In their article ‘Enzymes and bio-inspired electrocatalysts in solar fuel devices’ (Energy Environ. Sci., 2012, 5, 7470), Woolerton et al. use a generic Frost diagram (Fig. 6.21) to illustrate the concept of chemical energy storage and release. So-called ‘energy-rich substances’ are compounds (fuels and oxidants) in which chemical energy is stored: fuels (such as hydrocarbons, boranes, metal hydrides) lie towards the upper left whereas strong oxidants such as O₂ lie towards the upper right. ‘Energy-poor substances’ are stable, reduced compounds (e.g. H₂O) lying towards the lower left and oxidized compounds (e.g. CO₂, components of ash) lying towards the lower right. Energy release (whether by combustion or in a fuel cell or battery) occurs when a species from the upper left reacts with a species from the upper right to give products positioned diagonally down their respective arrows. During energy storage (such as photosynthesis or charging a battery) compounds from the lower left and lower right are transformed using sunlight or electricity into products that are positioned diagonally upwards along their respective arrows. Using data from Resource section 3 assess the usefulness of this concept for a methanol/oxygen system and a lead–acid battery.

Figure 6.21.

Transcribed Image Text:

Free energy Fuels Reduced Energy rich Oxidants Oxidized Energy poor Oxidation number, N