Uncovering Rose Petal-like Wettability as a Way to Improve Oxygen Reduction Reaction

Gaining a deeper understanding of how electrode wettability influences the triple contact between gas, electrolyte and electrode in electrochemical reactions represents a promising avenue to improve electrochemical processes. In particular, extreme wettability regimes display alluring results for energy transition reactions but are often obtained through complex surface patterning, a hard to scale approach. Here, the wettability of non-modified commercial carbon blacks (Vulcan XC-72, Acetylene Blacks Y70 and YS), largely used as catalyst supports inside fuel cells, is investigated as a way to enhance the performance of Oxygen Reduction Reaction (ORR). Advanced wettability measurement techniques (static and dynamic contact angle measured with different liquids, especially the electrolyte) and Atomic Force Microscopy were combined to interpret electrochemical area measurements such as Capacity Double Layer and Levich equation. Extreme wettability regimes (i.e. Lotus Leaf and Rose Petal effects) were obtained from the coupling of carbons film morphology and Nafion chemistry. To further investigate how the polymer affects wettability and electrochemical performance, different quantities of Nafion were added to Vulcan XC-72 films. Results show that Vulcan XC-72 films, in the presence of a suitable quantity of Nafion in the deposit, presented a Rose Petal-like wettability, characterized by very high contact angles and high adhesion. Here we show the first time that this wetting regime maximises the three-point contact area, fundamental for the electrochemical reaction. In fact, Rose Petal-like films showed a higher percentage of working area for ORR, as determined by electrochemical measurements, both compared to the geometrical area of the electrode and to the real film area, obtained by AFM measurements. Meanwhile, hydrophilic and Lotus Leaf samples displayed a lower working area due to shortage of, respectively, gas and electrolyte at the interface. These results underly the importance of pairing electrochemical studies with wettability analysis in conditions close to application (actual film composition including also polymers, actual electrolyte, dynamic measurements).