Alkali‐Driven Assembly of Protein‐Rich Biomass Boosts the Electrocatalytic Activity of the Derived Carbon Materials for Oxygen Reduction

High-Efficiency Oxygen Reduction Reaction Revived from Walnut Shell

The development of inexpensive and efficient electrocatalysts for oxygen reduction reactions (ORR) remains a challenge with respect to renewable energy technologies. In this research, a porous, nitrogen-doped ORR catalyst is prepared using the hydrothermal method and pyrolysis with walnut shell as a biomass precursor and urea as a nitrogen source. Unlike past research, in this study, urea is not directly doped; instead, a new type of doping is carried out after annealing at 550 °C. In addition, the sample's morphology and structure are analyzed and characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). A CHI 760E electrochemical workstation is used to test NSCL-900's performance in terms of oxygen reduction electrocatalysis (ORR). It has been found that the catalytic performance of NSCL-900 is significantly improved compared with that of NS-900 without urea doping. In a 0.1 mol/L KOH electrolyte, the half-wave potential can reach 0.86 V (vs. RHE) and the initial potential is 1.00 V (vs. RHE). The catalytic process is close to four-electron transfer and there are large quantities of pyridine nitrogen and pyrrole nitrogen.

Recent Advancements in the Synthesis and Application of Carbon-Based Catalysts in the ORR

Fuel cells are a promising alternative to non-renewable energy production industries such as petroleum and natural gas. The cathodic oxygen reduction reaction (ORR), which makes fuel cell technology possible, is sluggish under normal conditions. Thus, catalysts must be used to allow fuel cells to operate efficiently. Traditionally, platinum (Pt) catalysts are often utilized as they exhibit a highly efficient ORR with low overpotential values. However, Pt is an expensive and precious metal, posing economic problems for commercialization. Herein, advances in carbon-based catalysts are reviewed for their application in ORRs due to their abundance and low-cost syntheses. Various synthetic methods from different renewable sources are presented, and their catalytic properties are compared. Likewise, the effects of heteroatom and non-precious metal doping, surface area, and porosity on their performance are investigated. Carbon-based support materials are discussed in relation to their physical properties and the subsequent effect on Pt ORR performance. Lastly, advances in fuel cell electrolytes for various fuel cell types are presented. This review aims to provide valuable insight into current challenges in fuel cell performance and how they can be overcome using carbon-based materials and next generation electrolytes.

Acid-directed preparation of micro/mesoporous heteroatom doped defective graphitic carbon as bifunctional electroactive material: Evaluation of trace metal impurity

Extensive research to explore cost-effective carbon materials as electrocatalysts and electrode materials for energy conversion and storage has been conducted in the recent literature. This raised a crucial question regarding the origin of this electrocatalytic activity from the heteroatom doping/ hierarchical porous defect-rich architecture and/ or the trace metal impurities introduced during synthesis/ inherent to the precursor. In this work, an insight into this issue is provided by considering a lignocellulosic biowaste, Euryale Ferox (foxnut) shells as a precursor to derive micro/ mesoporous defective graphitic carbon sheets by the phosphoric acid (H₃PO₄) dictated in-situ carbonization for oxygen reduction reaction (ORR) and supercapacitor applications. The sample synthesized at 900 °C (FP900) shows an onset potential of 0.98 V vs. reversible hydrogen electrode (RHE), ORR current density of 5.5 mA cm^-2, and current stability of 93% (in 10 h measurement) in 0.1 M KOH. In addition, a symmetric supercapacitor device is assembled using the prepared material and the specific capacitance of 207.5 F g^-1 at 1 A g^-1 is obtained. An attempt to explain the origin of the electrochemical performance is made by establishing parallels with the physicochemical characterizations. The inherently doped heteroatoms give rise to electroactive functionalities and the wide enough pore size distribution improves the active sites utilization efficiency by enhancing the accessibility to electrolytic ions resulting in better electrochemical performance. Furthermore, the contribution from the intrinsic trace metal impurities is evaluated using X-ray fluorescence (XRF) spectroscopy. The presented research clarifies the non-contributing nature of trace metal species owing to the inaccessibility of active sites and lower abundance in F900 and FP900, respectively.

Ultrasound-assisted transformation from waste biomass to efficient carbon-based metal-free pH-universal oxygen reduction reaction electrocatalysts

Efficient carbon-based nitrogen-doped electrocatalysts derived from waste biomass are regarded as a promising alternative to noble metal catalysts for oxygen reduction reaction (ORR), which is crucial to fuel cell performance. Here, coconut palm leaves are employed as the carbon source and a series of nitrogen-doped porous carbons were prepared by virtue of a facile and mild ultrasound-assisted method. The obtained carbon material (ANDC-900-10) conveys excellent pH-universal catalytic activity with onset potentials (E_onset) of 1.01, 0.91 and 0.84 V vs. RHE, half-wave potentials (E_1/2) of 0.87, 0.74 and 0.66 V vs. RHE and limiting current densities (J_L) of 5.50, 5.45 and 4.97 mA cm^-2 in alkaline, neutral and acidic electrolytes, respectively, prevailing over the commercial Pt/C catalyst and, what's more, ANDC-900-10 displays preeminent methanol crossover resistance and long-term stability in the broad pH range (0-13), thanks to its abundant hierarchical nanopores as well as effective nitrogen doping with high-density pyridinic-N and graphitic-N. This work provides sonochemical insight for underpinning the eco-friendly approach to rationally designing versatile metal-free carbon-based catalysts toward the ORR at various pH levels.