Stability of Prussian Blue Films for Sensing H2O2 in a PEM-fuel Cell Environment
Developing a suitable hydrogen peroxide (H2O2) sensor able to measure small concentrations of H2O2 in-operando within a fuel cell is crucial to understanding the degradation mechanisms that take place in a Membrane-Electrode-Assembly of a PEM-fuel cell. Recently, fiber optic sensing probes based on Prussian blue (PB) have shown reliable response to H2O2 concentrations [1, 2], paving the way for further investigation inside operating PEM-fuel cells. This requires deposition of a robust PB film able to sustain the harsh environment of PEM-fuel cells. In this work, Prussian blue films have been deposited at different synthesis temperatures, and using different precursors including potassium ferrocyanide, potassium ferricyanide, iron chloride (III), and tetraethyl orthosilicate. The samples were immersed and left in a Phosphate Buffer Solution (PBS) of pH 2 at 80 °C for 21 hours and, in thereafter at 90 °C for 3 hours. These PB films were characterized using Fourier Transform Infrared Spectroscopy to analyze their adhesion properties following PBS processing at operating temperature and pH corresponding to an operating PEM-fuel cell.
PB films were deposited on microscopic glass slides using three different processes including; (a) the sol-gel process and dip coating technique, (b) chemical deposition of PB using potassium ferrocyanide and iron chloride (III), and (c) single-source-precursor deposition of PB using potassium ferricyanide. All three processes were performed in an aqueous solution of 0.1 mol L-1hydrochloric acid at 60 °C and the prepared samples were dried first at room temperature and then at 100 °C for 15 min.
To study the effect of synthesis temperature on adhesion properties of PB films, several more samples were prepared through the single-source-precursor route at temperatures of 40 °C, 60 °C, and 80 °C.
Two samples were prepared under each condition, with one sample subjected to PBS processing and followed by characterization of both samples using FTIR to study the effect of deposition process and synthesis temperature on adhesion properties of PB films.
Results and Discussion
Fig. 1 shows the FTIR spectra of the samples synthesized through different processes before (red lines) and after (blue lines) PBS processing in the range of 2500 cm-1 to 1500 cm-1. The peak at 2083 cm-1 is the characteristic absorption peak of PB which is assigned as the stretching vibration of C≡N group in PB structure . The sample prepared through the third process, (PB (III)) has the strongest peak at 2083 cm-1 both before and after PBS processing, indicating that the most stable and thickest PB film were prepared through this process. Conversely, the samples prepared through the first process (PB-Silica) exhibits the weakest peak at 2083 cm-1 before PBS processing and almost no characteristic peak after PBS processing which means the PB film was completely leached into the PBS. Also, the sample which was prepared through the second process, (PB (II)) does not suffer from complete leaching in PBS; however the characteristic peak of PB at 2083 cm-1is not as strong as the peak for the third process (PB (III)). We conclude that less PB is deposited onto the substrate through the second process (PB (II)) compared to the third one (PB (III)).
Fig. 2 depicts the FTIR spectra in the range of 2500 cm-1 to 1500 cm-1 for the samples synthesized through the third process at different temperatures ranging from 40 °C to 80 °C. The red lines and blue lines are the spectra of the samples before and after PBS processing, respectively. The sample which was prepared at 40 °C does not have any distinct peak at 2083 cm-1 after PBS processing which indicates the PB film was completely leached into PBS because of its poor adhesion properties. On the other hand, the PB films prepared at 60 °C and 80 °C have a strong peak at 2083 cm-1both before and after PBS processing which confirms the good adhesion properties of PB films prepared at these temperatures. Since there is no specific advantage between the PB films prepared at 60 °C and 80 °C, a temperature of 60 °C can be chosen as the optimum synthesis temperature to achieve the most stable PB film through this technique.
This work was supported by the German Academic Exchange Service (DAAD), and through a Strategic Research Grant from the Natural Science and Engineering Research Council (NSERC) of Canada.
 Akbari Khorami et al., Electrochimica Acta 115 (2014) 416- 424.
 Botero-Cadavid et al., Sensors and Actuators B: Chemical 185 (2013) 166-173.