Exploiting the high surface-area-to-volume ratio of nanomaterials to store energy in the form of electrochemical alloys is an exceptionally promising route for achieving high-rate energy storage and delivery. Nanoscale palladium hydride is an excellent model system for understanding how nanoscale-specific properties affect the absorption and desorption of energy carrying equivalents. Hydrogen absorption and desorption in shape-controlled Pd nanostructures does not occur uniformly across the entire nanoparticle surface. Instead, hydrogen absorption and desorption proceed selectively through high-activity sites at the corners and edges. Such a mechanism hinders the hydrogen absorption rates and greatly reduces the benefit of nanoscaling the dimensions of the palladium. To solve this, we modify the surface of palladium with an ultrathin platinum shell. This modification nearly removes the barrier for hydrogen absorption (89 kJ/mol without a Pt shell and 1.8 kJ/mol with a Pt shell) and enables diffusion through the entire Pd/Pt surface.

Materials that are capable of adsorbing and desorbing gases near ambient conditions are highly sought after for many applications in gas storage and separations. While the physisorption of typical gases to high surface area covalent organic frameworks (COFs) occurs through relatively weak intermolecular forces, the tunability of framework materials makes them promising candidates for tailoring gas sorption enthalpies. The incorporation of open Cu(I) sites into framework materials is a proven strategy to increase gas uptake closer to ambient conditions for gases that are capable of π-back-bonding with Cu. Here, we report the synthesis of a Cu(I)-loaded COF with subnanometer pores and a three-dimensional network morphology, namely Cu(I)–COF-301. This study focused on the sorption mechanisms of hydrogen, ethylene, and carbon monoxide with this material under ultrahigh vacuum using temperature-programmed desorption and Kissinger analyses of variable ramp rate measurements. All three gases desorb near or above room temperature under these conditions, with activation energies of desorption ( E des ) calculated as approximately 29, 57, and 68 kJ/mol, for hydrogen, ethylene, and carbon monoxide, respectively. Despite these strong Cu(I)–gas interactions, this work demonstrated the ability to desorb each gas on-demand below its normal desorption temperature upon irradiation with ultraviolet (UV) light. While thermal imaging experiments indicate that bulk photothermal heating of the COF accounts for some of the photodriven desorption, density functional theory calculations reveal that binding enthalpies are systematically lowered in the COF–hydrogen matrix excited state initiated by UV irradiation, further contributing to gas desorption. This work represents a step toward the development of more practical ambient temperature storage and efficient regeneration of sorbents for applications with hydrogen and π-accepting gases through the use of external photostimuli.

Understanding the role of oxygen vacancy–induced atomic and electronic structural changes to complex metal oxides during water-splitting processes is paramount to advancing the field of solar thermochemical hydrogen production (STCH). The formulation and confirmation of a mechanism for these types of chemical reactions necessitate a multifaceted experimental approach, featuring advanced structural characterization methods. Synchrotron X-ray techniques are essential to the rapidly advancing field of STCH in part due to properties such as high brilliance, high coherence, and variable energy that provide sensitivity, resolution, and rapid data acquisition times required for the characterization of complex metal oxides during water-splitting cycles. X-ray diffraction (XRD) is commonly used for determining the structures and phase purity of new materials synthesized by solid-state techniques and monitoring the structural integrity of oxides during water-splitting processes (e.g., oxygen vacancy–induced lattice expansion). X-ray absorption spectroscopy (XAS) is an element-specific technique and is sensitive to local atomic and electronic changes encountered around metal coordination centers during redox. While in operando measurements are desirable, the experimental conditions required for such measurements (high temperatures, controlled oxygen partial pressures, and H2O) practically necessitate in situ measurements that do not meet all operating conditions or ex situ measurements. Here, we highlight the application of synchrotron X-ray scattering and spectroscopic techniques using both in situ and ex situ measurements, emphasizing the advantages and limitations of each method as they relate to water-splitting processes. The best practices are discussed for preparing quenched states of reduction and performing synchrotron measurements, which focus on XRD and XAS at soft (e.g., oxygen K-edge, transition metal L-edges, and lanthanide M-edges) and hard (e.g., transition metal K-edges and lanthanide L-edges) X-ray energies. The X-ray absorption spectra of these complex oxides are a convolution of multiple contributions with accurate interpretation being contingent on computational methods. The state-of-the-art methods are discussed that enable peak positions and intensities to be related to material electronic and structural properties. Through careful experimental design, these studies can elucidate complex structure–property relationships as they pertain to nonstoichiometric water splitting. A survey of modern approaches for the evaluation of water-splitting materials at synchrotron sources under various experimental conditions is provided, and available software for data analysis is discussed.

Understanding and controlling the alloying properties of nanomaterials under electrochemical conditions are critically important for fields ranging from energy storage and catalysis to electrochromic window technology. Hydrogen-absorbing nanomaterials, like palladium, are especially interesting due to their ability to reversibly absorb and store hydrogen in their lattice at near-stoichiometric amounts. Palladium's work function is also significantly deeper than that of most transition metals, which enables electrochemical underpotential deposition of conformal monolayer and submonolayer amounts of transition metals onto the palladium surface. The simultaneous existence of these two properties is unique and opens new and exciting avenues for electrochemical applications. However, the intersection of surface-modified, hydrogen-alloyed palladium nanomaterials is poorly understood, and specifically, how these structures evolve during electrochemical operating conditions remains an open question. Here, we synthesize {100}-terminated palladium nanocubes and deposit between 0.5 and 22 monolayers of copper at the palladium surface. We then electrochemically alloy these surface-modified structures with hydrogen. Using a combination of analytical electrochemistry, spectroscopy, and microscopy, we track the positional evolution of the Cu at the surface of Pd, its impact on palladium's ability to absorb hydrogen, and copper's effect on hydrogen evolution electrocatalysis. We show that Cu readily alloys into the palladium nanocube at potentials more negative than the Cu 2+/0 deposition, but a 0.5 monolayer thick copper layer remains at the Pd surface regardless of potential. Finally, we discuss the implications of these findings within the framework of CO 2 reduction catalysis for carbon−carbon bond-forming chemistry.