Layered Hydroxides Research Articles

Inducing aluminum oxide growth at room temperature and atmospheric pressure through low dose gamma-ray irradiation

The corrosion resistance of the aluminum is well-known and it is attributed to passivation, a process that occurs when the metal surface reacts to the environment to form a spontaneously thin oxide film. Basically, the natural aluminum oxide film is composed of two layers, an internal layer composed entirely by Al2O3, and a second one made by Al(OH)3. In normal conditions of temperature and pressure, the Al(OH)3 film takes a long time to grow, something around several weeks or months. However, in the present work, through the use of low doses gamma-ray irradiation in an environment composed by air and water vapor, it was possible, in a few days, to increase the layer of Al(OH)3 on a sample of pure aluminum. Using a Cs-137 disk as a gamma-ray source, three aluminum samples were irradiated during 14, 8 and 5 days, receiving absorbed doses of 2.85 mGy, 1.49 mGy and 0.94 mGy, respectively. Comparing the Raman spectrum of the two irradiated samples with another non-irradiated sample, it was possible to verify that the sample irradiated during 14 days had the greatest values of peak intensity and peak area, indicating a higher concentration of aluminum hydroxide when compared to other samples. Moreover, the polarization curves of samples were obtained to analyze the nature of the film formed on the surface. From the results, it was possible to observe that the gamma-ray irradiation led to an improvement in passive film quality (higher resistance), once the irradiation resulted in a lower passivation current density (ipass). To confirm the consistence of previous analysis, more two samples were examined through X-ray photoelectron spectroscopy (XPS), where a sample irradiated during five days showed an increase in the percentage of aluminum hydroxide (related to XPS O 1s peak) in comparison to a non-irradiated sample. Then, it can be pointed that the irradiation, in a low dose, induced the growth of the aluminum hydroxide layer of passive film and improved the film's resistance in a few days. Moreover, the results show the utility of applying low doses of ionizing irradiation to change metal oxide surfaces.

High-Entropy Layered Rare Earth Hydroxides.

New high-entropy layered rare earth hydroxides ─ (Y,Eu,Gd,Er,Sm)2(OH)5NO3, (Y,Eu,Gd,Er,Tb)2(OH)5NO3, (Y,Eu,Gd,Er,Yb)2(OH)5NO3, (Y,Eu,Gd,Er,Nd)2(OH)5NO3, and (Y,Eu,Gd,Er,Nd,Sm,Tb)2(OH)5NO3 ─ were obtained using a hydrothermal microwave method. The annealing of layered rare earth hydroxides at 900 °C resulted in the corresponding high-entropy rare earth oxides. Based on inductively coupled plasma atomic emission spectroscopy data, the values for configurational entropy for both rare earth hydroxides and oxides were estimated, confirming the formation of high-entropy compounds. Energy-dispersive X-ray spectroscopy mapping, including mapping in the scanning transmission microscopy mode, showed no signs of chemical segregation and confirmed uniform rare earth elements' distribution both in the particles of high-entropy layered basic nitrates and in the particles of high-entropy oxides. The ratios of rare earth cations in the initial aqueous solutions of mixed nitrates were close to the ratios of cations in the resulting high-entropy layered rare earth basic nitrates and high-entropy rare earth oxides.

Low-crystalline Ni(OH)2 ultrathin nanosheets upcycled from solid wastes for efficient photoconversion of low-concentration CO2

Low-crystalline ultrathin nanosheets (UNSs) hold great potential for efficient photoreduction of anthropogenic CO2, while developing green synthetic methodologies and elucidating the structure-performance relationship remain significant challenges. Here, via a CaO-mediated strategy, we demonstrate a facile strategy for the large-scale production of low-crystalline Ni(OH)2 UNSs from industrial wastes. The resultant Ni(OH)2 UNSs deliver a superior CO formation rate of 9.2 μmol h−1 with a high selectivity of 93.1 % in low-concentration CO2 (10 %), which remarkably outperforms that of Ni(OH)2 NSs with high crystalline. Experimental and theoretical investigations demonstrate that the boosted performance originates from the fully exposed oxygen vacancies and low-coordination Ni sites, which facilitates the separation of photogenerated charge pairs and the adsorption and activation of diluted CO2. This work demonstrates a sustainable strategy for the large-scale synthesis of low-crystalline layered metal hydroxides, paving new avenues for the high-value utilization of industrial wastes in mitigating climate change.

Rose-like ZnNiCo layered double hydroxide grown on Co3O4 nanowire arrays for high energy density flexible supercapacitors

Ternary double layer hydroxides are considered as potential electrode materials for high-performance supercapacitors due to their unique layered structure and synergistic effect among their active components. However, the poor stability of these materials during the redox reaction seriously hinders their practical application. Therefore, it is of practical significance to design highly efficient and facile strategy to solve these issues. Herein, rose-like CC@Co3O4/ZNC-LDH hierarchical structure nanowire arrays are successfully synthesized on carbon cloth by a two-step hydrothermal method combined with a calcination process. Due to the unique structural design and the synergistic effect between Zn, Ni and Co elements, the prepared CC@Co3O4/ZNC-LDH nanocomposites exhibit excellent electrochemical properties (2084 mF cm−2 at 1 mA cm−2) and ideal capacitance retention (86.5 % after 10,000 cycles). In addition, the ASC device assembled with CC@Co3O4/ZNC-LDH nanocomposites as the positive electrode and activated carbon as the negative electrode can provide an energy density of 41 Wh kg−1 and a corresponding power density of 958 W kg−1 in a voltage window of 0–1.7 V. A multifunctional display can be lit up for more than 25 min by connecting two ASC devices in series. Therefore, the prepared rose-like CC@Co3O4/ZNC-LDH electrode with hierarchical structure should have a satisfactory practical application prospect.

Modification of spinel MnCo2O4 nanowire with NiFe-layered double hydroxide nanoflakes for stable seawater oxidation

Currently, direct electrolysis of seawater-based electrolytes rather than fresh water based ones for hydrogen production is gaining more and more attentions for creating a sustainable society. However, using seawater remains more challenges owing to the existence of competitive reactions between chlorine evolution reaction (ClER) or hypochlorite generation reaction and oxygen evolution reaction (OER) and electrode erosion. In this study, a MnCo2O4 nanowire coated with NiFe-Layered Double Hydroxide (NiFe-LDH) layer (MnCo2O4@NiFe-LDH) composite electrocatalyst prepared by a simple two-step hydrothermal method was applied for the seawater electrolysis, which exhibited low overpotentials of 219 and 245 mV at a relatively high current density of 100 mA cm−2 in alkaline simulated and natural seawaters, respectively, as the anode electrocatalyst. It is found that the NiFe-LDH layer on the MnCo2O4 nanowire can serve as Cl- protective layer to hinder the ClER and anode erosion and simultaneously improve the active surface area and intrinsic properties of MnCo2O4 nanowires, allowing for faster kinetics. While, the high valence states of Mn3+, Co3+, Ni3+and Fe3+ played a vital role for OER. In addition, when it was used as the bifunctional electrocatalyst for the overall real seawater splitting, the cell composed of MnCo2O4@NiFe-LDH (-) || MnCo2O4@NiFe-LDH (+) pair only required a low voltage of 1.56 V@10 mA cm−2 and simultaneously maintained excellent stability at a high current density of 100 mA cm−2. Such an electrocatalyst could be a promising candidate for long-term seawater splitting.

Insights into Chemical and Electrochemical Interactions between Zn Anode and Electrolytes in Aqueous Zn−ion Batteries

Aqueous Zn-ion batteries (ZIBs) have garnered significant interest in recent years due to their potential applications in large-scale stationary energy storage. Early ZIBs research has primarily focused on searching for better cathodes and understanding cathodic Zn2+ storage mechanisms. Only very recently has ZIBs research shifted to Zn anode. Here in this study, we report on insights into the interactions between Zn anode and aqueous Zn-salt electrolytes gained by a systematic investigation of bulk properties of electrolytes, surface properties of the reacted Zn, electrokinetics of Zn/Zn2+ redox reaction and cycle stability of Zn/electrolyte/Zn symmetrical cells. We found that Zn metal surface, regardless of electrolyte, are always covered by a layer of Zn-containing layered double hydroxides (Zn-LDHs) upon contact with aqueous Zn-electrolytes. We show that “OH− production” pathway resulted from the dissolved oxygen in Zn-electrolytes is the root cause for the Zn-LDHs formation. The electrokinetic studies reveal that Zn/Zn(ClO4)2 interface has the highest exchange current density, while the symmetrical cell tests show that Zn(OTf)2 is the most stable electrolyte for Zn-metal anode.

Synthesis of Solid Acid Catalysts of MgO–Al2O3 Mixed Oxide Containing Oxyanions by Thermal Decomposition of Anion-Exchanged Hydrotalcites

Solid acid catalysts of MgO–Al2O3 mixed oxides containing B4O72−, HPO42−, Mo7O246−, MoO42−, WO42−, and SO42− were synthesized by anion exchange with Cl− located in the space between anionic layers of hydrotalcite, followed by heat treatment at 773 K. The distance between the hydroxide layers was expanded by the intercalation of oxyanions larger than Cl−. The exchange of oxyanions in the interlayer space was confirmed by IR spectroscopy. Acid sites were generated on the obtained mixed oxides of MgO–Al2O3 by the electron withdrawing effect of the oxyanions. The acid catalyzed ethanol dehydration into ethylene and diethyl ether took place on the obtained catalysts. The effect of exchanged anions in the generation of acid sites was the largest in SO42−.

Surface active-site engineering in NiCoSe2/nitrogen-doped carbon dodecahedrons for efficient triiodide reduction in photovoltaics

Constructing Pt-free counter electrode (CE) catalysts with considerable catalytic activity, conductivity, and electrochemical stability are greatly significant for improving the overall power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). In this work, we focus on the uniformly distributing Ni and Co elements into layered hydroxide polyhedrons that stemmed from ZIF-67 to transform an active-site enriched composite of NiCoSe2 encapsulated by the derived nitrogen-doped carbon dodecahedrons (NiCoSe2/NC) through the simple pyrolysis procedure. Benefitting from the evenly distributed catalytic sites, unique structure, and positively synergy, the resultant NiCoSe2/NC displays excellent catalytic activity in triiodide reduction reaction (IRR). Consequently, DSSCs fabricated with the optimized NiCoSe2/NC yielded an impressive PCE of 9.92 %, superior to that of the traditional Pt-based devices (8.17 %). The intrinsic mechanism for superior IRR performance of NiCoSe2/NC was mainly ascribed to the favorable adsorption energy (Ead) for capturing I3−, the highly reactive metal catalytic sites at the interface, the efficient electron-transfer pathway, and the enhanced bonding interaction between I 5p and 3d states of the dual metals, as revealed by DFT calculation. It is worthy expecting to provide an extensible pathway for developing a cost-effective composite with profitable structure to guide the synthesis of efficient catalysts in catalytic fields.

Spider chrysanthemum-like Co flowers on a Ni foam for highly efficient H2O2 electroreduction in alkaline media

Transition metal oxides have shown potential as hydrogen peroxide (H2O2) reduction catalysts because of their low cost, good catalytic properties, and excellent stability in alkaline solutions. In this study, a CNF-0.25 composite electrode comprising spider chrysanthemum-like Co flowers on a Ni foam is synthesized as an efficient cathode for H2O2 electroreduction in alkaline media. CNF-0.25 exhibited superior stability and highly desirable electrocatalytic properties for H2O2 reduction with a high current density of 0.42 A cm−2 at − 0.5 V. The voltage of an Al-H2O2 semi-fuel cell prepared using CNF-0.25 as the cathode remains stable during a 50 h discharge test. The specific mechanism for the promotion of H2O2 electroreduction using CNF-0.25 is investigated according to the mechanism of H2O2 reduction over a cobalt-based catalyst. The spider chrysanthemum-like Co flowers are mainly composed of metallic Co, which improves the conductivity of the electrode. On the surface of the metallic Co, the presence of a layer of cobalt oxides or hydroxides provides sufficient catalytic active sites (Co2+/Co3+) and prevents further oxidation of internal Co. In addition, the unique spider chrysanthemum-like structure of CNF-0.25 increases the specific surface area of the electrode and facilitates electrolyte transport. This work offers another approach for designing highly efficient transition metal oxide electrodes in alkaline media.

Engineering redox active sites enriched 3D-on-2D bimetallic double layered hydroxide electrode for supercapatteries

The functionalization of structural nanoengineered battery-type electrodes has aided the emergence of supercapattery (SCp) subclass, which enables a wide range of applications. Herein, our research work provides a platform for two-step fabrication of nanoengineered 3D-on-2D structure as a promising approach to obtain high-performance battery-type electrodes. The hierarchical 2D NiCo bimetallic LDH NC(12)40 electrode was fabricated using electrodeposition, while the nanoengineered 3D ZIF-67 on 2D LDH electrode was achieved via pseudomorphic replication techniques. The fabricated 3D-on-2D NC(12)40-30 electrode reveals a maximum areal capacity of 1044 mC cm−2 at a current density of 4 mA cm−2 in 6 M KOH electrolyte. Furthermore, NC(12)40-30//AC was integrated as a SCp device, achieving a maximum specific capacitance of 63 F/g and maximum specific energy and power of 20.5 W/h/kg and 8522.7 W/kg, respectively, with improved capacitance retention (85%) even after 10,000 cycles. Thus, the assembled SCp coin cell displays 18-LED illumination in four different commercial LED colors, indicating the viability of the battery-type electrode for SCp development.

Single-Atomic Ir and Mo Co-Confined in a Co Layered Hydroxide Nanobox Mutually Boost Oxygen Evolution

Single-Atomic Ir and Mo Co-Confined in a Co Layered Hydroxide Nanobox Mutually Boost Oxygen Evolution

Radiolytic Gas Production from Aluminum Coupons (Alloy 1100 and 6061) in Helium Environments-Assessing the Extended Storage of Aluminum Clad Spent Nuclear Fuel.

Corrosion of aluminium alloy clad nuclear fuel, during reactor operation and under subsequent wet storage conditions, promotes the formation of aluminium hydroxide and oxyhydroxide layers. These hydrated mineral phases and the chemisorbed and physisorbed waters on their surfaces are susceptible to radiation-induced processes that yield molecular hydrogen gas (H2), which has the potential to complicate the long-term storage and disposal of aluminium clad nuclear fuel through flammable and explosive gas mixture formation, alloy embrittlement, and pressurization. Here, we present a systematic study of the radiolytic formation of H2 from aluminium alloy 1100 (AA1100) and 6061 (AA6061) coupons in “dry” (~0% relative humidity) and “wet” (50% relative humidity) helium environments. Cobalt-60 gamma irradiation of both aluminium alloy types promoted the formation of H2, which increased linearly up to ~2 MGy, and afforded G-values of 1.1 ± 0.1 and 2.9 ± 0.1 for “dry” and “wet” AA1100, and 2.7 ± 0.1 and 1.7 ± 0.1 for “dry” and “wet” AA6061. The negative correlation of H2 production with relative humidity for AA6061 is in stark contrast to AA1100 and is attributed to differences in the extent of corrosion and varying amounts of adsorbed water in the two alloys, as characterized using optical profilometry, scanning electron microscopy, Raman spectroscopy, and X-ray diffraction techniques.

Sodium dodecyl sulfate intercalated two-dimensional nickel-cobalt layered double hydroxides to synthesize multifunctional nanomaterials for supercapacitors and electrocatalytic hydrogen evolution

The controllable adjustment of layered double hydroxide layer spacing makes it possible as electrode material for energy storage and electrochemical hydrogen evolution at the same time. However, the lower conductivity and agglomeration significantly hinder its large-scale application as an active electrode material. Here, sodium dodecyl sulfate (SDS) intercalation modification was used to construct LDH derivatives and CoNi-SDS-LDH nanosheets were grown vertically on the nickel foam (NF) surface using in situ growth technique to construct a layered structure of nanosheets supported by nanowires, which effectively enhanced the ion transport efficiency and greatly reduced the agglomeration phenomenon of nanosheets. Thanks to the charge rearrangement in LDH and the controlled defects of the nanosheets, the electrode material has good electrochemical performance. The cycling stability of the CoNi-SDS-LDH self-supported electrode material is 91% after 10,000 cycles at 5 A g−1. Notably, the electrode material showed 98.1% performance retention after 10 h of hydrogen evolution test. It is shown that the anionic modification offers the possibility for large-scale applications of LDH in flexible supercapacitors and electrocatalytic hydrogen precipitation.

Hierarchically wood-derived integrated electrode with tunable superhydrophilic/superaerophobic surface for efficient urea electrolysis

Conferring surfaces with superhydrophilic/superaerophobic characteristics is desirable for synthesizing efficient gas reaction catalysts. However, complicated procedures, high costs, and poor interfaces hinder commercialization. Here, an integrated electrode with tunable wettability derived from a hierarchically porous wood scaffold was well designed for urea oxidation reaction (UOR). Interestingly, the outer surface of the wood lumen was optimized to the preferred wettability via stoichiometry to promote electrolyte permeation and gas escape. This catalyst exhibits outstanding activity and durability for UOR in alkaline media, requiring only a potential of 1.36 V (vs. RHE) to deliver 10 mA cm−2 and maintain its activity without significant decay for 60 h. These experiments and theoretical calculations demonstrate that the nickel (oxy)hydroxide layer formed through surface reconstruction of nickel nanoparticles improves the active sites and intrinsic activity. Moreover, the superwetting properties of the electrode promote mass transfer by guaranteeing substantial contact with the electrolyte and accelerating the separation of gaseous products during electrocatalysis. These findings provide the understanding needed to manipulate the surface wettability through rational design and fabrication of efficient electrocatalysts for gas-evolving processes.

Stabilizing efficient structures of superwetting electrocatalysts for enhanced urea oxidation reactions

Layered hydroxides have shown superior catalytic activity for the electrocatalytic organic compound oxidation reaction. However, metal leaching can lead to uncontrollable structural phase transformation. Here, we report a Cr-Ni(OH) 2 electrocatalyst as a model of a pre-catalyst for the identification of the structure-performance relationship. The optimized electrocatalyst delivered superb performances, i.e., a low potential of 1.38 V (versus reversible hydrogen electrode [RHE]) to reach 100 mA cm −2 and stable activity over 200 h at 10 mA cm −2 . In situ analyses and theoretical calculations demonstrate that well-tuned electronic structures and the superhydrophilic-superaerophobic surface can enable rapid urea oxidation reaction (UOR) kinetics, which reduces the specific adsorption OH − and significantly depresses Cr dopants leaching, and this helps to maintain high UOR performance. Furthermore, the crucial role of mass transfer improvement to alleviate the structural decay under high potentials is disclosed. • Catalyst with suitable wettability and optimized electronic structures was achieved • Rapid UOR kinetics significantly suppress Cr leaching • The improvement in mass transfer is crucial in stabilizing efficient structures The electrocatalytic organic compound oxidation reaction has been extensively employed to replace water electro-oxidation. However, most of their electrocatalysts suffer from sluggish kinetics with a large overpotential, further resulting in uncontrollable structural reconstruction of the catalyst. Here, a Cr-Ni(OH) 2 electrocatalyst with well-controlled superhydrophilicity-superaerophobicity was developed as a model to understand the structural stability of metal hydroxides during alkaline urea oxidation reaction (UOR). The rapid UOR kinetics of the Cr-Ni(OH) 2 electrocatalysts can ensure the high stability of efficient structures, resulting in best-in-class performances. Importantly, the discovery of structure and phase transformation at high potentials due to mass transfer limits created optimal electrocatalyst conditions for Cr-Ni(OH) 2 to promote urea-assisted hydrogen production, which is generally ignored by the field. A robust structure of electrocatalysts is critical for achieving a highly efficient urea oxidation reaction. Here, we demonstrate the regulation of electronic structures and the superwetting property of electrocatalysts in obtaining rapid UOR kinetics. Corroborated with in situ spectrum studies and theoretical calculations, rapid UOR kinetics can significantly suppress Cr dopants leaching, and mass transfer improvement plays a key role in stabilizing efficient structures at high potential.

Electrodeposited amorphous nickel–iron phosphide and sulfide derived films for electrocatalytic oxygen evolution

The preparation of inexpensive and efficient electrocatalysts for oxygen evolution reaction (OER) is crucial in the widespread application of water electrolyzers. A simple one-step aqueous electrodeposition method is utilized to prepare amorphous nickel-iron sulfide (Ni–Fe–S) and phosphide (Ni–Fe–P) films on Ni foam. The deposited films are highly porous, and can convert to active electrocatalysts for OER. In 1 M KOH, the Ni–Fe–S shows the highest OER activity, and requires only 230 mV overpotential to reach 0.05 A cm−2 OER current densities. The Fe–Ni–S also sustains the 30 h 0.05 A cm−2 galvanostatic OER test. Ex-situ characterizations show that sulfur in the Fe–Ni–S is oxidized and leached into the solution during OER, and that (oxy)hydroxide layer is formed at the surface. The adsorption energy of the hydroxyl group, an OER intermediate, is tuned by the electron interaction between the Ni and Fe, and the Ni–Fe–S exhibits the optimum hydroxyl group adsorption energy and the most facile OER kinetics. Also, higher intrinsic OER activity is observed for the electrodeposited amorphous nickel phosphide-derived film than the amorphous nickel sulfide-derived film.

Leveraging the Inductive Effect to Promote Oxygen Evolution on Oxides and Metal Hydroxide-Organic Frameworks

Late transition metal oxides are reported to be the most active non-precious catalysts for the oxygen evolution reaction (OER), but the most active oxides based on Ni or Co still have turnover frequencies per metal site at least one order of magnitude lower than that of oxygen-evolving complex (OEC) in biological systems.[1] The OEC contains an intricate manganese-calcium-oxo cluster, and its unparalleled OER activity can be attributed to the unique characteristics of Mn centers regulated by Ca2+ cations. Such electronic structure tuning can be generalized as the inductive effect.[2] Specifically, substituting metal oxides and complexes using foreign metals with higher electronegativity can increase their redox potentials and promote their activity. We have employed this concept to design bismuth-substituted strontium cobaltite, where the strongly electronegative Bi3+ ions give rise to Co centers with record intrinsic OER activity in alkaline solutions.[3] Unfortunately, metal substitution in oxides exhibits a much smaller tunability of electronic structures than that found in metal-oxo clusters. For instance, Ca2+ substitution of Mn4+ in synthetic [CaMn3O4] clusters can modulate the Mn3+/4+ redox potential by ~1 V,[4] whereas negligible changes of ~0.02 V are typically observed for oxides.[5] To tackle this limitation, we have designed metal hydroxide-organic frameworks (MHOFs) that combine the great tunability of enzymatic systems with known oxide-based chemistries.[6] A series of MHOFs were constructed by transforming layered hydroxides into 2D sheets composed of metal-octahedra chains cross-linked with neighboring chains using organic linkers. MHOFs can act as a tunable platform for the OER, where the nature of π-π-interactions between adjacent stacked linkers and the transition metals in the layered metal hydroxides dictate stability and activity, respectively. Substituting MHOF nanosheets with more electron-withdrawing cations increased their OER activity, where Fe-substituted Ni-based MHOFs exhibited three orders of magnitude enhancement in activity per metal site, rivaling those of state-of-the-art OER catalysts. This enhancement was correlated with the MHOF-based modulation of Ni redox potentials and the optimized binding of reaction intermediates. These results represent a step forward toward designing metal centers with ligand fields akin to those in homogenous/enzymatic systems, where MHOFs can act as a versatile platform to develop catalysts with unparalleled tunability.

Stability of Metal Hydroxide Organic Frameworks for Oxygen Evolution

The oxygen evolution reaction (OER) is central to storing electrical energy via chemical bonds in energy carriers and fuels through reactions such as electrochemical water splitting to produce hydrogen, CO2 reduction for CO and liquid hydrocarbons, and nitrogen to ammonia. While there has been considerable work towards the engineering of inexpensive yet highly active OER catalysts, current state-of-the-art materials are still at least an order of magnitude less active than oxygen evolving complexes found in biological systems that intricately combine inorganic metal-oxo clusters with organic ligands.1 As a result, metal organic frameworks (MOFs) have drawn considerable attention as hybrid organic-inorganic systems that can potentially mimic the unique structure of biological oxygen evolving complexes. Recently, metal hydroxide organic frameworks (MHOFs),2 a new class of MOFs that combine layered hydroxides with aromatic carboxylate linkers that stabilize the structure via π-π interactions, have been shown to display three times the tunability of layered hydroxides, offering extensive opportunities for further engineering of its electrochemical properties for numerous applications. However, the long-term stability of these material during OER is still unclear, which would be paramount to understand for further rational design of this class of material.In this study, we investigated Ni-based MHOFs with carboxylate linkers of varying π-π interaction strengths to understand how these differences affect the electrochemical stability of these materials during OER. We observed that the MHOFs all undergo activation during OER leading to two orders of magnitude increase in OER activity, where the MHOFs with weaker π-π interaction strengths tend to transform at a faster rate than the MHOFs with stronger π-π interaction strengths. We further characterized the MHOFs using a wide range of analytical techniques, including scanning transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and hard x-ray absorption spectroscopy, before and after extended OER cycling and galvanostatic tests to understand the transformed phase, which suggested that while the bulk structure largely remains unchanged, the surface undergoes significant restructuring into a Ni(OH)2-like phase during OER. Using operando UV-vis and Raman spectroscopy measurements on the MHOFs during OER to understand the factors that induce the transformation process, we found that there was a clear link between the Ni2+/3+, 4+ redox couple observed around 1.4 VRHE and a loss in the carboxylate organic linkers for the linkers with weak π-π interactions. However, for the MHOFs synthesized with linkers exhibiting strong π-π interaction strengths, there were smaller changes to the overall material during OER, suggesting that the bulk stability of these materials is largely dictated by the linker interaction strength and activation are primarily only surface transformations. These results directly demonstrate that linker selection also plays a key role in the stability MOFs under electrochemical conditions and are pertinent for rational design and understanding of the stability and activity of hybrid organic-inorganic materials as electrocatalysts.

(Invited) Local and Macroscopic Probes of Semiconductor/Electrocatalyst Photochemical Interfaces

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. These measurements are compared and contrasted to a new approach to “wirelessly” measure interfacial for different contact materials by analyzing shifts in element-specific shifts in x-ray photoelectron emission energies. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

(Invited) Ions Transport Properties and Carbonation Process Investigation of Nanocomposite Anion Exchange Membranes Containing Layered Double Hydroxide

Electrochemical energy conversion and storage (EECS) technologies such as fuel cells and electrolyzers are expected to play a pivotal role owing to their compatibility with the environment and high energy conversion efficiency. Anion-exchange membranes (AEMs) have received increased interest in recent years as the electrolyte separator in different EECS devices.[1] They have numerous advantages respect to the proton-exchange membranes, such as operation at high pH conditions, significantly lower cost (based on low-cost raw materials), less corrosive environment, and significantly lower fuel crossover (one order of magnitude lower than the acidic counterparts). AEM fuel cells and electrolyzers devices can reach the performance level required by applications with electrocatalysts that do not require a high loading of platinum-group metals (PGMs) due to the alkaline environment at the electrodes.However, the AEM based EECS development and implementation is significantly hindered by the anion exchange membrane (AEM) stability during cell operation, their low OH- conductivity and the low kinetics of the electrocatalysts. In fact, the electrolytic anion-exchange membrane (AEM) is a crucial component which should allow good charge and water transport, i.e. high ionic conductivity, but should also guarantee good chemical, thermal and mechanical stability and high durability. [2]We report on an extensive study on nanocomposite AEMs based on tetramethylammonium Polysulfone ionomer (PSU) and Layered Double Hydroxide (LDH) as nanofiller. [3] PSU is a thermoplastic polymer with high thermal stability, good chemical resistance and mechanical properties. However, since it has two activated positions per repetition unit for electrophilic aromatic substitution, it can undergo high degrees of functionalisation, which reduce the mechanical properties. We developed a two steps procedure, with allow a functionalization degree of about 80%, preserving its stability. Concerning the LDHs, these mineral anionic clays are excellent inorganic anionic conductors with elevated ion exchange capacity (IEC). They consist of the positively charged metal hydroxide layers with anions located in the interlayer space. This allows strong hydration of the material together with a large number of hydroxyls on the host layers forming a dense network of hydrogen bonds along the two-dimensional surface, therefore facilitating OH- ion conduction by diffusion mechanism.PSU/LDH AEMs were investigated in both OH- and forms, comparing swelling capacity, ionic conductivity and water diffusion. The latter was studied by NMR spectroscopy, measuring the self-diffusion coefficient by the Pulse Field Gradient (PFG) NMR techniques. The nanocomposite membranes are able to maintain good hydration at high temperatures, and to create an adequate nanostructure with the polymer chains, which favour the Grotthuss diffusion mechanism for the OH- ions. Such feature is reflected in the ionic conductivity and in the alkali stability, where they demonstrated the highest conductivity and a reduced membrane degradation rate.In addition, the 13C-NMR technique was used to investigate carbonation processes in the presence of CO2, showing that the presence of the LDH platelets into the AEM remarkably reduces the conversion rate of hydroxyl groups, as much as slow down the diffusion of carbonate ions.Finally, electrolysis cell tests were conducted on MEAs based on these hybrid membranes, and preliminary results showed very promising performance. Acknowledgments This work has been supported by the Italian Ministry for University and Research (MUR) for funding through the FISR 2019 project AMPERE (FISR2019_01294). References Vincent, I. and Bessarabov, D. (2018) Renewable and Sustainable Energy Reviews 81, 1690–704G. Arges, L. Zhang, ACS Appl. Energy Mater. 1 (2018) 2991–3012,Simari, C.,...and Nicotera, I. (2022) Electrochimica Acta, 403, p. 139713.