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Villeda-Hernandez et al. Soft Sci 2024;4:14 https://dx.doi.org/10.20517/ss.2023.52 Page 15 of 35

Table 4. Fermentation GERs
Fermentation reactions Bacteria
C H O (l) → 3CH (g) + 3CO (g) Methanogens
6
6
12
2
4
C H O (l) → CH (CHOH)COOH (l) + CH (CH )OH (l) + CO (g) Lactobacillus
6
3
2
2
12
6
3
C H O (l) → 2CH (CH )OH (l) + 2H O (l) + 2CO (g) Saccharomyces
6 12 6 3 2 2 2
C H O (l) + H O (aq) → C H OH (l) + 4CO (g) Escherichia coli
11
2
5
12
2
2
22
C H O (l) → 2[CH (CH )COOH] (l) + CH COOH (l) + H O (aq) + CO (g) Clostridium
9 18 9 3 2 3 2 2
C H O (l) → CH (CH ) COOH (l) + 2(CH COOH) (l) + 4CO (g) + 6H (g) Clostridium
12 24 12 3 2 2 3 2 2
GERs: Gas evolution reactions.
Table 5. Displacement reactions for gas evolution [103]
Displacement reactions
Zn (s) + 2HCl (aq) → ZnCl (aq) + H (g)
2
2
2HCl (aq) + MnO (s) → MnCl (aq) + Cl (g) + H O (l)
2
2
2
2
Fe (s) + 2H SO (aq) → FeSO (aq) + 2H O (l) + SO (g)
2 4 4 2 2
2Al (s) + 6H O (l) → 2Al(OH) (s) + 3H (g)
3
2
2
Ni (s) + 2HCl (aq) → NiCl (aq) + 2H (g)
2
2
Cu (s) + 4HNO (aq) → Cu(NO )2 (aq) + 2NO (g) + 2H O (l)
3 3 2 2
2KMnO (s) + 3H SO (aq) + 5H O (l) → K SO (aq) + 2MnSO (aq) + 8O (g) + 6H O (l)
2
4
2
2
2
4
4
2
4
The decomposition of H O in nature is typically catalyzed by enzymes such as catalase or peroxidase. These
2
2
enzymes catalyze the breakdown of H O into H O and O gas through:
2
2
2
2
The reaction, when not catalyzed by enzymes, is highly exothermic and releases substantial amounts of
energy, making it useful in generating heat and power. Among the most common catalysts used for this
reaction are MnO , silver (Ag), platinum (Pt), palladium (Pd), and iron (Fe).
2
MnO is inexpensive, stable, and easy to obtain, making it a popular choice in various H O -related fields.
2
2
2
Ag is an effective catalyst for the H O decomposition and is used in medical and industrial settings owing
2
2
to its high catalytic activity. Pt is another highly active catalyst for the decomposition of H O and is
2
2
frequently employed in industrial contexts. Pd acts as a replacement for platinum in industrial usage. Fe, as
a cheap and widely available catalyst, sees frequent use in industrial applications and water treatment.
The decomposition of H O is also employed in the field of soft robotics for the actuation of pneumatic
2
2
systems. This reaction serves as a means for inflating and deflating soft actuators, thus enabling motion in
soft robotic devices. The reaction is safe, as it produces no toxic or harmful by-products. However, the
storage of H O , especially in concentrated form, poses risks to consider; lower concentrations are safer but
2
2
result in decreased reactivity, potentially affecting the gas evolution performance.
Combustion
As mentioned in Section “POSITIVE PRESSURE”, combustion reactions have already been explored in soft
robotics as a power source for pressure-driven soft actuators. Their working principle, instead of relying on
the quantity of gas produced by the reaction, is explained by the localized and momentary elevated