Electrocatalytic CO2 reduction (ECO2R) to high-value chemicals is a promising method to upcycle emitted CO2, but it is also a fascinating scientific challenge. Catalyst materials, as well as cell configurations, play a pivotal role in the efficacy and efficiency of the ECO2R reaction, which also dictates reaction pathways and product selectivity. In this work, we employ the isotopological Zr- and Ce-based UiO-67 metal-organic frameworks (MOFs) that contain Pd species in a zero-gap gas diffusion cathode electrode configuration, where the water content, i.e., relative humidity (RH) level, in the CO2 gas stream can be varied. We show that only UiO-67-based MOFs containing Pd embedded in their pores can produce syngas, while the product selectivity can be controlled by varying the RH levels in the gas stream. The pristine MOFs (precatalysts) undergo chemical and structural transformation during the ECO2R reaction, forming the active catalysts toward CO2 electroreduction to syngas. Our work highlights the effect of water content on the selectivity during ECO2R, but also the need for predictive catalyst design for effective electroreduction of CO2 to high-value chemicals.

Single crystals and polycrystals of the novel compound Ca2TlO3Cl can be obtained by gas-phase and solid-state reactions, respectively. The title compound's crystal structure is hexagonal (a = 3.8806(2) Å, c = 18.6168(7) Å, P63/mmc) as measured by single crystal X-ray diffraction. A Rietveld refinement on powder X-ray diffraction data confirms this crystal structure. The atomic arrangement can be described as a cation ordered anti-structure of ScAl3C3, with a homoleptically, five-fold oxygen coordinated Tl3+ that consitutes corner-sharing trigonal bipyramids. Between these Tl-O sheets, a Cl-layer is found, making the whole structure quasi-2D. Thermal expansion coefficients in the range 15-290 K, as estimated from X-ray powder diffraction data, reveal that the relative length of the crystallographic c-axis (α33) is more temperature dependent than that of the a-axis (α11) lengths. The brown color of the polycrystalline title compound was investigated by spectroscopic reflectometry measurements. These data suggest an optical band gap of either 1.4 eV (indirect) or about 2.4-2.7 eV (direct), depending on interpretation model. However, density funciton theory calculations deliver a band structure that indicates that indirect band gaps are most likely, but with a size of about 2.7 eV. This discrepancy is discussed as a result of further local excitation phenomena, like charge-transfers and crystal defects. Photo-electrochemical investigations suggest that the title compound has an n-type character and is electrochemically activated by solar light.

Photoelectrochemical (PEC) water splitting is a promising method for sustainable hydrogen production. Among potential materials, tantalum nitride (Ta3N5) has emerged as a leading candidate due to its favorable band gap and high theoretical efficiency. This review highlights recent advancements in the synthesis, doping, and surface modification of Ta3N5 photoanodes, which have enabled photocurrent densities approaching the material's theoretical limit of 12.9 mA/cm2 at 1.23 V vs. RHE. Despite these advancements, significant challenges remain, particularly in achieving long-term stability. We critically evaluate the feasibility of meeting the U.S. Department of Energy's targets and provide insights into more achievable and realistic goals for PEC systems based on Ta3N5, focusing on efficiency, lifetime, and cost competitiveness.

Surface restructuring and the formation of amorphous layers during the initial stages of the oxygen evolution reaction (OER) is a common phenomenon in perovskite-based catalysts. It has also been observed that catalysts following the lattice oxygen mechanism (LOM) are prone to amorphization as the rates of lattice oxygen evolution and oxygen vacancy replenishment are not equal. This is accompanied by a significant A-site cation loss in the electrolyte, ultimately leading to the collapse of the crystalline phase and the appearance of an amorphous catalytic surface of a thickness of around 10 nm. Typically, the originally crystalline material is termed as precatalyst. In this work, we hypothesize that A-site deficient perovskite precatalysts of the family of Ba _x Gd _y La _z Co _2 O _3- _δ (BGLCxyz), which operate under LOM, will have a reduced degree or no surface amorphization under OER conditions in alkaline media. For this reason, A-site deficient BGLC587 with a 20% deficiency is synthesized to validate our main hypothesis. Furthermore, thermal reduction of the A-site deficient BGLC587 precatalyst is expected, to form embedded nanoparticles on its surface through the process of exsolution, thus inducing higher surface area and improved electrocatalytic activity, while still suppressing surface amorphization.

Perovskite oxides have great flexibility in their elemental composition, which is accompanied by large adjustability in their electronic properties. Herein, we synthesized twelve perovskite oxide-based catalysts for the oxygen evolution reaction (OER) in alkaline media. The catalysts are based on the parent oxide perovskite Ba0.5Gd0.8La0.7Co2O6-δ (BGLC587) and are synthesized through the sol-gel citrate synthesis route. To reduce the demand on cobalt (Co), but also increase the intrinsic catalytic activity of BGLC587 for the OER, we substitute Co on the B-site with certain amounts of Fe and Ni, synthesizing catalysts of the general formula Ba0.5Gd0.8La0.7Co2-x-yFexNiyO6-δ. A plethora of physicochemical and electrochemical methods suggest that an Fe content between 30 % and 70 % increases the intrinsic catalytic activity of BGLC587, while Tafel slopes in combination with in-situ Raman spectroscopy suggest the rate determining step is likely a proton-exchange reaction, progressing possibly through the lattice oxygen mechanism (LOM). We apply one of the optimized, Co-substituted perovskites in a monolithic, photovoltaic (PV)-driven electrolysis cell and we achieve an initial solar-to-hydrogen (STH) conversion efficiency of 10.5 % under one sun solar simulated illumination.

Bimetallic Metal-Organic Frameworks (MOFs) of silver, copper and the ligand benzene 1,3,5-tricarboxylate (AgCu-BTC MOFs), derived from Cu-BTC (HKUST-1), have been synthesized by fast co-precipitation method and investigated for CO2 reduction reaction (CO2RR). Three AgCu-BTC MOF variants were synthesized with varying Ag content: AgCu-1 (9.4 at.%), AgCu-2 (12.5 at.%), and AgCu-3 (16.5 at.%). A range of structural characterization techniques, including SEM-EDS, XRD, FTIR, and XPS, were utilized, revealing the formation of low-crystalline AgCu-BTC MOF with Ag+1 in an ionic state coordinated to the BTC framework. The investigation focused on CO2 reduction using humidified CO2 gas with bimetallic AgCu MOFs as electrocatalysts in a zero-gap MEA setup. The setup included a gas diffusion electrode (GDE) with a Sustainion Anion exchange membrane and bicarbonate as the anolyte. Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV) showed that the AgCu-3 MOF, with the highest silver content (16.5 at.%), exhibited a lower onset potential at -0.65 V vs Ag/AgCl compared to pristine Cu-BTC MOF owing to the better activity with Ag inclusion. Constant potential (CP) experiments combined with product analysis indicated that AgCu-3 MOF predominantly produced CO and H2 as the main products, achieving a faradaic efficiency of approximately 60% for CO production and 5% for hydrogen production. Moreover, reducing the humidity level in the inlet CO2 gas stream from 80% to 20% RH increased CO production by 2-fold, simultaneously suppressing the HER. This reduction in humidity resulted in an increased local concentration of CO2 at the catalyst site, leading to an enhanced CO2RR rate. SEM and FTIR investigations after the CP experiment demonstrated the instability of the AgCu-3 MOF, revealing substantial morphological changes under humid CO2 conditions.

Exsolution is a technique to create metal nanoparticles embedded within a matrix. The phenomenon has previously predominantly been studied in A-site deficient and stoichiometric perovskite powders. Here, we present a systematic study of an A-site excess perovskite oxide based on SrTiO3 thin films, doped with nickel and exsolved under different conditions. The study aims to shed light on particle formation in these novel systems, including the effects of (i) the thin film thickness, (ii) pre-exsolution annealing in an oxidative atmosphere, (iii) a reductive atmosphere during the exsolution step, and (iv) exsolution time on the particle size and particle density. Our results indicate that exsolution occurs quickly, forming nanoparticles both on the surface and in the bulk of the host perovskite. The findings indicate that pre-annealing in an ambient atmosphere leads to fewer but larger exsolved particles compared to samples without pre-annealing. Consequently, while crystallization of the thin film occurs in both atmospheres, the simultaneous crystallization of the thin film and formation of the nanoparticles leads to a smaller apparent average radius. Moreover, we present evidence that metal particles can be found beyond the originally doped region. These findings are a step towards realizing tunable functional materials using exsolution to create metallic nanostructures within a thin film in a predictable manner.

An oxide chloride, La5Cl7[TeO3]4, was synthesized using the conventional high-temperature solid-state synthesis technique in an inert atmosphere. This compound possesses a novel crystal structure that can be described with the triclinic space group P1̅ (No. 2) and unit cell parameters: a = 7.2634(3) Å, b = 8.1241(3) Å, c = 9.1993(3) Å, α = 79.373(1)°, β = 83.599(1)°, and γ = 82.511(1)°. The preference of Te(IV) to coordinate to oxygen and direct its lone pair toward the lower charged chlorine results in 2D layers of both oxygen and chlorine, alternating along the crystallographic b-direction. Homoleptic coordination, solely to oxygen, and heteroleptic coordination to oxygen and chlorine are observed for lanthanum, forming layers connected through edge-sharing polyhedra. In the crystal structure, two distinct tellurium positions are observed, with three close Te-O distances, emphasizing an active lone pair. The compound has been investigated by solid-state UV-vis measurements, and a band gap of 3.44 eV has been determined by DFT calculations. Detailed photoelectrochemical measurements clearly indicate that the title compound is photoelectrocatalytically active, showing an n-type behavior. Raman spectroscopy confirms that complex tellurite ions are present in the crystal structure; several observed bands can be assigned to Te-O stretching, reflecting the relatively low crystallographic symmetry of the title compound.

The traditional oxygen electrode in solid oxide electrolysis cells (SOECs), (La,Sr)(Co,Fe)O-3 (LSCF), suffers from high cost, evaporation at high temperatures, and societal aspects of the use of Co. In this work, a Co-free B-site multielement (so-called high-entropy) perovskite oxide, La0.6Sr0.4Cu0.2Fe0.2Ti0.2Mn0.2Ni0.2O3-delta (LSCuFTMN), has been synthesized and successfully applied as a novel oxygen electrode. X-ray photoelectron spectroscopy (XPS) data indicate that the multiple transition elements in the B-site exist in various valence states, leading to a spatially variable electron structure. Electrochemical measurements of LSCuFTMN suggest that the material exhibits extraordinary catalytic activity and stability under the studied working atmospheres and a decrease in polarization resistance by 24% compared to LSCF. By distribution of relaxation time (DRT) analysis, LSCuFTMN possesses better mass and charge transfer performance than traditional LSCF. An SOEC with LSCuFTMN as the oxygen electrode has been assembled and tested, and a current density of 1.2 A cm(-2) is obtained at 2.0 V and 800 degrees C in electrolysis of pure CO2, higher by nearly 50% compared to LSCF. The faradaic efficiency is over 95%. No clear recession is observed in the long term stability test. It is evident that multication - so-called high-entropy - oxides could be promising materials for improving the working performance of SOECs.

Plasmonically active nanoparticles offer a promising pathway to extend the absorption range of photocatalysts. While not necessarily catalytically active themselves, these particles allow the absorption of lower energy photons in wide band gap photocatalysts. Here, we present A-site excess SrTiO3 thin films, doped with Ni, where through a subsequent exsolution process we created well-socketed Ni nanoparticles in the surface of SrTiO3. These were galvanically replaced by Au, resulting in well-socketed Au nanoparticles with variable size on the surface, depending on the galvanic replacement time. Photoelectrochemical measurements and electron energy loss spectroscopy revealed the improved photoresponse of the thin films by plasmonic activity of the nanoparticles. The energy of the plasmon peak suggests that the main improvement results from the injection of hot charge carriers. Our study opens new avenues for the design and synthesis of the next generation of photocatalytic materials.

CeO2 surfaces play decisive roles in heterogeneous catalysis of important processes. Here, we investigate adsorption and dissociation of water and migration of protons on internal surfaces of nanoscopic porous CeO2. Sorption and thermogravimetry confirm literature suggestions that the surface is hydrogenated to Ce3+ ions and protons H+. The following chemisorption is dissociative, yet weak, and physisorption sets in only at the very highest relative humidities, reflecting hydrophobic behaviour. We link sample conductivities to surface protonic conductances via a brick layer model and show that behaviours at high, intermediate, and low temperatures with, respectively, positive, close to zero, and negative apparent activation energies and pH2O1/2, pH2O1, and pH2O3/2 dependences, can be attributed to different models of migration all within the chemisorbed layer, without contribution from physisorbed water. While CeO2 may special in this respect due to the effect of the hydrogenated surface, we believe the extended models of transport in the chemisorbed layer may apply also to other oxides. Unsaturated chemisorption may play an important role for CeO2 as catalyst in that the surface is left available for reactant molecules, still with availability of dissociated and mobile protons in the chemisorbed layer and electronic defects by Ce3+ in the surface.

Photoelectrochemical (PEC) water electrolysis is an important energy conversion (power-to-chemical) method, providing a solution to the intermittent nature of solar energy. However, as PEC systems usually suffer from low operational stability, they are seriously lagging in up-scaled demonstrations and viability. PEC systems are based on semiconductor/liquid interfaces, which have been extensively studied by experiments and theory, but there is a significant knowledge gap in the energetics of such interfaces during operation. In this work, operando ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been used to characterize the electrical and spectroscopic properties of a pristine Ta3N5 photoelectrode and a Ta3N5/NiOx protection/passivation layer system, which stabilizes an otherwise quickly corroding pristine photoelectrode. We directly observed Fermi-level pinning of Ta3N5 within the applied potential window under both dark and illumination conditions, detrimental to the performance and stability of the photoelectrode. Interestingly, in the Ta3N5/NiOx protection/passivation layer system, the Fermi level gets unpinned under illumination, allowing quasi-Fermi-level splitting and sustaining a significant PEC performance as well as high stability.

There is a growing need to control and tune nanoparticles (NPs) to increase their stability and effectiveness, especially for photo- and electrochemical energy conversion applications. Exsolved particles are well anchored and can be re-shaped without changing their initial location and structural arrangement. However, this usually involves lengthy treatments and use of toxic gases. Here, the galvanic replacement/deposition method is used, which is simpler, safer, and leads to a wealth of new hybrid nanostructures with a higher degree of tailorability. The produced NiAu bimetallic nanostructures supported on SrTiO3 display exceptional activity in plasmon-assisted photoelectrochemical (PEC) water oxidation reactions. In situ scanning transmission electron microscopy is used to visualize the structural evolution of the plasmonic bimetallic structures, while theoretical simulations provide mechanistic insight and correlate the surface plasmon resonance effects with structural features and enhanced PEC performance. The versatility of this concept in shifting catalytic modes to the hydrogen evolution reaction is demonstrated by preparing hybrid NiPt bimetallic NPs of low Pt loadings on highly reduced SrTiO3 supports. This powerful methodology enables the design of supported bimetallic nanomaterials with tunable morphology and catalytic functionalities through minimal engineering.

Hydrogenases are attractive biocatalysts for utilization in electrochemical devices as potential replacement for Pt in hydrogen evolving electrodes. In this work, we investigate the immobilization of ferredoxin tagged FeFe-hydrogenase (Fd-HydA1) on black TiO2 nanotubes (bTNTs), with uniform nanotube opening diameters of 140 nm. By utilizing an immunogold labelling method, we show that the enzymes attach on the top surface of the bTNTs film rather than on the inner nanotube walls, reflecting the difficulty to insert enzymes into high aspect ratio nanomaterials for O-2-shielding. Nevertheless, cyclic voltammetry demonstrates direct electron transfer between Fd-HydA1 and bTNTs for the hydrogen evolution reaction (HER) in neutral media. This work provides new insight towards design of new nanostructured electrodes for enzyme immobilization.

Optimized Pt-based methanol oxidation reaction (MOR) anodes are essential for commercial direct methanol fuel cells (DMFCs) and methanol electrolyzers for hydrogen production. High surface area Ti supports are known to increase Pt catalytic activity and utilization. Pt has been deposited on black titania nanotubes (bTNTs), Ti felts and, for comparison, Ti foils by a galvanic deposition process, whereby Pt(IV) from a chloroplatinate solution is spontaneously reduced to metallic Pt (at 65 °C) onto chemically reduced (by CaH2) TNTs (resulting in bTNTs), chemically etched (HCl + NaF) Ti felts and grinded Ti foils. All Pt/Ti-based electrodes prepared by this method showed enhanced intrinsic catalytic activity towards MOR when compared to Pt and other Pt/Ti-based catalysts. The very high/high mass specific activity of Pt/bTNTs (ca 700 mA mgPt−1 at the voltammetric peak of 5 mV s−1 in 0.5 M MeOH) and of Pt/Ti-felt (ca 60 mA mgPt−1, accordingly) make these electrodes good candidates for MOR anodes and/or reactive Gas Diffusion Layer Electrodes (GDLEs) in DMFCs and/or methanol electrolysis cells.