MOF-Derived 2D/3D Hierarchical N-Doped Graphene as Support for Advanced Pt Utilization in Ethanol Fuel Cell

A comparative study of α-Ni(OH)2 and Ni nanoparticle supported ZIF-8@reduced graphene oxide-derived nitrogen doped carbon for electrocatalytic ethanol oxidation

Ethanol electrooxidation is an important reaction for fuel cells, however, the major obstacle to ethanol electrocatalysis is the splitting of the carbon-carbon bond to CO2 at lower overpotentials. Herein, a ZIF-8@graphene oxide-derived highly porous nitrogen-doped carbonaceous platform containing zinc oxide was attained for supporting a non-precious Ni-based catalyst. The support was doped with the disordered α-phase Ni(OH)2 NPs and Ni NPs that are converted to Ni(OH)2 through potential cycling in alkaline media. The Ni-based catalysts exhibit high electroactivity owing to the formation of the NiOOH species which has more unpaired d electrons that can bond with the adsorbed species. From CV curves, the EOR onset potential of the α-Ni(OH)2/ZNC@rGO electrode is strongly shifted to negative potential (Eonset = 0.34 V) with a high current density of 8.3 mA cm-2 relative to Ni/ZNC@rGO. The high catalytic activity is related to the large interlayer spacing of α-Ni(OH)2 which facilitates the ion-solvent intercalation. Besides, the porous structure of the NC and the high conductivity of rGO facilitate the kinetic transport of the reactants and electrons. Finally, the catalyst displays a high stability of 92% after 900 cycles relative to the Ni/ZNC@rGO and commercial Pt/C catalysts. Hence, the fabricated α-Ni(OH)2/ZNC@rGO catalyst could be regarded as a potential catalyst for direct EOR in fuel cells.

Hierarchical copper-based metal-organic frameworks nanosheet assemblies for electrochemical ascorbic acid sensing

Noninvasive human health monitoring requires the development of efficient electrochemical sensors for the quantitative analysis of infinitesimal biomolecules. In this work, we reported a novel hierarchical nanosheet assemblies (HSA) of copper-based metal-organic frameworks (MOFs) as an electrochemical sensor for ascorbic acid (AA) detection. Copper 1,4-benzenedicarboxylate (CuBDC) HSA was constructed by three steps of in situ growth on stone paper, including hydrolysis, anion exchange, and heteroepitaxy growth. The monodispersed two-dimensional MOFs nanosheet units were aligned in an orderly manner and arranged into three-dimensional hierarchical assemblies. The CuBDC HSA-based AA sensor displayed a high sensitivity of 396.8 μA mM^-1 cm^-2 and a low detection limit of 0.1 μM. Excellent selectivity, stability and reproducibility were also obtained. Benefiting from the advantages of ultrathin nanosheets and nature-inspired hierarchy, this unique architecture facilitated reactant dispersion and maximized the accessible active sites and charge-transport capability and thus had superior catalytic ability for the electro-oxidation of ascorbic acid compared to bulk MOFs. Moreover, the CuBDC HSA sensor performed AA level detection in juice samples with acceptable accuracy and verified the feasibility for sweat AA sensing. This novel MOFs architecture holds great potential as an electrochemical sensor to detect AA for noninvasive human health monitoring in the future.

Pt-Co Electrocatalysts: Syntheses, Morphologies, and Applications

Pt-Co electrocatalysts have attracted significant attention because of their excellent performance in many electrochemical reactions. This review focuses on Pt-Co electrocatalysts designed and prepared for electrocatalytic applications. First, the various synthetic methods and synthesis mechanisms are systematically summarized; typical examples and core synthesis parameters are discussed for regulating the morphology and structure. Then, starting with the design and structure-activity relationship of catalysts, the research progress of the morphologies and structures of Pt-Co electrocatalysts obtained based on various strategies, the structure-activity relationship between them, and their properties are summarized. In addition, the important electrocatalytic applications and mechanisms of Pt-Co catalysts, including electrocatalytic oxidation/reduction and bifunctional catalytic reactions, are described and summarized, and their high catalytic activities are discussed on the basis of their mechanism and active sites. Moreover, the advanced electrochemical in situ characterization techniques are summarized, and the challenges and direction concerning the development of high-performance Pt-Co catalysts in electrocatalysis are discussed.

Exploiting the Synergistic Electronic Interaction between Pt-Skin Wrapped Intermetallic PtCo Nanoparticles and Co-N-C Support for Efficient ORR/EOR Electrocatalysis in a Direct Ethanol Fuel Cell

The development of low-Pt catalysts with high activity and durability is critical for fuel cells. Here, Pt-skin wrapped sub-5 nm PtCo intermetallic nanoparticles are successfully mounted on single atom Co-N-C support by exploiting the barrier effect of Co-anchor. According to a collaborative experimental and computational investigation, the increased oxygen reduction reaction activity of PtCo/Co-N-C arises from the direct electron transfer from PtCo to Co-N-C, and the resulting optimal d-band center of Pt. Owing to such unique electronic structure interaction and synergistic effect, the specific and mass activities of PtCo/Co-N-C are up to 4.20 mA cm^-2 and 2.71 A mg_Pt^-1 , respectively, with barely degraded stability after 40 000 CV cycles. The PtCo/Co-N-C also exhibits outstanding activity as an ethanol electrocatalyst. This work shows a new and effective route to boost the overall efficiency of direct ethanol fuel cells in acidic media by integrating intermetallic low-Pt alloys and single atom carbon support.

Self-Assembled Pt/MoCx/MWCNTs Nano Catalyst for Ethanol Electrooxidation of Fuel Cells

Direct ethanol fuel cells (DEFCs) have attracted more and more attention because of their unique advantages such as low cost and low toxicity. However, sluggish C-C bond cleavage during the ethanol electrooxidation reaction (EOR) in acidic media results in a lower energy yield and gravely hinders the commercialization of DEFCs. Therefore, it is very necessary to develop an anode catalyst with high performance, high stability and low cost to solve this problem. In this paper, Pt/MoC_x/MWCNTs nanocomposites with different mass ratios of PtMo were obtained through a molecular self-assembly technology. The structure and morphology of Pt/MoC_x/MWCNTs nanocomposites were characterized by several techniques such as XRD, FESEM, XPS, etc. The electrochemical performance and stability of Pt/WC_x/MWCNTs electrocatalysts toward EOR were investigated in acid electrolytes. The results show that PtMo exists in the form of alloy. The size of Pt/MoC_x nanoparticles is very uniform with an average size of ∼24 nm. The Pt/MoC_0.25/MWCNTs exhibits excellent electrocatalytic activities with an electrochemically active surface area of 37.1 m² g⁻¹, a peak current density of 610.4 mA mg_Pt⁻¹ and a steady-state current density of 39.8 mA mg_Pt⁻¹ after 7,200 s, suggesting that the Pt/MoC_0.25/MWCNTs is a very promising candidate for application in EOR of DEFCs.

Dual Inorganic Sacrificial Template Synthesis of Hierarchically Porous Carbon with Specific N Sites for Efficient Oxygen Reduction

It is still a challenge to achieve efficiently controlled preparation of functional oxygen reduction reaction (ORR) carbon electrocatalysts with multi-preferred structures (hierarchically porous networks and specific carbon-nitrogen bonds) from carbohydrate-containing small molecules via simple one-step pyrolysis. Based on the step-by-step spontaneous gas-foaming strategy, we successfully prepare 3D hierarchically porous networks with tunable N sites (N_P/N_G ≈ 1:1) by pyrolyzing diverse carbohydrates (glucose, maltose, and cyclodextrin) using nonmetal-metal dual inorganic sacrificial templates. In situ evaporation templates can simplify the procedure of the experiments and avoid the active site loss compared with traditional hard templates. Crucially, dual inorganic sacrificial templates can induce abundant defects and microscopic pore structures (the specific surface area increased from 922.403 to 1898.792 m²·g^-1) and tunable N sites compared with single nonmetal sacrificial templates. The regulatory mechanism of dual inorganic templates on N sites (N_P/N_G ≈ 1:1) is independent of the polymeric state of carbohydrate precursors or even the carbonization condition of the pyrolysis process. A series of carbon materials prepared by this strategy all have ORR-preferred structures and exhibit low ORR overpotentials compared with Pt/C. For instance, the Zn-air battery with βCD-DSC-950-1 exhibits an open-circuit potential of 1.51 V and a peak power density of 180.89 mW·cm^-2, higher than those of Pt/C (1.47 V, 174.94 mW·cm^-2). In general, the conversion of carbohydrate-containing small molecules to functional carbon materials provides a new strategy for the development of carbonaceous electrocatalysts.