Nanostructured Electrocatalysts Research Articles

Nanostructured Copper-Based Electrodes Electrochemically Synthesized on a Carbonaceous Gas Diffusion Membrane with Catalytic Activity for the Electroreduction of CO2.

In this work, four different 4 cm2-sized nanostructured Cu-based electrocatalysts have been designed by a one-step electrodeposition process of Cu metal on a three-dimensional carbonaceous membrane. One consisted of Cu0, and the other three were obtained by further simple oxidative treatments. Morphological, structural, and electrochemical investigations on the four materials were carried out by scanning electron microscopy, Raman spectroscopy, X-ray diffraction, linear sweep voltammetry, and potential-controlled electrolysis. All the electrocatalysts showed promising catalytic activities toward CO2 electroreduction in liquid phase, with a remarkable selectivity toward acetic acid achieved when using the oxidized materials. In particular, the best electrocatalytic activity was observed for the Cu2O-Cu0 catalyst, working at a relatively low potential (−0.4 V vs RHE), which exhibited a stable and low current density of 0.46 mA cm–2 and a productivity of 308 μmol gcat–1 h–1. These results were attributed to the nanostructured morphology that is characterized by many void spaces and by a high surface area, which should guarantee a large number of CuI and Cu0 catalytic active sites. Moreover, kinetic analyses and preliminary studies about catalyst regeneration highlighted the stability of the best-performing catalyst.

One-dimensional CoP/MnO hollow nanostructures with enhanced oxygen evolution reaction activity

Cobalt-based transition metal phosphides are flourishing as OER electrocatalysts. In this study, the CoP/MnO hollow nanofibers with the advantages of a more extensive contact interface were successfully synthesized. We found that the construction of hollow nanostructures and the composite of oxides are effective strategies to optimize the OER catalytic performance of transition metal phosphides. The template of the precursor can adjust the hollow nanostructure and keep it stable during the phosphating process. It is worth noting that the CoP/MnO composite material only needs an overpotential of 230 mV at a current density of 10 mA cm−2. In addition, it maintains the overpotential 263.5 mV after 5000 cycles of voltammetry measurements. In short, this research provides a simple solution for the design and realization of nanostructured electrocatalysts with excellent electrochemical performance.

2D layered nickel-cobalt double hydroxide nano sheets @ 1D silver nanowire-graphitic carbon nitrides for high performance super capacitors

Designing nanostructured electrocatalysts plays an important role in improving the electro-catalytic behaviors of energy storage devices. Achieving the high specific capacitance of these devices depends on the structure of nanomaterials and their synthesis process. Herein, we report a novel system consisting of 2D layered materials and 1D nanowire with desirable properties suitable for supercapacitor applications. Nickel-Cobalt layered double hydroxide (Ni-Co LDH)@Silver nanowire (Ag NWs)/graphitic carbon nitride (g-C3N4) nanomaterial was successfully synthesized using hydrothermal and ultra-sonication methods. The structural studies of the synthesized Ni-Co LDH based electrode materials are characterized by using different analytical techniques namely, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Field emission scanning electron microscopy (FE-SEM), High-resolution transmission electron microscopy (HR-TEM), Energy dispersive spectroscopy (EDAX). The electrochemical performance of Ni-Co LDH@Ag NWs/g-C3N4 hybrid electrode is investigated using Galvanostatic charge-discharge (GCD), cyclic and voltammetry (CV) techniques. The designed 3D nanostructured composites (Ni-Co LDH@Ag NWs/g-C3N4) showed high specific capacitance of 848.5 F/g at a current density at 1 A/g as good cyclic stability after 2000 cycles charge-discharge duration (93.5% at current density 5 A/g).

Platinum Nanoflowers: A New Class of Nanostructured Electrocatalysts for the Oxygen Reduction Reaction

Platinum-based nanomaterials are currently one of the most effective options as proton exchange membrane fuel cell (PEMFC) cathode catalysts for the oxygen reduction reaction (ORR). Throughout the last decade, advancements in catalyst morphologies have shifted from 2 to 3 nm nanoparticles towards complex shaped nanostructures (such as octahedra, nanowires, nanoframes...etc.) supported on carbon blacks.1 These advanced nanostructures take advantage of electronic and structural effects that allow for activity improvements of over 30 times when compared to traditional platinum nanoparticles in liquid electrolyte.2 We synthesized and explored a novel class of materials (platinum nanoflowers) for the oxygen reduction reaction which display a high level of crystallinity and porosity.3 The nanostructures form rapidly in solution when utilizing the microwave heating technique of an adapted polyol synthesis procedure and forms due to the altered kinetics of nanoparticle growth with the addition of iodine ions to allow for the overgrowth towards nanoflower structures. The nanoflowers appear monomorphic (similar size and shape), maintains its crystalline structure from petal to petal, and displays deep consistent pores throughout its structure. The platinum nanoflowers display a similar mass activity (~300 A/g at 0.90V vs. RHE) and a factor of 4 enhancement in specific activity (~1800 µA/cm2 at 0.90V vs. RHE) for the oxygen reduction reaction compared to a commercial platinum-carbon catalyst. Ly, A., Asset, T. & Atanassov, P. J. Power Sources 478, 228516 (2020).Chen, C. et al. Science 343, 1339–1343 (2014).Yin, J., Wang, J., Li, M., Jin, C. & Zhang, T. Chem. Mater. 24, 2645–2654 (2012). Figure 1

Visualizing the Spatial Heterogeneity of Electron Transfer on a Metallic Nanoplate Prism.

The involvement between electron transfer (ET) and catalytic reaction at the electrocatalyst surface makes the electrochemical process challenging to understand and control. Even ET process, a primary step, is still ambiguous because it is unclear how the ET process is related to the nanostructured electrocatalyst. Herein, locally enhanced ET current dominated by mass transport effect at corner and edge sites bounded by {111} facets on single Au triangular nanoplates was clearly imaged. After decoupling mass transport effect, the ET rate constant of corner sites was measured to be about 2-fold that of basal {111} plane. Further, we demonstrated that spatial heterogeneity of local inner potential differences of Au nanoplates/solution interfaces plays a key role in the ET process, supported by the linear correlation between the logarithm of rate constants and the potential differences of different sites. These results provide direct images for heterogeneous ET, which helps to understand and control the nanoscopic electrochemical process and electrode design.

Cover Feature: Metal‐Organic Framework Derived Nanostructured Bifunctional Electrocatalysts for Water Splitting (ChemElectroChem 20/2021)

The Cover Feature illustrates metal-organic framework-derived carbon/metal-based nanostructured electrocatalysts for electrochemical water splitting into hydrogen and oxygen. More information can be found in the Review by A. Aijaz and co-workers.

Carbon sustained SnO2-Bi2O3 hollow nanofibers as Janus catalyst for high-efficiency CO2 electroreduction

Electrochemical reduction of CO2 (CRR) into valuable products like formate is an appealing way to alleviate the pressure of the excessive CO2 emission and achieve carbon neutrality. However, exploitation of electrocatalysts with commercial application potentials remains challenging. Herein, we fabricate a hierarchical Janus electrocatalyst with SnO2 and Bi2O3 assembled outside and inside the hollow carbon nanofiber skeleton, respectively. Due to the nanostructuring effect, the catalytic performance of Janus active sites was magnified, achieving a remarkable current density of −220 mA cm−2 for formate production and 91% faradic efficiency at −1.29 V vs. RHE in a flow cell. Besides, the selectivity for formate was maintained over 48 h operation with an outstanding half-cell energy efficiency of 51%. In-situ FT-IR further prove the Janus active sites are conducive to the HCOO* reaction pathway rather than the *COOH way. This study may pave another avenue to prepare nanostructured Janus electrocatalysts for highly efficient CO2 reduction.

Reutilizing Methane Reforming Spent Catalysts as Efficient Overall Water-Splitting Electrocatalysts.

It is extremely prudent and highly challenging to design a greener bifunctional electrocatalyst that shows effective electrocatalytic activity and high stability toward electrochemical water splitting. As several hundred tons of catalysts are annually deactivated by deposition of carbon, herein, we came up with a strategy to reutilize spent methane reforming catalysts that were deactivated by the formation of graphitic carbon (GC) and carbon nanofibers (CNF). An electrocatalyst was successfully synthesized by in situ deposition of noble metal-free MoS2 over spent catalysts via a hydrothermal method that showed exceptional performance regarding the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). At 25 mA cm–2, phenomenal OER overpotentials (η25) of 128 and 154 mV and modest HER overpotentials of 186 and 207 mV were achieved for MoS2@CNF and MoS2@GC, respectively. Moreover, OER Tafel slopes of 41 and 71 mV dec–1 and HER Tafel slopes of 99 and 107 mV dec–1 were obtained for MoS2@CNF and MoS2@GC, respectively. Furthermore, the synthesized catalysts exhibited good long-term durability for about 18 h at 100 μA cm–2 with unnoticeable changes in the linear sweep voltammetry (LSV) curve of the HER after 1000 cycles. The carbon on the spent catalyst increased the conductivity, while MoS2 enhanced the electrocatalytic activity; hence, the synergistic effect of both materials resulted in enhanced electrocatalysts for overall water splitting. This work of synthesizing enhanced nanostructured electrocatalysts with minimal usage of inexpensive MoS2 gives a rationale for engineering potent greener electrocatalysts.

Controllable dry synthesis of binder-free nanostructured platinum electrocatalysts supported on multi-walled carbon nanotubes and their performance in the oxygen reduction reaction

Different platinum nanostructures were synthesized by pulsed laser ablation (PLA) of a pure Pt target, a by-product-free and solvent-free synthesis technique. The Pt nanostructures were directly deposited on multi-walled carbon nanotubes (MWCNT) pre-grown on a stainless steel (SS) mesh by chemical vapor deposition. Performing PLA at different background gas pressures (10−5 to 10 Torr, argon) and laser pulse energies (20 to 66 mJ pulse−1) led to a range of coating morphologies containing very low Pt loadings (14 to 35 μg cm−2), from the uniform and dense nanoparticle thin coatings at low pressure to highly porous granular coatings at higher pressure. Increasing the background gas pressure led to an increase in primary Pt nanoparticle size from 2.0 ± 0.2 nm to 2.8 ± 0.8 nm. XPS results showed a positive shift in the Pt 4f7/2 binding energies with lowering the PLA pressure and laser energy, which was linked to the formation of smaller Pt nanoparticles. The presence of Pt (111), (200), (220), and (311) was confirmed by SAED patterns in the Pt nanostructures. The electrocatalytic performance of these binder-free Pt/MWCNT/SS structures was evaluated in the oxygen reduction reaction (ORR) in an alkaline medium, in both the rotating disk electrode (RDE) and gas diffusion electrode (GDE) configurations. In the RDE setup, all Pt/MWCNT/SS electrocatalysts were found to yield a higher specific ORR activity than the commercial Pt/C thin film, while in the GDE setup the Pt/MWCNT/SS electrode with the lowest Pt loading and highest surface area yielded the highest ORR specific activity.

Novel MoSe2–Ni(OH)2 nanocomposite as an electrocatalyst for high efficient hydrogen evolution reaction

Nowadays, there is a great demand for low-cost and highly active electrocatalyst for the production of clean renewable energy. However, most of the electrocatalysts are noble metal-based which are very costly and unstable. To counter this, electrochemical water splitting in energy storage systems is been widely applied, using non-noble metal-based nanostructured electrocatalysts. In this work, a novel noble metal-free MoSe2–Ni(OH)2 nanocomposite electrocatalyst is synthesized using a multi-step hydrothermal technique for efficient hydrogen evolution reaction (HER). The morphology, structural, chemical composition, and functional features of the synthesized nanomaterials were characterized using different techniques that include scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), and Raman analysis. The new developed MoSe2–Ni(OH)2 nanocomposite combines a high active surface area with a high chemical stability, generating a novel material with a synergistic effect that enhances water splitting process performance. Thus, an outstanding low Tafel slope of 54 mV dec−1 is accomplished in the hydrogen evolution reaction.

On the activity and stability of Sb2O3/Sb nanoparticles for the electroreduction of CO2 toward formate

The development of new electrocatalysts with improved properties for the electrochemical reduction of CO 2 is being the objective of innumerable contributions. In this contribution, the electrochemical reduction of CO 2 to formate on novel carbon-supported antimony nanoparticles (Sb/C NPs) is studied. The carbon-supported Sb 2 O 3 /Sb nanoparticles were synthesised using a simple methodology at room temperature and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The Sb 2 O 3 /Sb/C electrodes were prepared by air-brushed onto a carbon paper and were characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The Sb 2 O 3 /Sb/C electrodes were also electrochemically characterized by cyclic voltammetry and displayed a clear activity for CO 2 electroreduction. To evaluate the properties of the electrodes for the reduction of CO 2 , electrolyses at different potentials were systematically conducted. Hydrogen and formate were the only products found. The faradic efficiency towards formate was found to be about 20%. This FE was in good agreement with previous finding obtained with comparable system. Interestingly, 24-hour tests were carried out to analyse the stability of the material under working conditions. The results indicate that Sb 2 O 3 /Sb-based electrodes display a remarkably better stability than other nanostructured electrocatalysts such as Sn or Bi.

Microchemical Engineering in a 3D Ordered Channel Enhances Electrocatalysis.

The kinetics of electrode reactions including mass transfer and surface reaction is essential in electrocatalysis, as it strongly determines the apparent reaction rates, especially on nanostructured electrocatalysts. However, important challenges still remain in optimizing the kinetics of given catalysts with suitable constituents, morphology, and crystalline design to maximize the electrocatalytic performances. We propose a comprehensive kinetic model coupling mass transfer and surface reaction on the nanocatalyst-modified electrode surface to explore and shed light on the kinetic optimization in electrocatalysis. Moreover, a theory-guided microchemical engineering (MCE) strategy has been demonstrated to rationally redesign the catalysts with optimized kinetics. Experimental measurements for methanol oxidation reaction in a 3D ordered channel with tunable channel sizes confirm the calculation prediction. Under the optimized channel size, mass transfer and surface reaction in the channeled microreactor are both well regulated. This MCE strategy will bring about a significant leap forward in structured catalyst design and kinetic modulation.

Self-Assembly of 0D-2D Heterostructure Electrocatalyst from MOF and MXene for Boosted Lithium Polysulfide Conversion Reaction.

The design of nanostructured electrocatalysts with high activity and long-term durability for the sluggish lithium polysulfide (LiPS) conversion reaction is essential for the development of high-performance lithium-sulfur (Li-S) batteries. Here, the self-assembly of bimetallic selenides on nitrogen-doped MXene (CoZn-Se@N-MX) based on the self-assembly of metal-organic framework and MXene is reported. A combination of 0D CoZn-Se nanoparticles and 2D N-MX nanosheet co-catalysts forms double lithiophilic-sulfifilic binding sites that effectively immobilize and catalytically convert LiPS intermediates. This 0D-2D heterostructure catalyst has a hierarchical porous architecture with a large active area and enables rapid Li ion diffusion, reduces the activation energy of Li2 S deposition, and lowers the energy barrier of Li2 S dissolution. In addition, an assembled CoZn-Se@N-MX hybrid synergistically prevents the aggregation of the CoZn-Se nanoparticles and restacking of the active areas of N-MX nanosheets during assembly and the LiPS conversion process. The Li-S battery with this 0D-2D catalyst delivers excellent rate capability, ultralong cycling life (over 2000 cycles), and a high areal capacity of 6.6 mAh cm-2 with a low electrolyte/sulfur ratio of 5µL mg-1 .

Recent advances in nanostructured electrocatalysts for hydrogen evolution reaction

Recent advances in nanostructured electrocatalysts for hydrogen evolution reaction

A Zeolitic-Imidazole Framework-Derived Trifunctional Electrocatalyst for Hydrazine Fuel Cells.

Hydrazine fuel cells are promising sustainable power sources. However, the high price and limited reserves of noble metal catalysts that promote the sluggish cathodic and anodic electrochemical reactions hinder their practical applications. Reflecting the enhanced diffusion and improved kinetics of nanostructured non-noble metal electrocatalysts, we report an efficient zeolitic-imidazole framework-derived trifunctional electrocatalyst for hydrazine oxidation, oxygen, and hydrogen peroxide reduction. Experimental results and theoretical calculations corroborate that the nanocarbon architecture with abundant Co-N species enhances the electronic interaction and optimizes the energy barriers of anodic hydrazine oxidation and cathodic oxygen reduction. The resulting assembled hydrazine-oxygen fuel cell yields a cell voltage and power density of 0.74 V and 20.5 mW cm-2, respectively. Moreover, benefiting from the liquid-liquid diffusion, the hydrazine-hydrogen peroxide cell shows a boosted cell voltage and power density, corresponding to 1.68 V and 41.0 mW cm-2. This work offers a highly active non-noble metal multifunctional electrocatalyst with a pioneering diffusion philosophy in the liquid electrochemical cells.

(Invited) Highly Stable Nanocarbon Catalysts for Reversible Energy Storage and Conversion Via Oxygen Electrocatalysis

Energy conversion and storage via direct electrochemical oxidation and reduction processes, such as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), can provide a clean, efficient, and economically viable means of meeting this sustainability challenge, by allowing stored renewable energy to be reliably delivered where and when it is needed. During the development of the highly demanded nonprecious metal catalysts to replace precious metals for these applications, carbon catalysts have many advantages over metal oxides, including excellent electrical conductivity, high surface area, and easy functionalization. However, most carbon-based catalysts suffer from poor activity and stability for the more challenging OER under a highly oxidative conditions. This represents one of grand challenges in energy conversion research societies.For the first time, this work demonstrated that a new class of large-diameter tubular nanocarbon (>500 nm in diameter) catalyst is capable of catalyzing both the ORR and the OER with activity comparable to Pt and exceeding Ir, respectively, for these two reactions. Most importantly, the tube catalyst exhibited good stability upon cycling across a wide potential window (0 to 1.9 V vs RHE) in 0.1 M NaOH electrolyte. The high OER activity was verified using well-designed electrochemical tests, demonstrating that the current generated during the OER arose solely from oxygen evolution, rather than carbon oxidation. This is the first example of such excellent OER activity and durability of a nanostructured carbon-based electrocatalyst across the complete ORR-OER potential window. The newly developed carbon tubes were synthesized via a one-step template-free graphitization process using inexpensive dicyandiamide (DCDA) precursor as carbon/nitrogen source and a ternary FeCoNi alloy as a catalyst. Generation of a mixed metal catalyst, i.e., FeCoNi during the graphitization process is the key factor to yield the largest tube size for maximum electrochemically accessible surface areas, dominant graphitic nitrogen doping, and thicker tube walls, which are associated with the significantly improved activity and durability. The cost effective, easily scalable, single step and environmentally benign synthesis procedure further favors this nanocarbon’s application as a practical bifunctional electrocatalyst for the ORR and OER, with potential to advance the development and commercialization of regenerative fuel cell systems and rechargeable metal-air batteries. The use of nanostructured carbon as bifunctional ORR/OER catalysts will open up new research direction in this field and further advance the role of carbon nanomaterials in oxygen electrocatalysis for energy conversion and storage.

Electrolysis on a Chip with Tunable Thin Film Nanostructured PGM Electrocatalysts Generated from Self-Assembled Block Copolymer Templates.

Self-assembled block copolymers are promising templates for fabricating thin film materials with tuned periodic feature sizes and geometry at the nanoscale. Here, a series of nanostructured platinum and iridium oxide electrocatalysts templated from poly(styrene)-block-poly(vinyl pyridine) (PSbPVP) block copolymers via an incipient wetness impregnation (IWI) pathway is reported. Both nanowire and nanocylinder electrocatalysts of varying feature sizes are assessed and higher catalyst loadings are achieved by the alkylation of the pyridine moieties in the PVP block prior to IWI. Electrocatalyst evaluations featuring hydrogen pump and water electrolysis demonstrations are carried out on interdigitated electrode (IDE) chips flexible with liquid supporting electrolytes and thin film polymer electrolytes. Notably, the mass activities of the nanostructured electrocatalysts from alkylated block copolymer templates are 35%-94% higher than electrocatalysts from non-alkylated block copolymer templates. Standing cylinder nanostructures lead to higher mass activities than lamellar variants despite their not having the largest surface area per unit catalyst loading demonstrating that mesostructure architectures have a profound impact on reactivity. Overall, IDE chips with model thin film electrocatalysts prepared from self-assembled block copolymers offer a high-throughput experimental method for correlating electrocatalyst nanostructure and composition to electrochemical reactivity.

The dual role of the surface oxophilicity in the electro-oxidation of ethanol on nanostructured Pd/C in alkaline media

• Pd/C electrocatalysts are used for amperometric sensors and also for Fuel Cells. • Size-induced effects on Pd/C resulted in distinct tendency to interact with oxygen. • Electrocatalytic activity towards ethanol oxidation depends on the surface oxophilicity. • PdO acts both as a promoter and/or inhibitor towards ethanol oxidation. • The catalytic activity can be enhanced by 57% only by varying surface oxophilicity. The ability of a surface to interact with oxygen (oxophilicity) plays a very important role in several catalytic, electroanalytic and electrocatalytic reactions. To date, however, the role of the oxophilicity in the electrochemical oxidation of ethanol was never shown for pure nanostructured Pd electrocatalysts supported on carbon, a promising material to be used as a non-enzymatic amperometric sensors for ethanol and also as an electrocatalyst for the Direct Ethanol Fuel Cells. In this work, Pd nanoparticles were prepared by chemical reduction at room temperature in the presence of varying amounts of sodium citrate in order to produce particles with distinct sizes. Then, we used cyclic voltammetry in 0.1 mol L -1 KOH to obtain an oxophilicity trend among various nanostructured Pd/C samples. The oxidation of ethanol in alkaline media demonstrated that the surface oxophilicity has a dual role in the kinetics of the reaction: a promoter at low and medium oxygen coverages, and a spectator (inhibitor) species at high surface coverages. Thus, by a proper control of the surface oxophilicity the current density at a constant potential can be enhanced by 57%, highlighting the importance of that parameter in the electro-oxidation on ethanol on Pd-based materials. Our results have a great impact for the design of more active non-enzymatic ethanol sensors and also for the development of more active electrocatalysts for Direct Ethanol Fuel Cells.

Scalable Synthesis of Hollow MoS2 Nanoparticles Modified on Porous Ni for Improved Hydrogen Evolution Reaction

Developing an inexpensive non-noble metal catalyst is essential in the electrochemical water reduction. However, to choose the support for hydrogen-producing catalyst is still a problem that needs to be solved. Herein, we report a novel route combining powder metallurgy and hydrothermal synthesis to fabricate Ni/MoS2 catalysts for boosting the hydrogen evolution reaction performance. Powder metallurgy produces the suborbicular pores on the surface and inside Ni once the sintering temperature reaches 1000 °C. The hollow MoS2 nanoparticles are successfully modified on the surface of porous Ni by hydrothermal synthesis. The hollow structure of MoS2 nanoparticles provides more active sites for electrochemical reaction and the porous Ni matrix with the porosity of 16.5% exhibits higher electronic conductivity, which endows the Ni/MoS2−1000 catalyst with an excellent hydrogen evolution reaction activity. The Ni/MoS2−1000 shows a low overpotential of 229 mV at the current densities of 10 mA·cm−2 with a small Tafel slope of 76 mV·dec−1. This study provides guidelines on the large-scale synthesis of nanostructured electrocatalysts with porous Ni as the support.

Benchmarking of oxygen evolution catalysts on porous nickel supports

Summary Active and inexpensive oxygen evolution reaction (OER) electrocatalysts are needed for energy-efficient electrolysis applications. Objective comparison between OER catalysts has been blurred by the use of different supports and methods to evaluate performance. Here, we selected nine highly active transition-metal-based catalysts and described their synthesis, using a porous nickel foam and a new Ni-based dendritic material as the supports. We designed a standardized protocol to characterize and compare the catalysts in terms of structure, activity, density of active sites, and stability. NiFeSe- and CoFeSe-derived oxides showed the highest activities on our dendritic support, with low overpotentials of η100 ≈ 247 mV at 100 mA cm–2 in 1 M KOH. Stability evaluation showed no surface leaching for 8 h of electrolysis. This work highlights the most active anode materials and provides an easy way to increase the geometric current density of a catalyst by tuning the porosity of its support.