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Metal aerogels assembled from nanoparticles have captured grand attention because they combine the virtues of metals and aerogels and are regarded as ideal materials to address current environmental and energy issues. Among these aerogels, those composed of two metals not only display combinations (superpositions) of the properties of their individual metal components but also feature novel properties distinctly different from those of their monometallic relatives. Therefore, quite some effort has been invested in refining the synthetic methods, compositions, and structures of such bimetallic aerogels as to boost their performance for the envisaged application(s). One such use would be in the field of electrocatalysis, whereby it is also of utmost interest to unravel the element distributions of the (multi)metallic catalysts to achieve a ratio of their bottom-to-up design. Regarding the element distributions in bimetallic aerogels, advanced characterization techniques have identified alloys, core-shells, and structures in which the two metal particles are segregated (i.e., adjacent but without alloy or core-shell structure formation). While an almost infinite number of metal combinations to form bimetallic aerogels can be envisaged, the knowledge of their formation mechanisms and the corresponding element distributions is still in its infancy. The evolution of the observed musters is all but well understood, not to mention the positional changes of the elements observed in operando or in beginning- vs end-of-life comparisons (e.g., in fuel cell applications). With this motivation, in this Account we summarize the endeavors made in element distribution monitoring in bimetallic aerogels in terms of synthetic methods, expected structures, and their evolution during electrocatalysis. After an introductory chapter, we first describe briefly the two most important characterization techniques used for this, namely, scanning transmission electron microscopy (STEM) combined with element mapping (e.g., energy-dispersive X-ray spectroscopy (EDXS)) and X-ray absorption spectroscopy (XAS). We then explain the universal methods used to prepare bimetallic aerogels with different compositions. Those are divided into one-step methods in which gels formed from mixtures of the respective metal salts are coreduced and two-step approaches in which monometallic nanoparticles are mixed and gelated. Subsequently, we summarize the current state-of-knowledge on the element distributions unraveled using diverse characterization methods. This is extended to investigations of the element distributions being altered during electrochemical cycling or other loads. So far, a theoretical understanding of these processes is sparse, not to mention predictions of element distributions. The Account concludes with a series of remarks on current challenges in the field and an outlook on the gains that the field would earn from a solid understanding of the underlying processes and a predictive theoretical backing.

Due to its unique ability to reduce carbon dioxide (CO2) into CO or formate at high versus low overpotentials, respectively, palladium is a promising catalyst for the electrochemical CO2-reduction reaction (CO2RR). Further improvements aim at increasing its activity and selectivity toward either of these value-added species, while reducing the amount of hydrogen produced as a side product. With this motivation, in this work, we synthesized a range of unsupported, bimetallic PdPt aerogels and pure Pt or Pd aerogels and extensively characterized them using various microscopic and spectroscopic techniques. These revealed that the aerogels’ porous web consists of homogenous alloys of Pt and Pd, with palladium and platinum being present on their surface for all compositions. The subsequent determination of these aeorgels’ CO2RR performance unveiled that the high activity of these Pt surface atoms toward hydrogen evolution causes all PdPt alloys to favor this reaction over CO2 reduction. In the case of the pure Pd aerogel, although, its unsupported nature leads to a suppression of H2 evolution and a concomitant increase in the selectivity toward CO when compared to a commercial, carbon-supported Pd-nanoparticle catalyst.

In recent years, operando/in situ X-ray absorption spectroscopy (XAS) has become an important tool in the electrocatalysis community. However, the high catalyst loadings often required to acquire XA-spectra with a satisfactory signal-to-noise ratio frequently imply the use of thick catalyst layers (CLs) with large ion- and mass-transport limitations. To shed light on the impact of this variable on the spectro-electrochemical results, in this study we investigate Pd-hydride formation in carbon-supported Pd-nanoparticles (Pd/C) and an unsupported Pd-aerogel with similar Pd surface areas but drastically different morphologies and electrode packing densities. Our in situ XAS and rotating disk electrode (RDE) measurements with different loadings unveil that the CL-thickness largely determines the hydride formation trends inferred from spectro-electrochemical experiments, therewith calling for the minimization of the CL-thickness in such experiments and the use of complementary thin-film control measurements., Angewandte Chemie. International Edition, 62 (16), ISSN:1433-7851, ISSN:1521-3773, ISSN:0570-0833

Colloidal nanocrystals of metals, semiconductors, and other functional materials can self-assemble into long-range ordered crystalline and quasicrystalline phases, but insulating organic surface ligands prevent the development of collective electronic states in ordered nanocrystal assemblies. We reversibly self-assembled colloidal nanocrystals of gold, platinum, nickel, lead sulfide, and lead selenide with conductive inorganic ligands into supercrystals exhibiting optical and electronic properties consistent with strong electronic coupling between the constituent nanocrystals. The phase behavior of charge-stabilized nanocrystals can be rationalized and navigated with phase diagrams computed for particles interacting through short-range attractive potentials. By finely tuning interparticle interactions, the assembly was directed either through one-step nucleation or nonclassical two-step nucleation pathways. In the latter case, the nucleation was preceded by the formation of two metastable colloidal fluids.

With the merits of quantum dots (QDs) (e.g., high molar extinction coefficient, strong visible light absorption, large specific surface area, and abundant functional surface active sites) and aerogels (e.g., self-supported architectures, porous network), semiconductor QD aerogels show great prospect in photocatalytic applications. However, typical gelation methods rely on oxidative treatments of QDs. Moreover, the remaining organic ligands (e.g., mercaptoacids) are still present on the surface of gels. Both these factors inhibit the activity of such photocatalysts, hampering their widespread use. Herein, we present a facile 3D assembly of II−VI semiconductor QDs capped with inorganic (NH4)2S ligands into aerogels using H2O as a dispersion solvent. Without any sacrificial agents, the resulting CdSe QD aerogels achieve a high CO generation rate of 15 μmol g-1 h-1, which is 12-fold higher than that of pristine-aggregated QD powders. Our work not only provides a facile strategy to fabricate QD aerogels but also offers a platform for designing advanced aerogel-based photocatalysts.

The important role of structure homogeneity in three-dimensional network nanostructures serving as noble metal aerogels (NMAs) has attracted extensive attention in the field of electrochemistry in the last two decades, whereas a comprehensive study of tailoring skeleton units and element distributions in NMAs is still lacking. Herein, a new modulation strategy to easily prepare multiscale NMAs with tunable composition is developed by utilizing the electrostatic interaction between oppositely charged colloidal metal nanoparticles. The modulation rule of the chemical distribution in bimetallic aerogels leads to the construction of the as-tailored double skeleton aerogels for the first time. Considering their specific structures, the intrinsic and exceptional catalytic and electrocatalytic performances of NMAs were investigated. This study optimizes the structure homogeneity of noble metal aerogels by investigating nanoparticle–ligand interactions and provides further proof of their exceptional electrocatalytic capabilities.

Continuous advancements in science and technology in the field of flexible devices encourage researchers to dedicate themselves to seeking candidates for new flexible transparent conductive films (FTCFs). Our recently developed two-dimensional (2D) metal aerogels are considered as a new class of FTCFs. Here, we describe a new large-scale self-assembly synthesis of bimetallic Pt-Ni 2D metal aerogels with controllable morphology during the synthesis. The obtained 2D aerogels require only a low quantity of precursors for the synthesis of percolating nanoscale networks with areas of up to 6 cm2 without the need of an additional drying step. Stacks of the obtained monolayer structures display low sheet resistances (down to 270 Ω/sq), while decreasing the optical transparency. In perspective, the 2D bimetallic Pt-Ni aerogels not only enrich the structural diversity of metal aerogels but also bring forth new materials for further applications in flexible electronics and electrocatalysis with reduced costs of production.

In this paper we present a comparison of aerogels which are dried under different conditions. Of those, most important are the solvent, temperature, and pressure. Criteria of comparison rely mostly on results from analysis of nitrogen adsorption experiments, as well as transmission electron microscopy imaging. Platinum loaded indium oxide aerogels were picked as a model system for this study as they can be used as highly effective heterogeneous catalysts in methanol steam reforming. The compared drying methods include supercritical drying from CO2, supercritical CO2 - ethanol mixture, freeze drying from tert-butanol and ambient conditions drying from acetone and 1-Methoxyheptafluoropropane. High porosities and large specific surface areas can be achieved via supercritical, freeze- and ambient conditions drying, while retaining the original gel morphology in this system for most methods except freeze drying and ambient conditions drying from acetone.

Electrochemiluminescence (ECL) represents a widely explored technique to generate light, in which the emission intensity relies critically on the charge-transfer reactions between electrogenerated radicals. Two types of charge-transfer mechanisms have been postulated for ECL generation, but the manipulation and effective probing of these routes remain a fundamental challenge. Here, we demonstrate the design of quantum dot (QD) aerogels as novel ECL luminophores via a versatile water-induced gelation strategy. The strong electronic coupling between adjacent QDs enables efficient charge transport within the aerogel network, leading to the generation of highly efficient ECL based on the selectively improved interparticle charge-transfer route. This mechanism is further verified by designing CdSe-CdTe mixed QD aerogels, where the two mechanistic routes are clearly decoupled for ECL generation. We anticipate our work will advance the fundamental understanding of ECL and prove useful for designing next-generation QD-based devices.

Wavelength-discriminating systems typically consist of heavy benchtop-based instruments, comprising diffractive optics, moving parts, and adjacent detectors. For simple wavelength measurements, such as lab-on-chip light source calibration or laser wavelength tracking, which do not require polychromatic analysis and cannot handle bulky spectroscopy instruments, lightweight, easy-to-process, and flexible single-pixel devices are attracting increasing attention. Here, a device is proposed for monotonously transforming wavelength information into the time domain with room-temperature phosphorescence at the heart of its functionality, which demonstrates a resolution down to 1 nm and below. It is solution-processed from a single host-guest system comprising organic room-temperature phosphors and colloidal quantum dots. The share of excited triplet states within the photoluminescent layer is dependent on the excitation wavelength and determines the afterglow intensity of the film, which is tracked by a simple photodetector. Finally, an all-organic thin-film wavelength sensor and two applications are demonstrated where this novel measurement concept successfully replaces a full spectrometer.

Hydrogen storage in the form of small molecules and subsequent release are foreseen to play a fundamental role in future energy systems or carbon cycles. Methanol is an ideal hydrogen carrier due to the high H/C ratio, the lack of C–C bonds, and being liquid under ambient conditions. Methanol steam reforming is an advantageous reaction for the release of the chemically bound hydrogen. Pd- or Pt-based intermetallic compounds have shown to be CO2-selective and long-term stable catalytic materials. However, an intrinsic understanding of the underlying processes is still lacking. In this study, we show that the redox activity in the In–Pt system can be steered by gas-phase changes and leads to highly active catalytic materials at 300 °C [1500 mol (H2)/(mol (Pt) × h)] with an excellent CO2 selectivity of 99.5%, thus clearly outperforming previous materials. Reactive transformations between In2Pt, In3Pt2, and In2O3 have been identified to cause the high selectivity. Redox activity of intermetallic compounds as part of the catalytic cycle was previously unknown and adds an understanding to the concept of different adsorption sites.

Summary Armed with merits of the metals (e.g., electrical conductivity, catalytic activity, and plasmonic properties) and aerogels (e.g., monolithic structure, porous network, and large specific surface area), metal aerogels (MAs) have stood out as a new class of porous materials in the last decade. With unparalleled potential in electrocatalysis, plasmonics, and sensing, they are envisaged to revolutionize the energy- and detection-related application fields. However, MA development is severely retarded by the lack of a sufficient material basis. Suffering from the ambiguous understanding of formation mechanisms, big challenges remain for tailoring MAs for task-specific applications. By surveying state-of-the-art developments, this review strives to summarize design principles and arouse interest in broad scientific communities. Moreover, critical challenges and opportunities are highlighted to provide a research roadmap for this young yet promising field.

Noble metal aerogels (NMAs), one class of the youngest members in the aerogel family, have drawn increasing attention in the last decade. Featuring the high catalytic activity of noble metals and a 3D self-supported porous network of the aerogels, they have displayed profound potential for electrocatalysis. However, considerable challenges reside in the rapid fabrication of NMAs with a well-tailored architecture, constraining the manipulation of their electrochemical properties for optimized performance. Here, a disturbance-assisted dynamic shelling strategy is developed, generating self-supported Au–Pd core–shell gels within 10 min. Based on suitable activation and desorption energies of the involved species as suggested by theoretical calculations, the Au–Pd core–shell aerogel manifests outstanding CO selectivity and stability at low overpotential (faradaic efficiency > 98% at −0.5 V vs. RHE over 12 hours) for the electrochemical CO2 reduction reaction (CO2RR). The present strategy offers a new perspective to facilely design architecture-specified high-performance electrocatalysts for the CO2RR.

Methanol steam reforming provides clean hydrogen by onboard production, which can directly be used for fuel cell applications–while using appropriate catalysts. InxPdy/In2O3 aerogels exhibit excell...

Due to their self-supporting and nanoparticulate structure, metal aerogels have emerged as excellent electrocatalysts, especially in the light of the shift to renewable energy cycles. While a large number of synthesis parameters have already been studied in depth, only superficial attention has been paid to the solvent. In order to investigate the influence of this parameter with respect to the gelation time, crystallinity, morphology, or porosity of metal gels, AuxCuy aerogels were prepared in water and ethanol. It was shown that although gelation in water leads to highly porous gels (60 m2g−1), a CuO phase forms during this process. The undesired oxide could be selectively removed using a post-washing step with formic acid. In contrast, the solvent change to EtOH led to a halving of the gelation time and the suppression of Cu oxidation. Thus, pure Cu aerogels were synthesized in addition to various bimetallic Au3X (X = Ni, Fe, Co) gels. The faster gelation, caused by the lower permittivity of EtOH, led to the formation of thicker gel strands, which resulted in a lower porosity of the AuxCuy aerogels. The advantage given by the solvent choice simplifies the preparation of metal aerogels and provides deeper knowledge about their gelation.

As there is a great demand of 2D metal networks, especially out of gold for a plethora of applications we show a universal synthetic method via phase boundary gelation which allows the fabrication of networks displaying areas of up to 2 cm2. They are transferred to many different substrates: glass, glassy carbon, silicon, or polymers such as PDMS. In addition to the standardly used web thickness, the networks are parametrized by their fractal dimension. By variation of experimental conditions, we produced web thicknesses between 4.1 nm and 14.7 nm and fractal dimensions in the span of 1.56 to 1.76 which allows to tailor the structures to fit for various applications. Furthermore, the morphology can be tailored by stacking sheets of the networks. For each different metal network, we determined its optical transmission and sheet resistance. The obtained values of up to 97 % transparency and sheet resistances as low as 55.9 Ω/sq highlight the great potential of the obtained materials., Stacked fractals: 2D gold mesh‐like gels with areas of up to 2 cm2 are synthesized via phase boundary gelation. The structures are described as fractals. By variation of experimental details, web thicknesses between 4.1 nm and 14.7 nm are achieved. The networks can be stacked or transferred to different substrates.

We report on a near-infrared (NIR) emitting semiconductor nanocrystal (NC) heterostructure with tunable optical properties. This heterostructure is built of two different semiconductor sections in ...

Noble‐metal aerogels (NMAs) have drawn increasing attention because of their self‐supported conductive networks, high surface areas, and numerous optically/catalytically active sites, enabling their impressive performance in diverse fields. However, the fabrication methods suffer from tedious procedures, long preparation times, unavoidable impurities, and uncontrolled multiscale structures, discouraging their developments. By utilizing the self‐healing properties of noble‐metal aggregates, the freezing‐promoted salting‐out behavior, and the ice‐templating effect, a freeze–thaw method is crafted that is capable of preparing various hierarchically structured noble‐metal gels within one day without extra additives. In light of their cleanliness, the multi‐scale structures, and combined catalytic/optical properties, the electrocatalytic and photoelectrocatalytic performance of NMAs are demonstrated, which surpasses that of commercial noble‐metal catalysts., Coming in from the cold: By utilizing the freezing‐promoted salting‐out and the ice‐templating effect, a freeze–thaw method is developed to fabricate various noble‐metal hydrogels within one day. In light of their catalytically/optically active sites and multi‐scale structures, the impressive performance for electrocatalysis and photoelectrocatalysis is demonstrated for electro‐oxidation of ethanol.

The material-efficient monolayers of transition-metal dichalcogenides (TMDs) are a promising class of ultrathin nanomaterials with properties ranging from insulating through semiconducting to metallic, opening a wide variety of their potential applications from catalysis and energy storage to optoelectronics, spintronics, and valleytronics. In particular, TMDs have a great potential as emerging inexpensive alternatives to noble metal-based catalysts in electrochemical hydrogen evolution. Herein, we report a straightforward, low-cost, and general colloidal synthesis of various 2D transition-metal disulfide nanomaterials, such as MoS2, WS2, NiSx, FeSx, and VS2, in the absence of organic ligands. This new preparation route provides many benefits including relatively mild reaction conditions, high reproducibility, high yields, easy upscaling, no post-thermal annealing/treatment steps to enhance the catalytic activity, and, finally, especially for molybdenum disulfide nanosheets, high activity in the hydrogen evolution reaction. To underline the universal application of the synthesis, we prepared mixed CoxMo1-xS2 nanosheets in one step to optimize the catalytic activity of pure undoped MoS2, which resulted in an enhanced hydrogen evolution reaction performance characterized by onset potentials as low as 134 mV and small Tafel slopes of 55 mV/dec.

Noble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano‐sized highly‐active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser‐generated inorganic‐salt‐stabilized metal nanoparticles, various impurity‐free NMAs (Au, Pd, and Au‐Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond., Worth its weight in gold: Starting from laser‐generated inorganic‐salt‐stabilized metal nanoparticles, various impurity‐free noble metal aerogels are fabricated, allowing the investigation of their intrinsic properties. Moreover, prominent roles played by ligands, which are interpreted from electron transfer between NMAs and respective ligands, are demonstrated, offering a new dimension to engineer electrocatalytic properties of broad materials.

Tuning electrocatalysts via interface engineering is widely adopted as a valid way to manipulate the electrocatalytic activity. Herein, the interface engineering of PdCu aerogels is successfully achieved by the integration of an ionic liquid (IL), which not only accelerates the gelation kinetics but also leads to the modulation of the interface electronic properties, enabling IL functionalized PdCu aerogels (IL/PdCu) as advanced electrocatalysts for the methanol oxidation reaction (MOR) with splendid electrocatalytic activity. Density functional theory (DFT) calculations also demonstrate that IL-involved interface engineering dramatically reduces the chemisorption energy of CO-containing intermediates and thus significantly boosts the MOR performance. Furthermore, direct methanol fuel cells (DMFCs) using an IL/Pd3Cu1 anode catalyst exhibit a higher power density than those containing Pd3Cu1 and commercial Pd catalysts. This work highlights the superiority of designing advanced electrocatalysts by interface engineering.

Solar radiation is a versatile source of energy, convertible to different forms of power. A direct path to exploit it is the generation of heat, for applications including passive building heating, but it can also drive secondary energy‐conversion steps. We present a novel concept for a hybrid material which is both strongly photo‐absorbing and with superior characteristics for the insulation of heat. The combination of that two properties is rather unique, and make this material an optical superheater. To realize such a material, we are combining plasmonic nanoheaters with alumina aerogel. The aerogel has the double function of providing structural support for plasmonic nanocrystals, which serve as nanoheaters, and reducing the diffusion rate of the heat generated by them, resulting in large local temperature increases under a relatively low radiation intensity. This work includes theoretical discussion on the physical mechanisms impacting the system's balanced thermal equilibrium., White light, white heat: Gold nanorods (AuNRs) embedded into highly insulating alumina aerogel pellets give a hybrid system that has a tremendous light to heat conversion performance. The AuNRs withstand the high local temperatures (175 °C) with little to no melting. The system allows the design and modelling of novel concepts for photoheaters.

The efficient scale-up of CO

In this study, faceted mesocrystals have been assembled from the dispersion of truncated cubic-shaped iron oxide nanoparticles stabilized by oleic acid (OA) molecules using the non-solvent “gas phase diffusion technique” into an organic solvent. The effects of synthesis conditions as well as of the nanoparticle size and shape on the structure and morphogenesis of mesocrystals were examined. The interactions of OA capped iron oxide nanoparticles with solvent molecules were probed by analytical ultracentrifugation and double difference pair distribution function analysis. It was shown that the structure of the organic shell significantly depends on the nature and polarity of solvent molecules.

In this study, faceted mesocrystals have been assembled from the dispersion of truncated cubic-shaped iron oxide nanoparticles stabilized by oleic acid (OA) molecules using the non-solvent “gas phase diffusion technique” into an organic solvent. The effects of synthesis conditions as well as of the nanoparticle size and shape on the structure and morphogenesis of mesocrystals were examined. The interactions of OA capped iron oxide nanoparticles with solvent molecules were probed by analytical ultracentrifugation and double difference pair distribution function analysis. It was shown that the structure of the organic shell significantly depends on the nature and polarity of solvent molecules.

Noble metal hydro- and aerogels exhibit a variety of unique properties, including large surface areas, ultralow densities, and high porosities. While their potential use in various applications ran...

All inorganic lead halide perovskite nanocrystals (PNCs) typically suffer from poor stability against moisture and UV radiation as well as degradation during thermal treatment. The stability of PNC...

Investigation of the influence of the nanocrystal (NC) surface chemistry on the (opto)electronic properties of NC-based thin films is of paramount importance for their further application in variou...

Noble metal aerogels (NMAs), as the most important class of noble metal foams (NMFs), appear as emerging functional porous materials in the field of materials science. Combining the irreplaceable roles of noble metals in certain scenarios, as well as monolithic and porous features of aerogels, NMAs can potentially revolutionize diverse fields, such as catalysis, plasmonics, and biology. Despite profound progress, grand challenges remain in their fabrication process, including the efficient structure control, the comprehensive understanding of the formation mechanisms, and the generality of the fabrication strategies, thus inevitably retarding the material design and optimization. This Perspective focuses on the key progress, especially of the fabrication strategies for NMAs during the last two decades, while other NMFs are also succinctly introduced. Challenges and opportunities are summarized to highlight the unexploited space and future directions in expectation of stimulating the broad interest of interdisciplinary scientists.

We investigate the influence of the average molar mass (Mw) of the capping agent poly(N-vinylpyrrolidone) (PVP) on the conductivity of a silver nanowire (AgNW) network. During the polyol process, the chain length of PVP is known to influence the AgNW diameters and lengths. By altering the reaction temperature and time and using PVP of different chain lengths, we synthesized AgNWs with varying diameters, lengths and PVP coverage. The obtained plethora of AgNWs is the basis for conductivity investigations of networks made of AgNWs with a diameter of either 60 nm or 80 nm. The results show a negative influence of long-chain PVP on the conductivity of the subsequent network if 60 nm thick AgNWs are employed. Overall, we obtain well performing AgNW transparent electrodes on glass with RS = 24.4 Ω sq-1 at 85.5%T550nm.

We present a universal one-step approach to generate metal aerogels at room and at dry ice cold temperature, abbreviated as DICE. Performing the synthesis completely in an ethanolic phase allows for the formation of pure noble (Pt, Pd, Au and Ag) and less noble metal (Cu and Co) gels in a remarkably short period of time compared to the widespread aqueous synthesis strategies. Furthermore, three-dimensional disordered, porous networks were obtained for each system with high surface areas of 860 m2 mol−1 for Ag up to 5470 m2 mol−1 for Co. Additionally, it is shown that the low reaction temperature of −70 °C enables the control of the gelation time without losing the structural characteristics. Moreover, with respect to industrial applications, this facile, stabilizer-free synthesis yields large amounts of materials using highly concentrated precursor solutions.

Due to its potential to replace fossil fuels in chemical processes by converting carbon dioxide (CO2) to industrial base reactants, the electroreduction of CO2 is receiving tremendous attention. Among the large number of electrocatalysts currently under investigation for this reaction, palladium (Pd) is increasingly appealing due to its ability to produce both carbon monoxide (CO) and formate at high selectivities depending on the applied potential. This unique change in selectivity between CO and formate at high vs. low overpotentials (i.e., -0.5 to -1.0 vs. -0.1 to -0.4 V vs. the reversible hydrogen electrode (VRHE), respectively) has been described multiple times on different Pd-electrocatalysts [1-3]. Those mechanistic investigations mainly focused on understanding the ability of palladium to form hydride (PdHx) under CO2 electroreduction conditions. While β-PdH is usually found at high overpotentials, the formation of α-PdH is described at low overpotentials, and a mixed-phase between both compositions is generally reported at intermediate overpotentials [1,3-5]. As a result, the CO2 reduction reaction (CO2RR) is assumed to take place on β-PdH at the high overpotentials leading to CO production, while α-PdH or a mixed α/β-phase is believed to be the active phase at low overpotentials associated with high formate yields [1,3,4]. However, large differences between the potential of β-PdH formation, as well as the extent of the mixed hydride state (i.e., featured as a sudden transition between α- and β-phases [3,4], or extending over hundreds of millivolts [1,5]), have been reported. In parallel to this, Pd-surface poisoning with CO is known to influence palladium’s CO2RR selectivity [2,3] while also slowing down hydride formation [6]. However, the precise interplay between surface-adsorbed CO and the formation of PdH, along with their combined effects on the mechanism of CO2 electroreduction remains poorly understood. In light of this, in this work, we investigated these effects using an unsupported Pd-aerogel with a web thickness of ≈ 6 nm, synthesized via a novel ethanolic synthesis approach [7]. We first analyzed the influence of the electrolyte composition (i.e., presence vs. absence of CO2 and bicarbonate) on the formation of surface-adsorbed CO and PdH by performing potential holds followed by positive linear sweep voltammograms in a rotating disk electrode (RDE) setup. As shown in Fig. 1A, these voltammograms featured two oxidation peaks at ≈ 0.8 vs. ≈ 1.0 VRHE that we could assign to the desorption of absorbed hydrogen and the oxidation of adsorbed CO, respectively. This in turn allowed us to track the formation of both of these species in relation to the applied potential and duration of the potential holds. Under CO2 electroreduction conditions, a full monolayer of CO was found to form across the entire CO2 electroreduction potential range. Moreover, large differences in the rate of PdH formation as a function of the applied potential were found in the presence of CO2 and/or bicarbonate, while in CO2- and bicarbonate-free electrolytes hydride formation is quasi-instantaneous at all potentials relevant for CO2 electroreduction. Time-dependent RDE investigations at constant potentials also revealed an initial decrease in the hydride content of the PdH phase during the formation of the CO monolayer, hinting at the involvement of this hydride in a hydrogenation step previously described as part of the CO2 electroreduction mechanism at low overpotentials [2]. Finally, these experiments were complemented with time-resolved in-situ X-ray absorption measurements that confirmed the formation of full β-PdH at all applied overpotentials under CO2 electroreduction conditions (see Fig. 1B), however, differences in the time-dependence of PdH formation on the applied potential were revealed. In summary, this contribution showcases the potential- and time-dependent CO monolayer formation under CO2RR conditions and its influence on PdH-formation. Specifically, our results reveal the independence of the final PdH-stoichiometry on the applied potential, while exposing the effect of the latter variable on the rates of CO-monolayer and β-PdH formation. On this basis, the mechanisms of CO2 electroreduction on Pd that have been proposed so far should be revisited to consider the simultaneous presence of full β-PdH and a near-full monolayer of CO at all overpotentials unveiled by our results. References: [1] D. Gao et al., Nano Res. 2017, 10 (6), 2181-2191. [2] X. Min et al., J. Am. Chem. Soc. 2015, 137 (14), 4701-8. [3] W. Zhu et al., Adv. Energy Mater. 2019, 9 (9), 1802840. [4] W. Sheng et al., Energy Environ. Sci. 2017, 10 (5), 1180-1185. [5] J.H. Lee et al., Chem. Commun. 2020, 56 (1), 106-108. [6] B. Łosiewicz et al., J. Electroanal. Chem. 2007, 611 (1-2), 26-34. [7] M. Georgi et al., Mater. Chem. Front. 2019, 3 (8), 1586-1592. Figure 1