Phosphorus recovery from sewage sludge via Mg-air battery system

A pressing issue in contemporary society is the resource scarcity of phosphorus. Operating on the principle of electrochemical reactions between Mg as the anode and oxygen from air as the cathode, Mg-air batteries (MAB) have been employed to provide new prospects for phosphorus recovery in struvite form. Different phosphorus concentrations and reaction time impact struvite generation in MAB systems; however, the exact mechanism has rarely been investigated. We investigated how varying the initial phosphorus concentration and the reaction time affects phosphorus recovery, electricity generation, and the efficiency of struvite production in MAB. Additionally, we examine the impact of solid carbon sources on phosphorus transformation in sludge. The findings revealed that the incorporation of solid carbon sources facilitated the release of phosphate by changing phosphorus speciation. The electrolyte derived from the conditioned sludge filtrate exhibited a remarkable phosphorus removal efficiency of 91.7 % within 1 h, yielding the highest struvite purity of ∼70 %, whereas that using raw sludge filtrate or extending the reaction time was found to be less effective, even reducing struvite formation. Furthermore, different electrolytes influence the system's ability to passivate anode, and electrolytes with higher phosphorus concentrations have better electricity production performance. The results by Visual MINTEQ model confirmed that longer reaction times and lower initial phosphorus concentrations can negatively affect struvite formation by introducing Mg3(PO4)2 and Mg(OH)2. The integration of agricultural waste as carbon sources with MAB for phosphorus recovery represents a potential methodology for struvite recuperation from sewage sludge, thereby heralding a sustainable strategy for resource recovery.

All-Solid-State Mg-Air Battery Enhanced with Free-Standing N-Doped 3D Nanoporous Graphene

Nitrogen (N) doping of graphene with a three-dimensional (3D) porous structure, high flexibility, and low cost exhibits potential for developing metal-air batteries to power electric/electronic devices. The optimization of N-doping into graphene and the design of interconnected and monolithic graphene-based 3D porous structures are crucial for mass/ion diffusion and the final oxygen reduction reaction (ORR)/battery performance. Aqueous-type and all-solid-state primary Mg-air batteries using N-doped nanoporous graphene as air cathodes are assembled. N-doped nanoporous graphene with 50-150 nm pores and ≈99% porosity is found to exhibit a Pt-comparable ORR performance, along with satisfactory durability in both neutral and alkaline media. Remarkably, the all-solid-state battery exhibits a peak power density of 72.1 mW cm-2 ; this value is higher than that of a battery using Pt/carbon cathodes (54.3 mW cm-2 ) owing to the enhanced catalytic activity induced by N-doping and rapid air breathing in the 3D porous structure. Additionally, the all-solid-state battery demonstrates better performances than the aqueous-type battery owing to slow corrosion of the Mg anode by solid electrolytes. This study sheds light on the design of free-standing and catalytically active 3D nanoporous graphene that enhances the performance of both Mg-air batteries and various carbon-neutral-technologies using neutral electrolytes.

First-Principles Study of Magnesium Peroxide Nucleation for Mg-Air Battery

Recently, rechargeable non-aqueous Mg-air batteries have gained a lot of interest as the next-generation energy storage device due to the high theoretical volumetric density (3832 Ah L^-1 for Mg anode vs. 2062 Ah L^-1 for Li), low cost and safety. The field of Mg-air batteries is in the initial stage of development having a limited number of experimental and theoretical reports, in which mainly a carbon cathode is used; however, the information regarding the structural form of carbon is still missing. In this work, using first-principles density functional theory (DFT) calculations, we demonstrate the possibility of graphene and graphite as a cathode material towards Mg-air batteries by studying the initial MgO and MgO₂ nucleation processes on the surfaces of graphene and graphite. The calculated free energy diagrams for the redox reactions of oxygen are used to identify the rate-determining step controlling the overpotentials for initial nucleation of MgO and MgO₂ . We observe that graphene and graphite surfaces show similar reactivity towards the nucleation of MgO or MgO₂ , and the overpotential of the controlling steps for MgO₂ nucleation is comparatively less than that of MgO nucleation, which is supported by a recent experimental study, where a higher discharge voltage was observed in a cell having a mixed MgO/MgO₂ discharge product than MgO-based cells. Furthermore, the preferable formation of MgO₂ cluster compared to MgO on the graphene surface during the ab initio molecular dynamic (AIMD) simulations confirms the selectivity of MgO₂ formation over MgO as the final discharge product. We believe that our study will be helpful in understanding the initial nucleation processes during the oxygen reduction reaction (ORR) mechanism and development of suitable cathodes for the future Mg-air batteries.

Atomic Fe-Nx Coupled Open-Mesoporous Carbon Nanofibers for Efficient and Bioadaptable Oxygen Electrode in Mg-Air Batteries

The recently emerging metal-air batteries equipped with advanced oxygen electrodes have provided enormous opportunities to develop the next generation of wearable and bio-adaptable power sources. Theoretically, neutral electrolyte-based Mg-air batteries possess potential advantages in electronics and biomedical applications over the other metal-air counterparts, especially the alkaline-based Zn-air batteries. However, the rational design of advanced oxygen electrode for Mg-air batteries with high discharge voltage and capacity under neutral conditions still remains a major challenge. Inspired by fibrous string structures of bufo-spawn, it is reported here that the scalable synthesis of atomic Fe-Nx coupled open-mesoporous N-doped-carbon nanofibers (OM-NCNF-FeNx ) as advanced oxygen electrode for Mg-air batteries. The fabricated OM-NCNF-FeNx electrodes present manifold advantages, including open-mesoporous and interconnected structures, 3D hierarchically porous networks, good bio-adaptability, homogeneously coupled atomic Fe-Nx sites, and high oxygen electrocatalytic performances. Most importantly, the assembled Mg-air batteries with neutral electrolytes reveal high open-circuit voltage, stable discharge voltage plateaus, high capacity, long operating life, and good flexibility. Overall, the discovery on fabricating atomic OM-NCNF-FeNx electrode will not only create new pathways for achieving flexible, wearable, and bio-adaptable power sources, but also take a step towards the scale-up production of advanced nanofibrous carbon electrodes for a broad range of applications.