Synthesis and electrocatalytic activity of phosphorus-doped carbon xerogel for oxygen reduction

A 2D Nano-architecture (NPSMC@Ir-Ru@rGO) Derived from Graphene Enfolded Polyphosphazene Nanospheres Decorated Ir-Ru Metals (PZS@Ir-Ru@GO) towards Bifunctional Water Splitting

A leap-forward approach has been successfully devised to synthesize a novel hierarchical binary metal modified heteroatom doped 2D micro-/mesporous carbon-graphene nanostructure (NPSMC@Ir-Ru@rGO) for overall water splitting application. To investigate the role of decorating metals, different electrolcatalysts like NPSMC, NPSMC@rGO, NPSMC@Ir@rGO, and NPSMC@Ru@rGO were also synthesized and structural changes were compared and investigated by physiochemical techniques. All of the samples have shown electrocatalytic activities attributed to the presence of heteroatom (N, P, S) doped micro-/mesoporous carbonaceous matrix, amorphous carbon in the coexistence of graphitic lattice carbons, presence of active metal NPs (Ir and/-or Ru), an even distribution of active sites, and graphene 2D interconnected channels to promote electron transfer ability, respectively. However, the Ir-Ru metal codeped nanocatalyst (NPCMS@Ir-Ru@rGO) is proved to be an excellent electrocatalyst based on the synergistic role of Ir-Ru metals that necessitates the low overpotentials of 181 mV and 318 mV to convey a current density of 10 mA cm-2 towards the electroctalytic application of HER and OER, respectively. Furthermore, exhibiting the corresponding Tafel slopes (132 and 70 mV dec-1 ) in an alkaline medium. This work is anticipated to open up new avenues for the development of promising electrocatalysts based on active metals modified heteroatom doped carbon nanomaterials for energy applications.

Resorcinol-Formaldehyde-Derived Carbon Xerogels: Preparation, Functionalization, and Application Aspects

Carbon xerogels (CXs) are materials obtained via the pyrolysis of resins prepared via the sol-gel polycondensation of resorcinol and formaldehyde. These materials attract great attention as adsorbents, catalyst supports, and energy storage materials. One of the most interesting features of CXs is the possibility of fine-tuning their structures and textures by changing the synthesis conditions in the sol-gel stage. Thus, the first part of this review is devoted to the processes taking place in the polycondensation stage of organic precursors. The formation of hydroxymethyl derivatives of resorcinol and their polycondensation take place at this stage. Both of these processes are catalyzed by acids or bases. It is revealed that the sol-gel synthesis conditions, such as pH, the formaldehyde/resorcinol ratio, concentration, and the type of basic modifier, all affect the texture of the materials being prepared. The variation in these parameters allows one to obtain CXs with pore sizes ranging from 2-3 nm to 100-200 nm. The possibility of using other precursors for the preparation of organic aerogels is examined as well. For instance, if phenol is used instead of resorcinol, the capabilities of the sol-gel method become rather limited. At the same time, other phenolic compounds can be applied with great efficiency. The methods of gel drying and the pyrolysis conditions are also reviewed. Another important aspect analyzed within this review is the surface modification of CXs by introducing various functional groups and heteroatoms. It is shown that compounds containing nitrogen, sulfur, boron, or phosphorus can be introduced at the polycondensation stage to incorporate these elements into the gel structure. Thus, the highest surface amount of nitrogen (6-11 at%) was achieved in the case of the polycondensation of formaldehyde with melamine and hydroxyaniline. Finally, the methods of preparing metal-doped CXs are overviewed. Special attention is paid to the introduction of a metal precursor in the gelation step. The elements of the iron subgroup (Fe, Ni, Co) were found to catalyze carbon graphitization. Therefore, their introduction can be useful for enhancing the electrochemical properties of CXs. However, since the metal surface is often covered by carbon, such materials are poorly applicable to conventional catalytic processes. In summary, the applications of CXs and metal-doped CXs are briefly mentioned. Among the promising application areas, Li-ion batteries, supercapacitors, fuel cells, and adsorbents are of special interest.

Progress of Nonmetallic Electrocatalysts for Oxygen Reduction Reactions

As a key role in hindering the large-scale application of fuel cells, oxygen reduction reaction has always been a hot issue and nodus. Aiming to explore state-of-art electrocatalysts, this paper reviews the latest development of nonmetallic catalysts in oxygen reduction reactions, including single atoms doped with carbon materials such as N, B, P or S and multi-doped carbon materials. Afterward, the remaining challenges and research directions of carbon-based nonmetallic catalysts are prospected.

Molten-Salt-Assisted Synthesis of Nitrogen-Doped Carbon Nanosheets Derived from Biomass Waste of Gingko Shells as Efficient Catalyst for Oxygen Reduction Reaction

Developing superior efficient and durable oxygen reduction reaction (ORR) catalysts is critical for high-performance fuel cells and metal–air batteries. Herein, we successfully prepared a 3D, high-level nitrogen-doped, metal-free (N–pC) electrocatalyst employing urea as a single nitrogen source, NaCl as a fully sealed nanoreactor and gingko shells, a biomass waste, as carbon precursor. Due to the high content of active nitrogen groups, large surface area (1133.8 m2 g−1), and 3D hierarchical porous network structure, the as-prepared N–pC has better ORR electrocatalytic performance than the commercial Pt/C and most metal-free carbon materials in alkaline media. Additionally, when N–pC was used as a catalyst for an air electrode, the Zn–air battery (ZAB) had higher peak power density (223 mW cm−2), larger specific-capacity (755 mAh g−1) and better rate-capability than the commercial Pt/C-based one, displaying a good application prospect in metal-air batteries.

Accelerating the Oxygen Reduction Reaction and Oxygen Evolution Reaction Activities of N and P Co-Doped Porous Activated Carbon for Li-O2 Batteries

Rechargeable lithium–oxygen (Li-O2) batteries represent state-of-the-art electrochemical energy storage devices that provide high energy densities. However, their commercialization is challenging owing to their low charging/discharging efficiencies, short battery lives, high overpotentials, and high cathode manufacturing costs. In this study, we prepared a metal-free, N,P co-doped, porous activated carbon (N,P-PAC) electrode via KOH activation and P doping for application as a Li-O2 battery cathode. When used in a rechargeable Li-O2 battery, the N,P-PAC cathode showed a high specific discharge capacity (3724 mA h g−1 at 100 mA g−1), an excellent cycling stability (25 cycles with a limit capacity of 1000 mA h g−1), and a low charge/discharge voltage gap (1.22 V at 1000 mA h g−1). The N,P-PAC electrode showed a low overpotential (EOER-ORR) of 1.54 V. The excellent electrochemical performance of the N,P-PAC electrode can mainly be attributed to its large active area and oxygen-containing functional groups generated via KOH activation and P-doping processes. Therefore, the N,P-PAC prepared in this study was found to be a promising eco-friendly and sustainable metal-free cathode material for Li-O2 batteries.

Fabrication of high B-doped ordered mesoporous carbon with 4-hydroxyphenylborate phenolic resin for supercapacitor electrode materials

The high B-doped ordered mesoporous carbon (HPB-OMC) was prepared by using 4-hydroxyphenylboronic acid-modified phenolic resin (HPBPF) as a boron and carbon precursor via the evaporation-induced self-assembly (EISA) approach. The chemical composite, mesoporous structure, and electrochemical properties of the as-prepared HPB-OMC are investigated. The results show that both highly boron-doped and well-ordered mesoporous structure are achieved for HPB-OMCs, owing to the improvement of solubility of the resins in ethanol, and the enhancement of thermal stability of pore channels during carbonization. Moreover, the HPB-OMCs exhibit an ideal electric double-layer capacitor performance. With the increase of the B-doped content, the specific capacitance of the HPB-OMC electrode rises gradually, then drops off a little. The HPB-OMC with a high B content (3.96 wt%) shows a much high specific capacitance of 183 F g-1 at a current density of 1 A g-1, suggesting its promising application in the field of supercapacitors.

N, P and S co-doped carbon materials derived from polyphosphazene for enhanced selective U(VI) adsorption

Herein, the precursor polyphosphazene was synthesized by the polymerization of hexachlorocyclotriphosphazene (HCCP) and bis(4-hydroxyphenyl) sulfone (BPS). The adsorbent which was codoped with N, P and S (amidate-CS) was developed from the precursor by using the carbonization method. The images of Scanning electron microscope (SEM) and Transmission electron microscope (TEM) indicate that the amidate-CS possessed porous graphene-like carbon lamellar structure. The excellent behaviors with respect to kinetics (120 min for equilibrium) and thermodynamics (maximum removal of 290 mg/g when pH was at 6.0) revealed the outstanding performance of amidate-CS in removing U(VI), which is due to the functional groups and strong covalent bonds between heteroatoms and uranyl ions. The adsorption of amidate-CS followed the pseudo-second-order kinetic and Langmuir adsorption model. The thermodynamic parameters indicate that the process was spontaneous and endothermic. The adsorption and desorption efficiency of amidate-CS had a slight decrease after five cycles, indicating excellent regeneration performance. Overall, the amidate-CS is a prospective candidate for highly selective U(VI) removing.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.

Recent advances in two-dimensional materials and their nanocomposites in sustainable energy conversion applications

Two-dimensional (2D) materials have a wide platform in research and expanding nano- and atomic-level applications. This study is motivated by the well-established 2D catalysts, which demonstrate high efficiency, selectivity and sustainability exceeding that of classical noble metal catalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and/or hydrogen evolution reaction (HER). Nowadays, the hydrogen evolution reaction (HER) in water electrolysis is crucial for the cost-efficient production of a pure hydrogen fuel. We will also discuss another important point related to electrochemical carbon dioxide and nitrogen reduction (ECR and N2RR) in detail. In this review, we mainly focused on the recent progress in the fuel cell technology based on 2D materials, including graphene, transition metal dichalcogenides, black phosphorus, MXenes, metal-organic frameworks, and metal oxide nanosheets. First, the basic attributes of the 2D materials were described, and their fuel cell mechanisms were also summarized. Finally, some effective methods for enhancing the performance of the fuel cells based on 2D materials were also discussed, and the opportunities and challenges of 2D material-based fuel cells at the commercial level were also provided. This review can provide new avenues for 2D materials with properties suitable for fuel cell technology development and related fields.

Glucose-derived carbon materials with tailored properties as electrocatalysts for the oxygen reduction reaction

Nitrogen-doped biomass-derived carbon materials were prepared by hydrothermal carbonization of glucose, and their textural and chemical properties were subsequently tailored to achieve materials with enhanced electrochemical performance towards the oxygen reduction reaction. Carbonization and physical activation were applied to modify the textural properties, while nitrogen functionalities were incorporated via different N-doping methodologies (ball milling and conventional methods) using melamine. A direct relationship between the microporosity of the activated carbons and the limiting current density was found, with the increase of microporosity leading to interesting improvements of the limiting current density. Regardless of the doping method used, similar amounts of nitrogen were incorporated into the carbon structures. However, significant differences were observed in the nitrogen functionalities according to the doping method applied: ball milling appeared to originate preferentially quaternary and oxidized nitrogen groups, while the formation of pyridinic and pyrrolic groups was favoured by conventional doping. The onset potential was improved and the two-electron mechanism of the original activated sample was shifted closer to a four-electron pathway due to the presence of nitrogen. Interestingly, the high pyridinic content related to a high ratio of pyridinic/quaternary nitrogen results in an increase of the onset potential, while a decrease in the quaternary/pyrrolic nitrogen ratio favors an increase in the number of electrons. Accordingly, the electrocatalyst with the highest performance was obtained from the activated sample doped with nitrogen by the conventional method, which combined the most appropriate textural and chemical properties: high microporosity and adequate proportion of the nitrogen functionalities.

Synthesis of P-Doped and NiCo-Hybridized Graphene-Based Fibers for Flexible Asymmetrical Solid-State Micro-Energy Storage Device

Fiber supercapacitors (FSCs) are promising energy storage devices in portable and wearable smart electronics. Currently, a major challenge for FSCs is simultaneously achieving high volumetric energy and power densities. Herein, the microscale fiber electrode is designed by using carbon fibers as substrates and capillary channels as microreactors to space-confined hydrothermal assembling. As P-doped graphene oxide/carbon fiber (PGO/CF) and NiCo₂ O₄ -based graphene oxide/carbon fiber (NCGO/CF) electrodes are successfully prepared, their unique hybrid structures exhibit a satisfactory electrochemical performance. An all-solid-state PGO/CF//NCGO/CF flexible asymmetric fiber supercapacitor (AFSC) based on the PGO/CF as the negative electrode, NCGO/CF hybrid electrode as the positive electrode, and poly(vinyl alcohol)/potassium hydroxide as the electrolyte is successfully assembled. The AFSC device delivers a higher volumetric energy density of 36.77 mW h cm^-3 at a power density of 142.5 mW cm^-3 . In addition, a double reference electrode system is adopted to analyze and reduce the IR drop, as well as effectively matching negative and positive electrodes, which is conducive for the optimization and improvement of energy density. For the AFSC device, its better flexibility and electrochemical properties create a promising potential for high-performance micro-supercapacitors. Furthermore, the introduction of the double reference electrode system provides an interesting method for the study on the electrochemical performances of two-electrode systems.

A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

Metal–air batteries (MABs), particularly rechargeable MABs, have gained renewed interests as a potential energy storage/conversion solution due to their high specific energy, low cost, and safety. The development of MABs has, however, been considerably hampered by its relatively low rate capability and its lack of efficient and stable air catalysts in which the former stems mainly from the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the latter stems from the corrosion/oxidation of carbon materials in the presence of oxygen and high electrode potentials. In this review, various carbon-composited bifunctional electrocatalysts are reviewed to summarize progresses in the enhancement of ORR/OER and durability induced by the synergistic effects between carbon and other component(s). Catalyst mechanisms of the reaction processes and associated performance enhancements as well as technical challenges hindering commercialization are also analyzed. To facilitate further research and development, several research directions for overcoming these challenges are also proposed.

Effects of N mono- and N/P dual-doping on H2O2, OH generation, and MB electrochemical degradation efficiency of activated carbon fiber electrodes

Pure N mono- and N/P dual-doped cotton-stalk-derived activated carbon fibers (CSCFs) were synthesized by steam, HNO₃(CSCF-N), NH₃(CSCF-A), and (NH₄)₃PO₄(CSCF-N/P) treatments. This study investigated how three different N/P modifiers affected the pore structure, chemical property, H₂O₂ generation ability, and electrocatalytic activity of methylene blue (MB) degradation of CSCFs in an electric-Fenton system. Results confirmed that the three employed treatments effectively doped N/P in the carbon lattice and slightly changed the pore structures. NH₃ and (NH₄)₃PO₄ were the most effective modifiers for the N mono-doping and N/P dual-doping of CSCFs, respectively. Among the fabricated CSCFs, the N/P dual-doped CSCF-N/P demonstrated the highest electrochemical activity in an electro-Fenton system, followed by the N mono-doped CSCF-A, the CSCF-N, and the raw CSCF. In contrast to the CSCF electrode, the CSCF-N/P electrode exhibited enhanced H₂O₂, OH generation, and MB degradation efficiency by 42%, 41%, and 35%, respectively. Under optimum conditions, the electrochemical decolorization efficiency of MB (initial concentration, 100 mg L^-1) of the CSCF-N/P reached 93% after 150 min and was 24.1% higher than that of the CSCF. By the tenth cycle, 82.2% of the MB could still be decomposed, suggesting the excellent stability and reusability of the N/P co-doped CSCF electrode. The outstanding electrocatalytic performance of the CSCF-N/P electrode is primarily due to the simultaneous doping of active N/P sites with low activation energy and introduction of mesopores with strong trapping forces for MB. The MB reduction catalyzed by CSCF electrodes followed pseudo-first-order kinetics, and the reaction rate depended on the modifiers.

Polymerizable ionic liquid as a precursor for N, P co-doped carbon toward the oxygen reduction reaction

Preparing N, P co-doped carbon as a metal-free catalyst for ORR with a PIL as a precursor.

Natural phosphorus-doped honeycomb carbon materials as oxygen reduction catalysts derived from Pulsatilla chinensis (Bunge) Regel

Facile phosphorus (P)-doped carbon materials based on Pulsatilla chinensis (Bunge) Regel (PCR) were directly used in oxygen reduction reaction (ORR).

Construction of Highly Catalytic Porous TiOPC Nanocomposite Counter Electrodes for Dye-Sensitized Solar Cells

Developing low-cost, durable, and highly catalytic counter electrode (CE) materials based on earth-abundant elements is essential for dye-sensitized solar cells (DSSCs). In this study, we report a highly active nanostructured compositional material, TiOPC, which contains titanium, oxygen, phosphorus, and carbon, for efficient CE in I₃^-/I^- electrolyte. The TiOPC nanocomposites are prepared from carbon thermal transformation of TiP₂O₇ in an atmosphere of nitrogen at high temperature, and their catalytic performance is regulated by changing the carbon content in the nanocomposites. The TiOPC with appropriate 24.6 wt % carbon and porous structure exhibits an enhanced electrocatalytic activity in the reduction of I₃^-, providing a short-circuit current density of 16.64 mA cm^-2, an open-circuit potential of 0.78 V, and an energy conversion efficiency of 8.65%. The photovoltaic performance of TiOPC CE-based DSSC is even superior to that of a Pt CE-based cell (13.80 mA cm^-2, 0.79 V, and 6.66%). The enhanced catalytic activity of TiOPC is attributed to the presence of predominant Ti-O-P-C structure along with the continuous conductive carbon network and the porous structure.

Co-supported catalysts on nitrogen and sulfur co-doped vertically-aligned carbon nanotubes for oxygen reduction reaction

A high-performance ORR catalyst of Co supported on nitrogen and sulfur co-doped VACNTs was fabricated by cobalt sputtering and subsequent annealing in decomposition atmosphere of NH₄SCN.

Synthesis of B-doped hollow carbon spheres as efficient non-metal catalyst for oxygen reduction reaction

Synthesis scheme and morphology of B-doped hollow carbon spheres applied as an efficient non-metal catalyst for oxygen reduction reaction.

Heterogeneous carbon gels: N-doped carbon xerogels from resorcinol and N-containing heterocyclic aldehydes

Direct, acid (HCl) initiated sol-gel polycondensation of resorcinol with pyrrole-2-carboxaldehyde or its derivative N-methyl-2-pyrrolecarboxaldehyde yields thermosetting phenolic organic gels with N-content of up to 8.4 wt %. After carbonization, sturdy monoliths of N-doped carbon xerogels with N-content of up to 8 wt % are produced. The morphology and porosity of the doped carbons can be tuned by the solvent composition and the amount of polymerization catalyst used. An increase in carbonization temperature from 600 to 1000 °C strongly affects the carbon gels' microporosity, resulting in a decrease in N2 adsorption capacity, but a significant increase in H2 adsorption capacity (at -196 °C). The growing H2 sorption capacity with the decreasing specific surface area (measured by N2) is related to the gradual shrinkage of the carbon xerogel matrix and narrowing of the small micropores. In addition, it is demonstrated that pyridine-based heterocyclic aldehydes, that is, 2- or 4-pyridinecarboxaldehyde, condensate with resorcinol in basic conditions (KOH, NH4OH). However, in this case, monoliths cannot be produced and powders/rigid solid precipitates are obtained instead. If NH4OH is used as a sol-gel polycondensation catalyst, N-doped foams are obtained as a final carbonaceous product.