Artificial photosynthesis steps up Photosynthesis fixes CO 2 from the air by using sunlight. Industrial mimics of photosynthesis seek to convert CO 2 directly into biomass, fuels, or other useful products. Improving on a previous artificial photosynthesis design, Liu et al. combined the hydrogen-oxidizing bacterium Raistonia eutropha with a cobalt-phosphorus water-splitting catalyst. This biocompatible self-healing electrode circumvented the toxicity challenges of previous designs and allowed it to operate aerobically. When combined with solar photovoltaic cells, solar-to-chemical conversion rates should become nearly an order of magnitude more efficient than natural photosynthesis. Science, this issue p. 1210

Abstract Artificial photosynthetic systems can store solar energy and chemically reduce CO 2 . We developed a hybrid water splitting–biosynthetic system based on a biocompatible Earth-abundant inorganic catalyst system to split water into molecular hydrogen and oxygen (H 2 and O 2 ) at low driving voltages. When grown in contact with these catalysts, Ralstonia eutropha consumed the produced H 2 to synthesize biomass and fuels or chemical products from low CO 2 concentration in the presence of O 2 . This scalable system has a CO 2 reduction energy efficiency of ~50% when producing bacterial biomass and liquid fusel alcohols, scrubbing 180 grams of CO 2 per kilowatt-hour of electricity. Coupling this hybrid device to existing photovoltaic systems would yield a CO 2 reduction energy efficiency of ~10%, exceeding that of natural photosynthetic systems.

Sunlight and its renewable counterparts are abundant energy sources for a sustainable society (1, 2). Photosynthetic organisms harness solar radiation to build energy-rich organic molecules from water and CO 2 . Numerous energy conversion bottlenecks in natural systems limit the overall efficiency of photosynthesis (3). Most plants do not exceed 1%, and microalgae grown in bioreactors do not exceed 3%; however, efficiencies of 4% for plants and 5 to 7% for microalgae in bubble bioreactors may be achieved in the rapid (short-term) growth phase (3). Artificial photosynthetic solar-to-fuels cycles may occur at higher intrinsic efficiencies (4–7), but they typically terminate at hydrogen (8), with no process installed to complete the cycle via carbon fixation. This limitation may be overcome by interfacing H 2 -oxidizing autotrophic microorganisms to electrodes that generate hydrogen or reducing equivalents directly (9–14).

We recently developed a hybrid inorganic-biological system that uses the catalysts of the artificial leaf (15, 16) in combination with the bacterium Ralstonia eutropha (17) to drive an artificial photosynthetic process for carbon fixation into biomass and liquid fuels (18). In this system, water is split to oxygen by a cobalt phosphate (CoP i ) catalyst and hydrogen is produced by a NiMoZn alloy at applied voltages of E appl = 3.0 V. Because the maximum energy efficiency is limited by the value of E appl relative to the thermodynamic potential for water splitting (= E appl /1.23 V), a reduction in E appl leads to biomass and liquid fuel efficiencies that surpass those of previous integrated bioelectrochemical systems and are commensurate with natural photosynthetic yields (18). However, reactive oxygen species (ROS) produced at the cathode were detrimental to cell growth. Because hydrogen peroxide (H 2 O 2 ), as well as short-lived superoxide (O 2 ⦁–) and hydroxyl radical (HO⦁) species, are thermodynamically favored against H 2 production at pH = 7, ROS production dominated at or below the potential to generate H 2 . When E appl reached a sufficient overpotential to drive water splitting, H 2 production to support cell growth outweighed the toxic effects of ROS (18). In addition, leaching of Ni from the NiMoZn alloy into solution inhibited microbial growth.

To develop a biocompatible catalyst system that is not toxic to the bacterium and lowers the overpotential for water splitting, we used a ROS-resistant cobalt-phosphorus (Co-P) alloy cathode (Fig. 1A, pathway 1). This alloy drives the hydrogen evolution reaction (HER) while the self-healing CoP i anode (19, 20) drives the oxygen evolution reaction (OER). The electrode pair works in concert to maintain extraneous cobalt ions at low concentration and to deliver low E appl that splits water to generate H 2 for R. eutropha, which supports CO 2 reduction into complex organic molecules at high efficiency. The Co-P alloy, which is known to promote HER under alkaline solutions (21), exhibits high HER activity in water at neutral pH with minimal ROS production. X-ray photoelectron spectroscopy of Co-P thin films supports the elemental nature of the alloy (fig. S1), and energy-dispersive x-ray spectroscopy (fig. S2) establishes a phosphorus composition of 6 weight percent, which we have found to exhibit optimal HER activity in water at neutral pH with a faradaic efficiency of 99 ± 2% (fig. S3). Moreover, the activity of this Co-P alloy surpasses the activity of the Earth-abundant NiMoZn and stainless steel (SS) cathodes used previously (18) (Fig. 1B). At constant voltage, a stable HER current is maintained for at least 16 days (Fig. 1C). Negligible H 2 O 2 is produced during HER (Fig. 1D), in contrast to that of simple metallic cathodes of Pt and SS.

Fig. 1 Active water-splitting catalyst pair with minimal biological toxicity. (A) Reaction diagram and scanning electron microscopy images for Co-P alloy cathode and CoP i anode. The main water-splitting reaction is shown in black; the side reactions that yield toxicants are in red. Scale bars, 10 μm. (B) Current-voltage (I-V) characteristics of different HER catalysts (10 mV/s). (C) Stability of Co-P cathode, as demonstrated by 16-day chronoamperometry. (D) Assay of H 2 O 2 accumulation for various cathodes combining with CoP i anode: yellow, Pt; blue, stainless steel (SS); red, Co-P alloy. E appl = 2.2 V. Error bars denote SEM; n = 3. (E) Cyclic voltammetry of Co2+ and Ni2+ in the presence of phosphate (P i ). Metal concentrations are both 0.5 mM; 50 mV/s. The current for Ni2+ is magnified by a factor of 50.

The Co-P HER and CoP i OER catalysts work in synergy to form a biocompatible water-splitting system that salvages Co2+ cations leached from the electrodes (Fig. 1A, pathway 2). In the cyclic voltammogram of Co2+ in the phosphate buffer (pH = 7) (Fig. 1E), a pre-wave to the catalytic water-splitting current corresponds to the oxidation of Co2+ to Co3+, which drives deposition of the catalyst. The CoP i catalyst is also known to exhibit a deposition rate that is linearly proportional to Co2+ concentration (22). The self-healing property of CoP i is derived from this interplay of the potential at which OER occurs versus the potential at which the catalyst deposits (20). In concert, the Co-P and CoP i catalysts preserve extremely low concentrations of Co2+ in solution through activity derived from the self-healing process. Inductively coupled plasma mass spectrometry (ICP-MS) analysis of a Co-P|CoP i catalyst system (E appl = 2.2 V) (23) reveals submicromolar levels of Co2+ in solution after 24 hours. This concentration of Co2+ (0.32 ± 0.06 μM) is well below the concentration of Co2+ (half-maximal inhibitory concentration IC 50 ≈ 25 μM) that is toxic to R. eutropha (fig. S4). When diffusion between the two electrodes is impeded by a porous glass frit, Co2+ concentrations rise to ~50 μM. We note that for the NiMoZn cathode, Ni2+ concentrations are not regulated by self-healing, as NiP i cannot form from P i (24), and the deposition to NiO x occurs at >1.5 V versus normal hydrogen electrode (NHE) (Fig. 1E; see fig. S5 for comparison with potentials of relevant redox processes).

Interfacing the biocompatible Co-P|CoP i water-splitting catalysts with R. eutropha results in a system capable of CO 2 fixation. The CoP i catalyst was deposited on a high–surface area carbon cloth as the electrode support (Fig. 1A and fig. S6), resulting in high currents (fig. S7) and a faradaic efficiency of 96 ± 4% (fig. S8). CO 2 reduction proceeded under a constant voltage within a batch reactor (fig. S9), which was half-filled with a solution containing only inorganic salts (mostly phosphate) and trace metal supplements (23).

The CoP i |Co-P|R. eutropha hybrid system can store more than half its input energy as products of CO 2 fixation at low E appl (Fig. 2A and table S1). Entries 1, 2, 3, and 5 show that η elec increases with decreasing E appl under 100% CO 2 until E appl < 2.0 V. Below E appl = 2.0 V (entry 8), a higher salt concentration (108 mM phosphate buffer) is required to facilitate mass transport and attendant current (fig. S10). However, high salt concentrations are undesirable for R. eutropha metabolism. Thus, a concentration of 36 mM phosphate and E appl = 2.0 V resulted in optimal η elec ; the highest η elec achieved for biomass production was 54 ± 4% (entry 5, n = 4) over a duration of 6 days. Our CO 2 reduction efficiency from H 2 is comparable to the highest demonstrated by R. eutropha during H 2 fermentation (25). This biomass yield is equivalent to assimilating ~4.1 mol (180 g) of CO 2 captured at the cost of 1 kWh of electricity. The amount of captured CO 2 is 10% of the amount caught by amine-based carbon capture and storage (~2000 g at the cost of 1 kWh) (26), whose processed product cannot be used as fuel. Enlarging the batch reactor volume by a factor of 10 did not perturb the efficiency (entries 4 and 6), indicating that the system is scalable and the reactor volume does not pose immediate limits. Note that η elec under air (400 ppm CO 2 ) is 20 ± 3% (entry 7, n = 3), which is lower than for pure CO 2 by a factor of only 2.7, although the partial pressure of CO 2 is reduced by a factor of 2500. This indicates that CO 2 is not a limiting reagent (see below). The ~20% of η elec for biomass is equivalent to assimilating ~1.5 mol of CO 2 captured from about 85,200 liters of air at ambient condition with the cost of 1 kWh of electricity.

Fig. 2 Energy efficiencies η elec and kinetics of the hybrid CO 2 reduction device. (A) η elec values for the production of biomass and chemicals at different values of E appl and various configurations (table S1). Solid bars are 5- to 6-day averages; hatched bars are 24-hour maxima. Error bars denote SEM; n ≥ 3. (B and C) Optical density at 600 nm (OD 600 ; indicator of biomass accumulation) and amounts of electric charges that were passed, plotted versus the duration of experiments with 100% CO 2 (B) and air (C) in the headspace at 1 atm pressure. E appl = 2.0 V. Error bars denote SEM; n = 4 for (B) and n = 3 for (C). (D) A microbial growth model predicts linear correlation between electric charges and biomass accumulation, when the H 2 generation rate by water splitting (I/2FV) is smaller than the maximum rate of H 2 consumption by active biomass (r max X a ) (23) (fig. S12). Dashed line indicates Michaelis constant of hydrogenase for H 2 . (E) Real-time monitor of biomass accumulation under “day”/“night” cycle test.

We also isolated a ROS-resistant variant of R. eutropha from one SS|CoP i water-splitting reactor after 11 consecutive days of operation (E appl = 2.3 V) with a H 2 O 2 generation rate of ~ 0.6 μM/min. Genome sequencing found several mutations between the strain (BC4) and the wild type (H16) (table S2). In the presence of paraquat as a ROS inducer (27), the IC 50 of paraquat for BC4 is almost one order of magnitude higher than that of the wild type (fig. S11). There is no obvious benefit of the BC4 strain with regard to η elec (table S1), further confirming the absence of ROS in our system (see above). Nonetheless, BC4 should find great utility for achieving high η elec in systems where ROS is problematic.

We found that biomass accumulation scales linearly with the amount of charge passed under pure CO 2 (Fig. 2B) or ambient CO 2 levels (Fig. 2C). The linear growth is accounted for by a model that combines governing equations for H 2 generation from water splitting and biomass accumulation from carbon fixation (23). The model predicts a linear correlation between biomass and charge passed after an induction period of low population density of bacteria and high H 2 concentration (Fig. 2D and fig. S12), which is consistent with the data shown in Fig. 2, B and C, where the induction period is too short to be observed. Gas chromatography measurements revealed a H 2 concentration in the reactor headspace of 0.19 ± 0.04% (n = 3) in 100% CO 2 and 0.10 ± 0.05% (n = 3) in air, corresponding to 1.5 ± 0.3 μM and 0.8 ± 0.4 μM, respectively, in water. These concentrations of H 2 are well below the Michaelis constant of ~6 μM for membrane-bound hydrogenases in R. eutropha (28), which suggests that H 2 is facilely consumed by R. eutropha. Moreover, similar linear growth conditions for both pure and ambient CO 2 atmospheres provide evidence that H 2 oxidation rather than CO 2 reduction is rate-limiting for biosynthesis. Lastly, R. eutropha halted growth during “night” cycles and continued CO 2 reduction 12 hours later upon resumption of the water-splitting reaction (Fig. 2E), confirming the intrinsic dependence of R. eutropha on H 2 generation. These data also reveal that the CoP i |Co-P|R. eutropha hybrid system is compatible with the intermittent nature of a solar energy source. Direct CO 2 reduction from air highlights the relatively high affinity of R. eutropha for CO 2 at low pressures and at high O 2 concentrations, in contrast to results reported for synthetic catalysts (29), individual enzymes (30, 31), and strictly anaerobic organisms such as acetogens and methanogens (11–14) (table S3).

Metabolic engineering of R. eutropha enables the renewable production of an array of fuels and chemical products (17). When R. eutropha confronts nutrient constraints coupled with carbon excess, the biosynthesis of poly(3-hydroxybutyrate) (PHB) is triggered in the wild-type H16 strain as an internal carbon storage pathway (17). As such, digestion is necessary for PHB collection (23). Under a constant rate of water splitting, PHB synthesis was not manifest until nitrogen became limiting (~2 days), as indicated by the cessation of biomass accumulation (Fig. 3A) as well as the η elec measured every 24 hours (Fig. 3B and fig. S13). With a titer of ~700 mg/liter, the 6-day average for PHB synthesis was η elec = 36 ± 3% (Fig. 2A, entry 9) with a 24-hour maximum of η elec = 42 ± 2% (n = 3) (Fig. 3B). In engineered strains (32, 33), this PHB pathway could be modified to excrete the fusel alcohols isopropanol (C 3 ), isobutanol (C 4 ), and 3-methyl-1-butanol (C 5 ), which possess energy densities of 24, 28, and 31 MJ/liter, respectively. The culture supernatant was then analyzed to quantify the secreted alcohols (23). The accumulation of these liquid fuels followed trends similar to those observed for PHB synthesis. As shown in Fig. 3, C and E, biomass production reached a plateau while isopropanol titers grew to ~600 mg/liter and C 4 + C 5 alcohol titers grew to ~220 mg/liter. An engineered R. eutropha strain produced isopropanol with a 6-day average η elec = 31 ± 4% (Fig. 2A, entry 10) and a 24-hour maximum of η elec = 39 ± 2% (n = 4) (Fig. 3D); a strain engineered to produce C 4 + C 5 alcohols averaged a 6-day η elec = 16 ± 2% (Fig. 2A, entry 11) with a 24-hour maximum of η elec = 27 ± 4% (n = 3) (Fig. 3F). The achieved titers are higher than previous reported values, and η elec values have increased by a factor of at least 20 to 50 (10, 18). R. eutropha has demonstrated tolerance toward isopropanol (fig. S14), allowing for enriched product concentrations under extended operation.

Fig. 3 Efficient synthesis of selectively produced chemicals from CO 2 and water. (A to F) PHB [(A) and (B)], isopropanol (C 3 ) [(C) and (D)], and C 4 and C 5 alcohols [(E) and (F)] were selectively produced from the hybrid device. In (A), (C), and (E), the OD 600 values, concentrations of selective chemicals, and charges passed through the electrodes are plotted versus the duration of experiments. Shown in (B), (D), and (F) are averaged η elec values for different products, measured at 24-hour intervals. Also shown are overall η elec values combining biomass and chemical formation. The η elec values for biomass, defined as intracellular organics excluding PHB, have been corrected to exclude the PHB interference in (B) (23) (see fig. S13 for values before correction). Error bars denote SEM; n = 3.

Our combined catalyst design mitigates biotoxicity at a systems level, allowing water-splitting catalysis to be interfaced with engineered organisms to realize high CO 2 reduction efficiencies that exceed natural photosynthetic systems. Because E appl required for water splitting is low (1.8 to 2.0 V), high η elec values are achieved that translate directly to high solar-to-chemical efficiencies (η SCE ) when coupled to a typical solar-to-electricity device (η SCE = η solar × η elec ). For a photovoltaic device of η solar = 18%, the Co-P|CoP i |R. eutropha hybrid system can achieve η SCE = 9.7% for biomass, 7.6% for bioplastic, and 7.1% for fusel alcohols. This approach allows for the development of artificial photosynthesis with efficiencies well beyond that of natural photosynthesis, thus providing a platform for the distributed solar production of chemicals.

Supplementary Materials www.sciencemag.org/content/352/6290/1210/suppl/DC1 Methods Tables S1 to S3 Figs. S1 to S14 References (34–49)

Acknowledgments: We thank N. Li for ICP-MS measurement and reagents, and J. Torella, C. Myhrvold, C. Lemon, and M. Huynh for helpful discussions. C.L. acknowledges X. Ling at Nanyang Technological University. Supported by a Lee Kuan Yew Postdoctoral Fellowship (C.L.), a predoctoral fellowship from the NSF Graduate Research Fellowships Program (B.C.C.), Office of Naval Research Multidisciplinary University Research Initiative award N00014-11-1-0725 (P.A.S.), Air Force Office of Scientific Research grant FA9550-09-1-0689 (D.G.N.), the Wyss Institute for Biologically Inspired Engineering (P.A.S.), and the Harvard University Climate Change Solutions Fund. This work was performed under the First 100 W Program at Harvard University. C.L., B.C.C., M.Z., P.A.S., and D.G.N. are inventors on patent applications (62/218,131) filed by Harvard University and Harvard Medical School related to the technology described in this paper. The genome sequences are accessible in the NCBI SRA database under accession number SRP073266.