Realizing Two-Electron Transfer in Ni(OH)₂ Nanosheets for Energy Storage

Abstract

The theoretical capacity of a given electrode material is ultimately determined by the number of electrons transferred in each redox center. The design of multi-electron transfer processes could break through the limitation of oneelectron transfer and multiply the total capacity but is difficult to achieve because multiple electron transfer processes are generally thermodynamically and kinetically more complex. Here, we report the discovery of two-electron transfer in monolayer Ni(OH) 2 nanosheets, which contrasts with the traditional one-electron transfer found in multilayer materials. First-principles calculations predict that the first oxidation process Ni 2+ → Ni 3+ occurs easily, whereas the second electron transfer in Ni 3+ → Ni 4+ is strongly hindered in multilayer materials by both the interlayer hydrogen bonds and the domain H structure induced by the Jahn−Teller distortion of the Ni 3+ (t 2g 6 e g 1 )-centered octahedra. In contrast, the second electron transfer can easily occur in monolayers because all H atoms are fully exposed. Experimentally, the as-prepared monolayer is found to deliver an exceptional redox capacity of ∼576 mA h/g, nearly 2 times the theoretical capacity of one-electron processes. In situ experiments demonstrate that monolayer Ni(OH) 2 can transfer two electrons and most Ni ions transform into Ni 4+ during the charging process, whereas bulk Ni(OH) 2 can only be transformed partially. Our work reveals a new redox reaction mechanism in atomically thin Ni(OH) 2 nanosheets and suggests a promising path toward tuning the electron transfer numbers to multiply the capacity of the relevant energy storage materials.

Crucial Role of Crystal Field on Determining the Evolution Process of Janus MoSSe Monolayer: A First-Principles Study

“…Under this regular crystal field, the five degenerated Mo 4d orbitals will spontaneously be separated into two groups, where three degenerated d z 2 , d xy , and d x 2 −y 2 orbitals will occupy the lower energy level area, and two degenerated d yz and d xz orbitals will occupy the higher energy level area. 42,43 Since the electronic configuration of the Mo atom is 4d 5 5s 1 , then four of the six outermost electrons are contributed to by the S atoms of the MoS 2 monomer, leaving the other two electrons filling the d z 2 orbit as shown in Figure 3d, consistent with a previous result. 44 To prove the above splitting rule, the partial projected electronic density of states (PDOS) of Mo 4d orbitals for perfect MoS 2 is investigated, and the result is shown in Figure 3e.…”

“…We can find that all Mo atoms are located at the center of a triangular prism crystal field, forming a 6-fold coordination as shown in Figure a. Under this regular crystal field, the five degenerated Mo 4d orbitals will spontaneously be separated into two groups, where three degenerated d z 2 , d xy , and d x 2 – y 2 orbitals will occupy the lower energy level area, and two degenerated d yz and d xz orbitals will occupy the higher energy level area. , Since the electronic configuration of the Mo atom is 4d 5 5s 1 , then four of the six outermost electrons are contributed to by the S atoms of the MoS 2 monomer, leaving the other two electrons filling the d z 2 orbit as shown in Figure d, consistent with a previous result…”

Stabilizing Pt Electrocatalysts via Introducing Reducible Oxide Support as Reservoir of Electrons and Oxygen Species

“…17,18 However, the stabilization mechanism and the electron interaction between Pt and metal oxides under dynamic potential cycling conditions remains elusive, not to mention the development of a further advancing strategy. 19,20 Herein, we propose an effective and scalable strategy to stabilize Pt by introducing reducible niobium oxide (Nb 2 O 5 ) as a reservoir for both electron and oxygen species. Although Nb 2 O 5 is a stable phase in the testing region according to the Pourbaix diagram, abundant electrons are stored in terms of Nb 4+ /oxygen vacancies (O v ) pairs.…”

“…However, H 2 O 2 as a byproduct is preferably generated at the Au surfaces and therefore may lead to a deterioration in the catalyst layer as well as the ionomer . Supporting Pt on metal oxides provides another viable route to suppress oxygenated species formation and stabilize low-coordinated Pt sites via the strong metal–support interactions (SMSI). , However, the stabilization mechanism and the electron interaction between Pt and metal oxides under dynamic potential cycling conditions remains elusive, not to mention the development of a further advancing strategy. , …”

Ultrathin Nanotube Structure for Mass-Efficient and Durable Oxygen Reduction Reaction Catalysts in PEM Fuel Cells

“…Hydrogen energy and its conversion devices, proton-exchange-membrane fuel cells (PEMFCs) have promising prospects in the fields of transportation and distributed power stations. − Currently, Pt-based nanocatalysts are the only practical catalysts for the sluggish oxygen reduction reaction (ORR) at the cathode. − To reduce Pt usage, the U.S. Department of Energy (DOE) has announced a mass activity (MA) target of 0.44 A/mg Pt @0.9 V iR ‑free and dual durability targets for platinum group metal (PGM) catalysts. The catalysts are required to show no more than 40% loss of MA and electrochemical surface area (ECSA) after passing two accelerated degradation tests (ADT-1 and -2), which simulate load cycling and start–stop conditions, respectively…”