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Carbon materials doped with sulfur (S) and nitrogen (N) are prospective alternatives to Pt-based catalysts as oxygen reduction reaction (ORR) catalyst since they are inexpensive and highly catalytic active. However, the further advancement of catalysts is greatly hindered for its unclear and equivocal catalytic sites. Here, using density functional theory studies, we find that those inactive carbon atoms next to graphitic N become ORR catalytic sites after S doping, which is distinctly different from N-doped carbon materials where catalytic sites are introduced by pyridinic N. The electrochemical results show that increasing the content of graphitic N can lead to higher catalytic activity for S and N dual-doped carbon materials, which exhibits higher turnover frequency towards ORR than that of N-doped carbon materials. This work provides insights for further advancement of electrocatalysts via heteroatom doping.

The development of commercial lithium–sulfur (Li–S) batteries is typically restricted by the intrinsic drawbacks of the dissolutiion and shuttling of lithium polysulfides (LPS) and the uncontrollable growth of lithium dendrites. To address these non-ignorable challenges, a multifunctional composite nanofiber-based separator consisting of an electrospinning polyimide (PI) substrate and a polyamide–polyvinyl alcohol (PA–PVA) top layer is designed and fabricated via interfacial polymerization (IP). In this situation, its superior features, such as desirable wettability, suitable porosity, excellent electrolyte uptake and retention, and high thermal tolerance, can be obtained. Besides, the architecture of the highly negatively charged PA–PVA top layer with a dense Turing structure can not only provide an electronegative environment to accelerate the Li+ transmembrane transfer as well as simultaneously repel the negatively charged polysulfide ions, but also enable ordered Li+ migration for homogeneous Li deposition on the anode. Due to these advanced structural characteristics, the Li–S battery assembled with the PI/PA–PVA separator gives a high initial specific capacity of 1499 mA h g−1 at 0.1C and a stable discharge capacity of 852 mA h g−1 with an ultralow fading rate of 0.1% per cycle after 500 cycles at 0.2C. More importantly, this PI/PA–PVA top layer has the functional characteristics of melting and self-closing at high temperatures, endowing the Li–S battery with excellent safety performance.

A better solid-liquid-gas three-phase boundary is vital for low energy cost oxygen evolution reaction (OER), making the designed regulation of interfacial atmosphere necessary. Herein, we find that the OER electrocatalysis can be dramatically improved by synergistically forming disordered electronic structures and superficial amorphous layers, as superficial oxyhydroxide, phosphorus-doped NiFe2O4 nanoarrays on nitrogen-doped carbon nanofibers (OP-NiFe2O4/NCNFs). Unveiled by the depth-profiling analysis from the X-ray photoelectron spectroscopy, the contents of phosphorous doping in the OP-NiFe2O4 nanoarrays change dynamically from outside to inside due to its in situ superficial reconstruction into the oxyhydroxide layer, thereby accelerating electron transfer between heterogeneous phases. As revealed by density functional theory calculations, this amorphous oxyhydroxide layer and dynamically varied phosphorous content would positively shift the d-band center of the NiFe2O4-related compounds, leading to lower adsorption energy towards water molecule. Thus, the OP-NiFe2O4/NCNF electrocatalyst displays a low overpotential of 260 mV at a current density of 10 mA cm-2 and a small Tafel slope of 44.8 mV dec-1 in an alkaline medium.

Fabrication of nitrogen-doped hollow carbon nanostructures is of great importance for achieving efficient electron and ion transport as a metal-free electrocatalyst. Herein, we report a step-wise polymerization and carbonization route to prepare N-doped hollow carbon nanoflowers (N-HCNFs) with a high nitrogen content up to 5.3 at%. A preformed covalent triazine framework (CTF) network assembled on a melamine-cyanuric acid (MCA) supermolecular crystal was achieved via a step-wise polymerization. The subsequent carbonization was conducted to obtain hollow and porous carbon materials with a unique flower-shape. Excellent electrochemical oxygen reduction reaction (ORR) performance with a positive half-wave potential of 0.84 V (vs. RHE) was achieved with excellent stability and methanol resistance in alkaline media. Furthermore, for the hydrogen evolution reaction (HER), a low overpotential of 243 mV at a current density of 10 mA cm−2 and a small Tafel slope of 111 mV dec−1 in acidic media were shown.

The exploration of a noble-metal-free and nitrogen-doped carbon (M-N/C) composite electrocatalyst for the oxygen reduction reaction (ORR) remains a great challenge. The activities of the M-N/C composite electrocatalysts are mainly affected by the metal active sites, pyridinic nitrogen, and graphitic nitrogen. In the present work, the iron-coordinated self-assembly is proposed for the preparation of iron-chelating pyridine nitrogen-rich coordinated nanosheet (IPNCN) composites as an electrocatalyst. Due to the highly conjugated structure of the IPNCN precursor, the pyridine nitrogen elements at both ends of the tetrapyrido [3,2-a:2',3'-c:3'',2''-h:2''',3'''-j] phenazine (TP) provide the multiple ligands, and the coordination interactions between the irons and the pyridine nitrogen further improve the thermodynamic stability, where the metal active sites and nitrogen elements are uniformly distributed in the whole structure. The resultant IPNCN composites exhibit excellent ORR performance with an onset potential of 0.93 V and a half potential of 0.84 V. Furthermore, the IPNCN composite electrocatalysts show the higher methanol resistance and electrochemical durability than the commercial Pt/C catalysts. It could be convinced that the as-designed IPNCN composite catalysts would be a promising alternative to the noble metal Pt-based catalysts in the practical applications.

Flexible paper electrodes with high conductivity and hierarchical porosity have been intensely required to develop next-generation supercapacitors with high energy density and power density. Herein, glucose-derived hydrothermal carbon nanospheres (CNSs) are chemically activated into highly porous carbon nanospheres (HPCNSs), which are uniformly dispersed by graphene oxide (GO) in water. Upon filtration, the resultant GO/HPCNS composites are assembled into a flexible composite paper of HPCNSs embedded in a parallelly arranged GO framework. Upon chemical reduction from GO to reduced graphene oxide (rGO), the rGO/HPCNS composite paper as a binder-free electrode facilitates the assembly of flexible supercapacitors. The rGO/HPCNS composite paper with optimized rGO/HPCNS ratio delivers high specific capacitance (235.3 F g −1 at 0.2 A g−1) and high cyclic stability (capacitance retention of 91% at 5 A g−1 after 5000 charge/discharge cycles), attributing to their excellent structural stability, 3D conductive network and hierarchical porosity.

Developing active and stable electrocatalysts of earth-abundant elements towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) still remains a crucial challenge. Herein, a cobalt-containing metal-organic framework using adenine as a ligand was synthesized and pyrolyzed without any other precursors, forming a cobalt nanoparticle-embedded nitrogen-doped carbon/carbon nanotube framework (Co@N-CNTF). Due to the abundant active sites of homogeneously distributed cobalt nanoparticles within nitrogen-doped graphitic layers, the resultant Co@N-CNTF catalysts exhibit an efficient and stable electrocatalytic performance as a tri-functional catalyst towards the ORR, OER and HER, including a high half-wave potential of 0.81 V vs. RHE for the ORR, and a low overpotential at 10 mA cm -2 for the OER (0.35 V) and HER (0.22 V). As a proof-of-concept, the Co@N-CNTF as an OER/HER bifunctional catalyst for full water splitting affords an alkaline electrolyzer with 10 mA cm -2 under a stable voltage of 1.71 V. Moreover, an integrated unit of a water-splitting electrolyzer using the Co@N-CNTF catalysts, which is powered with a rechargeable Zn-air battery using the Co@N-CNTF as an ORR/OER bifunctional catalyst on air electrodes, can operate under ambient conditions with high cycling stability, demonstrating the viability and efficiency of the self-powered water-splitting system.

Hydrogen evolution reaction (HER) has been severely suppressed by the first proton adsorption step, due to the “shackles” of the surrounding water in the form of hydronium ions (H13O6+). Here, it has been found anionic vacancies in NiCo2A4 (A = O, S, Se) can weaken the shackles of surrounding water in H13O6+, resulting into efficient capture of H+ to form a H+-enriched field. Taking selenium vacancy-rich NiCo2Se4 nanowires on nitrogen-doped carbon nanofibers as an example, it displays higher electrocatalytic activities with low overpotential of 168 mV at 10 mA cm−2 and a Tafel slope of 49.8 mV dec−1. The HER performance is strongly related to the anionic vacancy size, the larger the anionic vacancy size is (VSe > VS > VO), the higher the HER activity does. Guided by density functional theory calculations, it is found that the point defect structures can not only strengthen its interfacial linkage force with H+ by weakening the hydration force of contiguous hydronium ion, but also increase its charge density for efficient electron transfer to adsorbed H+. Therefore, this work creates a useful strategy to optimize the HER performance of traditional bimetallic compounds by engineering the solid-liquid-gas three-phase interfacial interaction.

The sodium-ion storage property in two-dimensional transition metal dichalcogenides (TMDs) is still seriously confined due to the lacking of efficient pathways for Na+ insertion, which enlightens the construction of multi-dimensional ion channels a necessary. Herein, we prepared interlayer defect, sulfur vacancy-contained ReS2 nanosheets on porous nitrogen-doped carbonized bacterial cellulose (dr-ReS2-x/NCBC). In such dual-defect configuration, the interlayer defects provide interconnected in-plane/interlamination channels for offering extra pathways of Na+ insertion/extraction and shortening ionic diffusion distance, while the sulfur vacancy could further enhance the electronic conductivity and induce more active sites for Na+ storage. Therefore, the dr-ReS2-x/NCBC anode displays an enhanced rate capacity (231.2 mAh g−1 at 5 A g−1) and a good cycling stability (187.3 mAh g−1 at 5 A g−1 after 500 cycles). Furtherly revealed by density functional theory calculations, the sodium-ion storage property is attributed to its negatively shifted binding energy for sodium ions (from −0.771 to −1.791/-1.244 eV), and alleviated structural change (from 5.8% to 3.4%/-1.6%) during sodiation/desodiation processes. The dr-ReS2-x/NCBC anode was also assembled coupling with Na3V2(PO4)3 cathode as a sodium-ion full cell for practical applications. This work is expected to offer an in-depth understanding between dual-defect engineering in TMDs-based anodes and as-enhanced sodium-storage performance.

The development of earth-abundance electrocatalyst with high performance for oxygen evolution reaction (OER) is of paramount importance in sustainable water splitting. Herein, the novel defect-induced nitrogen-doped carbon-supported Co3O4 nanoparticles is successfully fabricated as OER electrocatalyst (denoted as Co3O4/CN HNPs) through a wetness-impregnation treatment of Co/polyaniline (PANI) followed by a thermal annealing. This advanced architecture of Co3O4/CN HNPs can not only improve its conductivity and electrocatalytically active sites but also generate a large number of oxygen-vacancy defects and crystal defects, which effectively exert the preponderance in facilitating interfacial electronic transfer and optimizing the adsorption energy for intermediates, thus imparting the extraordinary activities in catalyzing OER. In addition, there are evidences demonstrating the formation of C-N coordination bonds through the strong interaction of the interconnected interface and the generation of pyridinic-N species after the annealing treatment, which enables the structural stability to get further strengthened and accelerates oxygen releasing for reduction of OER overpotential, respectively. Benefiting from the above desirable properties, the Co3O4/CN HNPs affords a lower overpotential of 290 mV at a current density of 10 mA cm−2 as compared to those of pure Co3O4 and PANI, outperforming commercial IrO2 and the representative Co3O4-based OER electrocatalysts as recently reported. Moreover, the Co3O4/CN HNPs also exhibits long durability with negligible activity degeneration at a current density of 10 mA cm−2 for 20 h. .

The lack of precise control of particle sizes is the critical challenge in the assembly of 3D interconnected transition-metal oxide (TMO) for newly-emerging energy conversion devices. A self-embedded templating strategy for preparing the TMO@carbon quasiaerogels (TMO@C-QAs) is proposed. By mimicking an aerogel structure at a microscale, the TMO@C-QA successfully assembles size-controllable TMO nanoparticles into 3D interconnected structure with surface-enriched carbon species. The morphological evolutions of intermediates verify that the self-embedded Ostwald ripening templating approach is responsible for the dual-channel TMO@C-QA formation. The general self-embedded templating strategy is easily extended to prepare various TMO@C-QAs, including the Co3O4@C-QA, Mn3O4@C-QA, Fe2O3@C-QA, and NiO@C-QA. Benefiting from the unparalleled 3D interconnected network of aerogels, the Co3O4@C-QA displays superior bifunctional catalytic activities for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), as well as high specific capacity and excellent long-term stability for lithium-ion battery (LIB) anode. A proof-of-concept battery-powered electrolyzer with Co3O4@C-QA cathode and anode powered by a full LIB with Co3O4@C-QA anode is presented. The battery-powered electrolyzer made of the state-of-the-art TMOs can exhibit great competitive advantages due to its supreme multifunctional energy conversion performance for future water electrolysis.