2 Formic Acid—The Stepping Stone Towards Carbon
Dioxide Utilisation
Formic acid was identified in fifteenth century as an acidic vapour in ant hills, from
where its name derivates—”formica”, the Latin word for ant. It was first synthesised
only in the nineteenth century from hydrocyanic acid by the famous French chemist
and physicist Joseph Gay-Lussac and also from carbon monoxide by another
French chemist, Marcellin Berthelot. However, formic acid received little industrial
attention until the last quarter of twentieth century, when it started to be used as a
preservative and antibacterial in livestock feed due to its low toxicity (LD 50 of
1.8 g/kg). More recently, formic acid regained a new interest, due to some features
that are key to the longed-for “post-fossil era” (Fig. 2).
(a) Formic acid is a stable product that can be formed by the “simple” two-electron
reduction of CO 2 (Eq. 1), what, noteworthy, resulted in a considerable attention
to the electrochemical CO 2 reduction in the last decade (see Sect. 3.).
(b) Besides formic acid, also carbon monoxide is a stable product of the
two-electron CO 2 reduction (Eq. 2). However, its high toxicity, low solubility
and low mass transfer rate make the carbon monoxide subsequent utilisation
challenging. In contrast, formic acid is a highly soluble and stable liquid, easy
to store and transport.
(c) Formic acid is also not explosive, what represents an important advantage
relatively to dihydrogen, an ideal “clean” fuel (see below).
(d) Formic acid is already used as a “building block” in chemical industry.
(e) Formic acid can be a substrate for further reduction to a carbon-based fuel,
methanol and methane, what might not be the most obvious option regarding
CO 2 consumption.
(f) Formic acid, formed from CO 2 and dihydrogen (Eq. 3), can be used as a
“storage form” of dihydrogen, an ideal “clean” fuel (potentially zero contribution to the global carbon cycle, with a high gravimetric energy density) [4–
13]. Although formic acid is not a perfect dihydrogen “storage medium”, due to
its relatively small hydrogen content (4.4%(m/m) or 5.3%(m/v)), it is currently
still one of the best options to circumvent the technical difficulties associated
with dihydrogen handling, storage and transport. For this purpose, formic acid
produced by CO 2 hydrogenation or any other approach is converted back to
dihydrogen when needed.
(g) Moreover, formic acid fuel cells are being the centre of a renewed interest [14–18].
(h) From a biotechnological point of view, formate can be both produced and
assimilated by many natural and biotechnologically engineered organisms and,
unlike dihydrogen (that is “just” oxidised to form reducing power), act as a
carbon source for “formatotrophic” organisms, thus, enabling considerably
higher biomass formation and VC and fuels production yields [19].
Carbon Dioxide Utilisation—The Formate Route
33
Dioxide Utilisation
Formic acid was identified in fifteenth century as an acidic vapour in ant hills, from
where its name derivates—”formica”, the Latin word for ant. It was first synthesised
only in the nineteenth century from hydrocyanic acid by the famous French chemist
and physicist Joseph Gay-Lussac and also from carbon monoxide by another
French chemist, Marcellin Berthelot. However, formic acid received little industrial
attention until the last quarter of twentieth century, when it started to be used as a
preservative and antibacterial in livestock feed due to its low toxicity (LD 50 of
1.8 g/kg). More recently, formic acid regained a new interest, due to some features
that are key to the longed-for “post-fossil era” (Fig. 2).
(a) Formic acid is a stable product that can be formed by the “simple” two-electron
reduction of CO 2 (Eq. 1), what, noteworthy, resulted in a considerable attention
to the electrochemical CO 2 reduction in the last decade (see Sect. 3.).
(b) Besides formic acid, also carbon monoxide is a stable product of the
two-electron CO 2 reduction (Eq. 2). However, its high toxicity, low solubility
and low mass transfer rate make the carbon monoxide subsequent utilisation
challenging. In contrast, formic acid is a highly soluble and stable liquid, easy
to store and transport.
(c) Formic acid is also not explosive, what represents an important advantage
relatively to dihydrogen, an ideal “clean” fuel (see below).
(d) Formic acid is already used as a “building block” in chemical industry.
(e) Formic acid can be a substrate for further reduction to a carbon-based fuel,
methanol and methane, what might not be the most obvious option regarding
CO 2 consumption.
(f) Formic acid, formed from CO 2 and dihydrogen (Eq. 3), can be used as a
“storage form” of dihydrogen, an ideal “clean” fuel (potentially zero contribution to the global carbon cycle, with a high gravimetric energy density) [4–
13]. Although formic acid is not a perfect dihydrogen “storage medium”, due to
its relatively small hydrogen content (4.4%(m/m) or 5.3%(m/v)), it is currently
still one of the best options to circumvent the technical difficulties associated
with dihydrogen handling, storage and transport. For this purpose, formic acid
produced by CO 2 hydrogenation or any other approach is converted back to
dihydrogen when needed.
(g) Moreover, formic acid fuel cells are being the centre of a renewed interest [14–18].
(h) From a biotechnological point of view, formate can be both produced and
assimilated by many natural and biotechnologically engineered organisms and,
unlike dihydrogen (that is “just” oxidised to form reducing power), act as a
carbon source for “formatotrophic” organisms, thus, enabling considerably
higher biomass formation and VC and fuels production yields [19].
Carbon Dioxide Utilisation—The Formate Route
33