How Thick Aqueous Alkali Should be Better for Aluminum-Air Batteries at Sub-Zero Temperatures: A Critical Anti-Freezing Concentration.

The application of portable aluminum-air batteries (AABs) in extreme environments is an inevitable demand for future development. Aqueous electrolyte freezing is a major challenge for low-temperature operations. Conventionally, enlightened by the organic system in metal ion batteries, blindly increasing the concentration is regarded as an efficient technique to reduce the freezing point (FP). However, the underlying contradiction between the adjusting mechanism of the FP and OH- transportation is ignored. Herein, the aqueous alkali solution of CsOH is researched as a prototype to disclose the intrinsic conductive behavior and related solvent structure evolution. Different from these inorganic electrolyte systems, the concept of a critical anti-freezing concentration (CFC) is proposed based on a specific temperature. The relationship between hydrogen bond reconstruction and de-solvation behavior is analyzed. A high conductivity is obtained at -30 °C, which is also a recorded value in an intrinsic aqueous AAB. The homogenous dissolution of the Al anode is also observed. As a general rule, the CFC concept is also applied in both the KOH and NaOH systems.

Metal-organic framework-derived metal-free highly graphitized nitrogen-doped porous carbon with a hierarchical porous structure as an efficient and stable electrocatalyst for oxygen reduction reaction.

Nitrogen-doped carbon materials are promising oxygen reduction reaction (ORR) electrocatalysts owing to high performance and stability. Herein, a three-dimensional porous bio-MOF-1, Zn8(Ad)4(Bpdc)6O·2Me2NH2 (Ad = adeninate; Bpdc = biphenyldicarboxylate), was used as precursor to fabricate N-doped porous carbon materials (NPC-1000-ts, where 1000 stands for the carbonization temperature and t represents the carbonization time, t = 2, 3 and 4 h) by simple carbonization under Ar atmosphere. The porous carbon materials had different contents of graphitic N and graphitization degrees of carbon. The catalytic activities of NPCs as metal-free ORR electrocatalysts were studied. The obtained NPC-1000-4 (pyrolysis under 1000 °C for 4 h) displayed outstanding ORR performance, with a positive onset potential (-0.012 V), a higher half-wave potential (E1/2) (-0.13 V) and a larger limiting current density (-5.76 mA/cm2) at -0.8 V (vs. Ag/AgCl) in KOH solution (0.1 M) than those of commercial Pt/C (20 wt%) catalyst (Eonset = -0.014 V, E1/2 = -0.14 V and -5.08 mA/cm2 at -0.8 V vs. Ag/AgCl). Obviously, the onset potential of NPC-1000-4 surpassed that of Pt/C, which was rare among currently available studies on metal-free nitrogen-doped porous carbon materials. Graphitic N significantly affected ORR catalytic performance besides graphitization degree of carbon. Meanwhile, NPC-1000-4 allowed an effective 4e--dominant ORR process, and most importantly, coupled with much higher long-term stability (89.5%) than that of commercial Pt/C (20 wt%, 65.8%) catalyst and higher resistance to methanol poisoning. The remarkable ORR activity of NPC-1000-4 can be ascribed to large surface area, considerable hierarchical porosity, high graphitization degree and synergism between enriched active sites and high portion of graphitic N. Overall, the findings guide the development of MOF-derived metal-free N-doped carbon materials as high-activity non-precious electrocatalysts for ORR.

Efficient Co-N/PC@CNT bifunctional electrocatalytic materials for oxygen reduction and oxygen evolution reactions based on metal-organic frameworks.

Cobalt-based, nitrogen-doped porous carbon materials with in situ grown carbon nanotubes (CNTs) were synthesized by the facile carbonization of porous 3D Bio-MOF-11 [Co2(ad)2(CH3COO)2]·2DMF·0.5H2O (ad = adenine). Co-N/PC@CNT-Ts inherit the octahedral shape from the precursor, and have a porous structure with in situ grown CNTs catalyzed by Co particles. Co-N/PC@CNT-T materials have excellent activities as bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in 0.1 M KOH electrolyte. Among the Co-N/PC@CNT-Ts, Co-N/PC@CNT-700 has the highest electrocatalytic activity. For ORR, Co-N/PC@CNT-700 has a higher onset potential of 0.92 V vs. reversible hydrogen electrode (RHE), high stability and methanol tolerance, which are even better than that of Pt/C. For OER, it has a low potential of 1.63 V at a current density of 10 mA cm-2. In addition, Co-N/PC@CNT-700 affords a low reversible overvoltage (bifunctional performance parameter) of 0.862 V between ORR and OER compared to the current advancing bifunctional catalysts. The superb bifunctional activity can be attributed to uniform CoNx active sites embedded in graphitized carbon, unique in situ grown CNT structure and ordered mesoporous structure. The synergistic effect enlarged the contact surface, exposed more active centers and provided many pathways, thereby boosting the electrocatalytic performance. In conclusion, this study provides a novel avenue for the application of stable transition metal-based, nitrogen-doped carbon materials as extremely efficient electrocatalysts for ORR and OER.

Cobalt and cobalt oxides N-codoped porous carbon derived from metal-organic framework as bifunctional catalyst for oxygen reduction and oxygen evolution reactions.

Metal-organic framework (MOF)-derived transition metal/metal oxide-carbon hybrids are promising cost-effective electrocatalysts to replace noble metal catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, Co@CoO@Co3O4-N/C was prepared by two-step thermal treatment of Co-MOF ([Co(INA)2]·0.5EtOH) (INA: isonicotinic acid). Firstly, Co-MOF, as precursor, was pyrolyzed at different temperatures in N2 atmosphere to obtain Co-N/C-T (T = 700, 800, 900 °C) materials among which Co-N/C-800 shows remarkably high ORR activity. After oxidation treatment, Co-N/C-800 is transformed into Co@CoO@Co3O4-N/C which exhibits enhanced electrocatalytic activities for both ORR and OER. The as-obtained Co@CoO@Co3O4-N/C has more positive onset potential (-0.136 V vs. Ag/AgCl) and higher limit current density (4.9 mA cm-2) than Co-N/C-800 (-0.143 V vs. Ag/AgCl and 3.9 mA cm-2), as well as better tolerance to methanol and stability (80.0%) than those of Pt/C (63.2%) for ORR. Co@CoO@Co3O4-N/C also displays outstanding OER performances, with lower overpotential (450 mV) than that of Co-N/C-800 (492 mV) at a current density of 10 mA cm-2. The excellent electrochemical performance of Co@CoO@Co3O4-N/C can be ascribed to uniformly dispersed Co-Nx active sites, strong synergistic effects between N-doped carbon support and Co@CoO@Co3O4 as well as ordered mesoporous structure, boosting mass transfer and accelerating electrocatalytic reaction.