Electrochemical Study of Nano-Iron/Iron Carbide As Rechargeable Electrodes
Authors: Paulraj Alagar Raj , Kiros Yohannes , Skårman Björn , Vidarsson Hilmar 
Published: February 2020

Fe–air system is considered to be of low-cost, environmentally friendly and mechanically stable among the available battery technologies. Iron electrodes can maintain more than 3000 cycles in Ni-Fe secondary batteries. However, the iron electrode possesses low charge efficiency and inferior rate capability. In order to circumvent the hydrogen evolution taking place during the reaction, which is the main contributor to the irreversibility, various additives both to the electrolyte and porous electrode have been employed to increase the charge/discharge performance (1, 2). A range of nano-iron materials were prepared to increase the surface area and thereby improve the utilization (3-5). Recently carbon grafted in iron as well as carbon/iron composite morphologies have shown increased discharge capacities (6, 7).
In this work, we have studied the surface morphology evolution upon electrochemical cycling of core-shell structured samples, i.e. 2 wt% Cu-doped nano-iron coated with a thin layer of iron carbide (NanoFeC-Cu). The electrodes were prepared by homogenizing iron powder, high surface area carbon black, bismuth sulfide and PTFE dispersion in a high speed mixer. This paste was then rolled on a nickel wire mesh. After this step, the electrode was sintered in nitrogen atmosphere for 30 min.Electrochemical measurements were carried out in a three cell configuration with nickel mesh as counter electrode and an Hg/HgO as reference electrode in 6M KOH. The electrochemical formation of the iron electrodes took about 30 to 40 cycles. The discharge/charge rate for respective steps was limited to C/10 and C/7 and the discharge cut-off voltage was -0.75V.
The Nano FeC-Cu achieved 350 mAh/g at ≈ 80% current efficiency, successfully running over 100 cycles as shown in Fig. 1.The electrode after galvanostatic and potentiodynamic polarization measurements showed Tafel slopes of 35 mV/decade and no passivation was observed at a current density of 40 mA/cm2 at -0.75V. SEM images of both the fresh and used electrode surfaces show that the nano particles are still intact with negligible particle agglomeration. The electrodes have shown stable performances with low capacity decay. The present core-shell structured material is, therefore, a promising anode candidate in alkaline-metal/air batteries.

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Acknowledgement

This work is funded by the Swedish Energy Agency.

Figure 1