Light­ing the path to recyc­ling car­bon dioxide

Semi­con­duct­ive pho­tocata­lysts that effi­ciently absorb sol­ar energy could help reduce the energy required to drive a bio­elec­tro­chem­ic­al pro­cess that con­verts CO₂ emis­sions into valu­able chem­ic­als, KAUST research­ers have shown. Recyc­ling CO₂ could sim­ul­tan­eously reduce car­bon emis­sions into the atmo­sphere while gen­er­at­ing use­ful chem­ic­als and fuels, explains Bin Bian, a Ph.D. stu­dent in Pas­cal Saikaly’s lab, who led the research. ​“Micro­bi­al elec­tro­syn­thes­is (MES), coupled with a renew­able energy sup­ply, could be one such tech­no­logy,” Bian says. MES exploits the capa­city of some microbes to take up CO₂ and con­vert it into chem­ic­als, such as acet­ate. In nature, chemo­lithoauto­troph microbes meta­bol­ize min­er­als as a source of energy in a pro­cess that involves the shut­tling of elec­trons. This cap­ab­il­ity can be exploited to turn CO₂ into value-added products if the microbes are sup­plied with a stream of elec­trons and pro­tons from anod­ic water split­ting in an elec­tro­chem­ic­al cell (see image). In their latest work, rather than focus on the CO₂-to-acet­ate step, the team worked on redu­cing the energy input for molecu­lar oxy­gen (O₂) pro­duc­tion at the anode, a reac­tion that keeps the over­all cell in bal­ance. ​“In MES sys­tems, the pro­cess that con­sumes the most energy is believed to be the oxy­gen evol­u­tion reac­tion (OER),” Bian explains. Research­ers have used light-cap­tur­ing anode mater­i­als, such as titani­um diox­ide, that har­ness energy from sun­light to help drive the OER. In their cur­rent work, the team invest­ig­ated a prom­ising altern­at­ive for the pho­toan­ode, the light-har­vest­ing mater­i­al, bis­muth vanadate. Bis­muth vanadate absorbed energy from a much broad­er range of the sol­ar spec­trum than titani­um diox­ide, mak­ing the whole MES cell more effi­cient, the team showed. ​“We obtained sol­ar-to-acet­ate con­ver­sion effi­ciency of 1.65 per­cent, which is the highest repor­ted so far,” Saikaly says. ​“This effi­ciency is around eight times high­er than the 0.2 per­cent effi­ciency of glob­al nat­ur­al pho­to­syn­thes­is, which is nature’s sol­ar-powered pro­cess for con­vert­ing CO₂ into energy-rich molecules,” Bian notes. So far the team has kept the microbe biocata­lysts sup­plied with a steady stream of elec­trons and CO₂ to sus­tain their growth. ​“The next step for us is to test our sys­tem under real sun­light and mon­it­or the resi­li­ence of the biocata­lysts under an inter­mit­tent renew­able energy source,” Saikaly says. Source: KAUST, Research High­light, 05.10.2020