Efficient and stable CO2 reduction to formic acid using PEM electrolyzer and recycled battery material


Electrocatalytic CO2 reduction

As the world economy grows and the society develops, people use more and more fossil fuels. Since the beginning of the second industrial revolution, carbon dioxide (CO₂) levels in the air have risen. CO₂ is a greenhouse gas. Increased CO₂ levels in the air causes serious problems with the climate and the environment.

CO₂ can be captured through various technologies, such as Direct Air Capture (DAC), and it can be electrochemically reduced to useful chemicals. This will contribute to a more sustainable and carbon-neutral future.

The electrolysis of CO₂ makes more than 20 different products. Formic acid is one of the most valuable. Many things are made from formic acid (HCOOH). It is an important liquid chemical raw material that is used in medicine, agriculture, energy, and other areas. On the other hand, formic acid, which is a key source of hydrogen, can help a lot with the problem of renewable energy being unstable. Therefore, getting high-purity formic acid from CO₂ reduction quickly and efficiently not only removes CO₂ from the air but also has economic benefits.

In current electrocatalytic CO₂ reduction technology, the most commonly used electrolytes are alkaline because effective CO₂ conversion usually requires alkaline conditions. However, a considerable part of CO₂ will be absorbed by the alkaline electrolyte and generate a large amount of carbonate (CO₃²⁻) precipitation deposited on the electrode surface. This significantly reduces the conversion efficiency and lifespan of the reactor. Strategies such as physical cleaning, pulse operation, and the use of dipole membranes can partially alleviate these problems but can’t completely resolve them.

CO2 reduction to formic acid in a PEM reactor

Carbonate formation occurs over a wide pH range. To avoid carbonate formation in formic acid production, CO₂ reduction in strong acids operating under a proton exchange membrane (PEM) system is a feasible solution.

Recently, researchers have developed a PEM reactor system that efficiently and stably reduces CO₂ to formic acid. In the PEM reactor, CO₂ gas is directly converted to formic acid at the cathode, and hydrogen gas is oxidized at the anode. This finally solves the problem of carbonate precipitation. The diagram below illustrates CO₂ reduction in the PEM reactor system.

CO2 reduction in a proton-exchange membrane (PEM) system produces formic acid.
CO2 reduction in a proton-exchange membrane (PEM) system produces formic acid.

The PEM reactor includes:

  • A Nafion 212 membrane used as the PEM.
  • One porous gas diffusion layer loaded with a cathode catalyst of lead compounds.
  • Another porous gas diffusion layer loaded with anode catalyst of Pt-Ru black.

During the cathode reaction, the rear of the gas diffusion electrode is permeated with high-purity CO₂ at a constant flow rate. The cathode electrolyte flows over the surface of the lead catalyst. CO₂ is converted to formic acid (HCOOH) on the surface of lead catalysts through the chemical reaction:

Cathode reaction: CO₂ + 2H⁺ + 2e⁻ → HCOOH

In the anode reaction, the rear of the gas diffusion electrode is permeated with high-purity humid hydrogen (H₂) gas at a constant flow rate. Hydrogen gas is oxidized according to the reaction:

Anode reaction: H₂ → 2H⁺ + 2e⁻

The overall reaction is:

CO₂ + H₂ → HCOOH

This new reaction mechanism allows 1 mole of CO₂ and 1 mole of H₂ to produce 1 mole of formic acid. Because the selectivity of CO₂-to-formic acid is less than 100%, more than 1 mole of CO₂ is needed.

Formic acid is purified and separated from water by pressurized extraction distillation. Conventional distillation methods fail to meet the practical separation requirements because the boiling point difference between formic acid and water is less than 1 ºC. BASF has previously employed liquid‒liquid extraction to purify formic acid to 95%.

Stable and efficient CO₂ reduction in strong acidic electrolytes is challenging because many catalysts are not resistant to acid corrosion.

Researchers have used recycled lead-acid batteries to obtain a lead-based, acid corrosion-resistant electrocatalyst. The waste lead–acid batteries were disassembled, and lead plates were obtained. The lead plates were washed, dried, crushed, and milled to obtain the recycled Pb catalyst nanoparticles. This Pb catalyst is a composite material of lead and lead sulfate (Pb–PbSO₄). More significantly, the catalysts can be prepared industrially on a kilogram or even ton scale to meet the needs of industrialization.

In this PEM reactor, formic acid is produced with over 93% Faradaic efficiency. The system is compatible with start-up/shut-down processes, achieves nearly 91% single-pass conversion efficiency for CO₂ at a current density of 600 mA cm⁻² and cell voltage of 2.2 V. This system can operate continuously for more than 5,200 h.

Production cost of formic acid

Researchers also analyzed the cost of producing one ton of formic acid in the PEM reactor, which is 1421 USD according to the sum of the following item costs:

The cost of producing one ton of formic acid in the PEM electrolyzer.
The cost of producing one ton of formic acid in the PEM electrolyzer.

When the PEM reactor system is powered by renewable wind energy, it can generate a profit of USD 235 per ton of formic acid produced, surpassing the market price of formic acid. Green hydrogen can be produced via renewable energy. Please view: how these startups produce green hydrogen.

Carbon neutral fuels

Converting CO₂ into fuels is a promising approach to mitigating climate change by recycling CO₂ into useful products. Recent advancements in this field have demonstrated various efficient methods for converting CO₂ into different types of fuels, including jet fuels, formate, methane, ethanol, and methanol.

We researched some early-stage startups working on the cutting-edge technologies of CO₂ conversion to fuels. Please view startups of carbon neutral fuels.

Patent

  • CN113564624B Method for preparing formate by recovering lead material through carbon dioxide reduction

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