No abstract

Electrocatalytic water splitting is a promising method to generate high-purity hydrogen, yet its efficiency is limited by the large energy barrier of the counter oxygen evolution reaction (OER). NiFe-layered double hydroxides (NiFe LDHs) are considered as promising electrocatalysts, and some studies have indicated that the involved cationic and anionic defects can further promote their electrocatalytic activities. Nevertheless, the inherent relationships between the defective structures and catalytic activities are still unclear. In this work, by alkaline corrosion of NiFeZn LDH and NaBH 4 activation, we involved cationic and anionic defects, respectively. In addition, they both promote the electrocatalytic activity of NiFe LDH in OER, and the co-defected NiFe LDH (cd-NiFe LDH−NaBH 4 ) only needs a low overpotential of 205 mV to obtain a current density of 10 mA cm −2 . We invested the exact effects of cationic and anionic defects by electrochemical characterizations and X-ray adsorption structure. The results indicate that the effects of cationic and anionic defects are different, and the cationic defects could promote the oxidation process of Ni species in OER, thus facilitating the favored lattice oxygen mechanism routine, while the anionic defects could enhance the reactivity of Fe sites by creating partially filled antibonding states of Fe−O. Our research reveals the origin of the accelerating effects brought by defective sites in NiFe LDH for OER and provides a facile approach to enhance the catalytic properties of noble metal-free electrocatalysts toward OER.

The Jahn–Teller effect (JTE) is one of the most important determinators of how much stress layered cathode materials undergo during charge and discharge; however, many reports have shown that traces of superstructure exist in pristine layered materials and irreversible phase transitions occur even after eliminating the JTE. A careful consideration of the energy of cationic distortion using a Taylor expansion indicated that second-order JTE (pseudo-JTE) is more widespread than the aforementioned JTE because of the various bonding states that occur between bonding and antibonding molecular orbitals in transition-metal octahedra. As a model case, a P2-type Mn-rich cathode (Na3/4MnO2) was investigated in detail. MnO6 octahedra are well known to undergo either elongation or contraction in a specific direction due to JTE. Here, the substitution of Li for Mn (Na3/4(Li1/4Mn3/4)­O2) helped to oxidize Mn3+ to Mn4+ suppressing JTE; however, the MnO6 octahedra remained asymmetric with a clear trace of the superstructure. With various advanced analyses, we disclose the pseudo-JTE as a general reason for the asymmetric distortions of the MnO6 octahedra. These distortions lead to the significant electrochemical degradation of Na3/4Li1/4Mn3/4O2. The suppression of the pseudo-JTE modulates phase transition behaviors during Na intercalation/deintercalation and thereby improves all of the electrochemical properties. The insight obtained by coupling a theoretical background for the pseudo-JTE with verified layered cathode material lattice changes implies that many previous approaches can be rationalized by regulating pseudo-JTE. This suggests that the pseudo-JTE should be thought more important than the well-known JTE for layered cathode materials.

Electrochemical production of H2O2 is a cost-effective and environmentally friendly alternative to the anthraquinone-based processes. Metal-doped carbon-based catalysts are commonly used for 2-electron oxygen reduction reaction (2e–ORR) due to their high selectivity. However, the exact roles of metals and carbon defects on ORR catalysts for H2O2 production remain unclear. Herein, by varying the Co loading in the pyrolysis precursor, a Co–N/O-C catalyst with Faradaic efficiency greater than 90% in alkaline electrolyte was obtained. Detailed studies revealed that the active sites in the Co–N/O-C catalysts for 2e–ORR were carbon atoms in C–O–C groups at defect sites. The direct contribution of cobalt single atom sites and metallic Co for the 2e–ORR performance was negligible. However, Co plays an important role in the pyrolytic synthesis of a catalyst by catalyzing carbon graphitization, tuning the formation of defects and oxygen functional groups, and controlling O and N concentrations, thereby indirectly enhancing 2e–ORR performance.