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...

No abstract

Iridium–nickel (Ir–Ni) and iridium–cobalt (Ir–Co) nanowires have been synthesized by galvanic displacement and studied for their potential to increase the performance and durability of electrolysis systems. Performances of Ir–Ni and Ir–Co nanowires for the oxygen evolution reaction (OER) have been measured in rotating disk electrode half-cells and single-cell electrolyzers and compared with commercial baselines and literature references. The nanowire catalysts showed improved mass activity, by more than an order of magnitude compared with commercial Ir nanoparticles in half-cell tests. The nanowire catalysts also showed greatly improved durability, when acid-leached to remove excess Ni and Co. Both Ni and Co templates were found to have similarly positive impacts, although specific differences between the two systems are revealed. In single-cell electrolysis testing, nanowires exceeded the performance of Ir nanoparticles by 4–5 times, suggesting that significant reductions in catalyst loading are possible without compromising performance.

Oxygen evolution reaction (OER) electrocatalysts with high activity, high stability, and low costs are needed for proton-exchange membrane (PEM) electrolyzers. Based on the high cost and limited supply of iridium, approaches that result in iridium-based OER catalysts with increased catalytic activity are of significant interest. We report a carbon-free, self-supported hydrous iridium−nickel oxide two-dimensional nanoframe structure synthesized by thermal treatment of iridium-decorated nickel oxide nanosheets under reducing conditions and subsequent chemical leaching in acid. The catalyst nanoarchitecture contains an interconnected network of metallic iridium−nickel alloy domains with hydrous iridium oxide and nickel oxide located in the surface region. The electrochemical oxidation step maintains the three-dimensional nanoarchitecture and results in a thin (∼5 Å) oxide/hydroxide surface layer. The temperature used for thermal reduction was found to strongly affect the catalyst surface structure and OER activity. Using a lower thermal reduction temperature of 200 °C was determined to provide a higher relative surface concentration of hydroxides and nickel oxide and result in higher OER activities compared with materials treated at 300 °C. The 200 °C-treated hydrous iridium−nickel oxide electrocatalyst showed 15 times higher initial OER mass activity than commercial IrO 2 , and the activity remained 10 times higher than IrO 2 after accelerated durability testing. Density functional theory (DFT) calculations and analysis of the experimental Tafel slopes support that the second electron transfer step is the rate-limiting step for the reaction. The DFT calculations demonstrate that Ni substitution on the IrO 2 surface lowers the activation energy for adsorbed intermediates of the second electron transfer step of the OER reaction. This work establishes that noble metal-decorated metal oxide nanosheets can be transformed into high surface area, carbon-free electrocatalyst nanostructures with high catalytic activities and molecular accessibility and reveals the importance of using controlled thermal reduction temperatures to alter the surface structure and OER activity.

Membrane electrode assembly durability is explored for polymer electrolyte membrane electrolyzers, focusing on catalyst (iridium, Ir) degradation at low loading and dynamic operation. Low catalyst loading and high cell potential are critical to observing durability losses over reasonably short experiments, regardless of test profile. While small losses are seen during steady operation, cycling greatly accelerates performance decreases. Ir dissolution mechanistically drives performance loss, thinning the anode catalyst layer and resulting in increasing kinetic losses during extended operation. While morphological changes to the catalyst layer are found, increasing polarization resistance suggests that degradation at the catalyst/ionomer/membrane interface may also contribute. Electrolyzer operation with model wind and solar profiles results in less severe performance losses compared to triangleand square-wave potential cycling due to the lower cycling frequency of the renewable profiles. However, in both cases kinetics dominated the loss, indicating that higher cycling rates accelerate loss and can be used to project the impact of intermittency on device lifetime. These results suggest that performance losses impact electrolyzers' abilities to operate with low catalyst loading and intermittent inputs, and that a combination of component development and system controls are needed to limit potential and performance loss.