| Massive emissions of carbon dioxide in the atmosphere caused serious greenhouse effect.The CO2 electrotreduction based on inorganic catalysts is a promising approach for CO2 conversion,while it is suffering from the low value of products and high energy consumption.An electrocatalysis/biosynthesis hybrid system can effectively combine the advantage of electrocatalysis and biocatalysis,and achieves the efficient conversion of CO2 to high value-added compounds.Intimate coupling of electrocatalysis and biosynthesis for the effective conversion of CO2 is a major bottleneck in the hybrid system.In this work,the highly efficient electrocatalysis/biosynthesis hybrid system was built,and the compatibility and interaction between electrocatalysis and biosynthesis was systematically studied to reveal the coupling mechanism and influence of main factors in the coupling process.Finally,the effective conversion of CO2 to poly-β-hydroxybutyrate was achieved.Firstlly,the system coupling CO2 electroreduction and syngas fermentation was developed,using Clostridium ljungdahlii as the biocatalyst that utilize CO and H2 generated from the CO2 electroreduction to convert CO2 to acetate.The results indicate that the yeast extract,vitamin,NiCl2 and CoCl2 in the bacterial growth medium significantly decreased the faraday efficiency of CO from 65.35 ± 5.65%to less than 1%.This led to the amount of CO could not satisfy the requirement of the syngas fermentation and the acetate production was only 269 mg L-1.So,the growth medium component changed the electron flow direction of the electrocatalysis in the hybrid system,and thus decreased the faraday efficiency of CO2 reduction.As a result,it was extremely difficult to efficiently couple the CO2 electroreduction and syngas fermentation.To solve the above problem,the water splitting based on Pt/C catalyst was further coupled with biosysthesis based on H2-oxidizing autotroph,Cupriavidus necator H16 to achieve the CO2 conversion to PHB.By investigating the optimal coupling conditions,it was found that the nitrogen was a decisive factor in the PHB systhesis,and the high nitrogen concentration(>0.5 g L-1(NH4)2SO4)inhibited the bacterial growth and PHB systhesis.The higher current density could obviously improve the bacterial growth rate and ability of PHB systhesis.In addition,the bacterial growth medium and high-density bacteria exihibited an inhibition on the performance of water spltting and electricity conversion efficiency,and this can be relieved by increasing the salinity of growth medium.Finally,A maximal PHB production of 423 ± 17 mg L-1 with the electricity conversion efficiency of 15.45 ± 1.36%can be achieved under the optimized conditions,and thus water splitting could be effectively coupled with biosysthesis.To develop the novel electrocatalyst with low cost,a Ni nanoparticles embedded nitrogen doped carbon nanotube(Ni@N-C)was synthesized by the rational structure desigen of catalyst,and was used as a hydrogen evolution reaction electrocatalyst in the water-splitting biosynthetic hybrid system.In the Ni@N-C,the Ni nanoparticles were encapsulated in N-doped carbon nanotubes,which prevented bacteria from direct contact with Ni and inhibited Ni2+ leaching.As a result,the Ni@N-C exhibited excellent biocompatibility and stability.What’s more,owing to the embedding of Ni and doping of N,the Ni@N-C showed a similar HER activity and stability to Pt/C in the hybrid system.So,a maximal PHB production of 386 ± 7 mg L-1with a maximum PHB production rate of 208± 4 g m-2 L-1 d-1 was achieved.So,by using this earth-abundant non-noble metal catalyst with low cost,the water splitting was also effectively coupled with biosynthesis for the CO2 conversion to PHB. |
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