# Barriers and Chemistry in a Bottle: Mechanisms in Today's Oxygen Barriers for Tomorrow's Materials
## 4 Active Barriers
### Pd/Pt-Based  
A recent development in the area of active oxygen barriers is based on fuel cell technology: Oxygen is reduced by hydrogen to form water [156]. The barrier consists of two distinct parts. First, hydrogen needs to be present. The atmosphere can be modified to hold high pressures of molecular hydrogen, but in most cases, a controlled release of hydrogen was chosen by incorporating hydrogen-evolving compounds. Examples of these compounds exist of calciumhydride or sodium borohydride, incorporated in the bottle cap [156; 157; 158]. As soon as the bottle is filled and water vapor enters the cap, hydrogen is set free through reaction (15) [159; 160]. The capacity of the barrier is again not very affected by the dry empty shelf life. In bottle technologies, the hydrogen-evolving compounds can be easily incorporated in the cap. For film applications however, especially for OLED-materials, its insertion in the system will provide more difficulty.  
\[\mathbf{NaBH_{4}+2\ H_{2}O\ \rightarrow NaBO_{2}+4\ H_{2}} \tag{15}\]The reaction between hydrogen and oxygen can be split into two reactions: the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR). Both require a catalyst that is usually a precious metal, such as for instance Pd and Pt that are the most recurring metals herein [161; 162; 163]. When compounded into a packaging solution, the catalyst is usually added as a divalent acetate salt, but it has been stated that at processing temperatures, the metal ion is reduced by its ligand to zero-valent metal particles and CO\({}_{2}\). The nano-sized metallic particles provide an active surface for reaction [164]. Ultimately, the metal can also be introduced as pre-formed nano-particles [165]. It has been widely examined how surface morphology of such particles effect the reactions, so well-defined structures would be able to improve the catalyst efficiency [166; 167; 168].  
The exact reaction mechanisms are still disputed, but are highly investigated in contact with watery solutions. For the HOR, it is assumed that hydrogen is adsorbed to the surface of the catalyst by dissociation, either to atoms (Equation (16)) or to an atom and an ion (Equation (17)). This is quickly followed by a charge transfer, releasing electrons in the conductance band of the metal (Equation (18)). These reactions are respectively referred to as the Tafel reaction, Heyrovsky reaction and the Volmer reaction [166; 167].  
\[\mathbf{2}\;\mathbf{Pt}+\;\mathbf{H_{2}}\;\rightarrow\mathbf{2}\;\mathbf{Pt}- \mathbf{H} \tag{16}\]  
\[\mathbf{Pt}+\;\mathbf{H_{2}}\;\rightarrow\mathbf{Pt}-\mathbf{H}+\mathbf{H}^{+ }+\mathbf{e}^{-} \tag{17}\]  
\[\mathbf{Pt}-\mathbf{H}\;\rightarrow\;\mathbf{Pt}+\mathbf{H}^{+}+\mathbf{e}^{-} \tag{18}\]  
On the side of ORR, the reaction is assumed to proceed either via a direct reduction with 4 electrons or through the formation of an intermediate hydrogen peroxide species (see Figure 10) [163; 167]. This species can form water with the addition of two more electrons, be decomposed on the surface or can be released as an oxidizing component in the solution.  
This system has been found to achieve very good performances, but the high cost of both the catalyst and the hydrogen source have limited the commercialization of the technology [169].