Lawrence Berkeley National Laboratory Recent Work Title Supported Oxygen Evolution Catalysts by Design: Toward Lower Precious Metal Loading and Improved Conductivity in Proton Exchange Membrane Water Electrolyzers Permalink https://escholarship.org/uc/item/7k52r7g7 Journal ACS Catalysis, 10(21) ISSN 2155-5435 Authors Regmi, YN Tzanetopoulos, E Zeng, G et al. Publication Date 2020-11-06 DOI 10.1021/acscatal.0c03098 Supplemental Material https://escholarship.org/uc/item/7k52r7g7#supplemental Peer reviewed eScholarship.org Powered by the California Digital Library University of California Supported Oxygen Evolution Catalysts by Design: Towards Lower Precious Metal Loading and Improved Conductivity in Proton Exc!ange Membrane Water Electrolyzers Yagya N. Regmi, Eden Tzanetopoulos, Guosong Zeng, Xiong Peng, Douglas I. Kushner, Tobias ". Kistler, Laurie A. King and Nemanja Danilovi&' Dr. Y. N. Regmi, E. Tzanetopoulos, Dr. X. Peng, Dr. D. Kushner, Dr. N. Danilovi& Energy (torage and Distributed Resources Division, #a)rence Berkeley National #aboratory, Berkeley, CA, 94720, U(" *E3mail4 ndanilovi&5lbl.gov Dr. Y. N. Regmi, Dr. L. A. King Department of Chemistry, 7ancheste 7etropolitan 2niversity, Cheste (t, 7anchester, 78 5GD, UK E. Tzanetopoulos College of Chemistry, University of California Berkeley, Berkeley, C", 94720, U(" Dr. G. Zeng, T. A. Kistle Chemi&al (&iences Division, #a)rence Berkeley National #aboratory, Berkeley, ,", 94720, 2(" Dr. G. Zeng, T. A. Kistle :oint Center for Arti;&ial Photosynthesis, La)rence Berkeley National Laboratory, Berkeley, California, U(" T. A. Kistle <alte S&hottky Institute and Physi&s Department, Te&hnis&he Universit=t München, Ga &hing, Germany 8 $eywords proton e?&hange membrane ele&trolyzers, )ate splitting, oxygen evolution rea&tion, supported ele&trocatalysts, iridium catalysts, photoreduction, membrane ele&trode assembly %bstract4 Reducing the pre&ious metal &ontent of )ate oxidation &atalysts for proton-e?&hange3membrane )ate ele&trolysers remains a &riti&al barrie to thei large3s&ale deployment. @erein, )e present an engineered a &hite&ture for supported iridium &atalysts, )hi&h enables de&reased pre&ious metal &ontent and improved a&tivity and &onductivity. The improvement in performance at lowe pre&ious metal loading is realized by the deposition of a &onformal laye of platinum nanoparti&les on titanium dioxide (TiB0C using a fa&ile photoreduction method to prepare &onductive laye &oated supports A,,(s). Platinum nanoparti&les are homogeneously dispersed on TiB0, and the &onductivity of the subsequent &atalysts )ith E- )tF pre&ious group metal loadings are signi;&antly highe than the &omme &ial /9 )tF loaded IrB0-TiB0 &atalysts. The &onformal &onductive laye also maintains enhanced &onductivity and ele&trochemi&al a&tivity upon thermal annealing )hen &ompared to &atalysts )ithout the &onductive laye and non- &onformal heterogeneous &onductive layer. The iridium mass a&tivity from hal63&ell studies shows 8.8F improvement for ,,( supported &atalysts at .0F lowe loadings &ompared to the &omme &ial &atalysts. The &onductive laye also improves the single &ell ele&trolyze performance at simila &atalyst loading in comparison to comme &ial state3of-the3art &atalyst. <e &orrelate the physi&al properties of the engineered &atalysts )ith thei ele&tro&hemi&al 0 performance in ele&trolyzers to understand structure3a&tivity relationships, and )e anti&ipate furthe performance improvements upon synthesis and materials optimizations. Introduction Ele&tro&hemi&al hydrogen generation has the potential to de&arbonize the energy se&tor, enabling high rene)able energy penetration through long-term energy storage and &onversion.830 "dditionally, deep de&arbonization of industrial processes such as food production (ammonia synthesis), oil re;ning, and steel manufa&turing &an be realized i6 the Ggreen” hydrogen from )ate splitting repla&es hydrogen generated via routes that releases &arbon dioxide and othe harmful byproducts.E3. "nnually, more than -9F of the /1 million tons of hydrogen is produced globally from hydrocarbons via steam methane reforming A(7R), )ate 3gas shift and &oal gasi;&ation, among othe processes.9 Ele&trolysis &urrently a&&ounts for only .F of hydrogen production. Proton-e?&hange3membrane APE7C based )ate splitting is one promising te&hnology to displa&e (7R, howeve several &ost3&utting measures are ne&essary to a&hieve the goal. Bne such measure is eithe to use &urtailed rene)able ele&tri&ity or to rea&h powe purchase agreements to reduce the feedstoc+ &ost (ele&tri&ity). Eventual )idespread deployment of PE7 )ate ele&trolyzers (PE7<EsC is proje&ted to rende the ele&trochemi&al sta&+ as the dominant cost, and the catalyst layers (CLs) the most expensive components in the sta&k.I Today, the ,#s a&&ount for about 9F of the total &ost of the sta&k, but this value is proje&ted to increase as ele&trochemi&al hydrogen production is s&aled up from +< to G<./ Thus, by simultaneously lowering the feedstoc+ &osts and eithe repla&ing or lowering the platinum group metal APG7C loadings in &atalyst layers A,#s), it )ill be possible to lowe the pri&e of green @0 to (7R range.9 The te&hno-e&onomi& barrie emanating from high PG7 loadings needs to be E surmounted to enable large s&ale (T<C ele&tro&hemi&ally generated hydrogen to be&ome e&onomi&ally &ompetitive )ith hydrogen generation from (7R./ The balance of plant and balance of &ell &osts are anti&ipated to de&rease )ith high throughput manufa&turing. Conversely, PG7 &osts are expe&ted to remain high due to thei limited abundance in the earth’s &rust, of whi&h Ir is far rare than Pt.K To reduce the pre&ious metal loading in PE7 ele&trolyzers, various strategies including the use of non-PG7 &atalysts, and engineering ,#s )ith lowe loadings of PG7 &atalysts supported on low3&ost substrates have been explored.-38E Lor example, King et al. re&ently reported use of CoP as the &athode &atalyst on PE7<E8. and Alia et al. have shown the promising stability of various low Ir-loaded anode ,#s.89 <hile a diverse library of non-PG7 materials for &athodi& hydrogen evolution rea&tion A@ER) in PE7<E has been developed,8I analogous materials for anodi& oxygen evolution rea&tion ABER) for PE7<E are yet to be identi;ed.8/38K <hile the ultimate goal is to mitigate the usage of PG7 &atalysts, the need for short and midterm strategies to lowe the PG7 loading represents a &riti&al development. To this end, one of the most promising strategies is to develop stable and &onductive ele&trocatalyst supports akin to the &arbon supports &urrently deployed in the ,#s of PE7 fuel cells. (upports employed in PE7<E anodes must )ithstand the highly &orrosive a&idi& (pHM8C and oxidizing AN0 OC &onditions.8- Lurthermore, the support needs to be &onductive. 2nde these &onditions, most metals )ill readily oxidize and form insulating layers )hile the fe) that &an )ithstand the harsh &onditions are $ust as expensive as Ir. <hile some metal oxides &an )ithstand the corrosive &onditions, most are poor &onductors.01308 (trategies to boost &onductivity of metal oxides such as doping titanium oxide (TiB0C and niobium oxide, have sho)n improved . &onductivity ove undoped oxidesP however, thei &onductivity is lowe than that of metals and the dopants are also known to lea&h out ove time, thereby furthe reducing &onductivity and stability.0030E "s depi&ted in Ligure 8a, for unsupported iridium the bul+ of the &atalyst parti&le functions as the &onductive support )ith only the surfa&e Ir atoms parti&ipating in &atalysis. Currently, PE7<E anodes typi&ally utilize unsupported Ir resulting in relatively large PG7 &atalyst parti&les and &lusters on the orde of K1 nm and larger, that are applied at high loading AN8 0 0.309 mgI Q&m C to maintain adequate ,# thi&kness, &onne&tivity, and porosity. " strategy to reduce PG7 loading on PE7<E anodes has been the use of non-PG7 &ores such as TiB0 )ith a shell of Ir &atalysts (Ir-TiB0C as illustrated in Ligure 8b. @owever, in orde to maintain &onductivity of the shell, Ir loadings in e?&ess of 91 )tF Ir are ne&essary,0I as demonstrated for hal63&ell BER and ele&trolyze performances.0, 88, 89, 0/30K Figure '( Schematic of: (a) bulk Ir catalyst (b) traditional -./ wt% Ir shell coated on TiO1 core (c) proposed catalyst with Pt or Au intermediate layer between Ir and TiO1 (d) nanostructuring strategy to reduce Ir loading without losing overall catalyst surface area (e) nanostructuring of both catalyst and conductive intermediate layer to lower PGM loading without losing catalyst surface area or conductivity. <e propose modifying the &ore3shell a &hite&ture by adding a very thin laye of &onductive and ele&trochemi&ally stable interfa&ial material, namely Pt or "u, bet)een the TiB0 &ore and the Ir 9 &atalyst ALigure 8&3e). (uch &onductive layers &an also provide the possibility to furthe nanostructure the Ir &atalyst and the &onductive laye itsel6 to reduce overall PG7 loading )ithout &ompromising spe&i;& a&tivity as depi&ted in Ligure 1d-e. This strategy represents a ne) pathway to reduce Ir loading )ithout &ompromising a&tivity and &onductivity. Repla&ing Ir )ith Pt or Au has e&onomi& advantages as )ell in the short and medium term, i.e. until earth abundant elements repla&e PG7s as BER &atalysts. Pt and "u are a magnitude more abundant than Ir on the earth’s crust.0- Bur inspiration &omes from the use of Pt as a &orrosion resistant prote&tive &oating in PE7<E anode porous transport layers and bipola plates, )here they are kno)n to )ithstand ove 50,000 E13EE h of servi&e. These pre&ious metal &oated TiB0 &onductive &atalyst supports A,,(sC also offe a pathway to enhance mass transport in the ,#s by &hanging the TiB0 parti&le size and thus improving the ,# porosity.