Investigation of the influence of electrolytes and the role of reduced graphene oxide as a support for metal catalysts on the catalytic activity toward the hydrogen evolution reaction

Abstract

Efficient hydrogen production is crucial due to the global energy crisis and environmental concerns, which necessitates identifying catalytic trends for informed choices in hydrogen generation technologies and materials. Hydrogen evolution reaction (HER) volcano plots, initially established for acidic solutions and later adapted for alkaline media, play a crucial role in identifying catalytic trends related to HER. The first part of the thesis focuses on conducting a comprehensive analysis of the HER catalytic trends on nine polycrystalline monometallic surfaces in seven solutions, encompassing a wide pH range from highly acidic to highly alkaline. Using theoretically calculated hydrogen binding energies (HBEs) on clean metallic surfaces and experimentally measured HER overpotentials it is shown, for the first time, that the volcano-like relationships are predominantly preserved across a broad spectrum of pH values, spanning from acidic to neutral and alkaline solutions. This signifies that HBE can be used as a descriptor for the identification of active electrocatalysts in a wide pH range, including pH neutral solutions. Accordingly, Cr was found to have high HER activity in pH-neutral solutions surpassing that of W, Fe, and Co. Furthermore, exposing metallic surfaces to high anodic potentials can lead to either enhanced or suppressed HER, depending on the metal and the electrolyte used. In view of the nontrivial impacts of surface oxidation, its effects might be seen either as a promotion in HER activity or as HER blocking (as in the case of Cr and W). The former case is specifically applicable to Ni and Co in alkaline and pH-neutral solutions, which can be attributed to the enhanced dissociation of water at the metal-oxide interface. This effect is more significant for Ni in NaCl solutions to the extent that, after undergoing oxidation, it becomes more active than Pt. Therefore, this section of the research work reveals that the shape of the volcano curve is largely preserved in the entire pH range, indicating that HBE can be employed to identify powerful catalysts for the HER regardless of pH. Supporting metal catalysts on various supports is another possible design strategy to boost the catalytic activity and stability and improve their utilization. Kinetic Monte Carlo (KMC) simulations were employed in the second part of the thesis work to illustrate that the HER can be enhanced through hydrogen spillover to the support material. This effect happens when the catalysts have a high surface coverage of adsorbed hydrogen atoms under the reaction conditions. Drawing from the insights obtained through KMC, a range of catalysts supported on reduced graphene oxide were synthesized, and their performance in promoting the HER was then compared to their respective pure metal counterparts in alkaline environments. While the support effect is negative for Ag, Au, and Zn, it boosts the HER activity for Pt, Pd, Fe, Co, and Ni. The HER volcano plot, a plot of calculated HBEs vs measured HER activities, is preserved and demonstrates a positive shift in the strong binding region. Thus, this part of the thesis work highlights the potential of metal-support interface engineering in making effective catalysts for HER and provides general guidelines for selecting suitable combinations of catalysts and supports for enhanced electrocatalytic hydrogen production.

Efikasna proizvodnja vodonika je ključna zbog globalne energetske krize i zabrinutosti za životnu sredinu, što zahteva identifikaciju katalitičkih trendova za informisane izbore u tehnologijama i materijalima za proizvodnju vodonika. Vulkanske krive reakcije izdvajanja vodonika (HER), prvobitno uspostavljene za kisele rastvore, a kasnije prilagođene za alkalne medije, igraju ključnu ulogu u identifikaciji katalitičkih trendova povezanih sa HER. Prvi deo ove doktorske disertacije fokusira se na sprovođenje sveobuhvatne analize HER katalitičkih trendova na devet polikristalnih monometalnih površina u sedam rastvora, obuhvatajući širok pH opseg od visoko kiselih do visoko alkalnih. Koristeći teorijskiizračunate energije vezivanja vodonika (HBE) na čistim metalnim površinama i eksperimentalno izmerene HER nadnapone, po prvi put je pokazanoda su relacije vulkanskog tipa pretežno očuvane u širokom spektru pH vrednosti, od kiselih do neutralnih i alkalnih rastvora. Ovo znači da se HBE može koristiti kao deskriptor za identifikaciju aktivnih elektrokatalizatora u širokom pH opsegu, uključujući pH neutralne rastvore. Shodno tome, otkriveno je da Cr ima visoku HER aktivnost u pH-neutralnim rastvorima, koja nadmašuje aktivnostV, Fe i Co. Štaviše, izlaganje metalnih površina visokim anodnim potencijalima može dovesti do pojačane ili potisnute HER, u zavisnosti od metala i korišćenog elektrolita. S obzirom na netrivijalne uticaje oksidacije površine, njeni efekti se mogu posmatrati ili kao promocija HER aktivnosti ili kao HER blokiranje (kao u slučaju Cr i V). Prvi slučaj je posebno primenljiv na Ni i Co u alkalnim i pH-neutralnim rastvorima, što se može pripisati pojačanoj disocijaciji vode na granici fazametal-oksid. Ovaj efekat je značajniji za Ni u rastvorima NaCl, do te mere da posle oksidacije postaje aktivniji od Pt. Stoga, ovaj deo istraživačkog rada otkriva da je oblik vulkanske krive u velikoj meri očuvan u čitavom pH opsegu, što ukazuje da se HBE može koristiti za identifikaciju pogodnih katalizatora za HER bez obzira na pH. Postavljanje metalnih katalizatora na različite nosače je još jedna moguća strategija dizajna za povećanje katalitičke aktivnosti i stabilnosti i poboljšanje njihovog korišćenja. Kinetičke Monte Karlo (KMC) simulacije su korišćene u drugom delu teze da bi se ilustrovalo da se HER može pospešitiprelivanjem vodonika na potporni materijal. Ovaj efekat se dešava kada katalizatori imaju visoku površinsku pokrivenost adsorbovanih atoma vodonika u uslovima reakcije. Na osnovu uvida dobijenih putem KMC- a, sintetisanje niz katalizatora podržanih na redukovanom grafen–oksidu, a njihov učinak u promovisanju HER je zatim upoređen sa odgovarajućim čistim metalima u alkalnim sredinama. Dok je efekat nosačanegativan za Ag, Au i Zn, on povećava HER aktivnost za Pt, Pd, Fe, Co i Ni. HER vulkanska kriva, dijagram izračunatih HBE u odnosu na izmerene HER aktivnosti, je očuvana i pokazuje pozitivan pomak u regionu jakog vezivanja. Stoga, ovaj deo teze naglašava potencijal inženjeringa granica fazametal-podloga u pripremiefikasnih katalizatora za HER i pruža opšte smernice za odabir odgovarajućih kombinacija katalizatora i nosača za poboljšanu elektrokatalitičku proizvodnju vodonika.