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Magnesium (Mg)-doped Fe3O4 nanoparticles represented as Mg x Fe3–x O4 (0 < x ≤ 1) were analyzed in comparison with zinc (Zn)-doped Fe3O4, Zn x Fe3–x O4. Magnetization versus applied magnetic field for Mg x Fe3–x O4 particles provided the maximum saturation magnetization (M s) with 69.37 emu/g at x = 0.1 as superparamagnetism, while the M s by Zn-doping was 80.93 emu/g at x = 0.2. The crystal unit volume (V c) by Mg doping at x ≤ 0.5 was constant, but the crystal size decreased with increasing x. Doping beyond x = 0.6 provided small amorphous power aggregates which offer universal dielectric response, implying a highly disordered system. In contrast, the V c by Zn doping expanded up to x = 0.4 as the acceptable limit, which was attributed to the large difference between doping ion radius and replaced Fe3+ ion radius. On the other hand, the Mg x Fe3–x O4 (0 ≤ x < 0.6) and Zn x Fe3–x O4 (0 ≤ x ≤ 0.4) formed by a crystal domain were analyzed by a three-circuit model with one normal parallel circuit and two circuits with resistance and a constant-phase element (CPE). The stability of capacity as the anode of lithium ion batteries was investigated for the composites prepared by adhering Mg2+, Zn2+, and Fe3+ on the sidewalls of as-modified multiwall carbon nanotubes. Among the ferrite composites, Zn0.2Fe2.8O4 provided the highest capacity with good stability under discharge and charge cycles.

High‐temperature proton‐exchange‐membrane fuel cells (HT‐PEMFCs) can offer improved energy efficiency and tolerance to fuel/air impurities. The high expense of the high‐temperature proton‐exchange membranes (HT‐PEMs) and their low durability at high temperature still impede their further practical applications. In this work, a phosphoric acid (PA)‐doped porous aromatic framework (PAF‐6‐PA) is incorporated into poly[2,2′‐(p‐oxydiphenylene)‐5,5′‐benzimidazole] (OPBI) to fabricate novel PAF‐6‐PA/OPBI composite HT‐PEMs through solution‐casting. The alkaline nitrogen structure in PAF‐6 can be protonated with PA to provide proton hopping sites, and its porous structure can enhance the PA retention in the membranes, thus creating fast pathways for proton transfer. The hydrogen bond interaction between the rigid PAF‐6 and OPBI can also enhance the mechanical properties and chemical stability of the composite membranes. Consequently, PAF‐6‐PA/OPBI exhibits an optimal proton conductivity of 0.089 S cm−1 at 200 °C, and peak power density of 437.7 mW cm−2 (Pt: 0.3 mg cm−2), which is significantly higher than that of the OPBI.   The PAF‐6‐PA/OPBI provides a novel strategy for the practical application of PBI‐based HT‐PEMs.

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