In the pursuit of CO₂ capture to ameliorate climate change, in addition to minimizing future emissions, the process of extracting the gas directly from the atmosphere has captured the imagination of scientists, engineers, entrepreneurs, and policy makers, around the world. It is a gold rush of opportunity for those who want to save humanity from a climate catastrophe, as well as those pulled in by unimaginable wealth when CO₂ can be exploited as the “new oil”.

Pulling CO₂ from the air using direct air capture (or DAC) technologies is a promising route forward, however, the big question on the mind of every stakeholder is, can it really meet the International Panel on Climate Change’s target of 10 G ton/year capacity by 2050? To achieve this goal, detractors of DAC will remind us that this will require about 1,000,000 plants, about 25% of the global electricity supply, and a land area twice the size of Switzerland.

Capturing CO₂ in the Earth’s oceans

There is a growing community of CO₂ capture enthusiasts who believe the answer lies in our oceans, which are currently the world’s greatest sink for CO₂.

Since the industrial revolution, around 40% of anthropogenic CO₂ has been absorbed by the oceans, with an estimated concentration of 2.1 mmol kg⁻¹ (0.095 kgm⁻³), which is 120 times larger than the current atmospheric concentration of CO₂.

Both DAC and oceanic CO₂ are subject to the carbon feedback cycle — a natural process that sees carbon exchanged between the atmosphere, land, ocean, and the Earth’s organisms. When judging the capacity of any technology to remove CO₂ from the atmosphere, scientists must consider a concept from physical chemistry called Henry’s law, which states that the amount and solubility of dissolved gas in a liquid is proportional to its partial pressure above the liquid.

To illustrate its relevance, think about soft drinks whose fizziness is produced with dissolved CO₂. Before opening a can, the gas above the liquid is almost pure CO₂ at a pressure higher than the normal atmosphere. After the can is opened, this gas above the liquid escapes, lowering the partial pressure of CO₂ above it. Over time, this results in degassing of the liquid as the dissolved CO₂ eventually escapes to the atmosphere, leaving behind flat soda.

The same phenomenon occurs with oceanic CO₂ capture. Removing CO₂ from the atmosphere results in a “degassing” from the ocean in the same way as it occurs in the soda can. Say we remove 16 gigatons/year using DAC for 30 years, ultimately removing 491 gigatons from the atmosphere, then the carbon feedback from the oceans as a result of the Earth’s carbon cycle would hypothetically kick back 51 gigatons of CO₂.  

This would translate into about 10-19% of captured CO₂ freed to the atmosphere from the oceans. Hence1.7-9.5 GtCO₂/year of additional CO₂ would have to be removed to meet current carbon budgets, something that DAC supporters ponder.

Off-Shore oceanic CO₂ capture

The Captura Corporation, a spin-off from Caltech, proposes a cost-effective alternative in the form of an offshore CO₂ capture system powered by renewable energy. Their system avoids the use of land, resides in the proximity of offshore CO₂ sequestration storage sites, and provides opportunities for offshore CO₂ enhanced oil recovery.

The technology uses electrodialysis, which is an electrochemical technique in which particles or molecules in a liquid are separated based on their ability to move through semipermeable membranes under the influence of an electric field.

The proposed electrodialysis cell is comprised of a bipolar membrane consisting of anion and cation exchange-layers, which are laminated together. On application of an electric field, the bipolar membrane dissociates water into hydroxide (OH^–) and protons (H⁺), which leads to a careful change in pH across the membrane and an equilibrium between dissolved CO₂, carbonate, bicarbonate.

The architecture of the proof-of-concept electrodialysis cell is suitable for scaling, making it practical for oceanic CO₂ capture, with an impressive electrochemical energy consumption of 155.4 kJ/mol (0.98 kWh/kg) of CO₂, with a CO₂ capture efficiency of 71%.

In an offshore capture site, oxygen and nitrogen would first be removed from a stream of oceanwater by three commercial membrane contactors. The acidified oceanwater would then be directed toward another three membrane contactors to remove CO₂ by a vacuum pump. A cold trap surrounded by dry ice will then be used to condense moisture from the gas output, and the basic effluent is disposed of in a collection tank.

It’s not simply the inverse of Henry’s law because the amount of dissolved CO₂ in native ocean water is tiny. The concentration of bicarbonate in the ocean is >120 times larger than CO₂ concentration in the air by volume. For every CO₂ molecule removed from ocean water, one molecule of OH^– is released, which then pulls more CO₂ from air over a 2–4-month time scale.

This set up has additional benefits, such as there is no separate air contactor or sorbent as the ocean is the contactor and seawater is the sorbent, reducing cost. The concentration of about 2 mM [HCO₃^–] in ambient ocean water represents a significantly increased volumetric concentration of carbon relative to 410 ppm CO₂ in the air, so pumping costs are expected to be lower, and the energy use for sorbent regeneration may turn out to be lower per mol/CO₂ compared to other DAC processes. Finally, the captured CO₂ from oceanic water can be coupled in tandem to an electrochemical cell to enable the offshore production of chemicals and fuels.

This out-of-the-box approach provides a fascinating adjunct to DAC, and it will be exciting to observe the progress of companies like Captura towards the practical implementation of this unique electrochemical technology for CO₂ capture.

Food for thought will be the capital and operating costs, as there is a long history of offshore technologies such as oil, gas, and wind being more expensive than onshore. The multiple for land versus sea for these established businesses will act as a guideline for the economics of air compared to oceanic CO₂ capture. 

Feature image: Vision of an offshore electrochemical technology for capturing CO₂ from oceanic waters using renewable energy. Credit: Captura Corp