Carbon dioxide (CO2) is a greenhouse and asphyxiant gas that is considered at least in part responsible for climate change effects such as global warming due to the increasing levels of CO2 in the earth's atmosphere. In addition, elevated CO2 levels are considered a health hazard when humans are exposed to such elevated levels, as such elevated levels of CO2 can cause several symptoms of unwellness including muscle stiffness, drowsiness, head ache, impaired vision, unconsciousness or even death depending on the level and duration of exposure. For instance, whereas atmospheric levels of CO2 are around 350-450 ppm, complaints of stiffness have been documented at increased levels in the range of 600-1,000 ppm and general drowsiness symptoms documented at increased levels in the range of 1,000-2,500 ppm. An increased risk of more severe adverse health effects is expected to occur at increased levels in the range of 2,500-5,000 ppm or above. These are increased ranges that may be encountered during normal daily life. More rarely, further elevated CO2 levels may be encountered, which may cause intoxication, breathing difficulties and palpitations for CO2 levels up to 30,000 ppm, in addition to which headaches and sight impairment may occur for CO2 levels up to 50,000 ppm, with unconsciousness resulting in death under prolonged exposure at risk of occurring for CO2 levels up to 100,000 ppm.
For at least these reasons, efforts have been made to remove CO2 from gas streams, e.g. industrial flue streams, air streams, e.g. to reduce the emission of greenhouse gases or to control the levels of CO2 in an occupied space such as an office space or a domestic dwelling. A variety of different solutions are available, ranging from absorbing materials, e.g. filters, to CO2 capturing devices. Such capturing devices may employ chemical entities to which the CO2 can be reversibly bound, such as amines, amidines or inorganic salts.
An example of an electrolyis-based industrial process is given by WO 2011/088515 A1, in which seawater is distilled to produce a concentrated brine solution. The concentrated brine solution is subsequently electrolysed to produce hydrogen and chlorine gas as well as a sodium hydroxide solution. The hydrogen and chlorine gas are subsequently applied to a volume of water to generate aqueous hydrochloric acid and sodium hydroxide solution is exposed to flue gases contaminated with CO2 in order to capture the CO2 from these flue gases. The aqueous hydrochloric acid stream and the sodium hydroxide solution stream including the captured CO2 are subsequently recombined to release the CO2 in a controlled manner and regenerate the original constituent gases. However, a problem with such a brine-based process is that it is ill-suited for use in small-scale application domains such as domestic or office spaces due to the generation of chlorine gas in the process, which is highly toxic to humans. Even if an apparatus is provided in which the chlorine gas is generated and managed in a well-controlled manner, accidental damage to the apparatus causing the unwanted release of chlorine gas due to the disruption of such control measures may be difficult to avoid.
US 2008/0248350 A1 discloses a CO2-negative process of manufacturing renewable H2 and trapping CO2 from air or gas streams. Direct current renewable electricity is provided to a water electrolysis apparatus with sufficient voltage to generate hydrogen and hydroxide ions at the cathode, and hydronium ions ((H3O)) and oxygen at the anode. These products are separated and sequestered and the base is used to trap carbon dioxide from the air or gas streams as bicarbonate or carbonate salts. These carbonate salts, hydrogen, and trapped carbon dioxide in turn can be combined in a variety of chemical and electrochemical processes to create carbon-based materials made from atmospheric carbon dioxide. However, a characteristic of such a process is that it also relies on the presence, continuous supply and consumption of suitable cations for the formation of the bicarbonate and carbonate salts, and it is difficult to run the process in a continuous mode. The need for continuous supply of suitable cations also makes the process less suitable for the aforementioned small-scale applications, which ideally should operate as a closed system requiring minimal user intervention.
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