Printed for Performance: Harnessing the Electrochemical Potential of Prussian White Thick Electrodes Via 3D Printing
Authors: Khanom Halima 
Published: February 2024

The development of Prussian White (PW) water-based inks for thick, structured electrodes will be presented. Thicker electrodes (>100 µm) have the potential to have both high power and higher energy, however there is a trade-off between both. When increasing the thickness of a standard two-dimensional (2D) electrode this lengthens the ionic pathways, limiting our rate of ionic diffusion. To overcome these restraints, we can introduce a 3D architecture within the electrode microstructure to maintain favourable, short ionic pathways, whilst increasing the active surface area of the electrode.
Here, we are investigating a high capacity, Na-ion material that is aqueous processed, to fabricate thick electrodes that can provide a high-capacity cell. PW is a Na-ion hexacyanoferrate crystal (NaxFe[Fe(CN)6]), where the redox properties of the iron atoms can be visualised in the voltage profile. PW has potential to be used for applications that Li-ion may not be necessarily required for, as it can reach a comparable capacity of 160 mAh/g, similar to that of LFP (Lithium iron phosphate). Moreover, this Na-ion material can overcome the supply and ethical issues that are currently associated with Lithium. Thus, PW here has the capability to become part of the next generation of batteries.
To fully understand the PW electrochemical behaviour, the material underwent a baseline process. From our initial research, the moisture sensitivity of PW was observed clearly in the electrochemistry where sloping voltages and poor cell capacities were seen. Therefore, an effective drying process was developed to prevent the leaching of water molecules, thus allowing the full usage of the PW and now achieving the theoretical capacity of 160 mAh/g.
PW was also examined via different mass loadings from 1-3 mAh/cm2, where we successfully demonstrated how thicker electrodes can provide a comparable cell capacity throughout, as shown by our electrochemical data. However, the limitations of thicker electrodes (3 mAh/cm2) were detected in the rate test data, where the PW experiences no capacity retention at high C rates, due to the limits of the elongated diffusion pathways in a 2D electrode. The GITT (galvanostatic intermittent titration technique) data visualised the diffusion mechanism of the thick PW electrodes, whilst from the EIS (electrochemical impedance spectroscopy) current density values were obtained.
Thus, PW is being investigated here to see whether it has potential to be 3D printed via extrusion to create our desired structured electrodes. To understand how effectively an electrode ink will be printed, the rheology properties of the ink is also characterised here. The different parameters of achieving a high viscous ink at low shear rates, will also be discussed to allow the fabrication of thick, structured PW electrodes.
With the initial 3D printed electrodes, the rate test data seen in Figure 1., the 3D printed electrodes hold a comparable capacity at low C rates, however, it is limited electrochemically at high C rates. It is clear that further optimisation of the ink rheology is needed and this can be done with the introduction of CNT (carbon nano tubes), higher solid content or using a secondary solvent to create an ink that is printable. Once structured electrodes are achieved through tuning the ink rheology and printing parameters, these electrodes will then be compared electrochemically against our standard 2D electrode, with hopes to overcome the ionic limits of thick 2D electrodes.
The wider impact is that 3D printing via extrusion can be used for other materials, once a rheological checklist is passed. Thus, 3D printing has the potential to produce a cell with both high energy and power. The combination of Prussian White and 3D printing offers a promising method to developing novel electrode materials with enhanced performance for a range of electrochemical applications.

Figure 1. Discharge capacities of PW9 (doctor blade) & 14.1 (3D printed) at different thicknesses, Na-half cell cycled between 1.3-3.8 V and discharged at C/5, C/2, C, 2C & 5C.

Figure 1