ESD 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 decarbonization policies typically assume an implementation of such large-scale negative emission technologies starting around the year 2030, ramped up to cause net negative emissions in the second half of the century and balancing earlier CO₂ release. On average, a contribution of −10 Gt CO₂ yr⁻¹ is expected by 2050 (Anderson and Peters, 2016). A viable approach for negative emissions should (i) rely on a scalable and sustainable source of energy (solar), (ii) result in a safely storable product, (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) feature large-scale feasibility and affordability.

Appendix A: Solar-to-carbon efficiency measure

Given a PEC device and a target sink product, we define the STC efficiency by the ratio of carbon atoms, which are chemically fixed, over the total incoming photon flux, j_ph, given by the integrated solar spectrum. The electronic current corresponding to this total photon flux would be the photocurrent that could be extracted from an ideal absorber with an infinitesimally small bandgap, where each photon contributes to one electron in the photocurrent. The STC efficiency limit for an ideal photoelectrochemical solar cell can then be calculated as follows: the Gibbs free energy difference per electron, ΔG, constitutes the electrochemical load of the cell. It limits, together with the terrestrial solar spectrum, n(λ), the electronic current density, j_e. Tandem solar cells are required for high efficiencies in photoelectrochemical energy conversion as they provide high currents and sufficient voltage to drive the reaction. The current density of an ideal tandem absorber under air mass 1.5 global illumination with very good catalysts can be calculated in the detailed-balance-scheme (May et al., 2017). Under the assumption of unity absorption above the bandgap, the top cell absorbs photons n(λ) in the range between the far UV (λ→0 nm) and the wavelength λ_i+1 corresponding to its bandgap; the bottom cell experiences the photon flux filtered by the top cell and therefore absorbs between the respective bandgaps of top and bottom cell. The smaller of the two values then gives the maximum photocurrent at zero load. The operational photocurrent is then obtained by intersecting the overall current-voltage curve of the solar cell with the curve of its load, given by the Gibb's free energy of the redox couple and the catalyst characteristics described by exchange current density and Tafel slope (see Asset). The selected product then defines the electron efficiency, η_e, i.e. the inverse of how many electrons are consumed for the formation of one product molecule from CO₂ and water. With the faradaic efficiency η_F, describing the efficiency of the conversion from current to desired product, the STC can be formulated as

$$\begin{array}{}\text{(A1)}& {\displaystyle }{\displaystyle }\mathrm{STC}={\mathit{\eta }}_{\mathrm{F}}{\mathit{\eta }}_{\mathrm{e}}{\min }_i\left[{\displaystyle \frac{\underset{{\mathit{\lambda }}_i}{\overset{{\mathit{\lambda }}_{i+\mathrm{1}}}{\int }}n\left(\mathit{\lambda }\right)\mathrm{d}\mathit{\lambda }}{\underset{\mathrm{0}}{\overset{\mathrm{\infty }}{\int }}n\left(\mathit{\lambda }\right)\mathrm{d}\mathit{\lambda }}}\right]={\displaystyle \frac{{\mathit{\eta }}_{\mathrm{F}}{\mathit{\eta }}_{\mathrm{e}}{j}_{\mathrm{e}}}{j_{\mathrm{ph}}}}.\end{array}$$

For formic acid (HCOOH, ΔG=1.4 eV, η_e=0.5), these idealized assumptions result in a maximum electronic photocurrent density of j_e≃26 mA cm⁻². For unity faradaic efficiency, we obtain a product current density equivalent of η_ej_e=13 mA cm⁻². It follows that ideally ca. 19 % of the incoming solar photons transform a CO₂ molecule to the liquid – and hence storable – product. The STC efficiency would therefore be 19 %. Taking into account photoconversion, faradaic, and system losses, values of 10 % STC or more appear feasible as 85 % of the material-specific and ca. 2∕3 of the overall theoretical efficiency limit were already demonstrated on a lab scale for the similar process of photoelectrochemical water splitting (Cheng et al., 2018). This is high compared to the currently achieved energetic efficiencies for natural photosynthesis of 2 %–3 %, which translate to roughly 1.5 %–2 % STC efficiency.

STC efficiencies are a function of the reaction path, similar to CO₂ reduction for fuel generation, where the obtainable efficiency depends on the Gibbs free energy (May et al., 2017). The distribution of energy over the chemical bonds varies for different products, yet for CO₂ removal, we are primarily interested in the number of converted CO₂ molecules. Therefore, the STC efficiencies can deviate significantly for products that have a similar energetic efficiency (Fig. 1c). Feasible products are associated with distinct storage requirements as well as different catalysts. Though the electronic photocurrent could be higher for acetic acid compared to formic acid due to a reduced electrochemical load, four electrons are required for the conversion of one CO₂ molecule, which in the end almost halves the efficiency. The theoretical efficiency limit, as shown in Fig. 1c, largely varies based on the number of electrons consumed per CO₂ molecule, which is one for oxalate, two for formic acid, four for acetic acid and formaldehyde, and six for methanol, ethanol, and 1-propanol. Therefore, using the carbon conversion rate as the benchmark for solar-driven negative emissions will result in a different choice of product compared to solar fuels, where energetic considerations dominate.

Appendix B: Cost estimate

To roughly estimate the costs of negative emissions by photoelectrochemical CO₂ reduction, we assume the module costs to be twice the module costs (Chang et al., 2018) of current crystalline silicon photovoltaics. With a depreciation period of 20 years, and running costs of 10 % of the investment sum, this would translate to EUR 55.60 per tonne of CO₂. Additional costs can arise from the diffusion limitation due to the high conversion rate, which might necessitate technically creating convection by means of mechanical fans. The energy costs of capturing atmospheric CO₂ are estimated to be about 30–88 kJ mol⁻¹ (Goeppert et al., 2012). With an average of 50 kJ mol⁻¹ and a current photovoltaic electricity price of EUR 30 per megawatt hour, this adds another EUR 9.50 per tonne, finally totalling EUR 65 per tonne. Transport costs to the storage location will vary with the chosen product and the vicinity between production and storage facility. If we assume, as a very rough estimate, similar transport costs of formic acid as for crude oil over a distance of 2000 km (Verma et al., 2017), this would result in additional EUR 24 per tonne of CO₂. The overall volume to be transported would be of the same order of magnitude as the present-day oil production. The costs for storage will vary with the sink product, the product volume, and the required post-processing. Some of the products could, in principle, be used as “plastic-based” construction materials, creating an economic value and hence reduce the overall costs. Considering the required scale, however, the market volume for such construction materials will probably not be significant.