Recent Progress on Fe/N/C Electrocatalysts for the Oxygen Reduction Reaction in Fuel Cells

In order to reduce the overall system cost, the development of inexpensive, high-performance and durable oxygen reduction reaction (ORR)N, Fe-codoped carbon-based (Fe/N/C) electrocatalysts to replace currently used Pt-based catalysts has become one of the major topics in research on fuel cells. This review paper lays the emphasis on introducing the progress made over the recent five years with a detailed discussion of recent work in the area of Fe/N/C electrocatalysts for ORR and the possible Fe-based active sites. Fe-based materials prepared by simple pyrolysis of transition metal salt, carbon support, and nitrogen-rich small molecule or polymeric compound are mainly reviewed due to their low cost, high performance, long stability and because they are the most promising for replacing currently used Pt-based catalysts in the progress of fuel cell commercialization. Additionally, Fe-base catalysts with small amount of Fe or new structure of Fe/Fe3C encased in carbon layers are presented to analyze the effect of loading and existence form of Fe on the ORR catalytic activity in Fe-base catalyst. The proposed catalytically Fe-centered active sites and reaction mechanisms from various authors are also discussed in detail, which may be useful for the rational design of high-performance, inexpensive, and practical Fe-base ORR catalysts in future development of fuel cells

Regeneration and Enhanced Catalytic Activity of Pt/C Electrocatalysts

By adding pure carbon support to improve the redispersion of platinum (Pt), a sintered Pt/C electrocatalyst for methanol electrooxidation was effectively regenerated in activity and doubled in amount on the basis of a one-step liquid oxychlorination. The apparent activity (mA mg_cata.^–1) of the optimal Pt/C regenerated (Pt 3.3 wt %) is close to the initial fresh Pt/C (Pt 10 wt %) and about two times that of fresh Pt/C (Pt 3.3 wt %), making Pt utilization doubled and then the resource-limited Pt potentially sustainable. The new nucleation of metal atoms on added pure support surface was found to be the key for both the improved redispersion of metal nanoparticles and the effective regeneration of catalytic activity in situ

Highly Efficient Regeneration of Deactivated Au/C Catalyst for 4‑Nitrophenol Reduction

In the present work, we proposed an effective method to regenerate sintered gold catalyst by improving the dispersion of gold nanoparticles. With the liquid oxychlorination reaction, a sintered carbon-supported gold (Au/C) nanocatalyst was effectively regenerated by improving the redispersion of Au nanoparticles with additional carbon support. The Au-catalyzed model reaction between 4-nitrophenol and sodium borohydride (NaBH₄) indicates that the apparent activity of the optimal Au/C regenerated (Au 0.45 wt %) exceeds that of the initial fresh Au/C (Au 1 wt %), making Au utilization tripled and potentially sustainable for their extensive application in industry

Theoretical Study of Resorufin Reduction Mechanism by NaBH₄

In the current work, the whole reduction mechanism of resorufin by sodium borohydride (NaBH₄) has been investigated completely using quantum chemical theory for the first time. The possible pathways for each step were considered as much as possible. The calculated results reveal that the reduction mechanism for resorufin undergoes a nucleophilic addition with BH₄^–, a synchronous proton abstraction from a carbon (C) atom, a protonation in a nitrogen (N) atom, and then a final hydrolysis process to obtain final reduced product dihydroresorufin. Interestingly, it was found that the protonation of N atom could induce a reduced product molecule with a Λ-type structure rather than a planar one, and the large alteration in geometry will induce different optical properties, such as fluorescent or nonfluorescent. More importantly, countercation Na⁺ and solvation effect of H₂O play important roles in reducing the activation energy in elementary steps, and their stabilization effect has been confirmed by NBO analysis. The detailed theoretical investigation for the reduction reaction of resorufin by NaBH₄ will support some guidance for the similar reduction reaction for organic compounds like aldehydes and ketones

Influence of bimodal non-basal texture on microstructure characteristics, texture evolution and deformation mechanisms of AZ31 magnesium alloy sheet rolled at liquid-nitrogen temperature

Cryogenic rolling experiments have been conducted on the AZ31 magnesium (Mg) alloy sheet with bimodal non-basal texture, which is fabricated via the newly developed equal channel angular rolling and continuous bending process with subsequent annealing (ECAR-CB-A) process. Results demonstrate that this sheet shows no edge cracks until the accumulated thickness reduction reaches about 18.5%, which is about 105.6% larger than that of the sheet with traditional basal texture. Characterization experiments including optical microstructure (OM), X-ray diffractometer (XRD), and electron backscatter diffraction (EBSD) measurements are then performed to explore the microstructure characteristics, texture evolution and deformation mechanisms during cryogenic rolling. Experimental observations confirm the occurrence of abundant {10–12} extension twins (ETs), twin-twin interactions among {10–12} ET variants and {10–12}-{10–12} double twins (DTs). The twinning behaviors as for {10–12} ETs are responsible for the concentration of c-axes of grains towards normal direction (ND) and the formation of transverse direction (TD)-component texture at the beginning of cryogenic rolling. The twinning behaviors with respect to {10–12}-{10–12} DTs are responsible for the disappearance of TD-component texture at the later stage of cryogenic rolling. The involved deformation mechanisms can be summarized as follows: Firstly {10–12} ETs dominate the plastic deformation. Subsequently, dislocation slip, especially basal slip, starts to sustain more plastic strain, while {10–12} ETs occur more frequently and enlarge continuously, resulting in the formation of twin-twin interaction among {10–12} ET variants. With the increasing rolling passes, {10–12}-{10–12} DTs incorporate in the plastic deformation and dislocation slip serves as the major one to sustain plastic strain. The activities of basal slip, {10–12} ETs and {10–12}-{10–12} DTs benefit in accommodating the plastic strain in sheet thickness, which contributes to the improved rolling formability in AZ31 Mg alloy sheet with bimodal non-basal texture during cryogenic rolling

Quantified effect of quench rate on the microstructures and mechanical properties of an Al–Mg–Si alloy

It is well known that lower quench rate leads to the formation of precipitate free zones (PFZs) adjacent to grain boundaries in aging hardening materials such as Al–Mg–Si alloys. However, the combined effect of PFZs and intragranular precipitates on the yield strength of Al–Mg–Si alloys is not systematically understood. To clarify this, an Al–Mg–Si alloy was water/oil/air quenched after solid solution treatment, then aged at 180 °C for 6 h. It was found that a lower quench rate led to coarser precipitates and wider PFZs. The effects of PFZs and precipitates on the yield strength of the Al–Mg–Si alloy were separated by quantitative characterization of multi-scale microstructures and mechanical simulations. Wide PFZs were found to have a notable effect on yield strength, amounting over half the strengthening effect of intragranular precipitates. This effect increases as the width of PFZ increases, due to more serious strain localization caused by PFZs. A corresponding equation has thus been proposed to describe the deleterious effects of PFZs on yield strength of Al–Mg–Si alloys

Growth Mechanism Deconvolution of Self-Limiting Supraparticles Based on Microfluidic System

The synthesis of colloidal supraparticles (SPs) based on self-assembly of nanoscopic objects has attracted much attention in recent years. Here, we demonstrate the formation of self-limiting monodisperse gold SPs with core–shell morphology based on the building blocks of flexible nanoarms in one step. A flow-based microfluidic chip is utilized to slow down the assembly process of the intermediates, which surprisingly allows for observation of ultrathin gold nanoplates as first intermediates. Notably, these intermediate cannot be observed in traditional synthesis due to their rapid rolling-up to form the second-order nanostructure of flexible hollow nanoarms. The growth mechanism of SPs can then be deconvoluted into two seed-mediated steps. Monte Carlo simulations confirm that the self-limiting growth of binary SPs is governed by a balance between electrostatic repulsion and van der Waals attraction

Morphology-Tuning-Induced Highly Efficient Regeneration of Pt/C Nanoelectrocatalysts

The sintering-induced irreversible deactivation of precious metal nanocatalysts is one of the main obstacles for their sustainable application. Here, by adding fluorine (F)-doped carbon support to improve the redispersion and tune the morphology of platinum (Pt) nanoparticles, a sintered Pt/C electrocatalyst for methanol electro-oxidation was completely regenerated in activity and doubled in amount of catalyst and tripled in metal utilization. The morphology-tuning-induced highly efficient regeneration based on F-doped carbon was further confirmed from the regeneration of a spent Pt/C for fuel cells. The quantum chemical calculation shows that the improved redispersion and the morphology transformation of Pt nanocatalyst could be mainly attributed to the strong interaction between <i>d</i> states of Pt and <i>p</i> states of F doped on carbon. The work presented here indicates that the morphology tuning of metal nanocatalysts is another way for highly efficient regeneration of precious metal nanocatalysts. It also opens a new pathway for people to get new functional materials with tunable morphology sustainably

Pt_0.61Ni/C for High-Efficiency Cathode of Fuel Cells with Superhigh Platinum Utilization

Exploring advanced electrocatalysts to accelerate the sluggish oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) is a promising route to alleviate the current challenges of fossil fuel exhaustion and environment pollution. Herein, a carbon-supported highly dispersed PtNi nanocatalyst (Pt_0.61Ni/C) with low platinum content of 2.76 wt % was prepared simply based on galvanic replacement for a high-efficiency ORR process. It presents a mass activity of ∼5 times of the conventional Pt-based catalyst at 0.9 V (vs reversible hydrogen electrode (RHE)) and a remarkable durability and methanol tolerance. The acidic fuel cell with such cathode catalyst (Pt_0.61Ni/C) presents a striking performance with maximum power density up to 1.1 W cm^–2 at 80 °C. Due to the extremely low platinum loading in the whole fuel cell, its Pt utilization (0.093 g_Pt kW^–1) is the highest reported ever in H₂/O₂ fuel cells. Such impressive performance of Pt_0.61Ni/C makes the obtained Pt_0.61Ni/C catalyst a very promising alternative to conventional Pt-based catalysts for their large-scale application in future