1.5 Catalysts development for DEFCs
1.5.2 Cathode catalyst material
Whilst the anode is primarily responsible for the EOR, the cathode is responsible for the oxygen reduction reaction (ORR). This is just as important as the EOR, if an anode with strong activity is paired with a cathode that reduces oxygen slowly, then it will reduce the performance of the fuel cell. As with the anode, the most popular catalysts are platinum for acid media and palladium for alkaline media. The selection of these metals is mainly due to their effectiveness in DMFCs, having high activity and good stability for the ORR and a lack of research into cathodes in a DEFC, even so, studies have shown Pt and Pd both exhibit average performance as a cathode catalyst[49-50].
Similar results have been seen when Sn and Ni were added to Pt. Beyhan et al. observed the activity of Pt-based catalysts and found that PtNi/C and PtSnNi/C had higher performance than the base catalyst. PtSn/C had a higher performance than both PtNi/C and PtSnNi/C, due to the optimal Sn oxides as an unalloyed state in PtSn/C[51].
PtNi/C however was found to have a lower performance than Pt/C for the ORR. The study also showed that PtSn/C had a higher oxygen reactivity than the other tested catalysts, making it easier for the O-O bond to be broken.
a. Platinum
Pt has shown only average performance as a cathode for ORR. Similarly, to the EOR, adding co-catalysts can help increase the catalysts performance. Alloying Pd to Pt enhances the oxygen reduction abilities of the catalyst. In ethanol, PtPd was found to have a larger overpotential compared to base Pt catalyst and showing a higher resilience to ethanol crossover. The addition of the palladium was found to decrease the affinity for OH- and CO species while increasing affinity for O
2 species[52]. It was also found
that cobalt (Co) was able to promote the oxygen reduction performance of a Pt catalyst, although it did not display an increase in ethanol tolerance.
b. Palladium
Palladium has good activity and chemical stability for use as the cathode catalyst, which is why Pd-based electrocatalysts are a popular option for the ORR in acid based DEFCs. The most widely researched binary palladium-based catalysts are PdCo, PdTi, PdCu and PdNi and the most widely researched ternary palladium-based catalysts are PdCoAu, PdCoMo, PdCoCe and PdCoNi. Although new research has shown that Pt coated Pd catalysts (Pt-Pd/C) with coverages of Pt being almost monolayer have a high activity towards ORR. These results have huge implications towards how catalyst surfaces are designed and nano-engineered for oxygen reduction[53].
c. Platinum vs Palladium
Both metals perform well as base catalysts for the ORR. Styven Lankiang et al. studied the performance of Pt, Pd and Au (gold) based binary and ternary catalysts in O2 saturated 0.1 M HClO4. The palladium led to a decrease in ORR activity each time
the ratio of PdPt was increased, although when gold was added to platinum the activity increased for ratios of up to 1:1. He concluded that catalysts based on Pd and Au were had lower activity towards ORR than Pt did in acid medium, because of the performance of the base catalysts in acid environments; Pt/C had a higher activity than both Pd/C and Au/C. Using data obtained from binary catalyst tests. It was determined that ternary catalysts Pt70Pd15Au15/C and Pt50Pd25Au25/C showed higher
catalytic activity for the ORR and both had a higher stability than the base catalysts. The main cause of degradation of the catalyst was aggregation of particles on the surface, therefore hampering its performance. It was shown however that the Au segregated from the bulk to the nanoparticle surface during the aging test, which is good for stability but reduces the catalyst activity[54].
d. Other viable catalysts
An article recently studied the oxygen reduction capacity of a non-noble metal catalyst Fe-N-C. Its behavior was tested in alkaline conditions using cyclic voltammetry and rotating disk electrode, looking at its reduction capability, selectivity and activity towards hydrogen peroxide. In the single cell DEFC test, after optimisation of the catalyst ink, the peak power density recorded was 62 mWcm-2 at low and mid-range
currents[55], comparing to some of the most prominent non-noble metal catalysts. The
results showed Fe-N-C is highly tolerant to ethanol, stable and has good selectivity for direct oxygen reduction to OH- with 4e-. However, at high current densities, the DEFC
test produced results showing steep decay occurring on the catalyst, seriously affecting the catalyst stability. Osmieri said this was due to “instability of the membrane
conductivity, the MEA fabrication procedure (absence of hot-pressing, poor compatibility between the membrane and the ionomer used for the catalyst ink preparation), mass transport issues (flooding of catalyst layer), and Ru electro- dissolution/ crossover.” Figure 1.13 shows the reduction in both polarisation and power density during the short-term durability test performed in an alkaline DEFC. The performance of the catalyst did improve after a purge drying, although not as greatly as other non-noble metal catalysts, attributed to, but not exclusively, the irreversible deactivation of the Fe-N-C catalyst. Zhang et al. reported similar findings in his paper ‘Highly active and stable non-noble metal catalyst for oxygen reduction reaction’ in which he studied the performance of Fe4N in alkaline solution and found
that the highest performing catalyst showed similar onset potential to Pt/C in alkaline conditions[56].
Figure 1.13 Polarisation (filled out points) and power density curves (hollow points) Short-term durability test in alkaline DEFC for Fe-N-C[55].