In order to decrease the global dependence on fossil fuels, high energy density, rechargeable batteries with high charge capacity are required for mobile applications and efficient utilization of intermittent sources of renewable energy. Metal-air batteries are promising due to their high theoretical energy density. In particular, the iron-air battery, with a maximum specific energy output of 764 W h kg−1Fe, represents a low cost possibility. This paper considers an iron-air battery with nanocomposite electrodes, which achieves an energy density of 453 W h kg−1Fe and a maximum charge capacity of 814 mA h g−1Fe when cycled at a current density of 10 mA cm−2, with a cell voltage of 0.76 V. The cell was manufactured by 3D printing, allowing rapid modifications and improvements to be implemented before an optimized prototype can be manufactured using traditional computer numerical control machining.

The development of air-breathing cathodes, which utilise atmospheric oxygen, enables the construction of lightweight, high energy density metal-air batteries and fuel cells. Air electrodes can be very lightweight and thin because the active material, oxygen, does not need to be stored inside the cell. However, air electrodes are restricted by poor reaction kinetics and low activity of many catalysts towards the oxygen evolution and reduction reactions. In addition, it is a challenge to maintain chemical and mechanical stability of the catalyst and supporting materials at oxidising currents under the strong alkaline conditions commonly used, and gas evolution. This paper reports a novel bifunctional oxygen electrode with remarkable stability, able to perform at current densities up to 1,000 mA cm -2 and withstand 3,000 cycles continuously. The electrode is catalysed by a mixture of Pd/C and mixed nickel-iron hexocyanoferrate, which have high activities towards the ORR and OER reactions, respectively.

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The characterization and improvement of a rectangular channel electrolyte flow compartment used in an iron-air flow battery was carried out by using an arrangement of copper electrodes to measure the current density distribution employing the limiting current technique. The present work addresses the hydrodynamics and mass transport distribution in the compartment and their improvement by an improved electrolyte compartment that results in a more uniform current distribution. The current distribution was evaluated as the ratio between the local and the averaged limiting current densities during the reduction of copper ions over a range of mean linear flow velocity across the electrode surface (2-30 cm s -1 ).The initial compartment, showed larger differences between the minimum and maximum currents than the electrolyte compartment that resulted as part of the design process and showed a higher pressure drop at a given mean linear flow velocity.Keywords: 3D printing, current distribution, electrolyte compartment, mass transport, pressure drop * Author for correspondence: E-mail: capla@soton.ac.uk 2 IntroductionThe energy demands of modern society, together with its supply and distribution, pose challenging problems. Finding a solution without compromising future generations will require our energy infrastructure to be transformed by allowing a greater, more managed contribution from renewable energy sources. At the same time, the present energy conversion systems should adopt new technologies to become more versatile and efficient. These crucial issues, together with the need of the automotive industry to develop electric and hybrid vehicles, have triggered new research approaches in energy storage technologies.Fuel cells, redox flow batteries and metal-air batteries have been highlighted due to their potential to deliver a high specific energy at moderate cost. In particular, metal air batteries, such as zinc-aluminium-and iron-air batteries, have received increasing attention due to the low cost and natural abundance of the metals and air together with their ease of electrical recharging. Iron-air batteries have advantages during the recharging cycle compared to zinc-air batteries, which have a tendency to form zinc dendrites at the negative electrode upon repeated cycling 1 . Figure 1 shows that the specific energy and theoretical capacity of the iron-air battery compare well with zincair batteries 2,3 . There remain many challenges for the development of the iron-air battery, including the low cell voltage due to slow reduction and passivation of the iron electrode due to the formation of insoluble and non-conductive oxides on discharge and slow evolution of oxygen on charge. Recent advances in nanotechnology have enabled lower oxygen reaction overpotentials by the use of catalytic nanostructured materials and improvements in the reversibility of iron 4,5,6 . 3When designing an electrochemical iron-air battery, it is important to consider that having an even current distribution is essential for effic...

Air electrode development is one of the most challenging steps in the design of lightweight and efficient metal-air batteries and fuel cells. The best performing oxygen catalysts often contain precious metals at a high manufacturing cost. In this paper, two low-cost catalysts for the oxygen reduction (ORR) and evolution reactions (OER), based on LSFCO perovskite and Ni-Fe hexacyanoferrate, were compared with a precious metal palladium catalyst on carbon (Pd/C). LSFCO/C showed the best all-round performance as a single bifunctional catalyst but Pd/C had the strongest ORR activity. Ni-Fe hexacyanoferrate is straightforward to manufacture in industrial quantities, and is more active for the OER than palladium and LSFCO perovskite at small loadings < 5 mg cm −2 . By mixing a small loading of Pd/C with Ni-Fe hexacyanoferrate, lower overpotentials for both the ORR and OER can be reached, with the difference in potential between the two reactions being only 0.62 V at a current density of 20 mA cm −2 . The effect of catalyst loading of each catalyst on the gas-diffusion electrode was studied, and rotating disk voltammetry was used to study the catalytic behavior of the Ni-Fe hexacyanoferrate catalyst.