Nafion membrane, hydrogen peroxide

Tatapudi and Fenton [69] explored the synthesis of ozone in a proton exchange membrane (PEM) electrochemical flow reactor as part of an overall scheme to study the paired synthesis of ozone and hydrogen peroxide in the same PEM reactor. A mixture of commercially available lead dioxide powder and Teflon , deposited on a Nafion 117 membrane, was used as the anode. Current efficiencies ranged from 2.5% at an applied potential of 3.0 V to 5.5% at 4.0 V. The low current efficiencies were attributed to inefficient reactor design. A decrease in ozone concentrations, observed at higher applied potentials (> 4.0 V) was attributed to the disintegration of lead dioxide at high anodic potentials. 

The same authors also demonstrated the feasibility of synthesizing ozone and hydrogen peroxide simultaneously using pure water and oxygen as the reactants [71]. Commercially available yS lead dioxide and graphite powders were used as the anode and cathode, respectively. While the lead dioxide was deposited directly onto a solid polymer electrolyte (Nafion 117), the graphite powder was deposited onto a carbon fiber paper and pressed against the membrane. At an applied potential of 4.5 V, with a current density of 2 A/cm, a 4.5% current efficiency was obtained for ozone evolution while the current efficiency for peroxide production was 0.8%. 

Application of transition metal hexacyanoferrates for development of biosensors was first announced by our group in 1994 [118]. The goal was to substitute platinum as the most commonly used hydrogen peroxide transducer for Prussian blue-modified electrode. The enzyme glucose oxidase was immobilized on the top of the transducer in the polymer (Nafion) membrane. The resulting biosensor showed advantageous characteristics of both sensitivity and selectivity in the presence of commonly tested reductants, such as ascorbate and paracetamol. 

The classic peroxidase immobilization has been extended to electrodes, e.g., determination of hydrogen peroxide based on a peroxidase/ferrocene-embedded carbon paste electrode covered with a Nafion-coated cellulose acetate membrane. The system was successfully used for glucose and urate in serum. 

Selective oxidation of n-paraffins has been carried out on catalytic iono-mer membranes with Fe +/H202 Fenton.The three-phase membrane reactors were constituted by a proton-conducting membrane (based on Nafion), fed on one side with a gaseous n-paraffins and, on the other side, with an aqueous solution of hydrogen peroxide and Fe " ions. The paraffin is activated by... 

The proton exchange membrane can be a source of fluoride ions as well [143]. Hydroxyl radicals, formed via crossover gases or reactions of hydrogen peroxide with Fenton-active contaminants (e.g., Fe +), could attack the backbone of Nafion, causing the release of fluoride anions these anions in turn promote corrosion of the fuel cell plates and catalyst, and release transition metals into the fuel cell [143]. Transition metal ions, such as Fe, then catalyze the formation of radicals within the Nafion membrane, resulting in a further release of fluoride anions. On the other hand, transition metal ions also can cause decreased membrane and ionomer conductivity in catalyst layers, as discussed in section 2.4 of this chapter. 

Membrane chemical degradation directly results from attack by active radicals. Hydrogen peroxide formed in fuel cells can result in radical formation. Some researchers add radical or HjOj scavengers to eliminate radicals or reduce the formed HjOj to harmless species. Aoki et al. (2006) confirmed that an appreciable fraction of HjOj or HO radicals was easily scavenged at Pt particles in the CL or in Pt-dispersed Nafion membranes. Danilczuk et al. (2009) found that Ce(III) in Nafion membranes with low concentrations can efficiently scavenge HO radicals because of the Ce(lll)/Ce(lV) couple redox... 

Figures 10.18 through 10.20 show fluoride emissions from CCMs in hydrogen peroxide. Both the original and the modified CCM had increasing fluoride emissions over time, but the modified CCM emitted more. Fluoride in a fuel cell solution can originate from the degradation of Nafion in the catalyst layers and/or in the Nafion membranes (Li, 2010).

Kodama, K., Miura, F., Hasegawa, N., Kawasumi, M., Morimoto, Y. (2005) Degradation of Nafion membranes in hydrogen peroxide. In 208th Electrochemical Society National Meeting, Los Angeles, CA, October 2005, Paper 1185. 

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