/NiFe2O4-CNL catalyst was remarkable in terms of stability during the reuse tests due to the strong interaction formed between the catalytically active metal and its support (the activity was retained during four cycles of 120 min at 333 K). Furthermore, despite the long-lasting mechanical stress, no significant palladium loss (only 0.08 wt%) was detected.

Two-dimensional template-directed synthesis of one-dimensional kink-rich Pd3Pb nanowires for efficient oxygen reduction

Developing facile synthetic strategies toward ultrafine one-dimensional (1D) nanowires (NWs) with rich catalytic hot spots is pivotal for exploring effective heterogeneous catalysts. Herein, we demonstrate a two-dimensional (2D) template-directed strategy for synthesizing 1D kink-rich Pd₃Pb NWs with abundant grain boundaries to serve as high-efficiency electrocatalysts toward oxygen reduction reaction (ORR). In this one-pot synthesis, ultrathin Pd nanosheets were initially generated, which then served as self-sacrificial 2D nano-templates. A dynamic equilibrium growth was subsequently established on the 2D Pd nanosheets through the center-selected etching of Pd atoms and edge-preferred co-deposition of Pd/Pb atoms. This was followed by the oriented attachment of the generated Pd/Pb alloy nanograins and fragments. Thus, kink-rich Pd₃Pb NWs with rich grain boundary defects were obtained in high yield, and these NWs were used as electrocatalytic active catalysts. The surface electronic interaction between Pd and Pb atoms effectively decreased the surface d-band center to weaken the binding of oxygen-containing intermediates toward improved ORR kinetics. Specifically, the kink-rich Pd₃Pb NWs/C catalyst delivered outstanding ORR mass activity and specific activity (2.26 A⋅mg_Pd^-1 and 2.59 mA⋅cm^-2, respectively) in an alkaline solution. These values were respectively 13.3 and 10.8 times those of state-of-the-art commercial Pt/C catalyst. This study provides an innovative strategy for fabricating defect-rich low-dimensional nanocatalysts for efficient energy conversion catalysis.

A Systematic Theoretical Study on Electronic Interaction in Cu-based Single-Atom Alloys

A meticulous understanding of the electronic structure of catalysts may provide new insight into catalytic performances. Here, we present a d-d interaction model to systematically study the electronic interaction in Cu-based single-atom alloys. We refine three types of electronic interactions according to the position of the antibonding state relative to the Fermi level. Moreover, we also find a special phenomenon in Mn-doped single-atom alloys in which no obvious electronic interaction is found, and the doped Mn metal seems to be a free atom. Then, taking Hf/Mn-doped single-atom alloys as an example, we discuss the electronic structure based on the density of states, charge transfer, crystal orbital Hamilton population, and wavefunctions. To support the proposed model and help analyze the data, we perform an energetic analysis of water dissociation in the water-gas shift reaction. The calculation results well confirm the d-d interaction model, where alloys with the position of the antibonding state close to the Fermi level exhibit excellent water dissociation ability in the water-gas shift reaction. However, the catalytic performance of the Mn-doped alloy is unsatisfactory, which is caused by its own special phenomenon.

Optimization of SnPd Shell Configuration to Boost ORR Performance of Pt-Clusters Decorated CoOx@SnPd Core-Shell Nanocatalyst

Fuel cells are expected to bring change to the whole human race when commercialized, however, the sluggish kinetics of oxygen reduction reaction (ORR) severely hampers their commercial viability. Thus far, platinum (Pt) based catalysts are nearly inevitable due to the harsh redox environment of fuel cells. Thus, minimizing Pt metal loading and increasing Pt utilization is a paramount factor for realizing fuel cell technologies. In this context, herein, we developed a multi-metallic nanocatalyst (NC) comprising Pt-clusters (1 wt.%) decorated SnPd composite shell over cobalt-oxide core crystal underneath (denoted as CSPP). For optimizing the ORR performance of the as-prepared NC, we further modulated the configuration of the SnPd shell. In the optimum case, when the Sn/Pd ratio is 0.5 (denoted as CSPP 1005), the ORR mass activity (MA) is 3034.7 mA mgPt−1 at 0.85 V vs. RHE in 0.1 M KOH electrolyte, which is 45-times higher than the commercial Johnson Matthey-Pt/C (J.M.-Pt/C; 20 wt.% Pt) catalyst (67 mA mgPt−1). The results of physical inspections along with electrochemical analysis suggest that such high performance of CSPP 1005 NC can be attributed to the synergistic collaboration between Pt-clusters, PtPd nanoalloys, and adjacent SnPd domains, where Pt-clusters and PtPd nanoalloys promote the O2 adsorption and subsequent splitting, while the SnPd shell favours the OH− relocation step. We believe that the obtained results will open a new avenue for further exploring the high-performance Pt-based catalysts with low Pt-loading and high utilization.

Shape-Controlled Photochemical Synthesis of Noble Metal Nanocrystals Based on Reduced Graphene Oxide

The fabrication of supported noble metal nanocrystals (NCs) with well-controlled morphologies have been attracted considerable interests due to their merits in a wide variety of applications. Photodeposition is a facile and effective method to load metals over semiconductors in a simple slurry reactor under irradiation. By optimizing the photodeposition process, the size, chemical states, and the geometrical distribution of metal NCs have been successfully tuned. However, metal NCs with well-controlled shapes through the photodeposition process have not been reported until now. Here, we report our important advances in the controlled photodeposition process to load regular noble metal NCs. Reduced graphene oxide (rGO) is introduced as a reservoir for the fast transfer of photoelectrons to avoid the fast accumulation of photogenerated electrons on the noble metals which makes the growth process uncontrollable. Meanwhile, rGO also provides stable surface for the controlled nucleation and oriented growth. Noble metal NCs with regular morphologies are then evenly deposited on rGO. This strategy has been demonstrated feasible for different precious metals (Pd, Au, and Pt) and semiconductors (TiO₂, ZnO, ZrO₂, CeO₂, and g-C₃N₄). In the prototype application of electrochemical hydrogen evolution reaction, regular Pd NCs with enclosed {111} facets showed much better performance compared with that of irregular Pd NCs.

Electronic and Potential Synergistic Effects of Surface-Doped P-O Species on Uniform Pd Nanospheres: Breaking the Linear Scaling Relationship toward Electrochemical Oxygen Reduction

Developing efficient oxygen reduction reaction (ORR) electrocatalysts is critical to fuel cells and metal-oxygen batteries, but also greatly hindered by the limited Pt resources and the long-standing linear scaling relationship (LSR). In this study, ∼6 nm and highly uniform Pd nanospheres (NSs) having surface-doped (SD) P-O species are synthesized and evenly anchored onto carbon blacks, which are further simply heat-treated (HT). Under alkaline conditions, Pd/^SDP-O NSs/C-HT exhibits respective 8.7 (4.3)- and 5.0 (5.5)-fold enhancements in noble-metal-mass- and area-specific activity (NM-MSA and ASA) compared with the commercial Pd/C (Pt/C). It also possesses an improved electrochemical stability. Besides, its acidic ASA and NM-MSA are 2.9 and 5.1 times those of the commercial Pd/C, respectively, and reach 65.4 and 51.5% of those of the commercial Pt/C. Moreover, it also shows nearly ideal 4-electron ORR pathways under both alkaline and acidic conditions. The detailed experimental and theoretical analyses reveal the following: (1) The electronic effect induced by the P-O species can downshift the surface d-band center to weaken the intermediate adsorptions, thus preserving more surface active sites. (2) More importantly, the potential hydrogen bond between the O atom in the P-O species and the H atom in the hydrogen-containing intermediates can in turn stabilize their adsorptions, thus breaking the ORR LSR toward more efficient ORRs and 4-electron pathways. This study develops a low-cost and high-performance ORR electrocatalyst and proposes a promising strategy for breaking the ORR LSR, which may be further applied in other electrocatalysis.

Sublayer Stable Fe Dopant in Porous Pd Metallene Boosts Oxygen Reduction Reaction

Engineering the morphology and electronic properties simultaneously of emerging metallene materials is an effective strategy for enhancing their performance as oxygen reduction reaction (ORR) electrocatalysts. Herein, a highly efficient and stable ORR electrocatalyst, Fe-doped ultrathin porous Pd metallene (Fe-Pd UPM) composed of a few layers of 2D atomic metallene layers, was synthesized using a simple one pot wet-chemical method and characterized. Fe-Pd UPM was measured to have enhanced ORR activity compared to undoped Pd metallene. Fe-Pd UPM exhibits a mass activity of 0.736 A mg_Pd^-1 with a loss of mass activity of only 5.1% after 10 000 cycles at 0.9 V versus the reversible hydrogen electrode (vs RHE) in 0.1 M KOH solution. Density functional theory (DFT) calculations reveal that the stable Fe dopant in the inner atomic layers of Fe-Pd UPM delivers a much smaller overpotential during O* hydrogenation into OH*. The morphology, porous structure, and Fe doping were verified to have enhanced ORR activity. We believe that the rational design of metallene materials with porous structures and interlayer doping is promising for the development of efficient and stable electrocatalysts.

Ultrathin Twisty PdNi Alloy Nanowires as Highly Active ORR Electrocatalysts Exhibiting Morphology-Induced Durability over 200 K Cycles

Even though the anion exchange membrane fuel cells have many advantages, the stability of their electrocatalysts for oxygen reduction reaction (ORR) has remained remarkably poor. We report here on the ultrathin twisty PdNi-alloy nanowires (NWs) exhibiting a very low reaction overpotential with an E_1/2 ∼ 0.95 V versus RHE in alkaline media maintained over 200 K cycles, the highest ever recorded for an electrocatalyst. The mass activity of the used NWs is >10 times higher than fresh commercial Pt/C. Therein, Ni improves the Pd d-band center for a more efficient ORR, and its leaching continuously regenerates the surface active sites. The twisty nanowire morphology imparts multiple anchor points on the electrode surface to arrest their detachment or coalescence and extra stability from self-entanglement. The significance of the NW morphology was further confirmed from the high-temperature durability studies. The study demonstrates that tailoring the number of contact points to the electrode-surface may help realize commercial-grade stability in the highly active electrocatalysts.

Recent Advances in Electrocatalysts for Proton Exchange Membrane Fuel Cells and Alkaline Membrane Fuel Cells

The rapid progress of proton exchange membrane fuel cells (PEMFCs) and alkaline exchange membrane fuel cells (AMFCs) has boosted the hydrogen economy concept via diverse energy applications in the past decades. For a holistic understanding of the development status of PEMFCs and AMFCs, recent advancements in electrocatalyst design and catalyst layer optimization, along with cell performance in terms of activity and durability in PEMFCs and AMFCs, are summarized here. The activity, stability, and fuel cell performance of different types of electrocatalysts for both oxygen reduction reaction and hydrogen oxidation reaction are discussed and compared. Research directions on the further development of active, stable, and low-cost electrocatalysts to meet the ultimate commercialization of PEMFCs and AMFCs are also discussed.

Seeded Synthesis of Unconventional 2H-Phase Pd Alloy Nanomaterials for Highly Efficient Oxygen Reduction

Crystal phase engineering of noble-metal-based alloy nanomaterials paves a new way to the rational synthesis of high-performance catalysts for various applications. However, the controlled preparation of noble-metal-based alloy nanomaterials with unconventional crystal phases still remains a great challenge due to their thermodynamically unstable nature. Herein, we develop a robust and general seeded method to synthesize PdCu alloy nanomaterials with unconventional hexagonal close-packed (hcp, 2H type) phase and also tunable Cu contents. Moreover, galvanic replacement of Cu by Pt can be further conducted to prepare unconventional trimetallic 2H-PdCuPt nanomaterials. Impressively, 2H-Pd₆₇Cu₃₃ nanoparticles possess a high mass activity of 0.87 A mg^-1_Pd at 0.9 V (vs reversible hydrogen electrode (RHE)) in electrochemical oxygen reduction reaction (ORR) under alkaline condition, which is 2.5 times that of the conventional face-centered cubic (fcc) Pd₆₉Cu₃₁ counterpart, revealing the important role of crystal phase on determining the ORR performance. After the incorporation of Pt, the obtained 2H-Pd₇₁Cu₂₂Pt₇ catalyst shows a significantly enhanced mass activity of 1.92 A mg^-1_Pd+Pt at 0.9 V (vs RHE), which is 19.2 and 8.7 times those of commercial Pt/C and Pd/C, placing it among the best reported Pd-based ORR electrocatalysts under alkaline conditions.

Improvement of Oxygen Reduction Performance in Alkaline Media by Tuning Phase Structure of Pd-Bi Nanocatalysts

Tuning the crystal phase of bimetallic nanocrystals offers an alternative avenue to improving their electrocatalytic performance. Herein, we present a facile and one-pot synthesis approach that is used to enhance the catalytic activity and stability toward oxygen reduction reaction (ORR) in alkaline media via control of the crystal structure of Pd-Bi nanocrystals. By merely altering the types of Pd precursors under the same conditions, the monoclinic structured Pd₅Bi₂ and conventional face-centered cubic (fcc) structured Pd₃Bi nanocrystals with comparable size and morphology can be precisely synthesized, respectively. Interestingly, the carbon-supported monoclinic Pd₅Bi₂ nanocrystals exhibit superior ORR activity in alkaline media, delivering a mass activity (MA) as high as 2.05 A/mg_Pd. After 10,000 cycles of ORR durability test, the monoclinic structured Pd₅Bi₂/C nanocatalysts still remain a MA of 1.52 A/mg_Pd, which is 3.6 times, 16.9 times, and 21.7 times as high as those of the fcc Pd₃Bi/C counterpart, commercial Pd/C, and Pt/C electrocatalysts, respectively. Moreover, structural characterizations of the monoclinic Pd₅Bi₂/C nanocrystals after the durability test demonstrate the excellent retention of the original size, morphology, composition, and crystal phase, greatly alleviating the leaching of the Bi component. This work provides new insight for the synthesis of multimetallic catalysts with a metastable phase and demonstrates phase-dependent catalytic performance.

Lattice-strain and electron-density modulation of palladium nanocatalysts for highly efficient oxygen reduction

Designing efficient electrocatalysts for the oxygen reduction reaction (ORR) is crucial to enhance the energy efficiencies of metal-air batteries and fuel cells. Palladium (Pd) catalysts show great potential due to their high intrinsic activity towards ORR but suffer from inferior durability. Here, we aim to employ tin oxide (SnO₂) supports to tailor the lattice strain and electron density of Pd catalysts to enhance their ORR performance. By using electrospinning and solvothermal techniques, a hierarchical Pd/SnO₂ hybrid catalyst was facilely synthesized with Pd nanoparticles anchored onto both the inside and outside walls of nanotube-like SnO₂ supports. Owing to the SnO₂ supports and the endowing metal-support interactions, tensile-strain and electron-rich features were both verified for the Pd nanoparticles in the Pd/SnO₂ catalyst. In comparison, no such features were found for the Pd nanoparticles in the Pd/C catalyst. As a consequence, the Pd/SnO₂ hybrid catalyst exhibits 2.5-times higher mass activity than the Pd/C catalyst and greatly improved durability with a current decay of 4% loss over 50 h compared with that (18%) of the Pd/C catalyst.

Phase-Controlled Synthesis of Pd-Se Nanocrystals for Phase-Dependent Oxygen Reduction Catalysis

Searching for highly efficient oxygen reduction reaction (ORR) electrocatalysts for fuel cell technology, in which the crystal structure plays a powerful role in regulating the electrocatalysis, is urgent yet challenging. Herein, we have explored the active and stable Pd-Se alloy electrocatalysts with controlled phase toward alkaline ORR. The phase-controlled Pd-Se nanoparticles (NPs) show interesting phase-dependent electrocatalytic performance, in which the Pd₁₇Se₁₅ NPs/C exhibits much better ORR performance than its counterpart, Pd₇Se₄ NPs/C, and the commercial Pd/C and Pt/C. Based on the detailed analysis, Pd in Pd₁₇Se₁₅ possesses more Se atom coordination and a higher valence state, thus providing a stronger capacity for the absorption of oxygenated species. DFT further reveals more charge transfer from the Pd₁₇Se₁₅ surface to the *OOH intermediate, which is the reason for the activity enhancement.

Atomic Crystal Facet Engineering of Core-Shell Nanotetrahedrons Restricted under Sub-10 Nanometer Region

Simultaneously engineering the size and surface crystal facets of bimetallic core-shell nanocrystals offers an effective route to not only reduce the extravagance of innermost core metal and maximize the utilization efficiency of shell atoms but also strengthen the core-to-shell interaction via ligand and/or strain effects. Herein, we systematically study the architecture transition and crystal facet engineering at the atomic level on the surface of sub-5 nm Pd(111) tetrahedrons (Ths), aimed at embodying how the variations in the local facet and shape of a sub-10 nm core-shell structure affect its surface geometrical properties and electronic structures. Specifically, surface atomic replication is predominant when the shell metal deposits less than five atomic layers, thus forming a series of Pd@M (M = Pt, Ru, and Rh) core-shell Ths enclosed by (111) facets (∼6.8 nm), while over five atomic layers, spontaneous facets tropism of each metal is predominant, where Pt atoms still follow fcc-(111) packing, Ru atoms select hcp-phase stacking, and Rh atoms choose fcc-(100) crystallization, respectively. In particular, Pt atoms take a seamless geometrical transformation from Pd@Pt Ths into Pd@Pt truncated octahedrons (TOhs, ∼7.6 nm). As a proof-of-concept application, such sub-10 nm core-shell architectures with Pt skin show a component-dependent relationship toward oxygen reduction reaction (ORR), where the catalytic activity follows the order of Pd@Pt(111) TOhs (E_1/2 = 0.916 V, 1.632 A mg_Pt^-1) > Pd@Pt(111) Ths > Pt black. Meanwhile the Ru skin show a facet-dependent relationship toward acidic hydrogen evolution reaction (HER) where the catalytic activity follows the order of Pd@Ru(111) Ths > Pd@Ru(hcp) Ths > Pd Ths.

Pt distribution-controlled Ni–Pt nanocrystals via an alcohol reduction technique for the oxygen reduction reaction

The catalytic performance and durability of Ni–Pt alloy nanoparticles synthesized using an alcohol reduction technique were enhanced by controlling the metallic Pt distribution.

Ultrasmall PdPtCo trimetallic nanorings with enriched low-coordinated edge sites and optimized compositions for effective oxygen reduction electrocatalysis

Ultrasmall two-dimensional Pd₅₅Pt₁₈Co₂₇ trimetallic alloy nanorings with enriched low-coordinated edge sites exhibit greatly enhanced ORR electrocatalytic activity.

Metallenes as functional materials in electrocatalysis

2D metals, metallenes, feature exciting opportunities at the forefront of electrocatalysis. We bring to attention metallene preparation techniques and modification strategies for the derivation of highly functional metallenes in key electrocatalytic applications.

Synthesis of Pd nanonetworks with abundant defects for oxygen reduction electrocatalysis

The Pd nanonetworks with abundant defects were synthesized by a one-pot method for enhanced oxygen reduction reaction performance.

Controlled morphology and dimensionality evolution of NiPd bimetallic nanostructures

Controlling the morphology of noble metal-based nanostructures is a powerful strategy for optimizing their catalytic performance. Here, we report a one-pot aqueous synthesis of versatile NiPd nanostructures at room temperature without employing organic solvents or surfactants. The synthesis can be tuned to form zero-dimensional (0D) architectures, such as core-shell and hollow nanoparticles (NPs), as well as nanostructures with higher dimensionality, such as extended nanowire networks and three-dimensional (3D) nanodendrites. The diverse morphologies were successfully obtained through modification of the HCl concentration in the Pd precursor solution, and the reaction aging time. An in-depth understanding of the formation mechanism and morphology evolution are described in detail. A key factor in the structural evolution of the nanostructures was the ability to tune the reduction rate and to protonate the citrate stabiliser by adding HCl. Spherical core-shell NPs were formed by the galvanic replacement-free deposition of Pd on Ni NPs which can be transformed to hollow NPs via a corrosion process. High concentrations of HCl led to the transition of isotropic spherical NPs into anisotropic wormlike nanowire networks, created through an oriented attachment process. Aging of these nanowire networks resulted in the formation of 3D porous nanodendrites via a corrosion process. The diverse structures of NiPd NPs were anchored onto acid treated-activated carbon (AC) and exhibited improved catalytic efficiency towards the hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP).

Active Site Engineering in Porous Electrocatalysts

Electrocatalysis is at the center of many sustainable energy conversion technologies that are being developed to reduce the dependence on fossil fuels. The past decade has witnessed significant progresses in the exploitation of advanced electrocatalysts for diverse electrochemical reactions involved in electrolyzers and fuel cells, such as the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), the CO₂ reduction reaction (CO₂ RR), the nitrogen reduction reaction (NRR), and the oxygen evolution reaction (OER). Herein, the recent research advances made in porous electrocatalysts for these five important reactions are reviewed. In the discussions, an attempt is made to highlight the advantages of porous electrocatalysts in multiobjective optimization of surface active sites including not only their density and accessibility but also their intrinsic activity. First, the current knowledge about electrocatalytic active sites is briefly summarized. Then, the electrocatalytic mechanisms of the five above-mentioned reactions (HER, ORR, CO₂ RR, NRR, and OER), the current challenges faced by these reactions, and the recent efforts to meet these challenges using porous electrocatalysts are examined. Finally, the future research directions on porous electrocatalysts including synthetic strategies leading to these materials, insights into their active sites, and the standardized tests and the performance requirements involved are discussed.

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.

Engineering bunched Pt-Ni alloy nanocages for efficient oxygen reduction in practical fuel cells

Development of efficient and robust electrocatalysts is critical for practical fuel cells. We report one-dimensional bunched platinum-nickel (Pt-Ni) alloy nanocages with a Pt-skin structure for the oxygen reduction reaction that display high mass activity (3.52 amperes per milligram platinum) and specific activity (5.16 milliamperes per square centimeter platinum), or nearly 17 and 14 times higher as compared with a commercial platinum on carbon (Pt/C) catalyst. The catalyst exhibits high stability with negligible activity decay after 50,000 cycles. Both the experimental results and theoretical calculations reveal the existence of fewer strongly bonded platinum-oxygen (Pt-O) sites induced by the strain and ligand effects. Moreover, the fuel cell assembled by this catalyst delivers a current density of 1.5 amperes per square centimeter at 0.6 volts and can operate steadily for at least 180 hours.

One-Pot Aqueous Synthesis of Icosahedral Au as Bifunctional Candidates for Enhanced Glucose Electrooxidation and Surface-Enhanced Raman Scattering

Bifunctional candidates, which could provide catalytic and plasmonic properties simultaneously, could activate a promising development for biomedicine. Here, we kinetically controlled and synthesized a penta-twinned icosahedral Au (Ih Au) by a facile wet-chemical protocol without assistance of stabilizers. Benefiting from icosahedral morphology and kinetic synthesis process, the Ih Au nanoparticles (NPs) incorporate three key advantages: (i) ample active sites/"hot spots" and surface strain, (ii) good stability/chemical inertness and easy functionalization, and (iii) biological compatibility and a clean surface, which could promote their electrocatalysis and photonic capacity. Ih Au NPs, as bifunctional nanomaterials, exert excellent electrocatalytic and surface-enhanced Raman scattering (SERS) performances. Ih Au delivers the highest glucose electrooxidation (GEO) peak current density with 6.87 mA cm^-2, which is 14 times larger than that of Turkevich Au (0.49 mA cm^-2) under the same condition. Moreover, the SERS signals of rhodamine 6G (R6G) on Ih Au are much stronger than that on the other corresponding Au counterparts. Particularly, the SERS intensity of R6G on Ih Au increases by about four times compared to that on Au NPs. This study motivates the great prospect for combining Ih Au's bifunctionalities and indicates the potential of bifunctional nanomaterials in biologically implanted devices.

Atomic-Level Construction of Tensile-Strained PdFe Alloy Surface toward Highly Efficient Oxygen Reduction Electrocatalysis

Exploring the high-performance non-Pt electrocatalysts for oxygen reduction reaction (ORR), the bottleneck process in fuel cells, is desirable but challenging. Here, we report the Pd@PdFe core-shell icosahedra as an active and durable electrocatalyst toward ORR in alkaline conditions, which feature a three-atomic-layer tensile-strained PdFe overlayer on Pd icosahedra. Our optimized catalyst shows 2.8-fold enhancement in mass activity and 6.9-fold enhancement in specific activity than commercial Pt/C catalyst toward ORR, representing one of the best non-Pt electrocatalysts. Moreover, the boosted ORR catalysis is strongly supported by the assembled fuel cell performance using Pd@PdFe core-shell icosahedra as the cathode electrocatalyst. The density functional theory calculations reveal that the synergistic coupling of tensile strain and alloy effects enables the optimum binding strength for intermediates, thus causing the maximum activity. The present work suggests the coupling between multiple surface modulations endows larger room for the rational design of remarkable catalysts.

Shape Control of Monodispersed Sub-5 nm Pd Tetrahedrons and Laciniate Pd Nanourchins by Maneuvering the Dispersed State of Additives for Boosting ORR Performance

It is a great challenge to simultaneously control the size, morphology, and facets of monodispersed Pd nanocrystals under a sub-5 nm regime. Meanwhile, quantitative understanding of the thermodynamic and kinetic parameters to maneuver the shape evolution of nanocrystals in a one-pot system still deserves investigation. Herein, a systematic study of the density functional theory (DFT)-calculated adsorption energy, thermodynamic factors, and reduction kinetics on Pd growth patterns is reported by combining theory and experiments, with a focus on the dispersed state of additives. As pure models, monodispersed Pd tetrahedrons enclosed by (111) facets with a narrow size distribution of 4.9 ± 1 nm and a high purity approaching 98% can be obtained when using 1,1'-binaphthalene (C₂₀ H₁₄ ) +2NH₃ as additives. Specifically, laciniate Pd nanourchins (Pd LUs) can evolve via anisotropic growth when replacing additive with dose-consistent 1,1'-binaphthyl-2,2'-diamine (C₂₀ H₁₆ N₂ , two NH₂ binding in C₂₀ H₁₄ ). Catalytic investigations show that the sub-5 nm Pd tetrahedrons exhibit higher activity in both the oxygen reduction (E_onset = 1.025 V, E_1/2 = 0.864 V) and formic acid oxidation reaction with respect to the Pd LUs and Pd black, which represents a great step for the development of well-defined Pd nanocrystals with size in the sub-5 nm regime as non-Pt electrocatalysts.

Au-Doped intermetallic Pd3Pb wavy nanowires as highly efficient electrocatalysts toward the oxygen reduction reaction

Au-Doped intermetallic Pd₃Pb wavy nanowires were synthesized and exhibited substantially enhanced properties for the oxygen reduction reaction relative to commercial Pt/C.

Niobium dioxide prepared by a novel La-reduced route as a promising catalyst support for Pd towards the oxygen reduction reaction

Electron transfer from NbO₂ to Pd enhances the ORR activity of Pd/NbO₂.

Synthesis of low- and high-index faceted metal (Pt, Pd, Ru, Ir, Rh) nanoparticles for improved activity and stability in electrocatalysis

Driven by the quest for future energy solution, faceted metal nanoparticles are being pursued as the next generation electrocatalysts for renewable energy applications. Thanks to recent advancement in solution phase synthesis, different low- and high-index facets on metal nanocrystals become accessible and are tested for specific electrocatalytic reactions. This minireview summarises the key approaches to prepare nanocrystals containing the most catalytically active platinum group metals (Pt, Pd, Ru, Ir and Rh) exposed with low- and high-index facets using solution phase synthesis. Electrocatalytic studies related to the different facets are highlighted to emphasise the importance of exposing facets for catalysing these reactions, namely oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), alcohol oxidation including methanol (MOR) and ethanol oxidation reactions (EOR), formic acid oxidation reaction (FAOR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The future outlook discusses the challenges and opportunities for making electrocatalysts that are even more active and stable.

A Pt-free graphenaceous composite as an electro-catalyst for efficient oxygen reduction reaction

Use of Pt-based electro-catalysts for the oxygen reduction reaction (ORR) is a major hindrance in large-scale application of proton exchange membrane fuel cells (PEMFCs). Hence, new, cost-effective and high performance electro-catalysts are required for the commercial success of PEMFCs. In this work, a Pt-free magnesium oxide (MgO) decorated multi-layered reduced graphene oxide (MLGO) composite is tested as an electro-catalyst for the ORR. The ORR activity of MgO/MLGO in terms of diffusion-controlled current density is found to be superior (6.63 mA per cm2-geo) than that of in-house prepared Pt/rGO (5.96 mA per cm2-geo) and commercial Pt/C (5.02 mA per cm2-geo). The applicability of less expensive MgO/MLGO not only provides a new electro-catalyst but also provides a new direction in exploring metal oxide-based electro-catalysts for the ORR.