Ru-based oxygen evolution reaction (OER) catalysts show significant promise for efficient water electrolysis, but rapid degradation poses a major challenge for commercial applications. In this work, we explore several Ru-based pyrochlores (A 2 Ru 2 O 7 , A = Y, Nd, Gd, Bi) as OER catalysts and demonstrate improved activity and stability of catalytic Ru sites relative to RuO 2 . Furthermore, we combine complementary experimental and theoretical analysis to understand how the A-site element impacts activity and stability under acidic OER conditions. Amongst the A 2 Ru 2 O 7 studied herein, we find that a longer Ru-O bond and a weaker interaction of the Ru 4d and O 2p orbitals compared to RuO 2 results in enhanced initial activity. We observe that the OER activity of the catalysts changes over time and is accompanied by both A-site and Ru dissolution at different relative rates depending on the identity of the A-site. Pourbaix diagrams constructed using density functional theory (DFT) calculations reveal a driving force for this experimentally observed dissolution, indicating that all compositions studied herein are thermodynamically unstable in acidic OER conditions. Theoretical activity predictions show consistent trends between A-site cation leaching and OER activity. These trends coupled with Bader charge analysis suggest that dissolution exposes highly oxidized Ru sites that exhibit enhanced activity. Overall, using the stability number (mol O 2 evolved /mol Ru dissolved ) as a comparative metric, the A 2 Ru 2 O 7 materials studied in this work show substantially greater stability than a standard RuO 2 and commensurate stability to some Ir mixed metal oxides. The insights described herein provide a path to further enhance Ru catalyst activity and durability, ultimately improving the efficiency of water electrolyzers.

Advances in the separation and functionalization of single walled carbon nanotubes (SWCNT) by their electronic type have enabled the development of ratiometric fluorescent SWCNT sensors for the first time. Herein, single chirality SWCNT are independently functionalized to recognize either nitric oxide (NO), hydrogen peroxide (H2O2), or no analyte (remaining invariant) to create optical sensor responses from the ratio of distinct emission peaks. This ratiometric approach provides a measure of analyte concentration, invariant to the absolute intensity emitted from the sensors and hence, more stable to external noise and detection geometry. Two distinct ratiometric sensors are demonstrated: one version for H2O2, the other for NO, each using 7,6 emission, and each containing an invariant 6,5 emission wavelength. To functionalize these sensors from SWCNT isolated from the gel separation technique, a method for rapid and efficient coating exchange of single chirality sodium dodecyl sulfate‐SWCNT is introduced. As a proof of concept, spatial and temporal patterns of the ratio sensor response to H2O2 and, separately, NO, are monitored in leaves of living plants in real time. This ratiometric optical sensing platform can enable the detection of trace analytes in complex environments such as strongly scattering media and biological tissues.

While photoelectrochemical (PEC) solar-to-hydrogen efficiencies have greatly improved over the past few decades, advances in PEC durability have lagged behind. Corrosion of semiconductor photoabsorbers in the aqueous conditions needed for water splitting is a major challenge that limits device stability. In addition, a precious-metal catalyst is often required to efficiently promote water splitting. Herein, we demonstrate unassisted water splitting using a non-precious metal molybdenum disulfide nanomaterial catalytic protection layer paired with a GaInAsP/GaAs tandem device. This device was able to achieve stable unassisted water splitting for nearly 12 hours, while a sibling sample with a PtRu catalyst was only stable for 2 hours, highlighting the advantage of the non-precious metal catalyst. In situ optical imaging illustrates the progression of macroscopic degradation that causes device failure. In addition, this work compares unassisted water splitting devices across the field in terms of the efficiency and stability, illustrating the need for improved stability. TOC GRAPHICThe primary strategy that has emerged to mitigate semiconductor surface corrosion is depositing thin films, such as titanium dioxide, that can act as protective barriers to prevent the electrolyte from coming into contact with the semiconductor surface. 7,8 These films have to be stable, thin enough to prevent significant light blocking, conformal, and conductive to create a stable and functional device. 9-11 Furthermore, if these films do not demonstrate intrinsic catalytic activity, an additional hydrogen and/or oxygen evolution catalyst is needed to promote efficient water splitting. Molybdenum disulfide nanomaterials have been shown to stabilize a variety of singlejunction Si and III-V PEC systems, functioning as a hydrogen evolution reaction (HER) catalyst and protection layer. [9][10][11][12][13][14][15] Because of the promising performance in single-junction photocathodes, it is of interest to use MoS2 with tandem semiconductor systems to improve the stability during unassisted solar water splitting.While most III-V-based unassisted water splitting devices to date have incorporated a Ga0.51In0.49P (hereafter GaInP2) (1.8 eV) top cell, device lifetimes have been limited to <100 h. 7,16 GaxIn1-xAsyP1-y (1.7 eV) has shown promise as a PV material and has been paired with a GazIn1-zAs bottom cell (1.1 eV) for efficient tandem PV systems, motivating efforts to incorporate GaxIn1-xAsyP1-y into PEC systems and investigate the stability of this quaternary top cell. [16][17][18][19] The composition of GaxIn1-xAsyP1-y (hereafter GaInAsP), nominally x ~ 0.68 and y ~ 0.34, gives the desired bandgap of 1.7 eV and a lattice constant matching that of GaAs. 16,18 A GaInAsP/GaAs (1.7/1.4 eV) pairing has a predicted maximum STH efficiency of ~12%, 16,19 far from the ideal combination of absorbers to achieve the highest of efficiencies, however sufficiently high to perform durability studies on active, unassisted water splitting systems, guiding t...

Thomas F (2020) Nitride or Oxynitride? Elucidating the Composition-Activity Relationships in Molybdenum Nitride Electrocatalysts for the Oxygen Reduction Reaction. Chemistry of Materials.

The separation of single-walled carbon nanotubes (SWNTs) by chirality is of great interest to enable the next generation of optical and optoelectronic devices. Many separation schemes employ the surfactant sodium dodecyl sulfate (SDS), with or without a bile salt surfactant such as sodium cholate (SC). In this study, we observe and explain the effect of these mixed surfactant systems on the hydrogel-based selective adsorption separation method. We find that sodium cholate outcompetes SDS more effectively on smaller diameter tubes and quantify this difference as the sodium cholate concentration is increased and (6,5) separation is diminished. These changes in separation efficiency with surfactant composition are understood using a theoretical model developed previously and predict that surfactant mixtures alter the charge per unit length of specific (n,m) SWNTs, altering the separation. This understanding of the chiral dependence of the surfactant binding will not only enable a greater understanding of surfactant coverage on the SWNT but also pave the path to further control the SWNT separation processes that depend on these surfactants.

Diverse Ag–MnOx surface sites/structures in Ag–Mn electrocatalysts afford robust local electronic structures tuned for efficient oxygen reduction.

Silicon has shown promise for use as a small band gap (1.1 eV) absorber material in photoelectrochemical (PEC) water splitting. However, the limited stability of silicon in acidic electrolyte requires the use of protection strategies coupled with catalysts. Herein, spin coating is used as a versatile method to directly coat silicon photoanodes with an IrO x oxygen evolution reaction (OER) catalyst, reducing the processing complexity compared to conventional fabrication schemes. Biphasic strontium chloride/iridium oxide (SrCl2:IrO x ) catalysts are also developed, and both catalysts form photoactive junctions with silicon and demonstrate high photoanode activity. The iridium oxide photoanode displays a photocurrent onset at 1.06 V vs reversible hydrogen electrode (RHE), while the SrCl2:IrO x photoanode onsets earlier at 0.96 V vs RHE. The differing potentials are consistent with the observed photovoltages of 0.43 and 0.53 V for the IrO x and SrCl2:IrO x , respectively. By measuring the oxidation of a reversible redox couple, Fe­(CN)6 3–/4–, we compare the charge carrier extraction of the devices and show that the addition of SrCl2 to the IrO x catalyst improves the silicon–electrolyte interface compared to pure IrO x . However, the durability of the strontium-containing photoanode remains a challenge, with its photocurrent density decreasing by 90% over 4 h. The IrO x photoanode, on the other hand, maintained a stable photocurrent density over this timescale. Characterization of the as-prepared and post-tested material structure via Auger electron spectroscopy identifies catalyst film cracking and delamination as the primary failure modes. We propose that improvements to catalyst adhesion should further the viability of spin coating as a technique for photoanode preparation.

Unique classes of active‐site motifs are needed for improved electrocatalysis. Herein, the activity of a new catalyst motif is engineered and isolated for the oxygen evolution reaction (OER) created by nickel–iron transition metal electrocatalysts confined within a layered zirconium phosphate matrix. It is found that with optimal intercalation, confined NiFe catalysts have an order of magnitude improved mass activity compared to more conventional surface‐adsorbed systems in 0.1 m KOH. Interestingly, the confined environments within the layered structure also stabilize Fe‐rich compositions (90%) with exceptional mass activity compared to known Fe‐rich OER catalysts. Through controls and by grafting inert molecules to the outer surface, it is evidenced that the intercalated Ni/Fe species stay within the interlayer during catalysis and serve as the active site. After determining a possible structure (wycherproofite), density functional theory is shown to correlate with the observed experimental compositional trends. It is further demonstrated that the improved activity of this motif is correlated to the Fe and water content/composition within the confined space. This work highlights the catalytic enhancement possibilities available through zirconium phosphate and isolates the activity from the intercalated species versus surface/edge ones, thus opening new avenues to develop and understand catalysts within unique nanoscale chemical environments.