Morphological Instability in Topologically Complex, Three-Dimensional Electrocatalytic Nanostructures

Meeting Program — March 2018

Yawei Li — Stu­dent Speak­er

Advi­sor: Joshua Sny­der
Depart­ment of Chem­i­cal and Bio­log­i­cal Engi­neer­ing
Drex­el Uni­ver­si­ty, Philadel­phia, Penn­syl­va­nia 19104

Abstract — Deal­loy­ing has shown increas­ing util­i­ty in the field of elec­tro­catal­y­sis as a tool for the syn­the­sis and devel­op­ment of nanoporous mate­ri­als pos­sess­ing high sur­face-to-vol­ume ratios with con­trolled mor­phol­o­gy and com­po­si­tion­al gra­di­ent (core-shell struc­ture). After elec­tro­chem­i­cal deal­loy­ing, the open, bicon­tin­u­ous, three-dimen­sion­al nanoporous nanopar­ti­cle elec­tro­cat­a­lysts exhib­it dra­mat­i­cal­ly enhanced elec­tro­cat­alyt­ic prop­er­ties.

In the devel­op­ment of effi­cient elec­tro­cat­a­lysts for oxy­gen reduc­tion reac­tion (ORR), dura­bil­i­ty is too often ignored in the pur­suit of high­er activ­i­ties. For 3-dimen­sion­al, nanoporous mate­ri­als, in addi­tion to the stan­dard mech­a­nisms of elec­tro­cat­a­lyst degra­da­tion includ­ing Pt dissolution/Ostwald ripen­ing and coalescence/aggregation, new modes of mor­pho­log­i­cal and com­po­si­tion­al evo­lu­tion must be con­sid­ered. Here we use a com­bi­na­tion of in-situ and ex-situ exper­i­men­tal tech­niques to devel­op insight into the struc­tur­al and com­po­si­tion­al evo­lu­tion of nanoporous PtNi nanopar­ti­cles (np-NiPt) formed through the deal­loy­ing of Pt 20 Ni 80 pre­cur­sor nanopar­ti­cles. We demon­strate that sur­face-dif­fu­sion facil­i­tat­ed coars­en­ing, dri­ven by the ten­den­cy to reduce the over­all sur­face free ener­gy of the sys­tem, is the dom­i­nant mech­a­nism of elec­tro­chem­i­cal active sur­face area (ECSA) loss, con­se­quent­ly result­ing in a decrease in activ­i­ty.

With a bet­ter under­stand­ing of the inter­play between nanoporous struc­ture coars­en­ing and tran­si­tion met­al loss, we have devel­oped strat­e­gy to mit­i­gate coars­en­ing and improve oper­a­tional cat­a­lyst sta­bil­i­ty by imped­ing step edge move­ment through the use of for­eign adsor­bates on the
sur­face. We show that par­tial mono­lay­er dec­o­ra­tion of np-NiPt with Ir, pos­sess­ing a sig­nif­i­cant­ly low­er rate of sur­face dif­fu­sion than Pt, acts to pin step edges and results in sig­nif­i­cant enhance­ment in cat­a­lyst dura­bil­i­ty as mea­sured by ECSA and ORR activ­i­ty reten­tion. With this strat­e­gy we will show how more detailed insight into the atom­ic process­es that gov­ern elec­tro­cat­alyt­ic mate­r­i­al insta­bil­i­ty can begin to break the inverse cor­re­la­tion between activ­i­ty and dura­bil­i­ty.