Formic acid fuel cells (direct formic acid fuel cells or DFAFCs) are a subcategory of direct liquid-feed fuel cells (DLFCs), in which the liquid fuel is directly oxidized (electrochemically) at the anode instead of reforming to produce hydrogen. Formic acid-based fuel cells represent a promising energy supply system in terms of high volumetric energy density, theoretical energy efficiency, and theoretical open-circuit voltage. They are also able to overcome certain problems inherent to traditional hydrogen (H2) feed fuel cells such as safe handling, storage, and H2 transportation.
There are 3 main types of DFAFCs:
Active DFAFCs, where a pump feeds the liquid fuel into the anode and oxygen in compressed air to the cathode. Active air-breathing DFAFCs, where the cathode is exposed to the oxygen present in ambient air. Passive air-breathing DFAFCs, where there are no mechanical components injecting fuels and oxygen into the cell.
The feeding of fuels and air into a cell increases its energy output, at the cost of price and size/portability.
Today, the main applications of DFAFCs include small, portable electronics, medical diagnostic devices, as well as larger fixed power applications and electric vehicles.
Fuel sourcing
FA is commonly produced by reacting CO with methanol in the presence of a strong base, followed by methyl formate hydrolysis, hydrolysis of formamide, and acidolysis of formate salts. However, FA can also be sustainably produced from the direct electroreduction of CO2, which neutralizes the impacts of the production of CO2 from the fuel cell itself, reducing the environmental impact. The reactions are as follows:
CO2 + 2H+ + 2e- → HCOO+ (in acidic environment)
CO2 + 2H+ + 2e- → HCOOH (in neutral/alkaline environment)
DFAFC comparison to other energy sources
While several fuels were tried for DLFC, formic acid (FA) has received the most interest due to its useful characteristics, such as high volumetric energy density (53g H2/L), open circuit voltage (1.48 V), high theoretical energy efficiency (58%). In addition, the storage of formic acid is easier and safer than that of pure hydrogen, and FA does not need to be kept at high pressures and/or low temperatures.
Similar to methanol, FA is a small organic molecule fed directly into the fuel cell, which removes the need for complicated catalytic reforming. However, compared to methanol, it has lower toxicity, better oxidation kinetics, and higher fuel cell efficiency, since formic acid does not cross over the polymer membrane. Because of its low cross over tendency, FA can be used at higher concentrations than methanol, mitigating the drawback of the lower volumetric energy density (4.4 kWh/dm3 vs 2.13 kWh/dm3).
A comparison of DFAFC with pure hydrogen cells, methanol cells, and gasoline for several characteristics is reported in the table below:
FA safety concerns
Formic acid in 85% concentration is flammable, and diluted formic acid is on the U.S. Food and Drug Administration list of food additives. The principal danger from formic acid is from skin or eye contact with the concentrated liquid or vapors.
Reactions
DFAFCs convert formic acid and oxygen into carbon dioxide and water to produce energy. Formic acid oxidation occurs at the anode on a catalyst layer. Carbon dioxide is formed and protons (H+) are passed through the polymer membrane to react with oxygen on a catalyst layer located at the cathode. Electrons are passed through an external circuit from anode to cathode to provide power to an external device.
The direct anode, cathode, and net reactions of DFAFC are reported below:
Anode: HCOOH → CO2 + 2 H+ + 2 e−
E0 = -0.25V (vs SHE)
Cathode: 1/2O2 + 2 H+ + 2 e− → H2O
E0 = 1.23V (vs SHE)
Net reaction: HCOOH + 1/2 O2 → CO2 + H2O
E0 = 1.48V (vs SHE)
Alternative reactive pathways
While the mechanism of reaction reported in the above section is commonly used for simple explicative purposes, the chemical pathways are actually more complicated, and are at the center of most of the research on DFAFCs to optimize their efficiency. The detrimental effect and poisoning caused by certain pathways can be mitigated by the use of catalysts with proper shape and morphology, such as Pt and/or Pt alloys.