Batteries employing water based electrolytes enable extremely low manufacturing costs and are inherently safer than Li-ion batteries. Batteries based on zinc, manganese dioxide, iron, and air have high energy relevancy, are not resource restricted, and can contribute to large scale energy storage solutions. Zinc has a rich history as electrode material for primary alkaline Zn–MnO₂ batteries. Historically, its use in secondary batteries has been limited because of morphological uncertainties and passivation effects that may lead to cell failure. Manganese dioxide electrodes are ineffective as rechargeable electrodes because of failure mechanisms associated with phase transformations during cycling. The irreversibility of manganese dioxide is strongly correlated to the formation of the electrochemically inactive spinel, Mn₃O₄/ZnMn₂O₄. The development of the iron electrode for Fe–air batteries was initiated in late the 1960s and these batteries still suffer from charging inefficiency, due to the unwanted hydrogen evolution reaction. Meanwhile, the air electrode is limited in long-term operation because of the sluggish oxygen evolution and reduction kinetics. These limitations of the Fe–air battery yield poor overall efficiencies, which bring vast energy losses upon cycling.

Herein, the limitations described above were countered for rechargeable Zn–MnO₂ and Fe–air batteries by synthesizing electrode materials and modifying electrolyte compositions. The electrolyte mixture of 1 M KOH + 3 M LiOH for rechargeable alkaline Zn–MnO₂ batteries limited the formation of the inactive spinels and improved their cycle life significantly. Further, the formation of the inactive spinels was overcome in mildly acidic electrolytes containing 2 M ZnSO₄, enabling the cells to cycle reversibly at lower pH via a distinctive reaction mechanism. The iron electrodes were improved with the addition of stannate, which suppressed hydrogen evolution. Furthermore, optimal charge protocols of the iron electrodes were identified to minimize the hydrogen evolution rate. On the air electrode, the synthesized NiCo₂O₄ showed excellent bifunctional catalytic activity for oxygen evolution and reduction, and was incorporated to a flow assisted rechargeable Fe–air battery, in order to prove the practicability of this technology. Studies of the electrode materials on the micro, macro, nano, and atomic scales were carried out to increase the understanding of the nature of and interactions between of these materials. This included both in operando and ex situ characterization. X-ray and neutron radiation, and analytical- and electrochemical methods provided insight to improve the performance and cycle life of the batteries.

Fe-air or Ni-Fe cells can offer low-cost and large-scale sustainable energy storage. At present, they are limited by low coulombic efficiency, low active material use, and poor rate capability. To overcome these challenges, two types of nanostructured doped iron materials were investigated: (1) copper and tin doped iron (CuSn); and (2) tin doped iron (Sn). Single-wall carbon nanotube (SWCNT) was added to the electrode and LiOH to the electrolyte. In the 2 wt. % Cu + 2 wt. % Sn sample, the addition of SWCNT increased the discharge capacity from 430 to 475 mAh g(-1), and charge efficiency increased from 83% to 93.5%. With the addition of both SWCNT and LiOH, the charge efficiency and discharge capacity improved to 91% and 603 mAh g(-1), respectively. Meanwhile, the 4 wt. % Sn substituted sample performance is not on par with the 2 wt. % Cu + 2 wt. % Sn sample. The dopant elements (Cu and Sn) and additives (SWCNT and LiOH) have a major impact on the electrode performance. To understand the relation between hydrogen evolution and charge current density, we have used in operando charging measurements combined with mass spectrometry to quantify the evolved hydrogen. The electrodes that were subjected to prolonged overcharge upon hydrogen evolution failed rapidly. This insight could help in the development of better charging schemes for the iron electrodes.

The Zn/MnO2 battery is safe, low cost and comes with a high energy density comparable to Li-ion batteries. However, irreversible spinel phases formed at the MnO2 electrode limits its cyclability. A viable solution to overcome this inactive phase is to use an aqueous ZnSO4-based electrolyte, where pH is mildly acidic leading to a different reaction mechanism. Most importantly, the addition of MnSO4 achieves excellent cyclability. How accessible Mn2+ ions in the electrolyte enhances the reversibility is presented. With added Mn2+, the capacity retention is significantly improved over 100 cycles. Zn2+ insertion plays an important role on the reversibility and a hydrated layered Zn-buserite structure formed during charge is reported. Furthermore, Zn4SO4(OH)(6) center dot 5H(2)O precipitates during discharge but is not involved in the electrochemical reaction. This precipitate both buffers the pH and partly insulates the surface. Described in operando study show how the phase transformations and the failure mechanisms depend on the presence of Mn2+-ions in the electrolyte. These results give insight necessary to improve this battery further to make it a worthy contender to the Li-ion battery in large scale energy storage solutions.

An experimental setup and routine is presented for evaluating potential catalysts for water splitting by means of measuring Faradaic efficiency in real time by coupled potentiometry and mass spectrometry. The aim was to simulate a potential industrial scale setup and generate results such as H2 production versus power input at a certain potential or current density in addition to electrochemical parameters. Three types of electrodes were tested: A) planar metal electrodes; B) metal foam based electrodes; C) porous electrodes with carbon additive. The results verify that the experimental routine yield desired accuracy, sensitivity and a negligible accumulation of gaseous products in the cell; thus the Faradaic efficiency is measured in real time. The metal based electrodes of category A and B proved to be durable with low overpotentials and high gas output to power input, whereas three tested metal oxide electrodes in C revealed (i) potential-dependent deviation in Faradaic efficiency, (ii) phase decomposition and (iii) an optimum operational power range.

Alkaline iron electrodes present some challenges for use in secondary batteries that are associated with low coulombic efficiency and discharge utilization. Low coulombic efficiency is correlated to the hydrogen evolution reaction that takes place during charge. In this work, we demonstrate rechargeable alkaline iron electrodes with significant capacity retention over 150 cycles with high efficiency by suppressing the hydrogen evolution with stannate. Adding stannate to the alkaline electrolyte when cycling the iron electrode drastically changes the electrochemistry. The additive brings on two advantageous attributes for the iron electrode: increased hydrogen evolution overpotential, and a flat and prolonged discharge curve at typical battery operation. These attributes were provided by a novel intermediate phase that was detected from in situ neutron diffraction measurements. This phase was only detected in situ while it decomposed ex situ, and indicated a solid solution constituted by some of the elements present in the electrode.

A Bi2O3 in beta-MnO2 composite cathode material has been synthesized using a simple hydrothermal method and cycled in a mixed KOH-LiOH electrolyte with a range of concentrations. We show that, at a KOH:LiOH molar ratio of 1:3, both proton insertion and lithium insertion occur, allowing access to a higher fraction of the theoretical capacity of the MnO2 while preventing the formation of ZnMn2O4. This enables a capacity of 360 mAh/g for over 60 cycles, with cycling limited more by anode properties than traditional cathodic failure mechanisms. The structural changes occurring during cycling are characterized using electron microscopy and in situ synchrotron energy-dispersive X-ray diffraction (EDXRD) techniques. This mixed electrolyte shows exceptional cyclability and capacity and can be used as a drop-in replacement for current alkaline batteries, potentially drastically improving their cycle life and creating a wide range of new applications for this energy storage technology.

Low cost rechargeable iron-air alkaline batteries have all essential attributes to adapt for large scale energy storage applications. To actualize this implementation needs to overcome the challenges including poor efficiency and short cycle lifetime. Herein, suitable synthesized catalysts for the air electrode were investigated prior to iron-air cell testing. NiCo2O4 as sole catalyst proved exceptional bifunctional OER/ORR activity and stability over 440 h operation in air. This catalyst fitted into an electrolyte and oxygen flow assisted rechargeable iron-air prototype and performed stable over 588 h and had an energy density of 377 Wh kg^-1 _Fe. Inadequate coulombic efficiencies of 75 – 85% and energy efficiencies around 50% hurt the performance of the cell though and needed further development. Nevertheless, the findings in this work reports the opportunities and obstacles of the rechargeable iron-air battery.

Fe-air or Ni-Fe cells can offer low-cost and large-scale sustainable energy storage. At present, they are limited by low coulombic efficiency, low active material use, and poor rate capability. To overcome these challenges, two types of nanostructured doped iron materials were investigated: (1) copper and tin doped iron (CuSn); and (2) tin doped iron (Sn). Single-wall carbon nanotube (SWCNT) was added to the electrode and LiOH to the electrolyte. In the 2 wt. % Cu + 2 wt. % Sn sample, the addition of SWCNT increased the discharge capacity from 430 to 475 mAh g⁻¹, and charge efficiency increased from 83% to 93.5%. With the addition of both SWCNT and LiOH, the charge efficiency and discharge capacity improved to 91% and 603 mAh g⁻¹, respectively. Meanwhile, the 4 wt. % Sn substituted sample performance is not on par with the 2 wt. % Cu + 2 wt. % Sn sample. The dopant elements (Cu and Sn) and additives (SWCNT and LiOH) have a major impact on the electrode performance. To understand the relation between hydrogen evolution and charge current density, we have used in operando charging measurements combined with mass spectrometry to quantify the evolved hydrogen. The electrodes that were subjected to prolonged overcharge upon hydrogen evolution failed rapidly. This insight could help in the development of better charging schemes for the iron electrodes.