Time Resolved Measurements of pH in Aqueous Magnesium‐Air Batteries during Discharge and Its Impact for Future Applications

In aqueous magnesium air batteries, the influence of the electrochemical behavior on pH of the electrolyte has not been investigated yet, which has a critical effect on the cell performance. We have monitored the evolution of the pH at various discharge current densities in situ in the Mg-air primary cells, which produce sparingly soluble magnesium hydroxide (Mg(OH)$_{2}$). These experiments show the temporal evolution of the pH of the electrolyte in the cell discharge, depending on the current density. The pH first increases rapidly to a maximum of pH 11 and then drops down slowly to the equilibrium at pH 10.7. At the peak pH oversaturation of Mg(OH)$_{2}$ is paramount, leading to the precipitation which balances the Mg(OH)$_{2}$ concentration in the electrolyte. This precipitation process coats both cathode and anode which leads to a decrease in cell efficiency and voltage. The results show that the cell design of Mg-air batteries is important for their lifetime and cell performance. The performance of the aqueous magnesium cell is increased several folds when the design is changed to a simple electrolyte flow cell

Capturing Nano‐Scale Inhomogeneity of the Electrode Electrolyte Interface in Sodium‐Ion Batteries Through Tip‐Enhanced Raman Spectroscopy

A prime challenge in the development of new battery chemistries is the fundamental understanding of the generation of the electrode–electrolyte interface (EEI) and its evolution upon cycling. Tip-enhanced Raman spectroscopy (TERS) under an inert gas atmosphere is employed to study the chemical components of the anode/cathode electrolyte interface in a sodium-ion battery. After the first cycle, TERS reveals that the EEI mostly consists of organic carbonate/dicarbonate, oligoethylene oxides, α,β-unsaturated vinyl ketones/acetates, and inorganic species ClO$_4$$^−$, ClO$_3$$^−$, and Na$_2$CO$_3$. Whereas after 5× cycling, the EEI composition has evolved to contain long chain monodentate or bridging/bidentate carboxylates and alkoxides. The TERS map reveals the nano-scale heterogeneity present in the EEI layers and elucidates a multilayered nano-mosaic coating structure. The sheer volume of Raman signature present in the TERS signal can completely unravel the mysteries regarding the chemical composition and may shed light to the physicochemical behavior of the EEI

Multi‐Component PtFeCoNi Core‐Shell Nanoparticles on MWCNTs as Promising Bifunctional Catalyst for Oxygen Reduction and Oxygen Evolution Reactions

The development of commercially viable fuel cells and metal-air batteries requires effective and cheap bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Multi-component Pt−Fe−Co−Ni nanoparticles on multi-walled carbon nanotubes (MWCNTs) were synthesized by wet chemistry route via NaBH$_4$ reduction of metal salts, followed by sintering at different temperatures. The catalyst demonstrates an excellent ORR activity and a promising OER activity in 0.1 m KOH, with a bi-functional over-potential, ΔE of 0.83 V, which is comparable to the values of Pt/C or RuO$_2$. Furthermore, it shows outstanding long-term stability in ORR and OER, namely diffusion limited current density at a potential of 0.3 V decreased just by 5.5 % after 10000 cycles in ORR. The results of the PFCN@NT$^{300}$ indicate a significant effect of the substitution of Pt by the transition metal (TM) and the formation of nanoparticles on the catalytic performance, especially in the OER

Anionen‐Einlagerungschemie organischer Kathoden für zweiwertige Metallbatterien mit hoher Energie und hoher Leistungsdichte

Multivalente Batterien sind sehr vielversprechend für nachhaltige Energiespeicheranwendungen der nächsten Generation. Hier berichten wir über eine Polytriphenylamin (PTPAn)‐Verbundkathode, die in der Lage ist, Tetrakis(hexafluorisopropyloxy)borat [B(hfip)₄]⁻ Anionen in sowohl Magnesium‐ (Mg) als auch Calcium‐ (Ca) Batteriesystemen hochreversibel zu speichern. Spektroskopische und Computerstudien zeigen den Redoxreaktionsmechanismus des PTPAn‐Kathodenmaterials. Die Mg‐ und Ca‐Zellen weisen eine Zellspannung von ∼3 V, eine hohe Leistungsdichte von ∼3000 W kg⁻¹ bzw. eine hohe Energiedichte von ∼300 Wh kg⁻¹ auf. Darüber hinaus könnte die Kombination der PTPAn‐Kathode mit einer Anode aus einer Calcium‐Zinn‐Legierung (Ca−Sn) eine lange Batterielebensdauer von 3000 Zyklen bei einer Kapazitätserhaltung von 60 % ermöglichen. Die Anionenspeicherchemie in Verbindung mit dem elektrochemischen Doppelionenkonzept demonstriert einen neuen gangbaren Weg zu Hochleistungsbatterien mit zweiwertigen Ionen

Multi‐Component PtFeCoNi Core‐Shell Nanoparticles on MWCNTs as Promising Bifunctional Catalyst for Oxygen Reduction and Oxygen Evolution Reactions

The development of commercially viable fuel cells and metal‐air batteries requires effective and cheap bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Multi‐component Pt−Fe−Co−Ni nanoparticles on multi‐walled carbon nanotubes (MWCNTs) were synthesized by wet chemistry route via NaBH₄ reduction of metal salts, followed by sintering at different temperatures. The catalyst demonstrates an excellent ORR activity and a promising OER activity in 0.1 m KOH, with a bi‐functional over‐potential, ΔE of 0.83 V, which is comparable to the values of Pt/C or RuO₂. Furthermore, it shows outstanding long‐term stability in ORR and OER, namely diffusion limited current density at a potential of 0.3 V decreased just by 5.5 % after 10000 cycles in ORR. The results of the PFCN@NT³⁰⁰ indicate a significant effect of the substitution of Pt by the transition metal (TM) and the formation of nanoparticles on the catalytic performance, especially in the OER

Anion Storage Chemistry of Organic Cathodes for High‐Energy and High‐Power Density Divalent Metal Batteries

Multivalent batteries show promising prospects for next-generation sustainable energy storage applications. Herein, we report a polytriphenylamine (PTPAn) composite cathode capable of highly reversible storage of tetrakis(hexafluoroisopropyloxy) borate [B(hfip)4] anions in both Magnesium (Mg) and calcium (Ca) battery systems. Spectroscopic and computational studies reveal the redox reaction mechanism of the PTPAn cathode material. The Mg and Ca cells exhibit a cell voltage >3 V, a high-power density of ~3000 W kg−1 and a high-energy density of ~300 Wh kg−1, respectively. Moreover, the combination of the PTPAn cathode with a calcium-tin (Ca-Sn) alloy anode could enable a long battery-life of 3000 cycles with a capacity retention of 60%. The anion storage chemistry associated with dual-ion electrochemical concept demonstrates a new feasible pathway towards high-performance divalent ion batteries