Recent Advancements and Future Prospects of Noble Metal-Based Heterogeneous Nanocatalysts for Oxygen Reduction and Hydrogen Evolution Reactions

Highly dispersed ultrafine PtCo alloy nanoparticles on unique composite carbon supports for proton exchange membrane fuel cells

The design of highly efficient and robust platinum-based electrocatalysts is pivotal for proton exchange membrane fuel cells (PEMFC). One of the long-standing issues for PEMFC is the rapid deactivation of the catalyst under working conditions. Here, we report a simple synthesis strategy for ultrafine PtCo alloy nanoparticles loaded on a unique carbon support derived from a zeolitic imidazolate framework-67 (ZIF-67) and Ketjen Black (KB) composite, exhibiting a remarkable catalytic performance toward the oxygen reduction reaction (ORR) and PEMFC. Benefitting from the N-doping and wide pore size distribution of the composite carbon supports, the growth of PtCo nanoparticles can be evenly restricted, leading to a uniform distribution. The Pt-integrated catalyst delivers an outstanding electrochemical performance with a mass activity that is 8.6 times higher than that of the commercial Pt/C catalyst. Impressively, the accelerated durability test (ADT) demonstrates that the hybrid carbon support can significantly enhance the durability. Theoretical simulations highlight the synergistic contribution between the supports and the PtCo nanoparticles. Moreover, hydrogen-oxygen fuel cells assembled with the catalyst exhibited a high power density of 1.83 W cm-2 at 4 A cm-2. These results provide a new opportunity to design advanced catalysts for PEMFC.

Multicomponent Metal Oxide- and Metal Hydroxide-Based Electrocatalysts for Alkaline Water Splitting

Developing cost-effective, highly catalytic active, and stable electrocatalysts in alkaline electrolytes is important for the development of highly efficient anion-exchange membrane water electrolysis (AEMWE). To this end, metal oxides/hydroxides have attracted wide research interest for efficient electrocatalysts in water splitting owing to their abundance and tunable electronic properties. It is very challenging to achieve an efficient overall catalytic performance based on single metal oxide/hydroxide-based electrocatalysts due to low charge mobilities and limited stability. This review is mainly focused on the advanced strategies to synthesize the multicomponent metal oxide/hydroxide-based materials that include nanostructure engineering, heterointerface engineering, single-atom catalysts, and chemical modification. The state of the art of metal oxide/hydroxide-based heterostructures with various architectures is extensively discussed. Finally, this review provides the fundamental challenges and perspectives regarding the potential future direction of multicomponent metal oxide/hydroxide-based electrocatalysts.

A DFT Study of Ruthenium fcc Nano-Dots: Size-Dependent Induced Magnetic Moments

Many areas of electronics, engineering and manufacturing rely on ferromagnetic materials, including iron, nickel and cobalt. Very few other materials have an innate magnetic moment rather than induced magnetic properties, which are more common. However, in a previous study of ruthenium nanoparticles, the smallest nano-dots showed significant magnetic moments. Furthermore, ruthenium nanoparticles with a face-centred cubic (fcc) packing structure exhibit high catalytic activity towards several reactions and such catalysts are of special interest for the electrocatalytic production of hydrogen. Previous calculations have shown that the energy per atom resembles that of the bulk energy per atom when the surface-to-bulk ratio < 1, but in its smallest form, nano-dots exhibit a range of other properties. Therefore, in this study, we have carried out calculations based on the density functional theory (DFT) with long-range dispersion corrections DFT-D3 and DFT-D3-(BJ) to systematically investigate the magnetic moments of two different morphologies and various sizes of Ru nano-dots in the fcc phase. To confirm the results obtained by the plane-wave DFT methodologies, additional atom-centred DFT calculations were carried out on the smallest nano-dots to establish accurate spin-splitting energetics. Surprisingly, we found that in most cases, the high spin electronic structures had the most favourable energies and were hence the most stable.

Pd-Co-Based Electrodes for Hydrogen Production by Water Splitting in Acidic Media

To realize the benefits of a hydrogen economy, hydrogen must be produced cleanly, efficiently and affordably from renewable resources and, preferentially, close to the end-users. The goal is a sustainable cycle of hydrogen production and use: in the first stage of the cycle, hydrogen is produced from renewable resources and then used to feed a fuel cell. This cycle produces no pollution and no greenhouse gases. In this context, the development of electrolyzers producing high-purity hydrogen with a high efficiency and low cost is of great importance. Electrode materials play a fundamental role in influencing electrolyzer performances; consequently, in recent years considerable efforts have been made to obtain highly efficient and inexpensive catalyst materials. To reach both goals, we have developed electrodes based on Pd-Co alloys to be potentially used in the PEMEL electrolyzer. In fact, the Pd-Co alloy is a valid alternative to Pt for hydrogen evolution. The alloys were electrodeposited using two different types of support: carbon paper, to fabricate a porous structure, and anodic alumina membrane, to obtain regular arrays of nanowires. The goal was to obtain electrodes with very large active surface areas and a small amount of material. The research demonstrates that the electrochemical method is an ideal technique to obtain materials with good performances for the hydrogen evolution reaction. The Pd-Co alloy composition can be controlled by adjusting electrodeposition parameters (bath composition, current density and deposition time). The main results concerning the fabrication process and the characterization are presented and the performance in acid conditions is discussed.

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.

Promoting the reduction of CO2 to formate and formaldehyde via gas-liquid interface dielectric barrier discharge using a Zn0.5Cd0.5S/CoP/multiwalled carbon nanotubes catalyst

A Zn_0.5Cd_0.5S (ZCS) solid solution was prepared using a hydrothermal method, in which CoP nanowires were added as a co-catalyst and co-deposited with multiwalled carbon nanotubes (MWNTs) on sponge to prepare a series of ZCS/CoP/MWNTs/sponge electrodes. The microstructures of catalysts were analyzed to confirm ZCS and CoP were successfully loaded in MWNTs/sponge. The CO₂ reduction products (formate and formaldehyde) produced via dielectric barrier discharge (DBD) using the different catalysts proved that the introduction of the CoP nanowires co-catalyst can enhance the catalytic activity of ZCS/MWNTs/sponge in the DBD system. Using 10% CoP and a ZCS/CoP concentration of 2.5 g·L^-1, the resulting ZCS/CoP/MWNTs/sponge catalyst exhibited the best catalytic of CO₂ reduction ability toward formate (7894.6 μmol·L^-1) and formaldehyde (308.5 μmol·L^-1) after 60 min of discharge, respectively. The proposed DBD catalytic mechanism for the reduction of CO₂ was analyzed according to the Tafel slope, density functional theory calculations, photocurrent density and plasma reaction process. Furthermore, the application of the DBD catalytic technology for CO₂ capture and reduction was shown to be efficient in a seawater system, and as such, it could be useful for marine CO₂ storage and conversion.

Co-Existence of Atomic Pt and CoPt Nanoclusters on Co/SnOx Mix-Oxide Demonstrates an Ultra-High-Performance Oxygen Reduction Reaction Activity

An effective approach for increasing the Noble metal-utilization by decorating the atomic Pt clusters (1 wt.%) on the CoO₂@SnPd₂ nanoparticle (denoted as CSPP) for oxygen reduction reaction (ORR) is demonstrated in this study. For the optimum case when the impregnation temperature for Co-crystal growth is 50 °C (denoted as CSPP-50), the CoPt nanoalloys and Pt-clusters decoration with multiple metal-to-metal oxide interfaces are formed. Such a nanocatalyst (NC) outperforms the commercial Johnson Matthey-Pt/C (J.M.-Pt/C; 20 wt.% Pt) catalyst by 78-folds with an outstanding mass activity (MA) of 4330 mA mg_Pt^-1 at 0.85 V vs. RHE in an alkaline medium (0.1 M KOH). The results of physical structure inspections along with electrochemical analysis suggest that such a remarkable ORR performance is dominated by the potential synergism between the surface anchored Pt-clusters, CoPt-nanoalloys, and adjacent SnPd₂ domain, where Pt-clusters offer ideal adsorption energy for O₂ splitting and CoPt-nanoalloys along with SnPd₂ domain boost the subsequent desorption of hydroxide ions (OH^-).

Noble Metal-Based Catalysts with Core-Shell Structure for Oxygen Reduction Reaction: Progress and Prospective

With the deterioration of the ecological environment and the depletion of fossil energy, fuel cells, representing a new generation of clean energy, have received widespread attention. This review summarized recent progress in noble metal-based core–shell catalysts for oxygen reduction reactions (ORRs) in proton exchange membrane fuel cells (PEMFCs). The novel testing methods, performance evaluation parameters and research methods of ORR were briefly introduced. The effects of the preparation method, temperature, kinds of doping elements and the number of shell layers on the ORR performances of noble metal-based core–shell catalysts were highlighted. The difficulties of mass production and the high cost of noble metal-based core–shell nanostructured ORR catalysts were also summarized. Thus, in order to promote the commercialization of noble metal-based core–shell catalysts, research directions and prospects on the further development of high performance ORR catalysts with simple synthesis and low cost are presented.

The performance of an atomically dispersed oxygen reduction catalyst prepared by γ-CD-MOF integration with FePc

Here, a series of Fe/N/C catalysts with different proportions and pyrolysis temperatures are prepared by co-pyrolysis of melamine with a γ-cyclodextrin metal-organic framework (γ-CD-MOF) containing iron(ii) phthalocyanine (FePc). Due to the restriction effect of the host and guest at the molecular level, γ-CD-MOF can effectively avoid the π-π stacking of FePc and restrain the agglomeration of Fe atoms during pyrolysis. The phases and structures of the catalysts are characterized, which proves that the obtained catalyst has a three-dimensional porous and internal cavity structure with abundant surface area (1055.317 m² g^-1) and Fe is atomically dispersed in nitrogen-doped carbon. The onset potential (0.988 V vs. RHE) and half-wave potential (0.846 V vs. RHE) of FePc@CD/M (1 : 20)-1000 are superior to those of a commercial 20% Pt/C catalyst. FePc@CD/M (1 : 20)-1000 also exhibits an approximately four-electron (3.84) transfer process, good stability and excellent methanol tolerance.

β-Arsenene Monolayer: A Promising Electrocatalyst for Anodic Chlorine Evolution Reaction

Materials innovation plays an essential role to address the increasing demands of gaseous chlorine from anodic chlorine evolution reaction (CER) in chlor-alkali electrolysis. In this study, two-dimensional (2D) semiconducting group-VA monolayers were theoretically screened for the electrochemical CER by means of the density functional theory (DFT) method. Our results reveal the monolayered β-arsenene has the ultralow thermodynamic overpotential of 0.068 V for CER, which is close to that of the commercial Ru/Ir-based dimensionally stable anode (DSA) of 0.08 V @ 10 mA cm−2 and 0.13 V from experiments and theory, respectively. The change of CER pathways via Cl* intermediate on 2D β-arsenene also efficiently suppresses the parasitical oxygen gas production because of a high theoretical oxygen evolution reaction (OER) overpotential of 1.95 V. Our findings may therefore expand the scope of the electrocatalysts design for CER by using emerging 2D materials.

PtAu Nanoparticles Supported by Reduced Graphene Oxide as a Highly Active Catalyst for Hydrogen Evolution

PtAu nanoparticles spontaneously deposited on graphene support, PtAu/rGO, have shown remarkably high catalytic activity for hydrogen evolution reaction (HER) in sulfuric acid solution. SEM images of the PtAu/rGO electrode surface showed that Pt nanoparticles that are non-uniform in size occupy both the edges of previously deposited uniform Au nanoparticles and the edges of graphene support. XPS analysis showed that the atomic percentages of Au and Pt in PtAu/rGO were 0.6% and 0.3%, respectively. The atomic percentage of Au alone on previously prepared Au/rGO was 0.7%. Outstanding HER activity was achieved for the PtAu/rGO electrode, showing the initial potential close to the equilibrium potential for HER and a low Tafel slope of −38 mV/dec. This was confirmed by electrochemical impedance spectroscopy. The chronoamperometric measurement performed for 40 min for hydrogen evolution at a constant potential indicated good stability and durability of the PtAu/rGO electrode.

In Situ Stabilisation of Silver Nanoparticles at Chitosan-Functionalised Graphene Oxide for Reduction of 2,4-Dinitrophenol in Water

This investigation reports the in situ growth of silver nanoparticles onto covalently bonded graphene oxide-chitosan, which serve as supported nanocatalysts for the NaBH4 reduction of 2,4-dinitrophenol in aqueous systems. Fumaryl chloride reacted with chitosan in an acidic environment to yield a tailored polymeric material. The latter was, in turn, treated with the pre-synthesised graphene oxide sheets under acidic conditions to generate the GO-functionalised membrane (GO-FL-CS). The adsorption of Ag+ from aqueous media by GO-FL-CS yielded a set of membranes that were decorated with silver nanoparticles (Ag NPs@GO-FL-CS) without any reducing agent. Various analytical tools were used to characterise these composites, including Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller surface area analysis, X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray analysis, inductively coupled plasma-mass spectrometry, and transmission electron microscopy. The silver-loaded materials were further used for the remediation of 2,4-dinitrophenol from aqueous solutions under batch operation. The BET analysis revealed that the functionalisation of GO with chitosan and Ag NPs (average size 20-60 nm) resulted in a three-fold increased surface area. The optimised catalyst (Ag mass loading 16.95%) displayed remarkable activity with an apparent pseudo-first-order rate constant of 13.5 × 10-3 min-1. The cyclic voltammetry experiment was conducted to determine the nitro-conversion pathway. The reusability/stability test showed no significant reduction efficiency of this metal-laden composite over six cycles. Findings from the study revealed that Ag NPs@GO-FL-CS could be employed as a low-cost and recyclable catalyst to convert toxic nitroaromatics in wastewater.