Rational design and facile preparation of non-noble trifunctional electrocatalysts with high performance, low cost and strong durability for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are highly demanded, but remain as a big challenge. Herein, we report a spontaneous gas-foaming method to prepare nitrogen doped ultrathin carbon nanosheets (NCNs) by simply pyrolysing a mixture of citric acid and NH₄Cl. Under the optimized pyrolysis temperature (carbonized at 1000 °C) and mass ratio of precursors (1 : 1), the synthesized NCN-1000-5 sample possesses an ultrathin sheet structure, an ultrahigh specific surface area (1793 m² g⁻¹), and rich edge defects, and exhibits low overpotential and robust stability for the ORR, OER and HER. By means of density functional theory (DFT) computations, we revealed that the intrinsic active sites for the ORR, OER and HER are the carbon atoms located at the armchair edge and adjacent to the graphitic N dopants. When practically used as a catalyst in rechargeable Zn–air batteries, a high energy density (806 W h kg⁻¹), a low charge/discharge voltage gap (0.77 V) and an ultralong cycle life (over 330 h) were obtained at 10 mA cm⁻² for NCN-1000-5. This work not only presents a versatile strategy to develop advanced carbon materials with ultrahigh specific surface area and abundant edge defects, but also provides useful guidance for designing and developing multifunctional metal-free catalysts for various energy-related electrocatalytic reactions.

The glorious charge transfer efficiency of photoanode is an important factor for efficient photoelectrochemical (PEC) water oxidation. However, it is often limited by slow kinetics of oxygen evolution reaction. Herein, a dual transition metal‐based metal–organic frameworks (MOF) cocatalyst, Fe@Ni–MOF, is introduced into a titanium‐doped hematite (Fe ₂ O ₃ :Ti) photoanode. The combination of Ni and Fe can optimize the filling of 3d orbitals. Moreover, the introduction of Fe donates electrons to Ni in the MOF structure, thus, suppressing the irreversible (long‐life‐time) oxidation of Ni ²⁺ into Ni ³⁺ . The resulting Fe@Ni–MOF/Fe ₂ O ₃ :Ti photoanode exhibits ∼threefold enhancement in the photocurrent density at 1.23 V versus the reversible hydrogen electrode. Kinetic analysis of the PEC water oxidation processes indicates that this performance improvement is primarily due to modulating the charge transfer efficiency of hematite photoanode. Further results show that a single transition metal‐based MOF cocatalyst, Ni–MOF, exhibits slow charge transfer in spite of a reduction in surface charge recombination, resulting in a smaller charge transfer efficiency. These findings provide new insights for the development of photoelectrodes decorated with MOFs.

The sluggish kinetics of air electrodes largely limits the practical applications of metal–air batteries. At present, preparing single-atom catalysts confined in ultrathin N-doped porous carbon (NPC) with a high surface area remains a challenge. Herein, a facile strategy by complexation of biomass and metal ions combined with the gas-foaming method was used to synthesize substantial Co–N₄ active sites on ultrathin NPC with a super-high specific surface area of 1977.9 m² g⁻¹. The catalyst owns brilliant oxygen reduction reaction properties with higher half-wave potential of 0.863 V and faster kinetics process (68.3 mV dec⁻¹) than those of Pt/C (0.856 V and 80.46 mV dec⁻¹, respectively). Remarkably, it exhibits distinguished reversibility with a high initial cycle efficiency of 60.8% and satisfactory stability when used in Zn–air batteries. Moreover, the assembled Al–air battery displays preeminent discharge performance with ultrahigh power density (494 mW cm⁻²) and energy density (2387 W h kg⁻¹) at 200 mA cm⁻². This study opens a new avenue to fully utilize biomass for constructing Co single-atom catalysts for metal–air batteries.