Acid anion electrolyte effects on platinum for oxygen and hydrogen electrocatalysis

Abstract

Platinum is an important material with applications in oxygen and hydrogen electrocatalysis. To better understand how its activity can be modulated through electrolyte effects in the double layer microenvironment, herein we investigate the effects of different acid anions on platinum for the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution/oxidation reaction (HER/HOR) in pH 1 electrolytes. Experimentally, we see the ORR activity trend of HClO4 > HNO3 > H2SO4, and the OER activity trend of HClO4$$ > $$ > HNO3 ∼ H2SO4. HER/HOR performance is similar across all three electrolytes. Notably, we demonstrate that ORR performance can be improved 4-fold in nitric acid compared to in sulfuric acid. Assessing the potential-dependent role of relative anion competitive adsorption with density functional theory, we calculate unfavorable adsorption on Pt(111) for all the anions at HER/HOR conditions while under ORR/OER conditions $${{{{{\rm{Cl}}}}}}{{{{{{\rm{O}}}}}}}_{4}^{-}$$ Cl O 4 − binds the weakest followed by $${{{{{\rm{N}}}}}}{{{{{{\rm{O}}}}}}}_{3}^{-}$$ N O 3 − and $${{{{{\rm{S}}}}}}{{{{{{\rm{O}}}}}}}_{4}^{2-}$$ S O 4 2 − . Our combined experimental-theoretical work highlights the importance of understanding the role of anions across a large potential range and reveals nitrate-like electrolyte microenvironments as interesting possible sulfonate alternatives to mitigate the catalyst poisoning effects of polymer membranes/ionomers in electrochemical systems. These findings help inform rational design approaches to further enhance catalyst activity via microenvironment engineering.

Effect of Surface‐Adsorbed and Intercalated (Oxy)anions on the Oxygen Evolution Reaction

“…[16,39] Thus, when too many oxyanions are present, they will block the OER active sites. [23,40,41] This competition is consistent with the observation that the addition of oxyanions to the electrolyte first leads to an improvement of the OER activity (see following paragraphs), but after a certain concentration (in the order of magnitude of 0.1 M) the overpotential increases. [15,16] In this regard, it is worth mentioning that anodic potentials can increase the local concentration of (oxy)anions in the (near-)surface area of the electrode, as oxyanion migration into the double-layer can take place to compensate the build-up positive charge on the electrode surface.…”

“…Density functional theory (DFT) calculations revealed that the adsorption energy is often negative (e.g., −1.9 eV for SeO 4 2− on NiOOH) [15] and occurs at the same transition metal sites where the OER intermediates are adsorbed (Ni preferred to Fe) [16, 39] . Thus, when too many oxyanions are present, they will block the OER active sites [23, 40, 41] . This competition is consistent with the observation that the addition of oxyanions to the electrolyte first leads to an improvement of the OER activity (see following paragraphs), but after a certain concentration (in the order of magnitude of 0.1 M) the overpotential increases [15, 16] .…”

Mixed Silver–Bismuth Oxides: A Robust Oxygen Evolution Catalyst Operating at Low pH and Elevated Temperatures

“…An increase in the H 2 SO 4 electrolyte concentration suppressed the electrodeposition of the [Ag + Bi]­O x catalyst as concluded from the more positive onset potential on the RHE scale and lower reductive stripping peak in voltammetry (Figure S11). It is also noted that increased concentrations of the H 2 SO 4 electrolyte were reported to suppress the OER kinetics due to specific surface adsorption. , Most importantly, the stability of the electrodeposited [Ag + Bi]­O x system was not affected by pH in any significant way within the examined range, as the catalytic material was able to form and remain stable for extended durations in both the presence and absence of metal precursors in the electrolyte solutions.…”

“…It is also noted that increased concentrations of the H 2 SO 4 electrolyte were reported to suppress the OER kinetics due to specific surface adsorption. 63,64 Most importantly, the stability of the electrodeposited [Ag + Bi]O x system was not affected by pH in any significant way within the examined range, as the catalytic material was able to form and remain stable for extended durations in both the presence and absence of metal precursors in the electrolyte solutions.…”

Modelling Anion Poisoning during Oxygen Reduction on Pt Near-Surface Alloys

“…The majority of recent studies have been carried out in perchloric acid electrolytes, as the adsorption of perchlorate anions is negligible compared to that of strongly adsorbing anions from other electrolytes such as sulfuric or phosphoric acid. 40,41 In particular, the anions from sulfuric and phosphoric acid electrolytes typically act as poisoning 'spectator' species for the ORR, as they block Pt active sites for this reaction. 40,[42][43][44][45] In particular, works by Adzic and Markovic 46 and Arenz 47,48 and their respective coworkers have elucidated distinct mechanisms in which anions from H 3 PO 4 (and H 2 SO 4 ) electrolytes block the Pt surface, as illustrated in Figure 1, thereby hindering ORR to proceed.…”