Activity and Durability of Iridium Nanoparticles in the Oxygen Evolution Reaction

Abstract

Unsupported iridium (Ir) nanoparticles, that serve as standard oxygen evolution reaction (OER) catalysts in acidic electrolyzers, were investigated for electrochemical performance and durability in rotating disk electrode (RDE) half-cells. Fixed potential holds and potential cycling were applied to probe the durability of Ir nanoparticles, and performance losses were found to be driven by particle growth (coarsening) at moderate potential (1.4 to 1.6 V) and Ir dissolution at higher potential (≥1.8 V Hydrogen is a major commodity chemical with approximately 2% of U. S. used energy going through a hydrogen pathway, primarily for ammonia production (agriculture) and the upgrading of crude oil (transportation). The majority of hydrogen in the US is produced from natural gas by steam methane reformation.1 While electrochemical water splitting currently represents a small percentage of hydrogen production, it is expected to have a growing role as costs decrease. 2Although the commercial competitiveness of electrolysis is currently limited by feedstock costs, catalyst cost and durability will become increasingly important as electrolyzers move toward low cost, intermittent, renewable sources of electricity such as wind and solar. 3,4 Acidic electrolyzers typically use iridium (Ir) in the oxygen evolution reaction (OER) as this material exhibits both reasonable activity and stability.5 Platinum and ruthenium have also been investigated as alternatives. Platinum, however, requires a higher overpotential (lower efficiency) and ruthenium has durability (dissolution) concerns. [6][7][8] Efforts to develop improved OER catalysts for acidic electrolyzers typically focus on supporting Ir oxide on titania 9-13 or alloying Ir with platinum, ruthenium, or other transition metal oxides [14][15][16][17][18][19][20][21][22][23] to improve durability and performance. Density functional theory studies have correlated trends in the OER activity of metal oxides to the adsorption energies of surface oxygen species, suggesting future directions for improving OER catalysts.24 Strasser et al. also examined the intrinsic activity of Ir, platinum, and ruthenium polycrystalline metals and nanoparticles in rotating disk electrode (RDE) half-cells, using carbon monoxide to determine catalyst surface areas.6 Efforts exploring OER catalysts, however, pale in comparison to the efforts expended in the pursuit of fuel cell catalysts for the oxygen reduction reaction (ORR). Specifically, the fuel cell community has established baselines and protocols for the performance and durability of ORR catalysts. [25][26][27][28] No such protocols or baselines currently exist for OER catalysts.This study presents data from several different commercial suppliers of unsupported and supported Ir and Ir oxide catalysts, and investigates the intrinsic activity of Ir in RDE half-cells, evaluating both performance and durability while presenting the data under standardized conditions. The modes of losses for Ir nanoparticles under specific testing protocols are present...

Using Instability of a Non-stoichiometric Mixed Oxide Oxygen Evolution Catalyst As a Tool to Improve Its Electrocatalytic Performance

“…There are several options given in the literature for how this can be achieved. Among them, the increase in surface area of Ir-based catalysts by using nanoparticles [8] or porous structures [9,10] is the most used approach for improving the gravimetric electrocatalytic activity. Another possibility is dilution of Ir by alloying with other metals.…”

Assessing the Potential of Co-Pt Bronze for Electrocatalysis in Acidic Media

“…Its OER activity is lower than that of IrO 2 below 1.62 V, but higher than that of IrO 2 (Furuya) over 1.6 V. Activities at higher potential with higher current, however, are affected by the conductivity OER activities are also compared with published data [10,11] using IrO 2 (Alfa Aesar) in Table 2. IrO 2 (Alfa Aesar) of this work showed lower OER activity than IrO 2 (Alfa Aesar) of published data [10,11], especially at 1.60 V. This difference could be due to difference in experimental conditions (e.g., IR-corrected [10,11] vs. not corrected in this work), or the difference in thin film formation (e.g., with Nafion [10,11] vs. without it in this work). In this screening study, which aims to reveal the potential of Pt-Co bronze as an electrocatalyst, further analyses were not carried out.…”

“…Therefore, the intrinsic ORR activity is very limited, but as the following section shows, potential cycles to the hydrogen potential regions can provide a significant increase in ORR activity, which makes this material attractive for reversible fuel cells. (e.g., IR-corrected [10,11] vs. not corrected in this work), or the difference in thin film formation (e.g., with Nafion [10,11] vs. without it in this work). In this screening study, which aims to reveal the potential of Pt-Co bronze as an electrocatalyst, further analyses were not carried out.…”

“…OER activities are also compared with published data [10,11] using IrO2(Alfa Aesar) in Table 2. IrO2 (Alfa Aesar) of this work showed lower OER activity than IrO2 (Alfa Aesar) of published data [10,11], especially at 1.60 V. This difference could be due to difference in experimental conditions * Mean diameter was calculated from BET surface area assuming monodispersed spherical particles with bulk density of respective material as: D(nm) = 6000/{BET(m 2 /g)*·ρ(g/cm 3 )}.…”

“…While various nonprecious metals can be used as OER catalysts for alkali electrolysis [8], only precious metals can be used for acidic polymer electrolyte membrane electrolysis, which allows much higher current density compared to alkali electrolysis, and therefore, arguably lower overall cost for hydrogen production. Among the OER catalysts in acidic media, iridium and/or ruthenium (mixed) oxides are regarded as the most active [9][10][11]. For stabilities, ruthenium metal and oxides are regarded as unstable, since ruthenium forms volatile RuO4.…”