Surface Chemistry Modulation of BaGd0.8La0.2Co2O6-δ As Active Air Electrode for Solid Oxide Cells.

Modulation of the surface chemistry of air electrodes makes it possible to significantly improve the electrocatalytic performance of solid oxide cells (SOCs). Here, the surface chemistry of BaGd0.8La0.2Co2O6-δ (BGLC) double perovskite is modulated by treatment in an acidic citric acid solution. The treatment leads to corrosion on the surface of BGLC particles, and the effect is dependent on the acidity of the solution. As the acidity of solution is low, Ba cations are selectively dissolved out of the BGLC surface, while as the acidity increases, the corrosion becomes more homogeneous. The Ba surface deficiency remarkably increases the concentration of surface oxygen vacancies and electrocatalytic activity of BGLC. To avoid the loss of Ba-deficient surface during the conventional high temperature sintering process, a sintering-free fabrication route is utilized to directly assemble the Ba-deficient BGLC powder into an air electrode. A single cell with the surface Ba-deficient BGLC electrode shows a peak power density of 1.04 W cm-2 at 750 °C and an electrolysis current density of 1.48 A cm-2 at 1.3 V, much greater than 0.64 W cm-2 and 1.02 A cm-2 of the cell with the pristine BGLC, respectively. This work provides a simple and effective surface chemistry modulation strategy for the development of an efficient air electrode for SOCs.

Development of Nanostructured Lanthanum Strontium Cobalt Ferrite/Gadolinian-Doped Ceria Composite Electrodes of Solid Oxide Cells Formed by In Situ Polarization.

In the development of nanoscale oxygen electrodes of high-temperature solid oxide cells (SOCs), the interface formed between the nanoelectrode particles and the electrolyte or electrolyte scaffolds is the most critical. In this work, a new synthesis technique for the fabrication of nanostructured electrodes via in situ electrochemical polarization treatment is reported. The lanthanum strontium cobalt ferrite (LSCF) precursor solution is infiltrated into a gadolinia-doped ceria (GDC) scaffold presintered on a yttria-stabilized zirconia (YSZ) electrolyte, followed by in situ polarization current treatment at SOC operation temperatures. Electrode ohmic and polarization resistances decrease with an increase in the polarization current treatment. Detailed microstructure analysis indicates the formation of a convex-shaped interface between the LSCF nanoparticles (NPs) and the GDC scaffold, very different from the flat contact between LSCF and GDC observed after heating at 800 °C with no polarization current treatment. The embedded LSCF NPs on the GDC scaffold contribute to the superior stability under both fuel cell and electrolysis operation conditions at 750 °C and a high peak power density of 1.58 W cm-2 at 750 °C. This work highlights a novel and facile route to in situ construct a stable and high-performing nanostructured electrode for SOCs.

Perovskite for Electrocatalytic Oxygen Evolution at Elevated Temperatures.

The development of advanced electrolysis technologies such as anion exchange membrane water electrolyzer (AEMWE) is central to the vision of a sustainable energy future. Key to the realization of such AEMWE technology lies in the exploration of low-cost and high-efficient catalysts for facilitating the anodic oxygen evolution reaction (OER). Despite tremendous efforts in the fundamental research, most of today's OER works are conducted under room temperature, which deviates significantly with AEMWE's operating temperature (50-80 °C). To bridge this gap, it is highly desirable to obtain insights into the OER catalytic behavior at elevated temperatures. Herein, using the well-known perovskite catalyst Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) as a proof of concept, the effect of temperature on the variation in OER catalytic activity and stability is evaluated. It is found that the BSCF's activity increases with increasing temperature due to enhanced lattice oxygen participation promoting the lattice oxygen-mediated OER process. Further, surface amorphization and cation leaching of BSCF become more pronounced as temperature increases, causing a somewhat attenuated OER stability. These new understandings of the fundamental OER catalysis over perovskite materials at industrial-relevant temperature conditions are expected to have strong implications for the research of OER catalysts to be deployed in practical water electrolyzers.

Enhanced charge-carrier dynamics and efficient solar-to-urea conversion on Si-based photocathodes.

Photoelectrochemical (PEC) coupling of CO2 and nitrate can provide a useful and green source of urea, but the process is affected by the photocathodes with poor charge-carrier dynamics and low conversion efficiency. Here, a NiFe diatomic catalysts/TiO2 layer/nanostructured n+p-Si photocathode is rationally designed, achieving a good charge-separation efficiency of 78.8% and charge-injection efficiency of 56.9% in the process of PEC urea synthesis. Compared with the electrocatalytic urea synthesis by using the same catalysts, the Si-based photocathode shows a similar urea yield rate (81.1 mg·h-1·cm-2) with a higher faradic efficiency (24.2%, almost twice than the electrocatalysis) at a lower applied potential under 1 sun illumination, meaning that a lower energy-consumption method acquires more aimed productions. Integrating the PEC measurements and characterization results, the synergistic effect of hierarchical structure is the dominating factor for enhancing the charge-carrier separation, transfer, and injection by the matched band structure and favorable electron-migration channels. This work provides a direct and efficient route of solar-to-urea conversion.

Electrochemically Assisted Construction of a La2NiO4+δ@Pt Core-Shell Structure for Enhancing the Performance and Durability of La2NiO4+δ Cathodes.

Ruddlesden-Popper oxide La2NiO4+δ (LNO) has a high ionic conductivity and good thermal match with the electrolyte of solid oxide fuel cells (SOFCs); however, LNO suffers from performance decay owing to the La surface segregation under the operation conditions of SOFCs. Herein, we report an in situ electrochemical decoration strategy to improve the electrocatalytic activity and durability of LNO cathodes. We show that the electrochemical polarization leads to in situ construction of the LNO@Pt core-shell structure, significantly suppressing the detrimental effect of La surface segregation on the LNO cathode. The initial peak power density of a single cell with the LNO cathode is 0.71 W cm-2 at 750 °C, increasing to 1.39 W cm-2 by the in situ construction of the LNO@Pt core-shell structure after polarization at 0.5 A cm-2 for 20 h. The LNO@Pt core-shell structure is also highly durable without noticeable performance degradation over the duration of the test for 180 h. The findings shed light on the design and fabrication of highly active and durable LNO-based cathodes for SOFCs.

Facile Construction of Nanostructured Cermet Anodes with Strong Metal-Oxide Interaction for Efficient and Durable Solid Oxide Fuel Cells.

Nanostructured anodes generate massive reaction sites to oxidize fuels in solid oxide fuel cells (SOFCs); however, the nonexistence of a practically viable approach for the construction of nanostructures and the retention of these nanostructures under the harsh operating conditions of SOFCs poses a significant challenge. Herein, a simple procedure is reported for the construction of a nanostructured Ni-Gd-doped CeO2 anode based on the direct assembly of pre-formed nanocomposite powder with strong metal-oxide interaction. The directly assembled anode forms heterointerfaces with the electrolyte owing to the electrochemical polarization current and exhibits excellent structural robustness against thermal ripening. An electrolyte-supported cell with the directly assembled anode produces a peak power density of 0.73 W cm-2 at 800 °C, while maintaining stability for 100 h, which is in contrast to the drastic degradation of the cermet anode prepared using the conventional method. These findings provide clarity on the design and construction of durable nanostructured anodes and other electrodes for SOFCs.

Electrocatalytic Urea Synthesis with 63.5 % Faradaic Efficiency and 100 % N-Selectivity via One-step C-N coupling.

Electrocatalytic urea synthesis via coupling N2 and CO2 provides an effective route to mitigate energy crisis and close carbon footprint. However, the difficulty on breaking N≡N is the main reason that caused low efficiencies for both electrocatalytic NH3 and urea synthesis, which is the bottleneck restricting their industrial applications. Herein, a new mechanism to overcome the inert of the nitrogen molecule was proposed by elongating N≡N instead of breaking N≡N to realize one-step C-N coupling in the process for urea production. We constructed a Zn-Mn diatomic catalyst with axial chloride coordination, Zn-Mn sites display high tolerance to CO poisoning and the Faradaic efficiency can even be increased to 63.5 %, which is the highest value that has ever been reported. More importantly, negligible N≡N bond breakage effectively avoids the generation of ammonia as intermediates, therefore, the N-selectivity in the co-electrocatalytic system reaches100 % for urea synthesis. The previous cognition that electrocatalysts for urea synthesis must possess ammonia synthesis activity has been broken. Isotope-labelled measurements and Operando synchrotron-radiation Fourier transform infrared spectroscopy validate that activation of N-N triple bond and nitrogen fixation activity arise from the one-step C-N coupling process of CO species with adsorbed N2 molecules.

Promoted hydrogen and acetaldehyde production from alcohol dehydrogenation enabled by electrochemical hydrogen pumps.

The dehydrogenation reaction of bioderived ethanol is of particular interest for the synthesis of fuels and value-added chemicals. However, this reaction historically suffered from high energy consumption (>260 °C or >0.8 V) and low efficiency. Herein, the efficient conversion of alcohol to hydrogen and aldehyde is achieved by integrating the thermal dehydrogenation reaction with electrochemical hydrogen transfer at low temperature (120 °C) and low voltage (0.06 V), utilizing a bifunctional catalyst (Ru/C) with both thermal-catalytic and electrocatalytic activities. Specifically, the coupled electrochemical hydrogen separation procedure can serve as electrochemical hydrogen pumps, which effectively promote the equilibrium of ethanol dehydrogenation toward hydrogen and acetaldehyde production and simultaneously purifies hydrogen at the cathode. By utilizing this strategy, we achieved boosted hydrogen and acetaldehyde yields of 1,020 mmol g-1 h-1 and 1,185 mmol g-1 h-1, respectively, which are threefold higher than the exclusive ethanol thermal dehydrogenation. This work opens up a prospective route for the high-efficiency production of hydrogen and acetaldehyde via coupled thermal-electrocatalysis.

Selective Cocatalyst Decoration of Narrow-Bandgap Broken-Gap Heterojunction for Directional Charge Transfer and High Photocatalytic Properties.

Narrow-bandgap semiconductors are promising photocatalysts facing the challenges of low photoredox potentials and high carrier recombination. Here, a broken-gap heterojunction Bi/Bi2 S3 /Bi/MnO2 /MnOx , composed of narrow-bandgap semiconductors, is selectively decorated by Bi, MnOx nanodots (NDs) to achieve robust photoredox ability. The Bi NDs insertion at the Bi2 S3 /MnO2 interface induces a vertical carrier migration to realize sufficient photoredox potentials to produce O2 •- and • OH active species. The surface decoration of Bi2 S3 /Bi/MnO2 by Bi and MnOx cocatalysts drives electrons and holes in opposite directions for optimal photogenerated charge separation. The selective cocatalysts decoration realizes synergistic surface and bulk phase carrier separation. Density functional theory (DFT) calculation suggests that Bi and MnOx NDs act as active sites enhancing the absorption and reactants activation. The decorated broken-gap heterojunction demonstrates excellent performance for full-light driving organic pollution degradation with great commercial application potential.

Compression Stress-Induced Internal Magnetic Field in Bulky TiO2 Photoanodes for Enhancing Charge-Carrier Dynamics.

Enhancing charge-carrier dynamics is imperative to achieve efficient photoelectrodes for practical photoelectrochemical devices. However, a convincing explanation and answer for the important question which has thus far been absent relates to the precise mechanism of charge-carrier generation by solar light in photoelectrodes. Herein, to exclude the interference of complex multi-components and nanostructuring, we fabricate bulky TiO2 photoanodes through physical vapor deposition. Integrating photoelectrochemical measurements and in situ characterizations, the photoinduced holes and electrons are transiently stored and promptly transported around the oxygen-bridge bonds and 5-coordinated Ti atoms to form polarons on the boundaries of TiO2 grains, respectively. Most importantly, we also find that compressive stress-induced internal magnetic field can drastically enhance the charge-carrier dynamics for the TiO2 photoanode, including directional separation and transport of charge carriers and an increase of surface polarons. As a result, bulky TiO2 photoanode with high compressive stress displays a high charge-separation efficiency and an excellent charge-injection efficiency, leading to 2 orders of magnitude higher photocurrent than that produced by a classic TiO2 photoanode. This work not only provides a fundamental understanding of the charge-carrier dynamics of the photoelectrodes but also provides a new paradigm for designing efficient photoelectrodes and controlling the dynamics of charge carriers.

Photoelectrochemical N2 -to-NH3 Fixation with High Efficiency and Rates via Optimized Si-Based System at Positive Potential versus Li0/.

As a widely used commodity chemical, ammonia is critical for producing nitrogen-containing fertilizers and serving as the promising zero-carbon energy carrier. Photoelectrochemical nitrogen reduction reaction (PEC NRR) can provide a solar-powered green and sustainable route for synthesis of ammonia (NH3 ). Herein, an optimum PEC system is reported with an Si-based hierarchically-structured PdCu/TiO2 /Si photocathode and well-thought-out trifluoroethanol as the proton source for lithium-mediated PEC NRR, achieving a record high NH3 yield of 43.09 µg cm-2 h-1 and an excellent faradaic efficiency of 46.15% under 0.12 MPa O2 and 3.88 MPa N2 at 0.07 V versus lithium(0/+) redox couple (vs Li0/+ ). PEC measurements coupled with operando characterization reveal that the PdCu/TiO2 /Si photocathode under N2 pressures facilitate the reduction of N2 to form lithium nitride (Li3 N), which reacts with active protons to produce NH3 while releasing the Li+ to reinitiate the cycle of the PEC NRR. The Li-mediated PEC NRR process is further enhanced by introducing small amount of O2 or CO2 under pressure by accelerating the decomposition of Li3 N. For the first time, this work provides mechanistic understanding of the lithium-mediated PEC NRR process and opens new avenues for efficient solar-powered green conversion of N2 -to-NH3 .

Balancing Mass Transfer and Active Sites to Improve Electrocatalytic Oxygen Reduction by B,N Codoped C Nanoreactors.

Mass transfer is critical in catalytic processes, especially when the reactions are facilitated by nanostructured catalysts. Strong efforts have been devoted to improving the efficacy and quantity of active sites, but often, mass transfer has not been well studied. Herein, we demonstrate the importance of mass transfer in the electrocatalytic oxygen reduction reaction (ORR) by tailoring the pore sizes. Using a confined-etching strategy, we fabricate boron- and nitrogen-doped carbon (B,N@C) electrocatalysts featuring abundant active sites but different porous structures. The ORR performance of these catalysts is found to correlate with diffusion of the reactant. The optimized B,N@C with trimodal-porous structures feature enhanced O2 diffusion and better activity per heteroatomic site toward the ORR process. This work demonstrates the significance of the nanoarchitecture engineering of catalysts and sheds light on how to optimize structures featuring abundant active sites and enhanced mass transfer.

Facile Approach for Improving the Interfacial Adhesion of Nanofiber Air Electrodes of Reversible Solid Oxide Cells.

Nanofibers have great promise as a highly active air electrode for reversible solid oxide cells (ReSOCs); however, one thorny issue is how to adhesively stick nanofibers to electrolyte with no damage to the original morphology. Herein, PrBa0.8Ca0.2Co2O5+δ (PBCC) nanofibers are applied as an air electrode by a facile direct assembly approach that leads to the retention of most of the unique microstructure of nanofibers, and firm adhesion of the nanofiber electrode onto the electrolyte is achieved by applying electrochemical polarization. A single cell with the PBCC nanofiber air electrode exhibits excellent maximum power density (1.97 W cm-2), electrolysis performance (1.3 A cm-2 at 1.3 V), and operating stability at 750 °C for 200 h. These findings provide a facile means for the utilization of nanofiber electrodes for high-performance and durable ReSOCs.

Ultrafine, Dual-Phase, Cation-Deficient PrBa0.8Ca0.2Co2O5+δ Air Electrode for Efficient Solid Oxide Cells.

Nanostructured air electrodes play a crucial role in improving the electrocatalytic activity of oxygen reduction and evolution reactions in solid oxide cells (SOCs). Herein, we report the fabrication of a nanostructured BaCoO3-decorated cation-deficient PrBa0.8Ca0.2Co2O5+δ (PBCC) air electrode via a combined modification and direct assembly approach. The modification approach endows the dual-phase air electrode with a large surface area and abundant oxygen vacancies. An intimate air electrode-electrolyte interface is in situ constructed with the formation of a catalytically active Co3O4 bridging layer via electrochemical polarization. The corresponding single cell exhibits a peak power density of 2.08 W cm-2, an electrolysis current density of 1.36 A cm-2 at 1.3 V, and a good operating stability at 750 °C for 100 h. This study provides insights into the rational design and facile utilization of an active and stable nanostructured air electrode of SOCs.

A critical review of the nano-structured electrodes of solid oxide cells.

Renewable energies from solar and wind power are playing an ever increasing role in meeting the tremendous global energy demand with substantially reduced carbon emissions; however, their intermittent nature poses a critical challenge for sustainability and practical applications. The capability of solid oxide cells (SOCs) to operate in both fuel cell and electrolysis modes makes them one of the most important candidates for efficient storage and regeneration of renewable energies. Here we present a critical review and prospects on the development of key fabrication techniques, e.g. wet infiltration, exsolution and direct assembly, for nanostructured electrodes, one of the most critical issues affecting the performance and stability of SOC technologies. The traits and challenges of these techniques are thoroughly discussed, with an aim to provide a critical guide for future design and development of more refined, high-performing nanostructured electrodes for SOCs.

Identifying and tailoring C-N coupling site for efficient urea synthesis over diatomic Fe-Ni catalyst.

Electrocatalytic urea synthesis emerged as the promising alternative of Haber-Bosch process and industrial urea synthetic protocol. Here, we report that a diatomic catalyst with bonded Fe-Ni pairs can significantly improve the efficiency of electrochemical urea synthesis. Compared with isolated diatomic and single-atom catalysts, the bonded Fe-Ni pairs act as the efficient sites for coordinated adsorption and activation of multiple reactants, enhancing the crucial C-N coupling thermodynamically and kinetically. The performance for urea synthesis up to an order of magnitude higher than those of single-atom and isolated diatomic electrocatalysts, a high urea yield rate of 20.2 mmol h-1 g-1 with corresponding Faradaic efficiency of 17.8% has been successfully achieved. A total Faradaic efficiency of about 100% for the formation of value-added urea, CO, and NH3 was realized. This work presents an insight into synergistic catalysis towards sustainable urea synthesis via identifying and tailoring the atomic site configurations.

New Undisputed Evidence and Strategy for Enhanced Lattice-Oxygen Participation of Perovskite Electrocatalyst through Cation Deficiency Manipulation.

Oxygen evolution reaction (OER) is a key half-reaction in many electrochemical transformations, and efficient electrocatalysts are critical to improve its kinetics which is typically sluggish due to its multielectron-transfer nature. Perovskite oxides are a popular category of OER catalysts, while their activity remains insufficient under the conventional adsorbate evolution reaction scheme where scaling relations limit activity enhancement. The lattice oxygen-mediated mechanism (LOM) has been recently reported to overcome such scaling relations and boost the OER catalysis over several doped perovskite catalysts. However, direct evidence supporting the LOM participation is still very little because the doping strategy applied would introduce additional active sites that may mask the real reaction mechanism. Herein, a dopant-free, cation deficiency manipulation strategy to tailor the bulk diffusion properties of perovskites without affecting their surface properties is reported, providing a perfect platform for studying the contribution of LOM to OER catalysis. Further optimizing the A-site deficiency achieves a perovskite candidate with excellent intrinsic OER activity, which also demonstrates outstanding performance in rechargeable Zn-air batteries and water electrolyzers. These findings not only corroborate the key role of LOM in OER electrocatalysis, but also provide an effective way for the rational design of better catalyst materials for clean energy technologies.

Boosting Electrocatalytic Activity of Single Atom Catalysts Supported on Nitrogen-Doped Carbon through N Coordination Environment Engineering.

Nonprecious group metal (NPGM)-based single atom catalysts (SACs) hold a great potential in electrocatalysis and dopant engineering has been extensively exploited to boost their catalytic activity, while the coordination environment of dopant, which also significantly affects the electronic structure of SACs, and consequently their electrocatalytic performance, have been largely ignored. Here, by adopting a precursor modulation strategy, the authors successfully synthesize single cobalt atom catalysts embedded in nitrogen-doped carbon, Co-N/C, with similar overall Co and N concentrations but different N types, that is, pyridinic N (NP ), graphitic N (NG ), and pyrrolic N (NPY ). Co-N/C with the Co-N4 moieties coordinated with NG displays far superior activity for oxygen reduction (ORR) and evolution reactions, and superior activity and stability in both zinc-air batteries and proton exchange membrane fuel cells. Density functional theory calculation indicates that coordinated N species in particular NG functions as electron donors to the Co core of Co-N4 active sites, leading to the downshift of d-band center of Co-N4 and weakening the binding energies of the intermediates on Co-N4 sites, thus, significantly promoting catalytic kinetics and thermodynamics for ORR in a full pH range condition.

A New Durable Surface Nanoparticles-Modified Perovskite Cathode for Protonic Ceramic Fuel Cells from Selective Cation Exsolution under Oxidizing Atmosphere.

A high-performance cathode of a protonic ceramic fuel cell (PCFC) should possess excellent oxygen reduction reactivity, high proton/oxygen-ion/electron conductivity, and sufficient operational stability, thus requiring a delicate tuning of both the bulk and surface properties of the electrode material. Although surface modification of perovskites with nanoparticles from reducing-atmosphere exsolution has been demonstrated effective at improving the electrochemical anodic oxidation, such nanoparticles would easily re-incorporate into the perovskite lattice causing a big challenge for their application as a cathode. Here, a durable perovskite-based nanocomposite cathode for PCFCs is reported, which is facilely prepared via the exsolution of nanoparticles in an oxidizing atmosphere. Through composition and cation nonstoichiometry manipulation, a precursor with the nominal composition of Ba0.95 (Co0.4 Fe0.4 Zr0.1 Y0.1 )0.95 Ni0.05 O3-δ (BCFZYN-095) is designed, synthesized, and investigated, which, upon calcination, gives rise to the formation of a perovskite-based nanocomposite comprising a major perovskite phase and a minor NiO phase enriched on the perovskite surface. The major perovskite phase enabled by the proper cation nonstoichiometry manipulation promotes bulk proton conduction while the NiO nanoparticles facilitate the oxygen surface exchange process, leading to a superior cathodic performance with a maximum peak power density of 1040 mW cm-2 at 650 °C and excellent operational stability of 400 h at 550 °C.

Toward an Understanding of the Reversible Li-CO2 Batteries over Metal-N4-Functionalized Graphene Electrocatalysts.

The lack of low-cost catalysts with high activity leads to the unsatisfactory electrochemical performance of Li-CO2 batteries. Single-atom catalysts (SACs) with metal-Nx moieties have great potential to improve battery reaction kinetics and cycling ability. However, how to rationally select and develop highly efficient electrocatalysts remains unclear. Herein, we used density functional theory (DFT) calculations to screen SACs on N-doped graphene (SAMe@NG, Me = Cr, Mn, Fe, Co, Ni, Cu) for CO2 reduction and evolution reaction. Among them, SACr@NG shows the promising potential as an effective electrocatalyst for the reversible Li-CO2 batteries. To verify the validity of the DFT calculations, a two-step method has been developed to fabricate SAMe@NG on a porous carbon foam (SAMe@NG/PCF) with similar loading of ∼8 wt %. Consistent with the theoretical calculations, batteries with the SACr@NG/PCF cathodes exhibit a superior rate performance and cycling ability, with a long cycle life and a narrow voltage gap of 1.39 V over 350 cycles at a rate of 100 µA cm-2. This work not only demonstrates a principle for catalysts selection for the reversible Li-CO2 batteries but also a controllable synthesis method for single atom catalysts.