Enhanced electrocatalysis of the oxygen reduction reaction based on patterning of platinum surfaces with cyanide

Abstract

The slow rate of the oxygen reduction reaction in the phosphoric acid fuel cell is the main factor limiting its wide application. Here, we present an approach that can be used for the rational design of cathode catalysts with potential use in phosphoric acid fuel cells, or in any environments containing strongly adsorbing tetrahedral anions. This approach is based on molecular patterning of platinum surfaces with cyanide adsorbates that can efficiently block the sites for adsorption of spectator anions while the oxygen reduction reaction proceeds unhindered. We also demonstrate that, depending on the supporting electrolyte anions and cations, on the same CN-covered Pt(111) surface, the oxygen reduction reaction activities can range from a 25-fold increase to a 50-fold decrease. This behaviour is discussed in the light of the role of covalent and non-covalent interactions in controlling the ensemble of platinum active sites required for high turn over rates of the oxygen reduction reaction.

Direct observation of the oxygenated species during oxygen reduction on a platinum fuel cell cathode

“…This analysis suggests that extending the stability range of the non-hydrated phase will prove beneficial in improving the overall ORR activity. The importance of non-hydrated OH for the ORR at high potentials explains the enhanced ORR activity of Pt in hydrophobic ionic liquids 8 , in the presence of certain aqueous ions 6,7 and on stepped surfaces with narrow terrace width 9,10 . The last is unexpected as step sites strongly bind OH, as indeed observed with a reduced activity for small Pt nanoparticles 27 .…”

“…In general, dehydrated environments 8 will by their very nature limit the hydration of any OH intermediates. Finally, as hydrogen bonding of H 2 O to OH can compete with direct ion-OH interactions 28 , specific ions in the electrolyte could limit the hydration of OH 6,7 . This new insight into the subtle interplay between non-covalent hydrophobic and hydrophilic interactions can inspire the design of new catalysts, not only for the ORR but also for other relevant electrochemical reactions.…”

“…Indeed, based on density functional theory (DFT) simulations of the oxygenated intermediates of the ORR, it has been proposed that the Pt-OH bond strength could be a key descriptor of ORR activity and could therefore guide catalyst design [3][4][5] . However, recent experiments have observed unusual ORR activity in the presence of various ions 6,7 , in dehydrated environments 8 and on stepped surfaces with limited terrace widths 9,10 . These indicate that experiments to observe and characterize OH species directly during the ORR are essential.…”

On the oxygen reduction reaction in phosphoric acid electrolyte: Evidence of significantly increased inhibition at steady state conditions

“…In this respect, the site blocking model gave a very elegant explanation for the different order in ORR activity when comparing low index Pt single crystal surfaces in sulfuric acid and perchloric acid electrolyte, respectively [1]. In addition it could explain the positive effect of adsorbed cyanide anions on the ORR activity in sulfuric and phosphoric acid electrolyte [21]. However, so far the experimental investigations have been performed under transient conditions, i.e.…”

Evaluation of temperature and electrolyte concentration dependent Oxygen solubility and diffusivity in phosphoric acid

“…Many studies have shown that the PA is a key factor for the observed lower current densities in HT-PEMFCs, while especially the oxygen reduction reaction (ORR) taking place on the cathode is severely affected by the PA electrolyte. The inhibiting effect of the electrolyte on the ORR activity has been often explained by strong adsorption of PA ions on active catalyst sites [5][6][7][8][9]. We have recently demonstrated using cyclic voltammetry that adding even trace amounts of PA to electrolyte solutions that weakly interact with Pt-catalyst surfaces can lower the ORR catalytic activity by more than one order of magnitude [10].…”