Applications In Energy Conversion Devices Research Articles

In situ Activation of Molecular Oxygen at Intermetallic Spacing-Optimized Iron Network-Like Sites for Boosting Electrocatalytic Oxygen Reduction.

The oxygen reduction reaction (ORR) catalyzed by transition-metal single-atom catalysts (SACs) is promising for practical applications in energy-conversion devices, but great challenges still remain due to the sluggish kinetics of O═O cleavage. Herein, a kind of high-density iron network-like sites catalysts are constructed with optimized intermetallic distances on an amino-functionalized carbon matrix (Fe-HDNSs). Quasi-in situ soft X-ray absorption spectroscopy and in situ synchrotron infrared characterizations demonstrate that the optimized intermetallic distances in Fe-HDNSs can in situ activate the molecular oxygen by fast electron compensation through the hybridized Fe 3d‒O 2p, which efficiently facilitates the cleavage of the O═O bond to *O species and highly suppresses the side reactions for an accelerated kinetics of the 4e- ORR. As a result, the well-designed Fe-HDNSs catalysts exhibit superior performances with a half-wave potential of 0.89V versus reversible hydrogen electrode (RHE)and a kinetic current density of 72mAcm-2@0.80V versus RHE, exceeding most of the noble-metal-free ORR catalysts. This work offers some new insights into the understanding of 4e- ORR kinetics and reaction pathways to boost electrochemical performances of SACs.

Tailoring hierarchically porous nanoarchitectured N-doped carbon decorated with FeIIN4 moiety and encapsulated Fe/Fe3C nanoparticles as a synergistic catalyst for ORR in Zn-air battery

Rational designing and exploiting non-noble metal electrocatalysts with desirable nanoarchitecture and abundant active sites are crucial but challenging requirements for the development of fuel cells and Zn-air batteries (ZABs). Herein, a highly efficient carbonous hybrid is developed using a facile and effective synthesis strategy via template-assisted and optimized post-pyrolysis processes. The optimized electrocatalyst (denoted as Fe@NC-700) has extremely large specific surface area (1262.8 m2 g–1), and presents a desirable hierarchically porous nanoarchitecture, including plenty micro-/meso-pores, which endows the catalyst with the enhanced mass transfer for the reaction species. More importantly, Fe@NC-700 possesses not only numerous FeIIN4 moiety uniformly dispersed in N-doped carbon matrix, but also encapsulated Fe/Fe3C nanoparticles, synergistically boosting the electrocatalytic activity towards oxygen reduction reaction (ORR) verified by density functional theory (DFT) calculation. Due to the beneficial microstructure and rich active sites, the catalyst has a positive half-wave potential (E1/2) of 0.865 V for ORR in alkaline electrolyte, intrinsic 4e– reaction path and robust stability (only 14 mV negative shift of E1/2 after 10,000 potential cycles), outperforming the Pt/C. Impressively, the aqueous and quasi-solid-state primary ZABs assembled with Fe@NC-700 as cathode catalyst demonstrate superior discharge performance and excellent durability, holding promising potential in practical application of energy conversion devices.

Unlocking the potential of proton conductivity in guanidinium-based ionic covalent organic nanosheets (iCONs) through pore interior functionalization

Recently, scientists have been exploring the incorporation of proton carriers such as water and phosphoric acid (PA) into the pores and channels of porous materials to enhance proton conduction performance. Ionic covalent organic nanosheets (iCONs) have been identified as promising functional materials due to their inbuilt ionic interfaces, which can facilitate strong interaction with counter ions present inside the pore structure and thus shorten ion transport pathways. However, there is a lack of research related to proton conductivity in iCONs loaded with PA. To address this, we prepared three functionalized guanidinium-based iCONs using a solvothermal condensation reaction between guanidinium amine (TG) and functionalized terephthaldehyde (Da, Dha, and Dma). PA was also incorporated into the iCON structure via ex situ loading to observe its effects on proton conduction performance. The results showed that both the iCONs and PA–iCONs were highly stable in water, organic solvents, acidic and basic media. Amongst these PA–iCONs, one with hydroxyl‑functionalization (PA–DhaTG) displayed high proton conductivity at 90 °C and 95% relative humidity due to a Grotthuss mechanism for protons. These functionalized guanidinium-based iCONs could prove useful for applications in energy conversion devices.

In-situ gas foaming synthesis of N, S-rich co-doped hierarchically ordered porous carbon as an efficient oxygen reduction reaction catalyst

The design and manufacture of cost-effective and efficient oxygen reduction reaction (ORR) catalysts is critical to the widespread application of multiple energy conversion devices. Herein, a combination of in-situ gas foaming and the hard template method is proposed to construct the N, S-rich co-doped hierarchically ordered porous carbon (NSHOPC) as an effective metal-free electrocatalyst for ORR via carbonizing a mixture of polyallyl thiourea (PATU) and thiourea in silica colloidal crystal template (SiO2-CCT) voids. Benefiting from the hierarchically ordered porous (HOP) architectures and the mass doping of N and S, NSHOPC displays excellent ORR activities (the half-wave potential of 0.889 V in 0.1 M KOH and 0.786 V in 0.5 M H2SO4) and long-term stability, which are all better than those of Pt/C. As the air cathode in a Zn-air battery (ZAB), NSHOPC exhibits a high peak power density of 174.6 mW·cm−2 and long-term discharge stability. The remarkable performance of the as-synthesized NSHOPC signifies broad prospects for actual applications in energy conversion devices.

Quantitative estimation of coercive field in a ferromagnetic grain using field sweep simulation

High coercivity is an important property of permanent magnets for application in energy conversion devices. The Nd magnet, ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$, is a typical material. Because coercivity is a long-time relaxation phenomenon, which originates from a strong metastable magnetic state, it is difficult to estimate coercive field (coercive force) studying the time evolution dynamics simulation of a model with atomistic parameters under the limitation of the simulation time. In our recent study [M. Nishino et al., Phys. Rev. B 102, 020413(R) (2020)], we presented a method to estimate coercivity using a statistical method to extend the limitation of simulation time and evaluated appropriately the coercive field of a single grain of the Nd magnet. In the present study, we propose an alternative method to estimate coercivity more conveniently using the field-dependent survival (nonreversal) probability generated by a time evolution simulation under a field sweep. We demonstrate that the coercive field of the single grain can be estimated. In this method, not only coercive field but also the zero-field energy barrier and field for the zero-energy barrier can be estimated. We discuss detailed features of the estimation of these quantities.

Iron phthalocyanine decorated porous biomass-derived carbon as highly effective electrocatalyst for oxygen reduction reaction

Searching for a low-cost, high-activity and stable nonprecious metal electrocatalyst for the oxygen reduction reaction (ORR) is critical to the application of energy conversion devices, such as zinc-air batteries and proton exchange membrane fuel cells. Herein, renewable and inexpensive black locust leaves were used as carbon and nitrogen precursors to prepare porous biomass-derived carbon (PBC) using K2FeO4 as the activator and then decorated by iron phthalocyanine (FePc) with a free-pyrolysis process to obtain the electrocatalyst FePcx/PBC. Notably, the as-obtained PBC possesses a large specific surface area and hierarchically porous structure, facilitating the exposure of active sites and diffusion of O2 as well as the electrolytes during the ORR process. Compared with commercial Pt/C, the half-wave potential (E1/2 =0.91 V) and limiting current density (JL=5.03 mA·cm−2) of FePc0.5/PBC positively shifted nearly 70 mV and increased 0.44 mA·cm−2, respectively, and an obvious enhancement in stability and tolerance to methanol can be observed. Moreover, the assembled zinc-air battery using FePc0.5/PBC as the air-cathode catalyst exhibits an open-circuit voltage of 1.452 V and a specific capacity of 812.1 mAh·g−1, much higher than those of Pt/C (1.435 V, 749.4 mAh·g−1). This work develops a feasible, convenient and economic strategy to fabricate effective Pt-free ORR electrocatalysts for relevant energy conversion devices.

Synthesis of graphene anchored with atomically isolated cobalt from a promising graphite-like supramolecule.

The application of heteroatom-doped graphene for photochemical and electrochemical reactions is primarily hindered by the lack of a controllable and facile synthesis strategy. In this work, few-layer CoN-graphene (1.8 nm thickness) with atomic Co has been fabricated via pyrolysis exfoliation. The half wave potential of CoN-graphene reaches 0.875 V vs. RHE, and the corresponding direct methanol fuel cell performance is 100% (higher than that of the commercial Pt/C catalyst), demonstrating potential for practical application in energy conversion devices.

Layered and hexagonal perovskites as novel classes of proton-conducting solid electrolytes. A focus review

Solid oxide electrolytes have attracted significant attention due to their possible applications in energy conversion devices, including solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs). Although a large amount of data has been accumulated to date, the design of new representatives of ionic electrolytes is of unquenchable interest. In this paper, a review of the new classes of proton-conducting solid electrolytes is provided. The physicochemical and transport properties of layered perovskites (BaNdInO4, BaNdScO4, SrLaInO4, BaLaInO4) and hexagonal perovskites (Ba7Nb4MoO20, Ba5Er2Al2ZrO13 and Ba5In2Al2ZrO13) were analyzed and summarized. Based on the performed analysis, the most promising compositions among the considered phases were identified and the effective approaches aimed at improving their functional characteristics were provided.

Effectiveness of a dual surface modification of metallic interconnects for application in energy conversion devices

A dual surface modification of an SOFC metallic interconnect with a Gd2O3 layer and an MnCo2O4 coating was evaluated. The tested samples were oxidized for 7000 h in air at 1073 K. Oxidation products were characterized using XRD, SEM-EDS, and confocal Raman imaging, and ASR was measured. The effect of gadolinium segregation at grain boundaries in Cr2O3 was evaluated using S/TEM-EDS. Area specific-resistance was measured and fuel cell tests investigating electrochemical performance and Cr contamination of electrodes were also performed. The results show that the proposed dual modification was more advantageous than either modification applied separately. The fuel cell tests performed after aging in humidified hydrogen at 1073 K and involving an actual interconnect made of this dual-modified material showed that after 250 h of aging its electrochemical parameters were nearly identical to those of the non-aged reference electrode. Moreover, the modification protected the electrodes from Cr poisoning.

Electrochemical Stability and Degradation of Commercial Pd/C Catalyst in Acidic Media

Palladium has attracted significant attention as a catalyst or co-catalyst for many electrochemical reactions in energy conversion devices. We have studied electrochemical stability of a commercial Pd/C sample in an acidic electrolyte by exposing it to an accelerated stress test (AST) to mimic potential spikes in fuel cells and electrolyzers during start/stop events. AST consisted of extensive rapid potential cycling (5000 cycles, 1 V/s) in two potential regions, namely AST1 was performed between 0.4 and 1.4 VRHE, while AST2 was performed between 0.05 and 1.4 VRHE. Degradation of Pd/C was monitored by the changes in Pd electrochemical surface area, while the hydrogen evolution reaction (HER) was used as a test reaction to observe the corresponding impact of the degradation on the activity of Pd/C. Significant Pd/C degradation and HER activity loss were observed in both potential regions. Coupling of the electrochemical flow cell with an inductively coupled plasma mass spectrometry device showed substantial Pd dissolution during both ASTs. Identical location scanning electron microscopy revealed that Pd dissolution is followed by redeposition during both ASTs, resulting in particle size growth. Particle size growth was seen as especially dramatic in the case of AST2, when particularly large Pd nanostructures were obtained on top of the catalyst layer. According to the results presented in this work, (in)stability of Pd/C and other Pd-based nanocatalysts should be studied systematically as it may present a key factor limiting their application in energy conversion devices.

ZIF-67-derived N-doped double layer carbon cage as efficient catalyst for oxygen reduction reaction

The slow kinetic of oxygen reduction reaction (ORR) hampers the practical application of energy conversion devices, such as fuel cells, metal-air batteries. Here, an efficient ORR electrocatalyst consists of Co, Ni co-decorated nitrogen-doped double shell hollow carbon cage (Ni–Co@NHC) was fabricated by pyrolyzing Ni-doped polydopamine wrapped ZIF-67. During the preparation, polydopamine served as a protective layer can effectively prevent the aggregation of Co and Ni nanoparticles during the pyrolysis process, and at the same time forming a carbon layer to grow a double layer carbon cage. This unique hollow structure endows the catalyst with a high specific surface area as well as more exposed active sites. Also benefited from the synergistic effect between Ni and Co nanoparticles, the Ni–Co@NHC catalyst leads to an outstanding ORR performance of half-wave potential (E 1/2, 0.862 V), outperforms that of commercial Pt/C catalyst. Additionally, when Ni–Co@NHC was used in the cathode for the zinc-air battery, the cell exhibits high power density (108 mW cm−2) and high specific capacity (806 mAh g−1) at 20 mA cm−2 outperforming Pt/C. This work offers a promising design strategy for the development of high-performance ORR electrocatalysts.

Alloying strategy for constructing multi-component nano-catalysts towards efficient and durable oxygen evolution in alkaline electrolyte

Exploring high-activity and durable 3d transition metal electrocatalyst for oxygen evolution reaction (OER) is one of the key issues in the water splitting reaction. Herein, a series of copper-based alloy (CuM, M = Fe, Co, Ni; FeCo, FeNi, CoNi; FeCoNi) nanoparticles have been synthesized by a rapid microwave-assisted solution phase method. Electrochemical measurements show that the catalysts possess outstanding OER activity as well as excellent long-term durability in alkaline electrolyte. The optimized samples of FeCoCu and FeCoNiCu exhibit superior activity with low overpotentials (265 and 269 mV at 10 mA cm-2), small Tafel slopes (49 and 48.9 mV dec-1), and low loss of activity after a 24 h stability test, which are much better than that of commercial RuO2. The significant OER activity and stability are mainly attributed to the synergistic effect of multiple metal elements. The alloying strategy promotes the combination of the highly active iron, cobalt and nickel elements with the excellent conductive copper element, which not only changes the electronic structure of the material surface, but also accelerates the electron transfer and provides more active sites, thereby improving the electrocatalytic performance of the material. This work demonstrates an efficient and rapid method to prepare low cost electrocatalysts, which is important for the applications in energy conversion devices.

Al, Fe-codoped CoP nanoparticles anchored on reduced graphene oxide as bifunctional catalysts to enhance overall water splitting

Rational design of high-efficiency and low-cost catalysts for water splitting electrolysis plays an important role in clean energy conversion technology. Herein, density functional theory simulations indicate that the codoping of p-metal Al and d-metal Fe could reasonably regulate the electronic structure of CoP, resulting in an optimal H adsorption free energy. Based on this finding, we successfully prepared a bifunctional catalyst of Al, Fe-codoped CoP nanoparticles/reduced graphene oxide, which displays superior HER performance in a wide pH range and OER performance in the alkaline medium with high H2/O2 evolution rates and Faradaic efficiencies. For overall water splitting, such a catalyst exhibits excellent activity (a low cell voltage of 1.66 V at 10 mA cm−2) and durability. The strategy of synergistic modulation of electronic structure by introducing different-type dopants into a hybrid proposed in this work is beneficial for designing novel electrocatalysts for applications in energy-conversion devices.

Hydrogel Electrolytes for Quasi-Solid Zinc-Based Batteries.

On account of high energy density depending on the utilized zinc metal anode of high theoretical capacity and its excellent security due to aqueous electrolytes that usually be locked in polymer hosts referred to as hydrogels, quasi-solid zinc-based batteries have been subjected to more and more interest from researchers. The good water retention and electrolyte load capacity of the hydrogel, contributing to the acquirement of high ionic conductivity and durability of the as-obtained quasi-solid electrolyte, play a significant role on the performance of the devices. Moreover, the chemistry of hydrogels can be tuned to endow quasi-solid electrolytes with additional functions in terms of application scenarios of solid-state batteries. Herein, the frontier disciplines of hydrogel electrolytes for Zn-based batteries were reviewed. The cross-linking process of the polymer networks for hydrogel materials with different functions, such as stretchability, compressibility, and self-healing, were also discussed to analyze the properties of the polymer electrolyte. Based on the merits of the functionalized hydrogel, the further application of hydrogel electrolytes in Zn-based batteries is the focus of this paper. The electrochemical performance and mechanical property of Zn-based batteries with functionalized hydrogel electrolytes under extreme conditions were presented to evaluate the crucial role of the polymer hydrogel electrolyte. Finally, the challenges of hydrogel electrolytes for currently developed Zn-based batteries are highlighted with the hope to boost their commercial application in energy conversion devices.

Nickel decorated bimetallic catalysts derived from metal-organic frameworks as cathode materials for rechargeable Zinc-Air batteries

The fabrication of efficient catalysts for the oxygen evolution reactions (OER) that are required to accelerate the sluggish kinetics of metal-air batteries has attracted significant attention for application in energy conversion devices. Metal-organic frameworks (MOFs) have been recognized as potential candidates for the formation of promising catalysts. In this study, we report the formation of Ni/CNT/CoNi using bimetallic-ZIF (CoNi-ZIF) as a precursor. Bimetallic CoNi encapsulated in carbon nanotubes (CNTs) was obtained by calcinating CoNi-ZIF-67 to yield CNT/CoNi. The resulting CNT/CoNi was highly porous with a significant specific surface area of 242 m2/g. After calcination, Ni deposition on the CNT was performed and the surface area was reduced to 168.7 m2/g. Ni/CNT/CoNi was applied as a cathode material in zinc-air batteries, exhibiting excellent electrochemical performance and cycling stability.

Mechanical and acoustic properties of a hybrid organic–inorganic perovskite, TMCM-CdCl3, with large piezoelectricity

Piezoelectric hybrid organic–inorganic perovskites have attracted significant attention due to their outstanding piezoelectricity comparable to perovskite oxides. However, their elastic properties, which determine the piezoelectric performance, have not yet been explored. Here, the elastic properties of a perovskite ferroelectric TMCM-CdCl3 (TMCM = trimethylchloromethyl ammonium) with a large piezoelectric effect are comprehensively investigated. High-pressure synchrotron powder x-ray diffraction experiments reveal that it exhibits a low bulk modulus of 14.58 GPa, which is an order of magnitude lower than those of perovskite oxides. In addition, Young’s moduli and shear moduli were calculated using density functional theory, and their values are significantly smaller than perovskite piezoceramics. Furthermore, the acoustic properties of TMCM-CdCl3 were calculated. Its longitudinal wave velocity is 3.86 km s−1–4.94 km s−1, and the corresponding specific acoustic impedance is 8.28 MRayl–10.21 MRayl. These maximum values are, respectively, 31.5%, 74.4%, and 8.4%, 73.1% lower than those from BaTiO3 and PbTiO3. Our findings disclose the soft nature and low acoustic impedance of hybrid perovskite piezoelectrics, which are critical to their applications in energy conversion devices.

Mechanistic Investigation into Efficient Water Oxidation by Co-Ni-Based Hybrid Oxide-Hydroxide Flowers.

Oxides are envisioned as promising catalysts to facilitate water oxidation, and the benign presence of hydroxide moieties can further enhance the catalyst performance. However, the nature of synergy between oxides and hydroxides remains elusive. In this study, we have designed a one-pot solution growth technique for the synthesis of flower-shaped N-doped-C-enveloped NiCo2O4/NixCo(1-x)(OH)y catalysts with varying oxide and hydroxide contents and investigated their water oxidation behavior. The correlation between performance-determining parameters involved in water oxidation, such as the onset potential and overpotential with oxide and/or hydroxide content, oxidation states (oxides), and elemental composition (Co/Ni content), and the possible ways to achieve their optimal values are discussed in detail. Our observations conclude that the onset potential and overpotential are minimal for the hybrid oxide-hydroxide bimetallic system compared with pristine hydroxide or oxide. The optimal hybrid catalyst shows excellent current density, low Tafel slope (82 mV/dec), and low onset potential (281 mV at 2 mA/cm2) and overpotential (348 mV at 10 mA/cm2), besides enduring operational stability in alkaline medium. The low Tafel slope suggests the preferable kinetics for water oxidation, and the poisoning study reveals the direct involvement of metal as active sites. The overall study unveils the synergy in the Co-Ni-based binary transition-metal oxide-hydroxide hybrid, which makes it a potential candidate for water oxidation catalysts, and hence, it is expected that the hybrid will find applications in energy conversion devices, such as electrolyzers.

Bimetallic CoNi Alloy Nanoparticles Embedded in Pomegranate-like Nitrogen-Doped Carbon Spheres for Electrocatalytic Oxygen Reduction and Evolution

Electrocatalysts for oxygen reactions play a vital role in the applications of energy conversion devices, such as in fuel cells, water splitting, and metal–air batteries. Developing novel nonprecio...

In situ conversion of iron sulfide (FeS) to iron oxyhydroxide (γ-FeOOH) on N, S co-doped porous carbon nanosheets: An efficient electrocatalyst for the oxygen reduction reaction and zinc–air batteries

For the development of metal-air batteries and fuel cells, highly efficient and inexpensive electrocatalysts are one of the main issues for scalable applications of these advanced energy devices. Herein, based on the in situ conversion of FeS to γ-FeOOH with a two-dimensional (2D) layered structure, a special nanohybrid, N, S co-doped porous carbon nanosheets loaded with γ-FeOOH, is fabricated by carbonization of Fe3+-coordinated polydopamine, which uniformly coats the surface of bacterial cellulose. The nanocomposites exhibit high activity towards the cathodic oxygen reduction reaction (ORR), characterized by a more positive half-wave potential (10 mV) than the commercial Pt/C (20 wt%) electrocatalyst in alkaline medium. Additionally, the nanohybrid has excellent stability and a high methanol tolerance towards the ORR. Further experiments and density functional theory (DFT) calculations demonstrate that the doped N, S in the carbon matrix and the (0 1 0) plane of γ-FeOOH are the ORR active sites. A zinc-air battery equipped with this nanohybrid presents better a power density (92 mW cm−2) and specific capacity (740 mA h g−1) than the Pt/C electrocatalyst, demonstrating its promising potential for application in energy conversion devices.

Simulation study on piezoelectric characteristics of two‐dimensional ZnO nanodiscs

Two-dimensional (2D) zinc oxide (ZnO) nanostructures have emerged as an attractive approach for constructing high-performance piezoelectric nanogenerators (NGs) because of their high rate of surface area to volume and good mechanical durability for applications in energy conversion devices. However, there has been to date no comprehensive study on piezoelectric characteristics of 2D ZnO nanostructures. In this paper, 2D nanodiscs have been prepared by aqueous solution method on flexible indium tin oxide coated polyethylene terephthalate substrates. Also, the piezoelectric characteristics are statistically demonstrated and analysed on the as-grown 2D ZnO nanodiscs by applying different external force with the finite element method (COMSOL Multiphysics). Besides, the size-dependent simulation analysis of piezopotentials is also conducted for further optimising output performance of NGs based on 2D ZnO nanostructures. Also, the simulation will provide a guideline for designing high-performance piezo-nanodevices.