Investigation on the electrocatalytic activity and stability of three-dimensional and two-dimensional palladium nanostructures for ethanol and formic acid oxidation

2D noble metals: growth peculiarities and prospects for hydrogen evolution reaction catalysis

High-performance electrocatalysts for the hydrogen evolution reaction are of interest in the development of next-generation sustainable hydrogen production systems. Although expensive platinum-group metals have been recognized as the most effective HER catalysts, there is an ongoing requirement for the discovery of cost-effective electrode materials. This paper reveals the prospects of two-dimensional (2D) noble metals, possessing a large surface area and a high density of active sites available for hydrogen proton adsorption, as promising catalytic materials for water splitting. An overview of the synthesis techniques is given. The advantages of wet chemistry approaches for the growth of 2D metals over deposition techniques show the potential for kinetic control that is required as a precondition to prevent isotropic growth. An uncontrolled presence of surfactant-related chemicals on a 2D metal surface is however the main disadvantage of kinetically controlled growth methods, which stimulates the development of surfactant-free synthesis approaches, especially template-assisted 2D metal growth on non-metallic substrates. Recent advances in the growth of 2D metals using a graphenized SiC platform are discussed. The existing works in the field of practical application of 2D noble metals for hydrogen evolution reaction are analyzed. This paper shows the technological viability of the "2D noble metals" concept for designing electrochemical electrodes and their implementation into future hydrogen production systems, thereby providing an inspirational background for further experimental and theoretical studies.

Substrate mediated properties of gold monolayers on SiC

In light of their unique physicochemical properties two-dimensional metals are of interest in the development of next-generation sustainable sensing and catalytic applications. Here we showcase results of the investigation of the substrate effect on the formation and the catalytic activity of representative 2D gold layers supported by non-graphenized and graphenized SiC substrates. By performing comprehensive density functional theory (DFT) calculations, we revealed the epitaxial alignment of gold monolayer with the underlying SiC substrate, regardless of the presence of zero-layer graphene or epitaxial graphene. This is explained by a strong binding energy (∼4.7 eV) of 2D Au/SiC and a pronounced charge transfer at the interface, which create preconditions for the penetration of the related electric attraction through graphene layers. We then link the changes in catalytic activity of substrate-supported 2D Au layer in hydrogen evolution reaction to the formation of a charge accumulation region above graphenized layers. Gold intercalation beneath zero-layer graphene followed by its transformation to quasi-free-standing epitaxial graphene is found to be an effective approach to tune the interfacial charge transfer and catalytic activity of 2D Au. The sensing potential of substrate-supported 2D Au was also tested through exploring the adsorption behaviour of NH3, NO2 and NO gas molecules. The present results can be helpful for the experimental design of substrate-supported 2D Au layers with targeted catalytic activity and sensing performance.

Nanoporous Gold for Energy Applications

Research activities using nanoporous gold (NPG) were reviewed in the field of energy applications in three categories: fuel cells, supercapacitors, and batteries. First, applications to fuel cells are reviewed with the subsections of proof-of-concept studies, studies on fuel oxidations at anode, and studies on oxygen reduction reactions at cathode. Second, applications to supercapacitors are reviewed from research activities on active materials/NPG composites to demonstrations of all-solid-state flexible supercapacitors using NPG electrodes. Third, research activities using NPG for battery applications are reviewed, mainly about fundamental studies on Li-air and Na-air batteries and some model studies on improving Li ion battery anodes. Although NPG based studies are the main subject of this review, some of meaningful studies using nanoporous metals are also discussed where relevant. Finally, summary and future outlook are given based on the survey on the research activities.

Hierarchical defective palladium-silver alloy nanosheets for ethanol electrooxidation

Tuning the chemical composition and surface structure of electrodes is demonstrated as a feasible and effective strategy to tailor advanced catalysts for energy electrocatalysis. In this work, hierarchical palladium-silver alloy nanosheets (PdAg NS) with the thickness ~7 atoms and rich atomic defects are successfully prepared, using the carbon monoxide (CO) confinement approach. The optimized Pd₇Ag₃ NS/C exhibits 8.8 times higher catalytic peak current density and much better stability toward ethanol electrooxidation than Pd NS/C catalyst. The catalytic enhancement mechanism could be attributed to the synergetic effects among optimized electronic structure of Pd, novel architecture, and rich atomic defects.

Insights into the Role of Poly(vinylpyrrolidone) in the Synthesis of Palladium Nanoparticles and Their Electrocatalytic Properties

Four types of palladium (Pd) nanoparticles were prepared from the systems containing PdCl₂ or Na₂PdCl₄ with or without the assistance of poly(vinylpyrrolidone) (PVP). Two types of Pd nanoparticles obtained in the absence of PVP were obviously larger than those synthesized with the assistance of PVP. The former large Pd particles showed typical features in cyclic voltammetry in H₂SO₄ solution, whereas two types of small Pd nanoparticles did not. However, small nanoparticles treated first in an electrochemical way in 0.5 M KOH solution displayed the adsorption and desorption peaks similar to those of typical Pd-modified electrodes in H₂SO₄ solution. Large Pd nanoparticles from the PdCl₂ synthesis system showed a catalytic specific current of 629 mA/mg in the electrocatalysis of ethanol, whereas large particles from the Na₂PdCl₄ system showed a current of 262 mA/mg. The maximum catalytic currents of small Pd nanoparticles without surface cleaning treatment were 1382 and 1019 mA/mg for samples from the Na₂PdCl₄ and PdCl₂ systems, respectively, higher than those being treated in KOH solution first, and the electrocatalytic stability of the two untreated samples was better. However, small nanoparticles after the electrochemical treatment can reach the maximum catalytic current faster. The synthesis and structure-property relation of four types of Pd nanoparticles have been discussed and analyzed on the basis of systematically experimental data.

Metal/graphene heterobilayers as hydrogen evolution reaction cathodes: a first-principles study

Rh atoms in the interaction region facilitate hydrogen evolution reaction, whereas others in the deformation and transition regions do not, due to the interlayer charge transfer between single-layer Rh sheet and graphene.