Hard carbon (HC) is widely regarded as one of the most promising anode materials for commercial sodium-ion batteries due to its excellent electrochemical performance and cost-effectiveness. Although organic polymers offer compositional homogeneity and structural tunability as HC precursors, their high raw material costs and uncontrollable carbonization processes limit large-scale applications. Here, we introduce a liquid-phase carbonization strategy to recycle waste polyethylene terephthalate (PET) into porous micro/nanostructured HC enriched with intrinsic carbon defects (LHC-3, LHC = liquid-phase-prepared hard carbon). These carbon defects and the morphological structures were modulated by bubbles generated from the decomposition of PET in the presence of N,N’-dimethylformamide and zinc acetate. The synergistic effects between intrinsic carbon defects and micro/nanostructure endow LHC-3 anode with high specific capacity (355 mAh·g⁻¹ at 0.1 A·g⁻¹), superfast charging capability (132.6 mAh·g⁻¹ input within 13 s of charging), and ultralong cycling stability (100,000 stable cycles at 50 A·g⁻¹). The sodium storage mechanism of LHC-3 anode was investigated by ex-situ Raman spectroscopy, X-ray photoelectron spectroscopy, and ion diffusion kinetics analysis. Theoretical calculations indicate that intrinsic carbon defects with non-zero curvature structure in LHC-3 enhance its ability to accommodate more Na⁺. These findings are expected to have broader applications in energy storage and waste management.

Heterojunction nanocomposite electrocatalysts with porous structures and large specific surface areas show great potential in improving their intrinsic activity and the number of accessible active sites for oxygen evolution reaction (OER). Herein, we describe an “exchanging sulfur for oxygen” protocol to fabricate a porous molybdate-based heterojunction electrocatalyst, Fe₂(MoO₄)₃/CoMoO₄, utilizing a sulfur-rich reagent, ammonium tetrathiomolybdate ((NH₄)₂MoS₄). During the calcination of the solid product formed from (NH₄)₂MoS₄ and CoCl₂/FeCl₃, the sulfur atoms of MoS₄²⁻ are oxidized into the acidic SO₂ gas plus HCl and NH₃ gases evolved in the system, which greatly facilitates the formation of macro/mesopores of the molybdate-based nanomaterial. It exhibits excellent electrocatalytic OER performance in alkaline media and only requires a low overpotential of 244 mV at a current density of 10 mA·cm⁻² with outstanding durability. Experimental examination and theoretical calculations reveal that its uniform interparticle porous structure enhances spatial connectivity and electrode–electrolyte contact, while strong electronic interactions at the heterointerface boost electrocatalytic activity. The phase combination increases interface electron concentration, accelerates charge transfer, and lowers free energy. This work provides a new strategy to construct the porous molybdate-based heterostructure electrocatalyst for remarkably boosting the OER performance.

The coexistence of multi-component active sites like single-atom sites, diatomic sites (DAS) and nanoclusters is shown to result in superior performances in the hydrogen evolution reaction (HER). Metal diatomic sites are more complex than single-atom sites but their unique electronic structures can lead to significant enhancement of the HER kinetics. Although the synthesis and identification of DAS is usually challenging, we report a simple access to a diatomic catalyst by anchoring Co-Ru DAS on nitrogen-doped carbon supports along with Ru nanoparticles (NPs). Experimental and theoretical results revealed the atomic-level characteristics of Co-Ru sites, their strong electronic coupling and their synergy with Ru NPs within the catalyst. The unique electronic structure of the catalyst resulted in an excellent HER activity and stability in alkaline media. This work provides a valuable insight into a widely applicable design of diatomic catalysts with multi-component active sites for highly efficient HER electrocatalysis.

The synergistic catalysis of heterojunction electrocatalysts for the multi-step process in hydrogen evolution reaction (HER) is a promising approach to enhance the kinetics of alkaline HER. Herein, we proposed a strategy to form nanoscale Ni/NiO heterojunction porous graphitic carbon composites (Ni/NiO-PGC) by reduction-pyrolysis of the preformed Ni-metal-organic framework (MOF) under H₂/N₂ atmosphere. Benefiting from low electron transfer resistance, increased number of active sites, and unique hierarchical micro-mesoporous structure, the optimized Ni/NiO-PGC_10-1-400 exhibited excellent electrocatalytic performance and robust stability for alkaline HER (η₁₀ = 30 mV, 65 h). Density functional theory (DFT) studies revealed that the redistribution of electrons at the Ni/NiO interface enables the NiO phase to easily initiate the dissociation of alkaline H₂O, and shifts down the d-band center of Ni and optimizes the H* adsorption–desorption process of Ni, thereby leading to extremely high HER activity. This work contributes to a further understanding of the synergistic promotion of the multi-step HER processes by heterojunction electrocatalysts.

Using simple methods to obtain efficient catalysts has been a long-standing goal for researchers. In this work, the employment of a one-pot pyrolysis reaction to achieve molecular confinement, has led to the preparation of ruthenium (Ru)-based nanoclusters in a carbon matrix. A unique feature of the synthetic approach employed is that solvent and substrates were calcined together. As solvent evaporates, during calcination, the substrates form a dense solid which has the effect of limiting the aggregation of Ru centers during the carbonization process. The catalyst prepared in this simple manner showed an impressively high activity with respect to the hydrogen/oxygen evolution reaction (HER/OER). The Ru nanoclusters (Ru NCs), as the hydrogen evolution reaction (HER) catalysts, require ultralow overpotentials of 5 mV and 5.1 mV at –10 mA·cm^–2 in 1.0 M KOH, and 0.5 M H₂SO₄, respectively. Furthermore, the catalyst prepared by the one-pot method has higher crystallinity, a higher Ru content and an ultrafine cluster size, which contributes to its exceptional electrochemical performance. Meanwhile, the RuO_X nanoclusters (RuO_X NCs), obtained by oxidizing the aforementioned Ru NCs, exhibited good oxygen evolution reaction (OER) performance with an overpotential of 266 mV at 10 mA·cm^–2. When applied to overall water splitting, Ru/RuO_X nanoclusters as the cathode and anode catalysts can reach 10 mA·cm^–2 at cell voltages of only 1.49 V in 1 M KOH.

Modifying electrocatalysts nanostructures and tuning their electronic properties through defects-oriented synthetic strategies are essential to improve the oxygen evolution reaction (OER) performance of electrocatalysts. Current synthetic strategies about electrocatalysts mainly target the single or double structural defects, while the researches about the synergistic effect of multiple structural defects are rare. In this work, the ultrathin NiFe layered double hydroxide nanosheets with a holey structure, oxygen vacancies and Ni³⁺ defects on nickel foam (NiFe-LDH-NSs/NF) are prepared by employing a simple and green H₂O₂-assisted etching method. The synergistic effect of the above three defects leads to the exposure of more active sites and significant improvement of the intrinsic activity. The optimized catalyst exhibits an excellent OER performance with an extraordinarily low overpotential of 170 mV at 10 mA·cm⁻² and a small Tafel slope of 39.3 mV·dec⁻¹ in 1 M KOH solution. Density functional theory calculations reveal this OER performance arises from pseudo re-oxidized metal-stable Ni³⁺ near oxygen vacancies (O_vac), which suppresses 3d-e_g of Ni-site and elevates d-band center towards the competitively low electron-transfer barrier. This work provides a new insight to fabricate advanced electrocatalysts for renewable energy conversion technologies.