Ideas : Photoelectrochemical carbon removal as negative emission technology

The pace of the transition to a low-carbon economy – especially in the fuels sector – is not high enough to achieve the 2◦C target limit for global warming by only cutting emissions. Most political roadmaps to tackle global warming implicitly rely on the timely availability of mature negative emission technologies, which actively invest energy to remove CO2 from the atmosphere and store it permanently. The models used as a basis for decarbonisation policies typically assume an implementation of such large-scale negative emission technologies starting around the year 2030, ramped up to cause net negative 5 emissions in the second half of the century and balancing earlier CO2 release. On average, a contribution of -10 Gt CO2/year is expected by 2050.(Anderson and Peters, 2016) A viable approach for negative emissions should (i) rely on an unlimited source of energy (solar), (ii) result in a safely storable product (e.g. liquid or solid, not gaseous), (iii) be highly efficient in terms of water and energy use, to reduce the required land area and competition with water and food demands of a growing world population and (iv) be large-scale feasibility and affordability. 10 Processes for the extraction of CO2 from the atmosphere are energy-intensive. This energy has to be supplied by lowor zero-carbon sources. At present, primarily direct air capture and biomass production are explored (Smith et al., 2016). Renewably driven direct air capture is believed to be expensive and has not yet demonstrated scalability. Therefore, the currently most feasible option appears to be the use of natural photosynthesis to generate biomass through afforestation or ocean fertil15 ization. Grown plants are then permanently stored, building a new stock of fossil fuels. Alternatively, the plant material can be combusted with carbon capture and storage to act as a low-carbon fuel. However, the efficiency of natural photosynthesis drops at high light conditions and because a significant fraction of the energy is used for the metabolism (Melis, 2012), the storage of solar energy in biomass is limited to 2-3% efficiency. Therefore large areas of agricultural land would be required for the achievement of the negative emission goals: The removal of one Gt CO2/year can demand more than 1 million km, 20 the combined area of Germany and France (Smith et al., 2016). Scaling biomass production to the required 10 Gt CO2/year is hence unlikely to be compatible with planetary constraints (Heck et al., 2018). 1 Earth Syst. Dynam. Discuss., https://doi.org/10.5194/esd-2018-53 Manuscript under review for journal Earth Syst. Dynam. Discussion started: 31 August 2018 c © Author(s) 2018. CC BY 4.0 License.

We suggest to employ photoelectrochemical CO 2 reduction, also called artificial photosynthesis : , :: to ::: this ::: end.As in its natural counterpart occurring in plants, photons in the artificial photosynthesis process excite charge-carriers, which then reduce (and oxidise) reactants in a liquid electrolyte to solar fuels.The photon energy is only briefly converted to electron energy, and then stored in molecular bonds.Light is absorbed in synthetic materials such as semiconductors or dyes and the chemical conversion typically takes place at (co)catalysts at the interface between electrolyte and light absorber.We primarily focus on tightly integrated photoelectrochemical systems, where the absorber is immersed into the electrolyte.While this approach imposes restrictions on the light absorber design, the tight integration also promises cost benefits (Kirner et al., 2016).
Therefore, the production of carbon-rich liquids, such as alcohols or (fatty) acids, is most promising.These could be stored in underground reservoirs such as depleted oil fields, but also used as precursors for organic construction materials.
Any competitive artificial approach should provide a significantly higher turnover than natural photosynthesis.To assess the technologies, their efficiency for carbon removal has to be estimated and compared.The typically used solar-to-fuel efficiency (May et al., 2017) is not suitable, as it only describes the relative fraction of incident solar radiation that is converted to chemical energy.Instead, negative emission technologies based on solar energy are better assessed by the solar-to-carbon (STC) efficiency, which we define as the ratio of converted CO 2 molecules to the incoming photon flux (Appendix A).
Our calculations in the following were performed under the -highly idealised -assumption that the overpotential is dominated by the oxygen evolution reaction for a very good catalyst, which can be justified for water splitting.CO 2 reduction with the currently available catalysts, on the other hand, is associated with significantly higher overpotentials.The direct impact of catalysis performance on achievable efficiencies can be seen in Figure 1(a-b), where obtainable STC efficiencies and resulting module areas are plotted as a function of Tafel slope and exchange current density.
Artificial solar energy conversion does -unlike natural photosynthesis -not suffer from an efficiency decrease due to high light conditions, as beneficial effects of light concentration on the solar cell and higher temperatures on catalysis can over- compensate the detrimental effect of temperature on the absorber.Hence, near-equatorial regions with high solar irradiation are viable target areas for its deployment.Under the assumption of 3500 kWh/m 2 available per year for a 2-axis tracker in the Sahara desert region (Amillo et al., 2014), we can estimate the required module area for the 2050 negative emission target of 10 Gt CO 2 /year.At a maximum STC efficiency of ca.19% ::: (for :::::: formic :::: acid, ::: see ::::::::: Appendix :: A), this would be approximately 13,500 km 2 .Under the conservative assumption that for a mature technology the overall system efficiency is half of the theo-5 retical efficiency, this translates to an areal requirement of about 27,000 km 2 (Fig. 1a :  c).The typical space factor for tracking photovoltaics of 0.2 , (Araki et al., 2016) :::::::::::::::: (Araki et al., 2016), : finally leads to a land footprint of ca.135,000 km 2 .Other desert areas such as the Gobi desert, or the Thar desert in north-western India, would also be interesting regions.In areas such as central Europe, a lower irradiance translates to larger footprints (Fig. 1b : d).The scale of such an effort, if one tried to realize it in a single project, would considerable and about one order of magnitude larger than the previously largest project for solar 10 electricity production in the Sahara desert :: be ::::::::::: considerable.However, it could be realized alongside with biomass approaches in other world regions, as it does not rely on agriculturally usable land.With the 2 • C target, there is a truly global incentive to realise such an undertaking.Especially if spread over several projects, the economic added value would be created in the regions that suffer most under global warming.
Carbon removal by artificial photosynthesis is water-efficient, compared to its natural counterpart, as water is only used as chemical precursor and not evaporated from the closed system.Considering formic acid as product to be stored, and the target of 10 Gt CO 2 /year to be removed, the water demand is about 4.1 Gt/year.This would be a substantial amount in dry regions.Desalination of seawater would be possible, albeit energetically inconvenient.However, the direct use of seawater was already demonstrated for electrochemical hydrogen production (Fukuzumi et al., 2017), and might therefore also be possible for CO 2 reduction.Another challenge is that high-efficiency carbon sinks concentrated in large-scale facilities could, in principle, suffer from mass transport limitations of dilute CO 2 in the atmosphere.This could be alleviated by selecting sites with high atmospheric convection rates, by spacing facilities sufficiently widely apart, or to combine them with solar updraft towers for electricity generation.

Figure 1 .
Figure 1.Theoretical efficiency limits and module area for the -10 Gt CO2/year scenario.(a) STC efficiency limit of a dual-junction absorber for formic acid (without system loss) as a function of exchange current density and Tafel slope.(b) Resulting module area at Sahara irradiance and 50% system loss.(c) STC efficiency and module area required under Sahara irradiance for different products at 50% system loss.Error bars indicate 40 and 60% loss, respectively.(d) Module area for formic acid production over the yearly irradiance at 50% (solid line), as well as 40 and 60% (dashed lines) system loss.Vertical lines mark typical irradiances accessible to a 2-axis tracker.