Selective Electrocatalytic Oxidation of Nitrogen to Nitric Acid Using Manganese Phthalocyanine.

Ammonia is produced through the energy-intensive Haber-Bosch process, which undergoes catalytic oxidation for the production of commercial nitric acid by the senescent Ostwald process. The two energy-intensive industrial processes demand for process sustainability. Hence, single-step electrocatalysis offers a promising approach toward a more environmentally friendly solution. Herein, we report a 10-electron pathway associated one-step electrochemical dinitrogen oxidation reaction (N2OR) to nitric acid by manganese phthalocyanine (MnPc) hollow nano-structures under ambient conditions. The catalyst delivers a nitric acid yield of 513.2 µmol h-1 gcat-1 with 33.9% Faradaic efficiency @ 2.1 V versus reversible hydrogen electrode. The excellent N2OR performances are achieved due to the specific-selectivity, presence of greater number of exposed active sites, recyclability, and long period stability. The extended X-ray absorption fine structure confirms that Mn atoms are coordinated to the pyrrolic and pyridinic nitrogen via Mn-N4 coordination. Density functional theory-based theoretical calculations confirm that the Mn-N4 site of MnPc is the main active center for N2OR, which suppresses the oxygen evolution reaction. This work provides a new arena about the successful example of one step nitric acid production utilizing a Mn-N4 active site-based metal phthalocyanine electrocatalyst by dinitrogen oxidation for the development of a carbon-neutral sustainable society.

Scalable Production of Cobalt Phthalocyanine Nanotubes: Efficient and Robust Hollow Electrocatalyst for Ammonia Synthesis at Room Temperature.

Electrocatalytic ammonia (NH3) synthesis through the nitrogen reduction reaction (NRR) under ambient conditions presents a promising alternative to the famous century-old Haber-Bosch process. Designing and developing a high-performance electrocatalyst is a compelling necessity for electrochemical NRR. Specific transition metal based nanostructured catalysts are potential candidates for this purpose owing to their attributes such as higher actives sites, specificity as well as selectivity and electron transfer, etc. However, due to the lack of a well-organized morphology, lower activity, selectivity, and stability of the electrocatalysts make them ineffective at producing a high NH3 yield rate and Faradaic efficiency (FE) for further development. In this work, stable ß-cobalt phthalocyanine (CoPc) nanotubes (NTs) have been synthesized by a scalable solvothermal method for electrochemical NRR. The chemically synthesized CoPc NTs show excellent electrochemical NRR due to high specific area, greater number of exposed active sites, and specific selectivity of the catalyst. As a result, CoPc NTs produced a higher NH3 yield of 107.9 µg h-1 mg-1cat and FE of 27.7% in 0.1 M HCl at -0.3 V vs RHE. The density functional theory calculations confirm that the Co center in CoPc is the main active site responsible for electrochemical NRR. This work demonstrates the development of hollow nanostructured electrocatalysts in large scale for N2 fixation to NH3.

Practical Aqueous Calcium-Ion Battery Full-Cells for Future Stationary Storage.

There is a pressing need for high-rate cycling and cost-effective stationary energy storage systems in concomitance with the fast development of solar, wind, and other types of renewable sources of energy. Aqueous rechargeable Ca-ion batteries have the potential to meet the growing demands of stationary energy storage devices because they are abundant and safe; they can also be manufactured at a low-cost and have a higher volumetric capacity. In this study, we have demonstrated a low-cost, safe, aqueous Ca-ion battery that is based on a low potential, lower specific weight, in situ polymerized polyaniline as an anode, and a high redox-potential open-framework structured potassium copper hexacyanoferrate as a cathode. The charge-discharge mechanism of this battery includes doping/dedoping of NO3- at the anode, and intercalation and deintercalation of Ca-ion at the cathode. This Ca-ion battery works successfully in a 2.5 M Ca(NO3)2 aqueous electrolyte that exhibits 70 Wh kg-1 specific energy at 250 W kg-1 and even maintains a high energy density of 53 Wh kg-1 at a higher rate of 950 W kg-1; this indicates a good rate capability (calculation based on anode active mass). At 0.8 A g-1, the battery provides an average specific capacity of 130 mA h g-1, exhibiting high Coulombic efficiency (∼96%), with 95% capacity retention of over 200 cycles across its life span, which is a new achievement in the electrochemical performance of aqueous Ca-ion batteries. Furthermore, the calcium-ion storage mechanism is investigated using high-end X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Thus, this significant electrochemical performance of the anode and the cathode renders the battery a promising candidate in grid-scale storage applications.