Scalable One-Pot Fabrication of Carbon-Nanofiber-Supported Noble-Metal-Free Nanocrystals for Synergetic-Dependent Green Hydrogen Production: Unraveling Electrolyte and Support Effects.

Electrocatalytic hydrogen evolution reactions (HER) are envisaged as the most promising sustainable approach for green hydrogen production. However, the considerably high cost often associated with such reactions, particularly upon scale-up, poses a daunting challenge. Herein, a facile, effective, and environmentally benign one-pot scalable approach is developed to fabricate MnM (M═Co, Cu, Ni, and Fe) nanocrystals supported over in situ formed carbon nanofibers (MnM/C) as efficient noble-metal-free electrocatalysts for HER. The formation of carbon nanofibers entails impregnating cellulose in an aqueous solution of metal precursors, followed by annealing the mixture at 550 °C. During the impregnation process, cellulose acts as a reactor for inducing the in situ reductions of MnM salts with the assistance of ether and hydroxyl groups to drive the mass production (several grams) of ultralong (5 ± 1 µM) carbon nanofibers ornamented with MnM nanoparticles (10-14 nm in size) at an average loading of 2.87 wt %. For better electrocatalytic HER benchmarking, the fabricated catalysts were tested over different working electrodes, i.e., carbon paper, carbon foam, and glassy carbon, in the presence of different electrolytes. All the fabricated MnM/C catalysts have demonstrated an appealing synergetic-effect-dependent HER activity, with MnCo/C exhibiting the best performance over carbon foam, close to that of the state-of-the-art commercial Pt/C (10 wt % Pt), with an overpotential of 11 mV at 10 mA cm-2, a hydrogen production rate of 2448 mol g-1 h-1, and a prolonged stability of 2 weeks. The HER performance attained by MnCo/C nanofibers is among the highest reported for Pt-free electrocatalysts, thanks to the mutual alloying effect, higher synergism, large surface area, and active interfacial interactions over the nanofibers. The presented findings underline the potential of our approach for the large-scale production of cost-effective electrocatalysts for practical HER.

Electronic and Geometric Effects Endow PtRh Jagged Nanowires with Superior Ethanol Oxidation Catalysis.

Complete ethanol oxidation reaction (EOR) in C1 pathway with 12 transferred electrons is highly desirable yet challenging in direct ethanol fuel cells. Herein, PtRh jagged nanowires synthesized via a simple wet-chemical approach exhibit exceptional EOR mass activity of 1.63 A mgPt-1 and specific activity of 4.07 mA cm-2 , 3.62-fold and 4.28-folds increments relative to Pt/C, respectively. High proportions of 69.33% and 73.42% of initial activity are also retained after chronoamperometric test (80 000 s) and 1500 consecutive potential cycles, respectively. More importantly, it is found that PtRh jagged nanowires possess superb anti-CO poisoning capability. Combining X-ray absorption spectroscopy, X-ray photoelectron spectroscopy as well as density functional theory calculations unveil that the remarkable catalytic activity and CO tolerance stem from both the Rh-induced electronic effect and geometric effect (manifested by shortened Pt─Pt bond length and shrinkage of lattice constants), which facilitates EOR catalysis in C1 pathway and improves reaction kinetics by reducing energy barriers of rate-determining steps (such as *CO → *COOH). The C1 pathway efficiency of PtRh jagged nanowires is further verified by the high intensity of CO2 relative to CH3 COOH/CH3 CHO in infrared reflection absorption spectroscopy.

Integration of multiple advantages into one catalyst: non-CO pathway of methanol oxidation electrocatalysis on surface Ir-modulated PtFeIr jagged nanowires.

Developing highly active methanol oxidation electrocatalysts with superior anti-CO poisoning capability remains a grand challenge. Herein, a simple strategy was employed to prepare distinctive PtFeIr jagged nanowires with Ir located at the shell and Pt/Fe located at the core. The Pt64Fe20Ir16 jagged nanowire possesses an optimal mass activity of 2.13 A mgPt-1 and specific activity of 4.25 mA cm-2, giving the catalyst a great edge over PtFe jagged nanowire (1.63 A mgPt-1 and 3.75 mA cm-2) and Pt/C (0.38 A mgPt-1 and 0.76 mA cm-2). The in-situ Fourier transform infrared (FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS) unravel the origin of extraordinary CO tolerance in terms of key reaction intermediates in the non-CO pathway. Density functional theory (DFT) calculations add to the body of evidence that the surface Ir incorporation transforms the selectivity from CO pathway to non-CO pathway. Meanwhile, the presence of Ir serves to optimize surface electronic structure with weakened CO binding strength. We believe this work will advance the understanding of methanol oxidation catalytic mechanism and provide some insight into structural design of efficient electrocatalysts.

Anti-dissolution Pt single site with Pt(OH)(O3)/Co(P) coordination for efficient alkaline water splitting electrolyzer.

As the most well-known electrocatalyst for cathodic hydrogen evolution in water splitting electrolyzers, platinum is unfortunately inefficient for anodic oxygen evolution due to its over-binding with oxygen species and excessive dissolution in oxidative environment. Herein we show that single Pt atoms dispersed in cobalt hydrogen phosphate with an unique Pt(OH)(O3)/Co(P) coordination can achieve remarkable catalytic activity and stability for oxygen evolution. The catalyst yields a high turnover frequency (35.1 ± 5.2 s-1) and mass activity (69.5 ± 10.3 A mg-1) at an overpotential of 300 mV and excellent stability. Mechanistic studies elucidate that the superior catalytic performance of isolated Pt atoms herein stems from optimal binding energies of oxygen intermediate and also their strong electronic coupling with neighboring Co atoms that suppresses the formation of soluble Ptx>4 species. Alkaline water electrolyzers assembled with an ultralow Pt loading realizes an industrial-level current density of 1 A cm-2 at 1.8 volts with a high durability.

PtIrM (M = Ni, Co) jagged nanowires for efficient methanol oxidation electrocatalysis.

It remains a huge challenge to develop methanol oxidation electrocatalysts with remarkable catalytic activity and anti-CO poisoning capability. Herein, PtIrNi and PtIrCo jagged nanowires are successfully synthesized via a facile wet-chemical approach. Pt and Ir components are concentrated in the exterior and Ni is concentrated in the interior of PtIrNi jagged nanowires, while PtIrCo jagged nanowires feature the homogeneous distribution of constituent metals. The PtIrNi and PtIrCo jagged nanowires exhibit mass activities of 1.88 A/mgPt and 1.85 A/mgPt, respectively, 3.24 and 3.19 times higher than that of commercial Pt/C (0.58 A/mgPt). In-situ Fourier transform infrared spectroscopy indicates that CO2 was formed at a very low potential for both nanowires, in line with the high ratio of forward current density to backward current density for PtIrNi jagged nanowires (1.30) and PtIrCo jagged nanowires (1.46) relative to Pt/C (0.76). Also, the CO stripping and X-ray photoelectron spectroscopy results substantiate the remarkable CO tolerance of the jagged nanowires. Besides, the two jagged nanowires possess exceptional activities toward ethanol and ethylene glycol oxidation reactions. This work provides a novel line of thought in terms of rational design of alcohol oxidation electrocatalysts with distinctive nanostructures.

Facile Synthesis of Surfactant-Induced Platinum Nanospheres with a Porous Network Structure for Highly Effective Oxygen Reduction Catalysis.

Developing a facile and eco-friendly method for the large-scale synthesis of highly active and stable catalysts toward oxygen reduction reaction (ORR) is very important for the practical application of proton exchange membrane fuel cells (PEMFCs). In this paper, a mild aqueous-solution route has been successfully developed for the gram-scale synthesis of three-dimensional porous Pt nanospheres (Pt-NSs) that are composed of network-structured nanodendrites and/or oval multipods. In comparison with the commercial Pt/C catalyst, X-ray photoelectron spectroscopy (XPS) demonstrates the dominant metallic-state of Pt and electrochemical impedance spectroscopy (EIS) indicates the substantial improvement of conductivity for the Pt-NSs/C catalyst. The surfactant-induced porous network nanostructure improves both the catalytic ORR activity and durability. The optimal Pt-NSs/C catalyst exhibits a half-wave potential of 0.898 V (vs. RHE), leading to the mass activity of 0.18 A mgPt -1 and specific activity of 0.68 mA cm-2 which are respectively 1.9 and 5.7 times greater than those of Pt/C. Moreover, the highly-active Pt-NSs/C catalyst shows a superior stability with the tenable morphology and the retained 78% of initial mass activity rather than the severe Pt aggregation and the only 58% retention of the commercial Pt/C catalyst after 10000 cycles.

Sub-Monolayer YOx /MoOx on Ultrathin Pt Nanowires Boosts Alcohol Oxidation Electrocatalysis.

A crucial issue restricting the application of direct alcohol fuel cells (DAFCs) is the low activity of Pt-based electrocatalysts for alcohol oxidation reaction caused by the reaction intermediate (CO*) poisoning. Herein, a new strategy is demonstrated for making a class of sub-monolayer YOx /MoOx -surface co-decorated ultrathin platinum nanowires (YOx /MoOx -Pt NWs) to effectively eliminate the CO poisoning for enhancing methanol oxidation electrocatalysis. By adjusting the amounts of YOx and MoOx decorated on the surface of ultrathin Pt NWs, the optimized 22% YOx /MoOx -Pt NWs achieve a high specific activity of 3.35 mA cm-2 and a mass activity of 2.10 A mgPt -1 , as well as the enhanced stability. In situ Fourier transform infrared (FTIR) spectroscopy and CO stripping studies confirm the contribution of YOx and MoOx to anti-CO poisoning ability of the NWs. Density functional theory (DFT) calculations further reveal that the surface Y and Mo atoms with oxidation states allow COOH* to bind the surface through both the carbon and oxygen atoms, which can lower the free energy barriers for the oxidation of CO* into COOH*. The optimal NWs also show the superior activities toward the electro-oxidation of ethanol, ethylene glycol, and glycerol.

Exclusive Strain Effect Boosts Overall Water Splitting in PdCu/Ir Core/Shell Nanocrystals.

Core/shell nanocatalysts are a class of promising materials, which achieve the enhanced catalytic activities through the synergy between ligand effect and strain effect. However, it has been challenging to disentangle the contributions from the two effects, which hinders the rational design of superior core/shell nanocatalysts. Herein, we report precise synthesis of PdCu/Ir core/shell nanocrystals, which can significantly boost oxygen evolution reaction (OER) via the exclusive strain effect. The heteroepitaxial coating of four Ir atomic layers onto PdCu nanoparticle gives a relatively thick Ir shell eliminating the ligand effect, but creates a compressive strain of ca. 3.60%. The strained PdCu/Ir catalysts can deliver a low OER overpotential and a high mass activity. Density functional theory (DFT) calculations reveal that the compressive strain in Ir shell downshifts the d-band center and weakens the binding of the intermediates, causing the enhanced OER activity. The compressive strain also boosts hydrogen evolution reaction (HER) activity and the strained nanocrystals can be served as excellent catalysts for both anode and cathode in overall water-splitting electrocatalysis.

Spiny Pd/PtFe core/shell nanotubes with rich high-index facets for efficient electrocatalysis.

The performance of fuel-cell related electrocatalysis is highly dependent on the morphology, size and composition of a given catalyst. In terms of rational design of Pt-based catalyst, one-dimensional (1D) ultrafine Pt alloy nanowires (NWs) are considered as a commendable model for enhanced catalysis on account of their favorable mass/charge transfer and structural durability. However, in order to achieve the noble metal catalysts in higher efficiency and lower cost, building high-index facets and shaping hollow interiors should be integrated into 1D Pt alloy NWs, which has rarely been done so far. Here, we report the first synthesis of a class of spiny Pd/PtFe core/shell nanotubes (SPCNTs) constructed by cultivating PtFe alloy branches with rich high-index facets along the 1D removable Pd supports, which is driven by the galvanic dissolution of Pd substrates concomitant with Stranski-Krastanov (S-K) growth of Pt and Fe, for achieving highly efficient fuel-cells-related electrocatalysis. This new catalyst can even deliver electrochemical active surface area (ECSA) of 62.7 m2 gPt-1, comparable to that of commercial carbon-supported Pt nanoparticles. With respect to oxygen reduction catalysis, the SPCNTs showcase the remarkable mass and specific activity of 2.71 A mg-1 and 4.32 mA cm-2, 15.9 and 16.0 times higher than those of commercial Pt/C, respectively. Also, the catalysts exhibit extraordinary resistance to the activity decay and structural degradation during 50,000 potential cycles. Moreover, the SPCNTs serve as a category of efficient and stable catalysts towards anodic alcohol oxidation.

Palladium Single Atoms on TiO2 as a Photocatalytic Sensing Platform for Analyzing the Organophosphorus Pesticide Chlorpyrifos.

Traditional methods for analyzing organophosphorus pesticide chlorpyrifos, usually require the tedious sample pretreatment and sophisticated bio-interfaces, leading to the difficulty for real-time analysis. Herein, we use palladium single-atom (PdSA)/TiO2 as a photocatalytic sensing platform to directly detect chlorpyrifos with high sensitivity and selectivity. PdSA/TiO2 , prepared by an in situ photocatalytic reduction of PdCl42- on the TiO2 , shows much higher photocatalytic activity (10 mol g-1 h-1 ) for hydrogen evolution reaction than Pd nanoparticles (1.95 mol g-1 h-1 ), and excellent stability. In the presence of chlorpyrifos, the photocatalytic activity of PdSA/TiO2 decreases. Through this inhibition effect the platform can realize a detection limit for chlorpyrifos of 0.01 ng mL-1 , much lower than the maximum residue limit (10 ppb) permitted by the U.S. Environmental Protection Agency.

Strain engineering of metal-based nanomaterials for energy electrocatalysis.

The strain effect, along with the ligand effect and synergistic effect, contributes primarily to the optimization of electrocatalytic activity and stability. The strain effect leads to a shift in the d-band center and alters binding energies toward adsorbates. Under electrocatalytic circumstances, the strain effect and ligand effect by and large function in combination; however, the decay and vanishing of the ligand effect precede the strain effect as the thickness of the shell in the core/shell structure or metallic overlayers on substrates increases. The strain effect on electrocatalytic activity can be well engineered by tuning the thickness of shells or atomic composition. Microstrain, or localized lattice strain, is another type of strain associated with structural defects such as grain boundaries and multi-twinning. In this review, we discuss the origin of the strain effect and how it affects electrocatalytic activity based on the d-band model. We present the structural characterization and quantitative determination of strain. Metal-based nanocrystals are basically grouped into two types of structures to which the strain engineering applies, i.e. lattice strain-associated structures (which include the general core/shell structure and solid solution alloy) and multiple defects-induced structures. Then analysis is performed on the correlation of strain and ligand effects and on the tuning strategies of the strain effect for electrocatalysis. After that, we use representative examples to demonstrate how strain engineering assists in typical electrocatalytic reactions on anodes and cathodes. Finally, we summarize and propose potential research areas in terms of enhancing electrocatalytic activities by strain engineering in the future.

MXene/Si@SiO x@C Layer-by-Layer Superstructure with Autoadjustable Function for Superior Stable Lithium Storage.

Despite its very high capacity (4200 mAh g-1), the widespread application of the silicon anode is still hampered by severe volume changes (up to 300%) during cycling, which results in electrical contact loss and thus dramatic capacity fading with poor cycle life. To address this challenge, 3D advanced Mxene/Si-based superstructures including MXene matrix, silicon, SiO x layer, and nitrogen-doped carbon (MXene/Si@SiO x@C) in a layer-by-layer manner were rationally designed and fabricated for boosting lithium-ion batteries (LIBs). The MXene/Si@SiO x@C anode takes the advantages of high Li+ ion capacity offered by Si, mechanical stability by the synergistic effect of SiO x, MXene, and N-doped carbon coating, and excellent structural stability by forming a strong Ti-N bond among the layers. Such an interesting superstructure boosts the lithium storage performance (390 mAh g-1 with 99.9% Coulombic efficiency and 76.4% capacity retention after 1000 cycles at 10 C) and effectively suppresses electrode swelling only to 12% with no noticeable fracture or pulverization after long-term cycling. Furthermore, a soft package full LIB with MXene/Si@SiO x@C anode and Li[Ni0.6Co0.2Mn0.2]O2 (NCM622) cathode was demonstrated, which delivers a stable capacity of 171 mAh g-1 at 0.2 C, a promising energy density of 485 Wh kg-1 based on positive active material, as well as good cycling stability for 200 cycles even after bending. The present MXene/Si@SiO x@C becomes among the best Si-based anode materials for LIBs.

Synthesis of Si-Induced MnO/Mn2SiO4@C Cuboids as High-Performance Anodes for Lithium-Ion Batteries.

The exploration of anode materials of lithium-ion batteries (LIBs) is still a great challenge because of their low electrical conductivity and poor durability. Transition-metal oxides are proposed as a potential alternative, even though their dimension and structure greatly affect their electrochemical properties. In this study, MnO/Mn2SiO4@C cuboids were prepared via the polymerization-pyrolysis process. Larger MnCO3 precursor particles embedded into a monolithic carbon framework and formed smaller nanoparticles owing to the inducing effect of Si element in phthalocyanino silicon (SiPc), giving MnO/Mn2SiO4@C cuboids. The micron-scaled cuboid composite can lead to higher tap density and greater electrical performance due to lower interparticle resistance. Therefore, the as-prepared MnO/Mn2SiO4@C electrode exhibits stable specific capacities of 585.9 and 423.9 mA h g-1 after 1000 discharge/charge cycles at 1 and 2 A g-1, respectively. Meanwhile, an excellent rate capacity of 246.3 mA h g-1 was achieved even at 30 A g-1. Additionally, this facile and economical strategy to improve electrode performance provides a commercially feasible way for the construction of high-performance LIBs.

One Step Synthesis of Uniform SnO2 Electrode by UV Curing Technology toward Enhanced Lithium-Ion Storage.

A uniform anode material composed of ultrasmall tin oxide (SnO2) nanoparticles with an excellent lithium-ion (Li-ion) storage performance is obtained for the first time through one step UV curing technology. The diameter of ∼3 nm-sized SnO2 particles is uniformly dispersed in the styrylpyridinium (SbQ) polymer because of its photo-cross-linking property. The in situ cross-linking of SbQ polymer not only assist synthesis of uniform ultrasmall SnO2, but act as a strong adhesion binder on SnO2 nanoparticles, thereby effectively accommodating the volume expansion of SnO2 anodes during cycling process. The uniform electrode exhibits substantially higher specific capacity and longer cycling stability compared with the SnO2 nanoparticles electrodes treated by traditional PVDF-mixing method. A stable specific capacity of 572.5 mA h g-1 of the SnO2 electrode derived from UV curing technology is obtained at a current density of 0.2 C (156.2 mA g-1) after 150 cycles. Even at high rate of 5 C (3905 mA g-1), the electrode still demonstrates specific capacity of 440.2 mA h g-1. Therefore, the scalable and low-cost synthetic approach described herein can readily be extended to other nanomaterials electrodes to improve their lithium-storage properties.

Layered Transition Metal Oxynitride Co3Mo2OxN6-x/C Catalyst for Oxygen Reduction Reaction.

Transition metal oxynitrides have now garnered growing interest in our quest for highly efficient alternatives to Pt in direct methanol alkaline fuel cells. Herein, carbon supported Co3Mo2OxN6-x was synthesized via a simple two-step approach wherein the reactants undergo refluxing and heat treatment in NH3. For the as-prepared Co3Mo2OxN6-x catalyst, uniformly dispersed on XC-72, with the particle size averaged at 5 nm, the catalytic activities toward oxygen reduction reaction in alkaline media are related to the commercial Pt/C, such as the comparable onset potential (0.9 V vs RHE), half-wave potential (0.8 V vs RHE), and even higher specific activity (82.7 mA cm-2 at 0.7 V). Significantly, the Co3Mo2OxN6-x catalyst was highly stable in terms of 95% current retention after 12 h chronoamperometry measurement, indicative of favorable prospect for the non-noble cathodic catalyst in alkaline fuel cell.