Us­ing the Sun to Re­cy­cle CO2 into Fuel and Plas­tic

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Sci­en­tists at the Depart­ment of En­ergy’s Lawrence Berke­ley Na­tional Lab­o­ra­tory (Berke­ley Lab) have har­nessed the power of pho­to­syn­the­sis to con­vert car­bon diox­ide into fu­els and al­co­hols at ef­fi­cien­cies far greater than plants. The achieve­ment marks a sig­nif­i­cant mile­stone in the ef­fort to move to­ward sus­tain­able sources of fuel.

Schematic of a so­lar-pow­ered elec­trol­y­sis cell which con­verts car­bon diox­ide into hy­dro­car­bon and oxy­genate prod­ucts with an ef­fi­ciency far higher than nat­u­ral pho­to­syn­the­sis. Power-match­ing elec­tron­ics al­low the sys­tem to op­er­ate over a range of sun con­di­tions. (Credit: Clarissa Towle/berke­ley Lab)

Many sys­tems have suc­cess­fully re­duced car­bon diox­ide to chem­i­cal and fuel pre­cur­sors, such as car­bon monox­ide or a mix of car­bon monox­ide and hy­dro­gen known as syn­gas. This new work, de­scribed in a study pub­lished in the jour­nal En­ergy and En­vi­ron­men­tal Sci­ence, is the first to suc­cess­fully demon­strate the ap­proach of go­ing from car­bon diox­ide di­rectly to target prod­ucts, namely ethanol and ethy­lene, at en­ergy con­ver­sion ef­fi­cien­cies ri­val­ing nat­u­ral coun­ter­parts. The re­searchers did this by op­ti­miz­ing each com­po­nent of a pho­to­voltaic-elec­tro­chem­i­cal sys­tem to re­duce volt­age loss, and cre­at­ing new ma­te­ri­als when ex­ist­ing ones did not suf­fice.

“This is an ex­cit­ing de­vel­op­ment,” said study prin­ci­pal in­ves­ti­ga­tor Joel Ager, a Berke­ley Lab sci­en­tist with joint ap­point­ments in the Ma­te­ri­als Sciences and the Chem­i­cal Sciences di­vi­sions. “As ris­ing at­mo­spheric CO2 lev­els change Earth’s cli­mate, the need to de­velop sus­tain­able sources of power has be­come in­creas­ingly ur­gent. Our work here shows that we have a plau­si­ble path to mak­ing fu­els di­rectly from sun­light.”

That sun-to-fuel path is among the key goals of the Joint Cen­ter for Ar­ti­fi­cial Pho­to­syn­the­sis (JCAP), a DOE En­ergy In­no­va­tion Hub es­tab­lished in 2010 to ad­vance so­lar fuel re­search. The study was con­ducted at JCAP’S Berke­ley Lab cam­pus.

The ini­tial fo­cus of JCAP re­search was tack­ling the ef­fi­cient split­ting of wa­ter in the pho­to­syn­the­sis process. Hav­ing largely achieved that task us­ing sev­eral types of de­vices, JCAP sci­en­tists do­ing so­lar-driven car­bon diox­ide re­duc­tion be­gan set­ting their sights on achiev­ing ef­fi­cien­cies sim­i­lar to those demon­strated for wa­ter split­ting, con­sid­ered by many to be the next big chal­lenge in ar­ti­fi­cial pho­to­syn­the­sis. Another re­search group at Berke­ley Lab is tack­ling this chal­lenge by fo­cus­ing on a spe­cific com­po­nent in a pho­to­voltaic-elec­tro­chem­i­cal sys­tem. In a study pub­lished to­day, they de­scribe a new cat­a­lyst that can achieve car­bon diox­ide to mul­ti­car­bon con­ver­sion us-

ing record-low in­puts of en­ergy.

Not just for noon For this JCAP study, re­searchers en­gi­neered a com­plete sys­tem to work at dif­fer­ent times of day, not just at a light en­ergy level of 1-sun il­lu­mi­na­tion, which is equiv­a­lent to the peak of bright­ness at high noon on a sunny day. They var­ied the bright­ness of the light source to show that the sys­tem re­mained ef­fi­cient even in low light con­di­tions.

When the re­searchers cou­pled the elec­trodes to sil­i­con pho­to­voltaic cells, they achieved so­lar con­ver­sion ef­fi­cien­cies of 3 to 4 per­cent for 0.35 to 1-sun il­lu­mi­na­tion. Chang­ing the con­fig­u­ra­tion to a high-per­for­mance, tan­dem so­lar cell con­nected in tan­dem yielded a con­ver­sion ef­fi­ciency to hy­dro­car­bons and oxy­genates ex­ceed­ing 5 per­cent at 1-sun il­lu­mi­na­tion.

Cop­per-sil­ver Cath­ode

At left is a sur­face view of a bimetal­lic cop­per-sil­ver nanoco­ral cath­ode taken from a scan­ning elec­tron mi­cro­graph. To the right is an en­ergy-dis­per­sive X-ray im­age of the cath­ode with the cop­per (in pink/red) and sil­ver (in green) high­lighted. (Credit: Gu­ru­dayal/berke­ley Lab)

“We did a lit­tle dance in the lab when we reached 5 per­cent,” said Ager, who also holds an ap­point­ment as an ad­junct pro­fes­sor at UC Berke­ley’s Ma­te­ri­als Sci­ence and Engi­neer­ing Depart­ment.

Among the new com­po­nents de­vel­oped by the re­searchers are a cop­per-sil­ver nanoco­ral cath­ode, which re­duces the car­bon diox­ide to hy­dro­car­bons and oxy­genates, and an irid­ium ox­ide nan­otube an­ode, which ox­i­dizes the wa­ter and cre­ates oxy­gen.

“The nice feature of the nanoco­ral is that, like plants, it can make the target prod­ucts over a wide range of con­di­tions, and it is very sta­ble,” said Ager.

The re­searchers char­ac­ter­ized the ma­te­ri­als at the Na­tional Cen­ter for Elec­tron Mi­croscopy at the Molecu- lar Foundry, a DOE Of­fice of Sci­ence User Fa­cil­ity at Berke­ley Lab. The re­sults helped them un­der­stand how the met­als func­tioned in the bimetal­lic cath­ode. Specif­i­cally, they learned that sil­ver aids in the re­duc­tion of car­bon diox­ide to car­bon monox­ide, while the cop­per picks up from there to re­duce car­bon monox­ide fur­ther to hy­dro­car­bons and al­co­hols.

Seek­ing bet­ter, low-en­ergy breakups

Be­cause car­bon diox­ide is a stub­bornly sta­ble mol­e­cule, break­ing it up typ­i­cally in­volves a sig­nif­i­cant in­put of en­ergy.

“Re­duc­ing CO2 to a hy­dro­car­bon end prod­uct like ethanol or ethy­lene can take up to 5 volts, start to fin­ish,” said study lead au­thor Gu­ru­dayal, post­doc­toral fel­low at Berke­ley Lab. “Our sys­tem re­duced that by half while main­tain­ing the se­lec­tiv­ity of prod­ucts.”

Notably, the elec­trodes op­er­ated well in wa­ter, a neu­tral ph en­vi­ron­ment.

“Re­search groups work­ing on an­odes mostly do so us­ing al­ka­line con­di­tions since an­odes typ­i­cally re­quire a high ph en­vi­ron­ment, which is not ideal for the sol­u­bil­ity of CO2,” said Gu­ru­dayal. “It is very dif­fi­cult to find an an­ode that works in neu­tral con­di­tions.”

The re­searchers cus­tom­ized the an­ode by grow­ing the irid­ium ox­ide nan­otubes on a zinc ox­ide sur­face to cre­ate a more uni­form sur­face area to bet­ter sup­port chem­i­cal re­ac­tions.

“By work­ing through each step so care­fully, these re­searchers demon­strated a level of per­for­mance and ef­fi­ciency that peo­ple did not think was pos­si­ble at this point,” said Berke­ley Lab chemist Frances Houle, JCAP deputy direc­tor for Sci­ence and Re­search In­te­gra­tion, who was not part of the study. “This is a big step for­ward in the de­sign of de­vices for ef­fi­cient CO2 re­duc­tion and test­ing of new ma­te­ri­als, and it pro­vides a clear frame­work for the fu­ture ad­vance­ment of fully in­te­grated so­lar-driven Co2-re­duc­tion de­vices.”

Other co-au­thors on the study in­clude James Bul­lock, a Berke­ley Lab post­doc­toral re­searcher in ma­te­ri­als sciences, who was in­stru­men­tal in engi­neer­ing the sys­tem’s pho­to­voltaic and elec­trol­y­sis cell pair­ing. Bul­lock works in the lab of study co-au­thor Ali Javey, Berke­ley Lab se­nior fac­ulty sci­en­tist and a UC Berke­ley pro­fes­sor of elec­tri­cal engi­neer­ing and com­puter sciences.

Photo by lon­n­ie­gam­ble, CC

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