Oxygen-vacant nanocrystalline MnO(2) has been prepared by the simple process of annealing pristine oxide in Ar or O(2) . Both experimental and computational studies indicate that the catalytic activity of MnO(2) towards oxygen reduction is enhanced by introducing a modest concentration of oxygen vacancies.

In this Letter, we reported on the preparation and Li-ion battery anode application of ultrasmall Sn nanoparticles (∼5 nm) embedded in nitrogen-doped porous carbon network (denoted as 5-Sn/C). Pyrolysis of Sn(Salen) at 650 °C under Ar atmosphere was carried out to prepare N-doped porous 5-Sn/C with the BET specific surface area of 286.3 m(2) g(-1). The 5-Sn/C showed an initial discharge capacity of 1014 mAh g(-1) and a capacity retention of 722 mAh g(-1) after 200 cycles at the current density of 0.2 A g(-1). Furthermore, a reversible capacity of ∼480 mAh g(-1) was obtained at much higher current density of 5 A g(-1). The remarkable electrochemical performance of 5-Sn/C was attributed to the effective combination of ultrasmall Sn nanoparticles, uniform distribution, and porous carbon network structure, which simultaneously solved the major problems of pulverization, loss of electrical contact, and particle aggregation facing Sn anode.

Perovskite oxides offer efficient and cheap electrocatalysts for both oxygen reduction reactions and oxygen evolution reactions (ORR/OER) in diverse oxygen-based electrochemical technologies. In this study, we report a facile strategy to enhance the electrocatalytic activity of CaMnO3 by introducing oxygen defects. The nonstoichiometric CaMnO(3-δ) (0 < δ ≤ 0.5) was prepared through thermal reduction of pristine perovskite microspheres and nanoparticles, which were synthesized from thermal-decomposition of carbonate precursors and the Pechini route, respectively. The as-prepared samples were analyzed by chemical titration, structural refinement, thermogravimetric analysis, and energy spectrometry. In 0.1 M KOH aqueous solution, the nonstoichiometric CaMnO(3-δ) with δ near 0.25 and an average Mn valence close to 3.5 exhibited the highest ORR activity (36.7 A g(-1) at 0.70 V vs RHE, with onset potential of 0.96 V), which is comparable to that of benchmark Pt/C. Density functional theory (DFT) studies and electrical conductivity measurement revealed that the enhanced ORR kinetics is due to facilitated oxygen activation and improved electrical properties. Besides high activity, the nonstoichiometric perovskite oxides showed respectable catalytic stability. Furthermore, the moderate oxygen-defective CaMnO(3-δ) (δ ≈ 0.25) favored the OER because of the improved electrical conductivity. This study makes nonstoichiometric CaMnO(3-δ) a promising active, inexpensive bifunctional catalytic material for reversible ORR and OER.

Metal organic frameworks (MOFs), an emerging class of nanoporous crystalline materials, have become increasingly attractive for solar energy applications. In this work, we report a newly designed mixed-node MOF catalyst, Co x Fe1–x -MOF-74 (0 < x ≤ 1), which acts as a highly efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline solution with remarkably low overpotential (280 mV at a current density of 10 mA/cm2), small Tafel slope (56 mV/dec), and high faradic efficiency (91%) and can deliver a current density of 20 mA/cm2 at 1.58 V for overall water splitting. Moreover, using the combination of multiple spectroscopic methods, including X-ray absorption, electron spin resonance, and X-ray photoelectron spectroscopy, etc., we unraveled the mechanistic origin of the enhanced catalytic performance of Co x Fe1–x -MOF-74 compared to its single-metal counterparts. We show the mixed-node MOF can provide more open metal sites and an enhanced electron-rich environment, which facilitates efficient charge transfer and results in significantly enhanced OER activity.

Well-ordered NiFe-MOF-74 is in situ grown on Ni foam by the induction of Fe2+ and directly used as an OER electrocatalyst. Benefited from the intrinsic open porous structure of MOF-74, the in situ formed MOF arrays and the synergistic effect of Ni and Fe, outstanding water oxidation activity is obtained in alkaline electrolytes with an overpotential of 223 mV at 10 mA cm-2.

A series of calcium-manganese oxides (Ca-Mn-O) were prepared through thermal decomposition of carbonate solid-solution precursors and investigated as electrocatalysts for oxygen reduction reaction (ORR). The synthesized crystalline Ca-Mn-O compounds, including perovskite-type CaMnO 3 , layered structured Ca 2 Mn 3 O 8 , post-spinel CaMn 2 O 4 and CaMn 3 O 6 , presented similar morphologies of porous microspheres with agglomerated nanoparticles. Electrochemical results, surface analysis, and computational studies revealed that the catalytic activities of Ca-Mn-O oxides, in terms of onset potential, reduction current, and transferred electron number, depended strongly on both the surface Mn oxidation state and the crystallographic structures. Remarkably, the as-synthesized CaMnO 3 and CaMn 3 O 6 exhibited considerable activity and enabled an apparent quasi 4-electron oxygen reduction with low yield of peroxide species in alkaline solutions, suggesting their potential applications as cheap and abundant ORR catalysts.

The electrochemical splitting of water, as an efficient and large-scale method to produce H, is still hindered by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. Considering the synergetic effect of the different metal sites with coordination on the surface of electrocatalysts, the hybrids of Co/Fe phosphides (denoted as Co-Fe-P) is prepared by one-step phosphorization of CoFe metal-organic frameworks for the first time as highly efficient electrocatalysts for OER. Benefiting from the synergistic effect of Co and Fe, the high valence of Co ions induced by strongly electronegative P and N and the large electrochemical active surface area (ECSA) originated from exposed nanowires on the surface of Co/Fe phosphides, the resultant Co-Fe-P-1.7 exhibits remarkable electrocatalytic performances for OER in 1.0 M KOH, affording an overpotential as low as 244 mV at a current density of 10 mA/cm, a small Tafel slope of 58 mV/dec, and good stability, which is superior to that of the state-of-the-art OER electrocatalysts. In addition, the two-electrode cell with Co-Fe-P-1.7 modified Ni foam as anode and cathode in an alkaline electrolyte, respectively, exhibits the decomposition potential of ca. 1.60 V at a current density of 10 mA/cm and excellent stability.

Molybdenum carbides are considered as one type of privileged noble-metal-free electrocatalysts for hydrogen evolution reactions (HER) due to their d-band electron structure, which is similar to Pt. Especially, the electronic structure of such materials can be further adjusted by elemental doping to improve their electrocatalytic activity. Herein, we selected the Anderson-type polyoxometalates (POMs) (NH4)n[TMMo6O24H6]·5H2O (TM = Ni2+, Co2+, n = 4; TM = Fe3+, Cr3+, n = 3) as precursors to prepare new transition-metal-doped Mo2C materials. When these POMs were mixed with dicyandiamide (DCA) by solid grinding, and carbonized at a high temperature, a series of Ni-, Co-, Fe-, and Cr-doped Mo2C composite nanoparticles covered by few-layer graphitic carbon shells (abbr. TM-Mo2C@C) were obtained. All these nanoparticles possess a similar size, morphology, and TM/Mo component ratio, and thus it is feasible to systematically investigate the influence of different TM dopants on the electrocatalytic activity of Mo2C for HER. Both electrocatalytic experiments and DFT calculations reveal that TM dopants have a significant effect on the hydrogen binding energy (ΔGH*) and the catalytic activity of Mo2C. The sequence of HER electrocatalytic activity is as follows: Ni-Mo2C > Co-Mo2C > Fe-Mo2C > Cr-Mo2C. As a result, Ni-Mo2C@C possesses the best HER performance, which required an overpotential of 72 mV at a current density of 10 mA cm-2 and the Tafel slope is 65.8 mV dec-1. This work suggests a shortcut to reasonably investigate the effects of elemental doping on molybdenum carbides and explore new high-efficient and low-cost electrocatalysts for HER.