Low Energy Research Articles

A Photovoltaic/Thermal Electrochemical Stripping System for Urine Nitrogen Recovery

Introduction Distributed production of urine-derived fertilizers offers a sustainable and equitable alternative to Haber-Bosch fertilizer. Lab-scale electrochemical stripping (ECS) recovers nitrogen as ammonium sulfate fertilizer from many complex wastewaters, including urine.1 Developing nations in Africa pay higher prices than the global average for Haber-Bosch-based ammonia due to accessibility issues.2 Using solar panels as a power source, ECS can be deployed for low-carbon, on-site ammonia production which can minimize transport, optimize energy, and lower fertilizer prices. This study aimed to (1) understand the ideal ranges of ECS operating parameters (current density and temperature) for efficient nitrogen recovery, and (2) provide proof-of-concept for a photovoltaic/thermal ECS system. Methods Experiments were performed in a three-chamber electrochemical reactor. Electrodialysis was used to remove nitrogen from urine in the anode chamber to the catholyte, while membrane stripping recovered nitrogen from the catholyte as ammonium sulfate in the trap chamber. Solar panels face diminishing efficiency as they heat up.3 On the other hand, membrane stripping becomes more efficient with increased catholyte temperature, increasing the nitrogen recovery rate.4 Thus, experiments varying the temperature of catholyte from 30°C to 60°C were performed to quantify the extent to which heat improves ECS efficiency. Current densities ranging from 5mA/cm2 to 20mA/cm2 were evaluated for their effect on nitrogen recovery rate and energy efficiency to inform the sizing of photovoltaic components. A photovoltaic/thermal ECS prototype was constructed and operated indoors using a solar simulator, and outdoors in real sunlight. Heat was transferred from the solar panel to the catholyte using a cold plate fixed to the backside of the solar panel. Energy production and usage of the solar-ECS system were compared across several conditions: with or without heat transfer, and with or without batteries and charge controllers (Figure 1). Results Higher temperatures led to higher nitrogen recovery rates, with a maximum rate of 5.4 mg N/min at 60°C as compared to 4.2 mg N/min at room temperature. Heat also decreased electrical resistance inside the reactor, resulting in lower energy consumption. However, higher temperatures increased the movement of water vapor from the catholyte to the trap chamber, causing a more dilute ammonia sulfate solution. Additionally, experiments were operated as an open system. As a result, ammonia evaporation out of the reactor was observed as the temperature increased. The primary effect of current density is on ammonia migration from the anode chamber to the cathode chamber (removal). Nitrogen removal occurs from the influent faster with higher current densities at a rate of 0.09 g N per mA/cm2 over 7 hours. However, the system experiences lower current efficiency and lower overall energy efficiencies, using an additional 2.2 kJ/g N per additional mA/cm2. Removal speed and energy efficiency have implications for the cost of ECS. For a single reactor to extract a set amount of nitrogen in a given period, a larger reactor can be built and operated at a low current density for a higher capital cost with lower operating costs, or a smaller reactor can be operated at a high current density for a converse cost scheme. A functional photovoltaic/thermal ECS prototype was built. The simple cold plate attachment effectively lowered the steady state temperature of the solar panel surface by 17°C and raised the temperature of the catholyte by 13°C. Integration of a battery, maximum power point tracker (MPPT), and DC/DC converters allowed for a 53% increase in the production of solar electricity and control over current input to the ECS reactor, although they increased the cost of the system. Using the outputs of this solar-ECS prototype as a benchmark, a techno-economic analysis will be completed to elucidate competitiveness. Conclusion Understanding relationships between electrical energy, heat, and ECS performance informs the scale-up of ECS for distributed ammonia production. Capturing nitrogen for reuse reduces reliance on Haber-Bosch ammonia, prevents aqueous pollution, and supports a circular nitrogen economy. Broad application of this technology would promote equitable access to fertilizer and support local food production. Integration with solar panels improves the applicability of ECS to remote and developing regions by avoiding the need for grid-based electricity. The joint solar-ECS system advances the United Nations’ Sustainable development goals for responsible production and consumption, clean energy, and clean water and sanitation access. References Tarpeh, W.A. et al. Environ. Science Technol. 2018Comer et al., Joule (2019)Fragassa, C. et al., Appl. Comput. Mech. 2021Liu, M. J. et al. Water Research 2020 Figure 1

Solving ZIB Challenges: The Dynamic Role of Water in Deep Eutectic Solvents Electrolyte

Zinc-Ion batteries (ZIBs) are considered one of the most promising technologies in the realm of post-lithium-ion batteries. Rechargeable chemistries with zinc anodes hold significant technological potential due to their high theoretical energy density, lower manufacturing costs, abundant raw materials, and inherent safety. ZIBs typically feature neutral or weakly acidic aqueous electrolytes and exhibit typical drawbacks of Zn anodes: hydrogen evolution, passivation, and shape changes.In aqueous electrolytes, two types of water molecules can be described: free water molecules and solvated water molecules that participate in Zn2+ solvation structure [Zn(H2O)6]2+. The free water easily reacts with metallic Zn at the electrode/electrolyte interface, leading to the aforementioned irreversibilities. Alternative electrolytes such as Deep Eutectic Solvents (DESs) can be used to modulate Zn solvation shell, preserving the green and safe characteristics of aqueous-based ones. DESs show relevant characteristics for battery applications, such as low vapor pressure, good thermal and chemical stability, non-flammability, easy moldability, wide electrochemical window, and structured interface. However, a main drawback for electrochemical applications is the high viscosity, low diffusivity, and sluggish kinetics.Water molecules can be added as a cosolvent to eutectic mixtures to increase the diffusion coefficient of electroactive species and reduce the electrolyte viscosity without destroying the eutectic nature by up to 40%. The hydration percentage of DES can modify the thickness of the electrolyte-electrode interface and the Zn2+ solvation structure. Thus, the tuning capability, on one hand, of the solution chemistry of Zn2+ containing electroactive species, and on the other hand, of the electrode–electrolyte interfacial structure and chemistry by the combined use of DES and water provides a flexible tool to control Zn plating and stripping processes.In this study, we investigate the electrochemistry of Zn in ethaline DES with different percentages of water (0%, 5%, 10%, 25%, 50%, 70%, respectively), transitioning from a water-in-salt structure to a salt-in-water one. Fundamental electrokinetic and electrocrystallization studies based on cyclic voltammetry and chronoamperometry at different cathodic substrates were complemented with galvanostatic cycling tests of Zn|Zn symmetric CR2032 coin cells, SEM imaging of electrodes, and in situ Surface Enhanced Raman spectroscopy (SERS).Moreover, the Zn solvation structure was investigated using spontaneous Raman spectroscopy . The employed methodology provided valuable insights into the Zn plating reaction. Water molecules are present in a "confined" state rather than a free one, directly interacting with choline cations and being trapped in the eutectic framework if the molar fraction is kept below 40%.When ZnSO4 is dissolved in anhydrous ethaline, the [ZnCl4]2− species is formed and stabilized by choline. The peculiarities of Zn2+ coordination were found to result in unusual voltammetric behavior, characterized by a cathodic peak in the cathodic branch of the anodic-going scan.Although tetrachlorozincate is the dominant species, it does not seem to be the complex from which Zn is deposited; indeed, [ZnCl4]2− has a very negative reduction potential. Mechanistic studies of Zn electrodeposition onto different cathodic materials have concluded that the metal is formed by the reduction of an intermediate Zn-containing species. The two organic components of the eutectic solvents can be dehydrogenated at a negative potential, forming RO-, which can replace one or more chloride ligands in [ZnCl4]2−.Hydrated DES exhibits faster kinetics, which, in turn, leads to a lower nucleation overpotential. This phenomenon has been explained with two different mechanisms occurring at the same time. Firstly, by increasing the hydration percentage in the eutectic framework, the coordination of Zn2+ changes, forming [ZnCl3(H2O)]-. The water molecule in the Zn2+ sheath has a lower energy barrier than chlorine in the step-by-step desolvation of Zn2+. Consequently, the [ZnCl3(H2O)]- complex exhibits a lower dissociation energy than [ZnCl4]2- present in anhydrous ethaline, resulting in a faster desolvation process that can reduce nucleation overpotentials.Secondly, water molecules hydrogen-bonded to DES components can be reduced at cathodic potential, forming H· radicals that can then participate in the decomposition of the organic component. This apparent side reaction can aid in the formation of RO- ligands that participate in the Zn-species undergoing reduction, thereby increasing the amount of reactants.This multi-technique approach, also applicable to the study of new electrolytes, leads to the optimization of DES hydration, reducing the presence of free water and addressing the main drawbacks associated with eutectic electrolytes. The nuanced understanding of the interplay between solvent composition, Zn solvation, and electrodeposition mechanisms paves the way for the development of more efficient and sustainable ZIB technologies. Figure 1

(Invited) Delayed Fluorescence from Inverted Singlet and Triplet Excited States

Hund’s multiplicity rule states that for a given electronic configuration, a higher spin state has a lower energy. Rephrasing this rule for molecular excited states predicts a positive energy gap between singlet and triplet excited states (∆E ST). Although there is general agreement that ∆E ST must be positive, the potential for negative ∆E ST in phenalene analogues, such as cycl[3.3.3]azine and heptazine has been discussed during the last two decades.1–4 In this presentation, we report a fluorescent heptazine analogue HzTFEX2 that disobeys Hund’s rule, possessing negative ∆E ST of –11 meV (Fig. 1).5 The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature due to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes using this molecule exhibit a fast transient electroluminescence decay with a maximum external quantum efficiency of 17%, demonstrating potential implications for optoelectronic devices.(1) W. Leupin, J. Wirz, J. Am. Chem. Soc. 102, 6068–6075 (1980).(2) W. Leupin, D. Magde, G. Persy, J. Wirz, J. Am. Chem. Soc. 108, 17–22 (1986).(3) J. Ehrmaier, E. J. Rabe, S. R. Pristash, K. L. Corp, C. W. Schlenker, A. L. Sobolewski, W. Domcke, J. Phys. Chem. A 123, 8099–8108 (2019).(4) P. de Silva, J. Phys. Chem. Lett. 10, 5674–5679 (2019).(5) N. Aizawa, Y.-J. Pu, Y. Harabuchi, A. Nihonyanagi, R. Ibuka, H. Inuzuka, B. Dhara, Y. Koyama, K. Nakayama, S. Maeda, F. Araoka, D. Miyajima, Nature 609, 502 (2022). Figure 1

Electrolyte-Induced Fe-Doping of LaNiO3 Model Surfaces for Water Electrolysis

Renewable energy needs to be store in different energy forms for the energy transition away from fossil fuels. One of most promising forms is chemicals such as H2 and C-based fuels obtained via electrochemical reactions. The common hinderance in such reactions is the sluggish oxygen evolution reaction (OER), which causes low energy efficiency in conversion devices. Developing highly active, earth-abundant OER electrocatalysts in alkaline media is necessary to boost the performance of such devices. Fe ion content in electrolyte is known to influence the OER-activity of metal oxide electrocatalysts. For example, the Fe incorporation in NiOOH is a well-established and important effect for OER.[1] Here, we present the effect of absorbed Fe on the OER activity of the epitaxial Ni-surface terminated LaNiO3 thin films. The correlation between surface species and OER activity of electrocatalysts was investigated using surface-sensitive low-energy ion scattering and X-ray photoemission spectroscopies. We observe more than an order-of-magnitude boost in OER activity during initial cycling, from 0.15 mAcm-2 to 3.40 mAcm-2 at 1.60 V vs. reversible hydrogen electrode. The activity increase was mainly caused by the incorporation of up to 5 % Fe at the top surface of LaNiO3 film (less than ~1 nm) even though only a very trace amount of Fe ion, 30 ppb, is present in the 0.1 M electrolyte of high purity KOH 99.99 %. Our results demonstrate that the disordered active surface phase, which develops even on single crystalline surfaces, is a complex interplay between the as-prepared (surface) crystalline structure, composition of the solid electrocatalyst and the liquid electrolyte, and has strong impact on the final OER activity, implying that we need to think differently about the crystalline electrocatalysts: crystalline bulk and dynamically changing surface layer.[1] L. Trotochaud, S.L. Young, J.K. Ranney, S.W. Boettcher, Nickel–Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation, J. Am. Chem. Soc. 136 (2014) 6744–6753.

Feasibility of Remotely Controlled Capsule to Enable Directionality and Higher Dose Rates for Conventional Low Dose Rate Brachytherapy Using Topas

With low dose radiation (LDR) brachytherapy, thus far, only low dose rates could be used with low energy level radiation sources at 30 keV or less due to the risk of radiation to healthy cells, tissue, and organs. The radiation seeds are placed in the location of the tumor and is left in the body for the total dose to be continuously administered throughout the decay period of the radiation seed. However, the amount of time to deliver the total dose can take up to several months depending on the type of radiation source, tumor size and location, due to the low dose rate of <2 Gy/hr. In contrast, high dose radiation (HDR) is defined as a dose delivered at a rate >12 Gy/hr, although usually much higher, often in excess of 1 Gy/min, with a higher energy radiation source close to 0.5 MeV.For the first time, a remotely controllable, implanted radiation capsule is introduced, to enable a very high dose rate of 10 Gy/min for 30 keV I-125 radiation source. Until now, this combination of low energy and high dose rate for implanted radiation seeds for LDR was not possible due to the risk of radiation. The implantable capsule is designed to control this risk of radiation by remotely controlling the opening of the capsule hole. In the open case, the high dose rate of I-125 is delivered for a duration of 1 minute and then closed to stop emission.For feasibility, a simulation study was conducted using TOPAS for Monte Carlo simulation of radiotherapy. In the top right figures, a cylindrical capsule of height 6 mm, diameter 3 mm, and wall thickness 0.9 mm is designed using gold to contain the radiation and prevent leakage radiation. An opening of 30 degrees was made in the middle of the capsule wall where the radiation was to be emitted. For the radiation source, a 30 keV I-125 was used with a specific activity of 6.42 x 1014 Bq/gram. With a wall thickness of 0.9 mm, calculations showed that 4.4 micrograms of I-125 was needed to increase the source activity for 1.454 x 1020 Bq per second for a high dose rate of 10 Gy/min.The yellow and red colors in the capsule are both gold with wall thickness of 0.9 mm. The red portion of the cylinder is where the 30 degree opening was defined. The green lines show the paths of the radiation particles.TOPAS simulation results successfully show that the capsule contains the radiation and the only direction of radiation is through the defined opening in the capsule. The bottom three graphs show the x, y, and z plane cuts showing a directional radiation beam that drops in intensity, from a range of 1.0 to 0, over distance and slightly widens over distance.To control the opening and closing of the hole in the radiation capsule, an inner cylinder and outer cylinder is used, where the inner cylinder can move up and down. A permanent magnet is attached to the bottom of the inner cylinder and an electromagnet is attached to the base at the bottom of the capsule. An implantable inductive coil receives an AC current from the external inductive coil, which is rectified using a diode and capacitor circuit. This DC current powers the electromagnet, which repels the inner cylinder, moving it up to align the windows of the inner and outer cylinders. This alignment opens the hole and radiation is emitted towards the target tumor. After the dose is delivered, the current is turned off, which turns off the electromagnet. With no repelling force from the electromagnet, the permanent magnet reattaches itself to the base, and the inner cylinder slides down and the windows are no longer aligned. This blocks the radiation from being emitted. The dimensions of the capsule and the windows depend on the radiation source and tumor size. Micromachining and micro 3D printing techniques can be applied for fabrication of the device.In conclusion, a feasibility study using TOPAS demonstrated that a 0.9 mm gold wall with a hole can contain a very high source activity of I-125 radiation for a very high dose rate of 10 Gy/min and emit a directional beam through the hole. A remotely controllable capsule can enable a high dose rate with LDR brachytherapy that combines the advantages of LDR and HDR, which is minimum radiation risk and minimum treatment time, simultaneously. Potential applications for this is intracavitary cancer treatment such as esophageal or cervical cancer. Figure 1

(Invited) Organic Memristors for Mimicking Synaptic Plasticity

Since the introduction of a concept of memristor by Leon Chua in 1971 as the fourth fundamental passive component in electronic circuits, and its functional prototype introduced by HP Lab in 2008, research has increasingly concentrated on exploring its application ability in electronic circuits. First, the memristor principles have been exploited in various memory devices within a classical von Neumann computer architecture. The properties of memristor were found particularly suitable for resistive random-access memories (ReRAM).1 Contrary to the classical von Neumann architecture based on the binary logic, advanced methods like neuromorphic computing require an electronic element with continuously varying states depending on previous input signals. The input signal consists of a sequence of voltage spikes that stimulate the device, similarly as a neural synapse does in biological systems. These trains of spikes change the output state of the device continuously to a lower resistance state, mimicking a learning process. Having these functionalities, memristors are considered to be possible building blocks of brain-inspired neuromorphic computing and artificial neural networks.2 Memristors with neurosynaptic functionality take a continuity of resistance values with synaptic weights modulated by the number and frequency of homogeneous spikes. In order to mimic the neural synapse, memristors must exhibit analog properties including non-abrupt switching transitions, memory loss, continuously variable resistance states, and predictable response. We present such functionality on thin films of two kinds of organic materials:(i) A newly synthesized poly(methacrylamide) derivative with carbazole charge transporting group separated from the polymer backbone by an alkyl chain, was synthesized recently and found to behave as a bistable resistive memory.3 Thin films of the polymer, sandwiched between Al and ITO electrodes, exhibit rewritable flash memory behavior with bistable conductivity, with the set switching voltage ranging from 2 to 4.5 V, and the current ON/OFF ratio exceeding 100. The device demonstrates a remarkable lifetime and remains persistent for more than 104 seconds under the static voltage of 0.5 V. The main physical mechanisms driving the resistive switching have been attributed to the electric field-induced reorientation of heterocycles, which modulates charge transport, and trapping/detrapping of charges in localized states within the bulk of the polymer. Memory persistence is strengthened by the physical crosslinking caused by hydrogen bonds between amide and carbonyl groups in the side chains. Depending on the layer thickness, electrode material and applied voltage range the electrical characteristics can change from bistable to analog behavior showing memristive properties.(ii) Ditopic π-conjugated bis-terpyridine (tpy-) ligands form supramolecular structures in which the ligands are interconnected by complexation with metal ions. These molecules offer a low energy operation with improved reproducibility of memristive characteristics. They possess low-lying molecular energy levels and relatively narrow bandgap enabling the low-power consuming resistive switching and longer stability. We used a newly synthesized bis-terpyridine ligands based on 9-phenyl carbazole, Cbz, and its complex with cobalt Co2+ as an active layer of a memristor device. A two-terminal memristor architecture was employed, consisting of the ligand or its cobalt complex deposited on an ITO substrate, and aluminum, gold, or gallium as a top electrode. We show that Cbz as a memristor active layer exhibits an excellent nonvolatile bistable memory behavior, while Co-Cbz exhibits both electronic memory and synaptic plasticity. The ligand exhibited bi-stable conduction and non-volatile memory effect with persistence over 18 hours, while its cobalt complex displayed a synaptic effect characterized by subsequent potentiation and depression cycles. Voltage-induced modulation of synaptic weight was observed with a low applied voltage below 500 mV and short pulse duration below 20 ms. Pair pulse facilitation and pair pulse depression demonstrated significant synaptic weight changes, showing the cobalt complex ability to emulate biological neurons. The operational mechanism was attributed to a combination of the redox effect in the ligand and complex, and to the voltage-induced migration of perchlorate counter ions and subsequent redox processes, altering the metal-to-ligand charge transfer band and changing the conducting state of the active layer.Acknowledgments: The work was financially supported by the Czech Science Foundation, project 24-10384S, and MEYS of the Czech Republic, project LUAUS24032. Strukov, D. B., Snider, G. S., Stewart, D. R. & Williams, R. S. The missing memristor found. Nature 453, 80–83 (2008).Chang, T., Jo, S. H. & Lu, W. Short-term memory to long-term memory transition in a nanoscale memristor. ACS Nano 5, 7669–7676 (2011).Panthi, Y.R, Pfleger, J., Vyprachticky, D., Pandey, A., Thottappali, M.A., Sedenkova, I., Konefal, M. & Foulger, S.H. Rewritable resistive memory effect in poly[N-(3-(9H-carbazol-9-yl)propyl)-methacrylamide] memristor. Mater. Chem. C 11, 17093 (2023).

(Invited) Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System

3-dimensional hybrid halide perovskites have been intensely researched because of their extraordinary optoelectronic properties with spectacular quantum efficiencies arising primarily from their suitable bandgaps to harvest the solar spectrum, low excitonic binding energies, and low-cost synthetic processes. However, stabilizing these materials under real-life operating conditions has proven to be a challenge, leading to extensive explorations of lower dimensional analogous hybrid halide materials that are often more stable, though generally having considerably higher bandgaps and excitonic binding energies. I shall present some of our investigations leading to the synthesis of a highly moisture-stable 2-dimensional hybrid lead halide material with the lowest bandgap and exciton binding energy within this class of compounds.1 I shall detail the origin of such a low bandgap and exciton binding energy, identifying the most critical control parameters to determine these properties. In addition, I shall also present preliminary results to suggest the beneficial impacts of incorporating the same organic ligand while making a solar photovoltaic device based on three-dimensional hybrid lead halide perovskites.Reference: Debasmita Pariari, Sakshi Mehta, Sayak Mandal, Arup Mahata, Titas Pramanik, Sujit Kamilya, Arya Vidhan, Tayur N. Guru Row, Pralay K. Santra, Shaibal K. Sarkar, Filippo De Angelis, Abhishake Mondal, and D. D. Sarma, Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System, J. Am. Chem. Soc. 145, 15896 (2023).

Electrospun Nano-Porous Cellulose Acetate Membrane Casted with Chitosan for Solid-State Sodium-Ion Batteries

Emerging as a safe and economically viable alternative to lithium-ion batteries, the solid-state sodium ion battery (ss-SIB) has captured increasing attention as a transformative technology for realizing a decarbonized economy and ensuring a sustainable energy supply. Here we report a nanoarchitecture strategy of biodegradable, biocompatible, and naturally abundant cellulose derivative (cellulose acetate, CA) and chitosan (CH) biopolymer-based nano-porous electrospun composite electrolyte (ECE) for flexible and wearable ss-SIBs. A simple combination of electrospinning and solution casting was utilized to fabricate mechanically robust (13.76 MPa), thin-film (0.067 mm), highly flexible, and high room temperature Na-ion conductive (1.04 x 10-4 S.cm-2) ECE. Lower activation energy (Ea = 0.13 eV) indicates easy charge carrier diffusion ability of as prepared ECE. The electrochemical stability window (ESW = 3.45 V) of ECE is studied by the linear sweep voltammetry (LSV) with respect to stainless steel and sodium metal electrodes. Uniform galvanostatic Na plating and stripping at room temperature is observed over 900 hrs at 0.5 mA•cm-2 current density in symmetric (Na|ECE|Na) cell performance, this underscores impressive electrochemical stability and compatibility with sodium metal. Using Na3V2(PO4)3 (NVP) as cathode, ECE, and Na metal as anode in the full cell, specific discharge capacity of 83 mA×h×g-1 at room temperature was attained at 0.1 C rate, highlighting the practical applicability of the nanostructured electrospun composite electrolyte for flexible and wearable ss-SIBs. Keywords Cellulose acetate, chitosan, electrospun, solid-state sodium-ion battery, specific discharge capacity.

Electrifying Chemistry: Characterizing TEMPO-Based Electrocatalysts for Isopropanol to Acetone Conversion in Novel Heat Pumps

Heating, ventilation, and air conditioning (HVAC) systems account for more than 40% of the U.S. electricity usage, primarily utilizing vapor compression system (VCS) heat pumps, raising environmental concerns. The phase down of high GWP refrigerants, targeting an 85% reduction by 2035, prompts the exploration of novel heat pump technologies. Electrochemical looping heat pumps (ELHP) show promise by employing redox reactions for fluid compression to replace conventional compression technologies. However, energy-intensive redox interconversion necessitates catalysts to lower activation energy. Inspired by electrosynthesis principles, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) emerged as a successful homogeneous electrocatalyst for selective isopropanol oxidation to acetone for use in ELHP. HPLC and 13C NMR confirmed remarkable selectivity (~100%) at practical isopropanol concentrations.To enhance TEMPO's catalytic activity, various electron-withdrawing and donating groups were explored. TEMPO-OCH3, among seven derivatives, exhibited superior efficiency with a notable rate constant (6 M-1s-1) and turnover frequency (3.1 s-1). This study innovatively applies concepts from molecular catalysis for the development of efficient catalyst materials.

Single Atom Catalyst Anchored on Nitrogen-Doped Porous Carbon As an Effective Sulfur Host for Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are considered highly promising for next-generation energy storage due to their high theoretical specific capacity and energy density. However, challenges such as the insulating nature of sulfur cathodes, the detrimental shuttle effect of lithium polysulfides (LiPSs), and sluggish conversion kinetics of LiPSs during charge/discharge cycles impede their commercial viability. In this study, we employed a facile ‘dissolution-carbonization’ approach to synthesize nitrogen-coordinated monometallic atomic catalysts anchored on mesoporous carbon to promote surface-mediated reactions of LiPSs. The hierarchical porous carbon support physically confines the sulfur species, while the introduction of single atoms efficiently captures polysulfide intermediates and facilitates their redox conversion kinetics with a lower activation energy barrier. The synergistic effect of the single atom and nitrogen-rich porous carbon (NC) significantly enhances reaction kinetics and sulfur species utilization. Consequently, LSBs incorporating single-atom catalysts (SACs) demonstrate remarkable performance metrics, including high-capacity retention (824.2 mAh g−1), superior Coulombic efficiency (>98.5%), low-capacity decay rate (0.042% per cycle) after 500 cycles at 1 C, and excellent rate capability (776 mA h g–1 at 3 C). This work presents an effective strategy that combines the functions of a nanoporous material host and SACs for lithium-sulfur batteries.

(Invited) Consideration on Gate Stack Formation Processes of Ultrawide Bandgap Ga2O3 with SiO2 Dielectric Layer

MOSFETs on β-Ga2O3 are expected to be a high-efficient and cost-effective alternative option of high-voltage power devices. Taking account of the reliability and the large conduction-band offset against β-Ga2O3, it would be reasonable to employ SiO2 as the gate dieletric. In this study we investigated the effects of post-deposition annealing (PDA) on SiO2/β-Ga2O3 MOS gate stacks, in terms of the MOS electrical characteristics and the band alignment.β-Ga2O3 (001) wafers with ~2×1016 cm-3 n-type doped epitaxial layers were cleaned in diluted HF solution, and SiO2 films were deposited by electron-beam (EB) evaporation of Si in O2 ambient of ~1×10-2 Pa, followed by PDA at various temperatures from 600 to 1000℃ in either O2 or N2 ambient. Au was evaporated as gate electrode to form MOS capacitors. X-ray photoelectron spectroscopy (XPS) was conducted on the stacks with ~3nm-thick SiO2 for the evaluation of band alignment. Ultraviolet photoelectron spectroscopy (UPS) was also employed to determine the band diagram of β-Ga2O3 (001).Nearly-ideal capacitance-voltage characteristics were obtained for the MOS capacitor fabricated with O2-PDA at 1000℃. The interface state density (Dit) determined by conductance method decreased down to ~1010 cm-3 at the energy level of EC−0.2 eV by PDA at higher temperature. The hysteresis width (ΔVhys) of C-V curves was smaller for the samples annealed at higher temperatures. The flatband voltage (VFB) shift after O2-PDA resulted in VFB close to the ideal value estimated from the physical properties of Au and Ga2O3, suggesting that fixed charge density at the interface would be minimized. However, PDA in N2 at 1000℃ resulted in a negative shift of VFB, which indicates the formation of positive fixed charges in the stack. The beneficial influences of O2-PDA on those characteristics suggest that an oxygen deficiency introduced near the interface is one of the origins of interface defect states. It is noteworthy that Ga3d XPS on the surface of Ga2O3 substrate always shows a tailing in lower binding energy side, which is indicating oxygen deficiency on the surface. The intensity of this lower-binding-energy component reduced significantly when a bare-Ga2O3 substate was annealed in O2, however, it did not decrease efficiently even after O2-PDA when the surface was covered with SiO2. This is probably due to the oxygen rearrangement at SiO2/Ga2O3 interface determined by the difference of material properties of those oxides. This fact suggests that the Ga2O3 surface easily forms oxygen vacancies when it is covered with SiO2. This would be the reason why the formation of interface defect states is not suppressed without O2-PDA.Next the band diagram of β-Ga2O3 (001) substrate was investigated by UPS analysis where the energy level of valence band edge referring to the vacuum level was evaluted from the energy difference between minimum (cut-off) and maximum edges of the obtained kinetic energy spectra of photoelectrons. As a result we found the valence band edge of of β-Ga2O3 (001) was ~8.2 eV below the vacuum level, where the influences of O2 annealing at 1000℃ on the valence band edge energy level was limited to the shift within ~0.1eV. This would tell us the valence band offset between SiO2 and Ga2O3 should be around ~1.1eV, and the conduction band offset between them is to be around 2.8-2.9eV, by taking account of the well-reported SiO2 band diagram and assuming the bandgap of β-Ga2O3 as 4.7eV. On the other hand, the valence band offset estimated from the deconvolution of the XPS valence band spectrum for a ~3nm-thick SiO2/β-Ga2O3 stack from the energy edge difference between the deconvoluted spectra corresponding to SiO2 and Ga2O3, was a few hundreds of mV smaller than the above expected value (~1.1eV). This discrepancy is indicating the formation of an interface dipole layer between SiO2 and Ga2O3 with a pair of nagative and positive charges aligned at the interface to cause a shift in the band alignment. The magnitude of such dipole was indicated to be more significant (~0.5eV) for the stack after PDA at 1000℃, whereas it was less (0.2~0.3eV) after PDA at 600℃.In conclusion, we demonstrated that SiO2/β-Ga2O3 (001) MOS capacitors showed nearly-ideal electrical characteristics with significantly reduced interface defect density and small fixed charge density by applying O2-PDA. The band alignment of SiO2/β-Ga2O3 (001) MOS stack was clarified by photoelectron spectroscopy to confirm a very large conduction band offset of the stack, but an influence of the formation of interface dipole layer between SiO2 and Ga2O3 on the band alighment was also indicated.

Understanding the Role of Strain on Electrochemical Reaction Kinetics: Oxygen Evolution on Mechanically Strained Rutile TiO2 As a Model System

Electrocatalysis offers a green alternative (when coupled with renewably-sourced electrons) to conventional carbon-intensive chemical fuel and commodity production processes. Electrochemical device efficiency, however, is limited by the sluggish kinetics of multi-electron transfer reactions, such as the oxygen evolution reaction (OER). Theoretical adsorption free energy scaling relationships predict large oxygen evolution overpotentials (>370 mV) for metal oxides performing the adsorbate evolution mechanism, a series of four proton-concerted electron transfer steps.1 Identifying catalyst design strategies that overcome or “break” scaling relations (i.e. strain,2 high entropy alloys,3 non-concerted proton and electron transfer mechanisms4,5) would provide a means to lower kinetic potential energy penalties and enhance the efficiency and economic incentives for further development of renewably-driven electrochemical reactions.Here we investigate the role of mechanically induced strain on the OER activity of polycrystalline rutile TiO2 thin films, grown on the super elastic alloy nitinol (NiTi) and mechanically strained by tensile tester. Our work characterizes how strain manifests in these complex polycrystalline systems with complementary physical characterization techniques (e.g. X-ray diffraction and Raman spectroscopy) and relates strain to OER performance in alkaline electrolytes. We contextualize our results within the framework of conventional adsorption scaling relationships to hypothesize how tensile strain influences reaction intermediate adsorption affinities, inferring implications for the OER kinetics of benchmark IrO2 and RuO2 catalysts that share the same crystal structure as the rutile TiO2 considered here.(1) Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J. Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem 2011, 3 (7), 1159–1165. https://doi.org/10.1002/cctc.201000397.(2) Khorshidi, A.; Violet, J.; Hashemi, J.; Peterson, A. A. How Strain Can Break the Scaling Relations of Catalysis. Nat Catal 2018, 1 (4), 263–268. https://doi.org/10.1038/s41929-018-0054-0.(3) Kante, M. V.; Weber, M. L.; Ni, S.; van den Bosch, I. C. G.; van der Minne, E.; Heymann, L.; Falling, L. J.; Gauquelin, N.; Tsvetanova, M.; Cunha, D. M.; Koster, G.; Gunkel, F.; Nemšák, S.; Hahn, H.; Velasco Estrada, L.; Baeumer, C. A High-Entropy Oxide as High-Activity Electrocatalyst for Water Oxidation. ACS Nano 2023, 17 (6), 5329–5339. https://doi.org/10.1021/acsnano.2c08096.(4) Grimaud, A.; Diaz-Morales, O.; Han, B.; Hong, W. T.; Lee, Y.-L.; Giordano, L.; Stoerzinger, K. A.; Koper, M. T. M.; Shao-Horn, Y. Activating Lattice Oxygen Redox Reactions in Metal Oxides to Catalyse Oxygen Evolution. Nature Chem 2017, 9 (5), 457–465. https://doi.org/10.1038/nchem.2695.(5) Yoo, J. S.; Rong, X.; Liu, Y.; Kolpak, A. M. Role of Lattice Oxygen Participation in Understanding Trends in the Oxygen Evolution Reaction on Perovskites. ACS Catal. 2018, 8 (5), 4628–4636. https://doi.org/10.1021/acscatal.8b00612.

Nickel-Based Metal-Organic Framework Materials with Mixed Ferrocene-Based Ligands As Anodic Catalysts for Water Electrolysis and Urea Electrolysis

Water electrolysis is a highly promising technology for hydrogen production, generating green hydrogen with no greenhouse gas emissions. The water electrolysis reaction involves two half-reactions: hydrogen evolution and oxygen evolution. While oxygen evolution in water electrolysis involves a four-electron transfer requiring higher overpotential and thus leading to energy consumption. Urea electrolysis offers lower theoretical energy consumption than water electrolysis, making them a key technology in future electrolytic hydrogen production processes. Conventional anodic catalysts, such as Ir and Ru, exhibit good performance in oxygen evolution, but being precious metals, they face limitations due to high cost and limited abundance. Its is crucial to find an active, durable and cost-effective anode catalyst for realistic application.Metal-organic framework (MOF) materials, due to their high porosity, large surface area, and unique structural features, have emerged as promising materials for water electrolysis. However, their relatively poor stability is a drawback that needs to be overcome. In herein, we design the mixed ligand strategy for synthesizing the MOF materials. Ferrocene-based ligands, ferrocenedicarboxylic acid (Fd), combing with terephthalic acid (H2BDC) as the mixed ligands for this study. The MOF catalysts, namely Ni-BDC and Ni-BDC-Fd, was synthesized using a simple one-step hydrothermal method with ferrocenedicarboxylic acid and terephthalic acid.Raman spectra revealed the presence of Fe-O, originating from the iron in Fd, demonstrating a structural transformation during the hydrothermal process. XRD analysis also confirmed the existence of FeO. The SEM images showed nanosheets and nanoneedles morphologies for Ni-BDC, while a dense nanofiber structure for Ni-BDC-Fd demonstrating the effect of 2nd ligand on the MOF structure.Electrochemical tests were conducted in 1 M KOH electrolyte and 1 M KOH + 0.5 M urea electrolyte, including oxygen evolution reaction (OER), urea oxidation reaction (UOR). The overall reaction comprising the same catalysts for the anode and the cathode side is performed to examine the real electrolysis performance.Results showed that the addition of Fd enhanced OER and UOR activity in alkaline and urea electrolytes. Ni-BDC-Fd achieved 1.483 V vs. RHE and 1.386 V vs. RHE at 65 mA/cm-2 driving potential for OER and UOR, respectively, requiring a smaller potential than Ni-BDC to achieve the same current density. To verify that the performance improvement was not solely due to iron, NiFe-BDC was synthesized by adding iron nitrate, showing inferior UOR performance compared to Ni-BDC-Fd but superior OER performance to NiFe-BDC. This indicated that the performance improvement resulted from structural changes rather than just the presence of iron atoms. Ni-BDC-Fd exhibited the lowest Tafel slope in OER, indicating a faster reaction rate due to the increased active surface area during OER. After continuous catalysis for 80 hours in the alkaline water electrolysis cell and urea electrolysis cell, a significant improvement in the loss of potential was observed.These results suggest that the addition of ferrocene dicarboxylic acid altered the structure of Ni-BDC, maintaining an appropriate interlayer distance. This not only enhanced its catalytic performance but also improved its catalytic stability, demonstrating a promising strategy for preparing metal-organic framework materials for electrocatalysis applications. Figure 1

Unravelling the Effect of Sodium Doping on Li2MnO3 nanostructured Cathode Materials

The quest to develop affordable, high-performance electrode materials for lithium-ion batteries in electric vehicles is driving scientific innovation forward. Li2MnO3, as a prospective high-capacity (459 mAh.g-1) cathode, suffers from capacity degradation and voltage decay during the cycling process. Nanostructuring and incorporating sodium ions into lithium sites can mitigate voltage decay by limiting transition metal migration, impeding oxygen loss, and improving lithium diffusion by lowering the activation energy of Li-rich layered host materials. Herein, sodium-incorporated nonospherical structures of the form Li2-xNaxMnO3 (0 ≤ x ≤ 2), generated through the amorphisation and recrystallisation (A+R) technique show remarkable structural stability and enhanced Li-ion mobility owing to the enlarged lithium lattice upon sodium entrance. Specifically, the Li1.95Na0.05NaO3 nanosphere, displays more stable Li+/Mn4+ layers which will greatly contribute to its capacity. The microstructures associated with this configuration show well-ordered layers with high lithium kinetics and lower activation energy compared to pristine Li2MnO3. Characterisation of the x-ray diffraction (XRD) patterns revealed peak broadening along with the shifting of peaks at 2Θ~38 to the right due to the enlarged lithium layers occupied by sodium ions to facilitate lithium diffusion. Moreover, the undesired phase transformation from layered to spinel was observed at a later stage of charge for the sodium-doped systems, suggesting that the presence of sodium stabilizes the structure and minimizes the migration of manganese into lithium layers. These findings may lead to solutions for problems such as structural instability, lattice oxygen loss, and capacity decay in layered lithium oxide materials.

Advancements in Additive Manufacturing of Protonic Ceramic Fuel Cells

Protonic ceramic fuel cells (PCFCs), also known as proton conducting solid oxide fuel cells, are a promising low-temperature (400-600oC) fuel cell technology. Compared to conventional oxygen-ion conducting ceramic materials, proton conducting ceramic electrolyte materials have lower activation energy (< 50 eV), which can enhance fuel cell efficiency and power output. Additive manufacturing has the potential to revolutionize PCFC manufacturing, as it enables fabrication of both dense and porous structures with desirable mechanical and electrochemical properties. In this study, we employed extrusion-based 3D printing to fabricate PCFCs using BaZr0.3Ce0.5Y0.15O3-δ and BaZr0.4Ce0.4Y0.1Yb0.1O3-δ as electrolytes, NiO as anode, and BaCo0.4Fe0.4Zr0.1Y0.1O3-δ and Ba0.5Sr0.5Co0.8Fe0.2O3-δ as cathode. For comparison, we also printed oxide conducting ceramic fuel cells using gadolinium-doped-ceria (GDC ) as electrolyte. We investigated the printable pastes' rheological properties using dynamic light scattering, viscometer, tensiometry, differential scanning calorimetry, and thermal gravimetric analysis. We also characterized the cells using current-voltage measurements, electrochemical impedance spectroscopy, and various spectroscopic and microscopic techniques (HR-TEM-EELS, SEM-EDX) to understand the underlying mechanisms. Finally, we conducted a systematic study to optimize the sintering temperature for optimal fuel cell performance and investigate degradation mechanisms to improve stability. The best printed fuel cells produced 926 mW/cm2 with 3-layer structure and 426 mW/cm2 with 1-layer structure at 550oC.

Crashworthiness Performance and Multi-Objective Optimization of Bi-Directional Corrugated Tubes under Quasi-Static Axial Crushing.

Longitudinal corrugated tubes (LCTs) exhibit stable platform force under axial compression but have low specific energy absorption. Conversely, circumferential corrugated tubes (CCTs) offer higher specific energy absorption but with unstable platform force. To overcome these limitations, this paper introduces a novel bi-directional corrugated tube (BCT) that amalgamates the strengths of both the CCT and LCT while mitigating their weaknesses. The BCT is formed by rolling a bi-directional corrugated structure into a circular tubular form. Numerical simulations of the BCT closely align with experimental results. The study further examines the influence of discrete parameters on the BCT's performance through simulations and identifies the tube's optimal design using the integral entropy TOPSIS method. A full factorial experimental approach is then employed to investigate the impact of radial amplitude, axial amplitude, and neutral surface diameter on the crushing behavior of the BCT, comparing it with the CCT and LCT. The results reveal that increasing Ai enhances the axial resistance of the structure, while increasing Aj reduces the buckling effect, resulting in a higher specific energy absorption and lower ultimate load capacity for the BCT compared to the CCT and LCT. A simultaneous multi-objective optimization of the CCT, LCT, and BCT confirms that the BCT offers superior specific energy absorption and ultimate load capacity. The optimal configuration parameters for the BCT have been determined, providing significant insights for practical applications in crashworthiness engineering.

Degradation and Protection Mechanisms of Platinum-Based Catalysts in Hydrogen-Bromine Flow Batteries

Cost-effective and large-scale electrical energy storage is pivotal in facilitating the assimilation of renewable energy sources like wind and solar, as well as enhancing the resilience of the future electricity grid. Hydrogen-Bromine flow battery (HBFB) technology is a candidate for a low cost and flexible system. Bromine is abundantly available on Earth. In addition, the reaction between bromine and bromide is characterized by rapid kinetic properties, thus no platinum group metals (PGMs) are required on the bromine side of the cell as a catalyst to sustain a high current density of >500 mA/cm2. Moreover, hydrogen-based flow batteries can be coupled with hydrogen pipelines & electrolyzers to boost the integration of new energy systems.However, the selection of bromine brings technical challenges that can limit the commercial deployment of such HBFB systems. We have proven a lifetime of more than 8,000 hours of continuous cycling at the lab scale. During this proof-of-concept validation, we found that bromine species (liquid) crosses through a dense proton exchange membrane to the hydrogen side of the cell. On this side, a Platinum-based catalyst layer is present to promote hydrogen oxidation-evolution reactions (HOR/HER). The crossover liquid interacts first with the catalyst layer before it crosses through the gas diffusion layer and exits the membrane electrode assembly (MEA). Chemical interactions between the crossover liquid and the catalyst can lead to catalyst migration, Ostwald ripening, or even permanent catalyst dissolution. This is a concern and must be addressed. The root of this concern is that the Platinum-based catalyst is highly reactive (high surface-active area respective to the loading, m2/g), and also the crossover bromine species are likely corrosive (highly acidic, pH <0, and contain polybromides). Therefore, chemical interaction cannot be fully avoided. Nevertheless, the cathodic protection reduces this interaction, which prolongs the lifetime of the electrochemical system. The cathodic protection consists of an applied positive potential and the presence of hydrogen gas on the catalyst nanoparticles. The catalyst degradation together with cathodic protection needs to be better understood and quantified to gain the confidence that the HBFB system fundamentally can last for more than 10 years of continuous operation. This confidence is essential to deliver a minimum viable product (MVP) of HBFB, e.g. on a 100 kW – 1 MW scale, to the market.To address the aforementioned scientific challenges, this work studies the liquid crossover phenomena and the interaction between the crossover liquid and platinum-based catalyst in depth. Liquid crossover rate, concentration, and bromine speciation from working electrochemical cells are quantified at different states of charge (SoCs) and different modes (charge or discharge). This information is used to set up further ex-situ experiments to understand and determine the chemical interaction between crossover liquid and catalyst. Electrochemical ex-situ tests for Pt dissolution measurement such as determination of the electrochemically active surface area (ECSA), are performed before and after soaking hydrogen electrodes in different concentrations of hydrogen bromide solutions that mimic the crossover liquid. These results are then combined with in-situ tests, where the catalyst dissolution rate is quantified in actual crossover liquid and electrolyte by the inductively coupled plasma (ICP) method. Finally, post-mortem analyses on the catalyst layer based on scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. will give quantitative and qualitative insights into the impact of cycling and liquid crossover on the catalyst performance, morphology, crystal structure, and chemical state of the catalyst.The goal of this work is to quantify the extrapolated lifetime of HBFB systems within a high confidence level. Then conclude if this technology is fundamentally sound and can be scaled up to an MVP to be deployed to the market. As a low cost energy storage solution, the HBFB’s lifetime should be at least 5 years and preferably more than 10 years to attract commercial attention. Moreover, the collective knowledge gained in this work about the catalyst degradation mechanisms will spark further ideas and guide the next generation of the system and its operating conditions to reduce the degradation rate. These presented ideas may also shine a light on catalyst degradation and protection mechanisms in other electrochemical applications that operate in corrosive conditions.Figure 1 Membrane electrode assembly architecture, incl. electrochemical reactions and species transport phenomena in the charge and discharge modes. Figure 1

A New High-Valent Fe Redox Couple in Intercalation Electrodes

The overwhelming majority of layered oxide cathode materials are based on the redox activity of expensive and scarce transition metals like Co and Ni. While the Fe-redox based material LiFePO4 has become increasingly attractive due to its low cost, its low gravimetric energy density and relatively low operating voltage limit its use cases. A high-voltage Fe-based layered oxide cathode would provide high energy density utilizing the redox-activity of the most abundant transition metal in Earth’s crust, but such a material that provides stable and reversible electrochemical performance remains elusive. Indeed, high-voltage Fe-redox in oxides is plagued by cation disorder and oxygen dimerization, which leads to capacity fading and large voltage hysteresis (> 1 V).[1]In contrast, the layered oxide cathode Li4FeSbO6 (equivalently, Li[Li1/3Fe1/3Sb1/3]O2, abbreviated LFSO) exhibits <0.2 V of hysteresis, and a discharge plateau at 4.2 V versus Li/Li+ with two-electron capacity.[2] Both the exceptional electrochemical reversibility of this material and its phase separating nature upon (de)intercalation raise questions about the origin of these properties that are unusual for layered Fe oxide cathodes. In this work, we will discuss our efforts to probe the electronic structure of LFSO with Fe L–edge x–ray absorption (XAS) and 57Fe Mössbauer spectroscopy, as well as O K–edge resonant inelastic x–ray scattering spectroscopy (RIXS). The combination of these element-specific and valence-state sensitive techniques, in conjunction with charge-transfer multiplet and density functional theory calculations, allows us to reveal the electronic structure changes at Fe and O as a function of state of charge for the first time. Correlating these electronic structure changes with synchrotron diffraction measurements, we reveal a unique redox compensation mechanism that readily rationalizes the exceptional electrochemical reversibility, unlocking a new direction for high-voltage, Fe-based redox systems.

Simulating the Effects of Ammonia Crossover and Alternative Ligands in Thermally Regenerative Flow Batteries

There is a significant amount of low-temperature heat (< 100 °C) available globally from various sources such as geothermal energy, industrial processes, and thermal power plants. Electrochemical and membrane-based methods for harnessing this heat are becoming of greater interest due to their ability to utilize the low temperature energy sources and because they can be sized modularly to fit the power, energy, and spatial requirements of the application. The all-aqueous thermally regenerative ammonia battery (Cuaq-TRAB) is a leading technology in the waste heat to electrical power production space due to recent results showing operational power densities of 25 mW cm-2 with an estimated thermal energy efficiency of 7%, outperforming most technologies in this space. Previous research on the Cuaq-TRAB has shown that the battery suffers from the crossover of ammonia through the membrane which adversely impacts battery performance.To investigate these impacts, we developed a simple numerical model that simulates discharge curves using one fitting parameter to account for parasitic crossover instead of modeling species transport through the membrane. The model was able to replicate experimental data from Cuaq-TRABs with different membrane materials and for multiple applied current densities with < 5% error for average power and energy capacity. Results showed that there is a 20% loss in Cuaq-TRAB energy capacity due to ammonia crossover when compared to a discharge with no parasitic crossover losses. The model was then extended to demonstrate the impact of different changes to the electrolyte chemistry on the resultant power and energy capacities for Cuaq-TRABs. Using this model, we estimate that if chloride was the ligand for the positive electrolyte instead of bromide, it would decrease the power and energy capacity of the battery by 30%. However, considering that an economic analysis showed that chloride is 80% cheaper than bromide, it may be beneficial to consider the use of chloride ligands in this technology despite its worse electrochemical performance.

Pure 3D Conducting Polymer Network for an Ultra-Sensitive and Flexible Hydrogen Gas Sensor at Room Temperature

Hydrogen (H2) is considered a next-generation energy source with diverse applications across valuable social, economic, and industrial sectors, including transportation, biomedical, power generation, and space exploration. Due to its low ignition energy and wide flammable range, there is an urgent global demand for ultrasensitive, lightweight, portable, wearable, and flexible hydrogen sensors to identify early gas leaks and prevent environmental and economic implications.In this study, we present a flexible hydrogen gas sensor based on conducting polymers, namely polypyrrole (PPY) with graphene nanoparticles, synthesized by the bi-continuous microemulsion polymerization method. The one-step scalable synthetic procedure enables the design of 1D, 2D, and 3D conducting polymers and their nanocomposites with tunable morphological and electrical properties. The unique multilayered oil-to-water nature of the bi-continuous microemulsion allows the fabrication of pure mesoporous 3D conducting polymers and their composites without the addition of any cross-linkers with excellent electrical properties. The 3D pure porous polymer network acts as a monolithic conducting framework that facilitates electron and ion transport and promotes the diffusion of molecules and ions more efficiently than 1D or 2D structures. The soft and porous nature of the polymer improves the interaction and adsorption of the target gas molecules and sensing material by increasing the number of active sites.This 3D PPY_graphene flexible network synthesized by the bi-continuous microemulsion method is successfully utilized to fabricate an ultra-sensitive flexible gas sensor. The performance of the sensor can be modified based on the reactor design. The sensor exhibits a notable response to 1 ppm of H2 gas. At room temperature, the fabricated sensor displayed 14 and 20 seconds of response and recovery time, respectively. To the best of our knowledge, no study has explored the application of 3D pure conducting polymers and their nanocomposites for hydrogen gas sensors using the bi-continuous microemulsion polymerization method, yielding such exceptional sensing results. Figure 1