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...

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.

For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 μA cm Pt –2 at 0.9 V) and high surface areas (>90 m 2 g Pt –1 ). Platinum–nickel (Pt—Ni) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m 2 g Pt –1 , a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochemical test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the Pt—Ni nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications.

Solid metal oxides for carbon capture exhibit reduced adsorption capacity following high-temperature exposure, due to surface area reduction by sintering. Furthermore, only low-coordinate corner/edge sites on the thermodynamically stable (100) facet display favorable binding toward CO, providing inherently low capacity. The (111) facet, however, exhibits a high concentration of low-coordinate sites. In this work, MgO(111) nanosheets displayed high capacity for CO, as well as a ∼65% increase in capacity despite a ∼30% reduction in surface area following sintering (0.77 mmol g @ 227 m g vs 1.28 mmol g @ 154 m g). These results, unique to MgO(111), suggest intrinsic differences in the effects of sintering on basic site retention. Spectroscopic and computational investigations provided a new structure-activity insight: the importance of high-temperature activation to unleash the capacity of the polar (111) facet of MgO. In summary, we present the first example of a faceted sorbent for carbon capture and challenge the assumption that sintering is necessarily a negative process; here we leverage high-temperature conditions for facet-dependent surface activation.

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This paper combines density functional theory calculations and electrochemical testing to study activity differences among iridium (Ir) surfaces in the oxygen evolution reaction. Ir metal/hydroxide is significantly more active than Ir oxide, which may be due to oxide skins at the surface weakening O-binding relative to pure metal or oxide surfaces. Here we report a disparity in activity between Ir and Ir oxide in half-cells not observed in single-cells. Extended operation at elevated temperature and potential were found to result in oxide growth, limiting how surface differences affect electrolyzer performance. Comparisons of half-and single-cell testing were used to assess how well rotating disk electrode testing predicts membrane electrode assembly performance and durability. Although oxygen evolution activities in half-cells can translate to single-cells, standard rotating disk electrode test procedures can exaggerate the activity benefit of a metal/hydroxide surface relative to membrane electrode assembly performance under typical operating conditions; it also appears that a half-cell test cannot reasonably accelerate activity loss from continual operation. While a variety of novel catalyst approaches, including alloying, faceting, morphology, and supports can improve oxygen evolution kinetics, these results suggest that Ir surfaces at different oxide states may struggle to improve performance at the device level.

The role of the interaction between doped carbon-based materials and ionic conductors is essential in multiple technologies, from fuel cells and energy storage devices to conductive polymer composites. In this paper, we report how the surface chemistry of transition metal−nitrogen−carbon (MNC) electrocatalysts affects catalyst−ionomer interaction and the resulting structure of cathodes. The cathode structure resulting from these interactions is directly related to the performance in membrane electrode assembly (MEA) fuel cells. To advance the development of platinum group metal (PGM)free electrodes for the oxygen reduction reaction it is necessary to understand the structure of the catalyst layers with focus on chemistry and distribution of active sites and ionomer morphology. To assess catalyst interaction with an ionomer, X-ray photoelectron spectroscopy is applied to study the chemistry of catalyst layers while density functional theory (DFT) is used to calculate adsorption energies of the ionomer side chain on different nitrogen species. We report that a high surface concentration of hydrogenated nitrogen at the surface of MNC catalysts causes inefficient ionomer morphology, while an abundance of surface oxides promotes both an efficient distribution of active sites and an optimal ionomer−catalyst interface. The critical role of protonation of nitrogen within catalytic layers in inhibiting proton transport during fuel cell operation is also suggested. This is the first report of the effect the surface chemistry of MNC catalysts, in the presence of the ionomer, has on the structure and performance of MEA electrodes.

The development of high-performance silicon anodes for the next generation of lithium ion batteries (LIBs) evokes increasing interest in studying its lithiated counterpart-lithium silicide (LiSi). In this paper we report a systematic study of three thermodynamically stable phases of LiSi (x = 4.4, 3.75, and 2.33) plus nitride-protected LiSi, which are synthesized via the high-energy ball-milling technique. All three LiSi phases show improved performance over that of unmodified Si, where LiSi demonstrates optimum performance with a discharging capacity of 3306 (mA h)/g initially and maintains above 2100 (mA h)/g for over 30 cycles and above 1200 (mA h)/g for over 60 cycles at the current density of 358 mA/g of Si. A fundamental question studied is whether different electrochemical paradigms, that is, delithiation first or lithiation first, influence the electrode performance. No significant difference in electrode performance is observed. When a nitride layer (LiNSi) is created on the surface of LiSi, the cyclability is improved to retain the capacity above 1200 (mA h)/g for more than 80 cycles. By increasing the nitridation extent, the capacity retention is improved significantly from the average decrease of 1.06% per cycle to 0.15% per cycle, while the initial discharge capacity decreases due to the inactivity of Si in the LiNSi layer. Moreover, the Coulombic efficiencies of all LiSi-based electrodes in the first cycle are significantly higher than that of a Si electrode (∼90% vs 40-70%).