Emre Burak Boz

Increasing penetration of renewable energy sources presents grid-level challenges, notably supply–demand imbalances driven by their inherent intermittency. To overcome this challenge, there is a compelling need for long-duration energy storage solutions with high round-trip efficiency, intrinsic safety, scalability and low levelized cost of storage (LCOS). To this effect, all-iron redox flow batteries (AIRFBs) present an appealing solution largely due to their environmentally benign nature, inexpensive raw materials (ca. $ 110 ton^-1) and high energy density^[1]. However, conventional AIRFBs are limited by the surface area and available open volume within porous electrodes, as it ultimately determines the amount of iron that can be plated on the negative electrode. Accumulation of the solid phase within the reactor prevents independent scaling of power and energy, resulting in decreased system flexibility and increased capital costs.

Redox flow batteries (RFBs) offer a promising solution for long-duration energy storage owing to their scalable design and decoupled power–energy architecture. While commercial RFBs predominantly employ vanadium-based chemistries, there is increasing interest in alternative chemistries that utilize earth-abundant and low cost materials, including iron^[1]. Conventional aqueous all-iron redox flow batteries (AIRFBs) employ Fe(II)/Fe(III) redox couple at the positive electrode, where the reactions take place in liquid form. Whereas the Fe(II)/Fe redox couple which undergoes phase transformation via plating/stripping reactions is used on the negative side^[2]. Given that the amount of iron plated depends on accessible pore volume of the electrode, it ultimately limits the battery capacity and prevents power-energy decoupling^[3].

Four nitrogen precursors are evaluated for iron–nitrogen–carbon catalysts in proton-exchange membrane fuel cells. Improved nitrogen incorporation and iron–nitrogen coordination enhance activity, linking synthesis to oxygen reduction performance.

Carbonaceous porous diffusion media play a critical role in electrochemical technologies sustaining multiphase (gas and liquid) flows, including low-temperature fuel cells and CO₂ electrolyzers. To resist liquid intrusion and preserve gas pathways, these materials are typically hydrophobized with polytetrafluoroethylene dispersions. However, the application of dispersion-based coatings limits uniformity and performance, while the persistence and toxicity associated with per- and polyfluoroalkyl compounds have prompted regulatory scrutiny. Here, we investigate polydimethylsiloxane (PDMS), a fluorine-free, low-surface-energy, and environmentally benign polymer, as a viable alternative. We assess four PDMS coating application strategies: (1) dip-coating, vapor deposition in (2) oxygen and (3) nitrogen atmospheres, and (4) electrografting of an amine-functionalized derivative. We perform spectroscopic, microscopic, wetting, and electrochemical double-layer measurements to correlate the surface chemical composition and morphology with the resulting wettability. Vapor deposition in an oxygen environment produces a superhydrophobic rough microstructure; however, the weak adhesion of the coating and substrate oxidation result in severe flooding when the substrate is exposed to an alkaline liquid flow. Notably, electrografting and vapor deposition in nitrogen yield thin uniform coatings with an ethanol-potassium hydroxide solution repellency in flow comparable to the polytetrafluoroethylene baseline. On the contrary, dip-coated samples feature thick unevenly distributed layers exhibiting poor liquid infiltration resistance. Under prolonged flow in the presence of 10 wt % ethanol, the desorption of the physisorbed layer vapor deposited in nitrogen led to a continuous increase in wetting, resulting in complete electrode flooding within 65 h. In contrast, the electrografted substrates showed enhanced durability under liquid flows with various compositions, retaining repellency for the same time scale comparably with the PTFE baseline, highlighting the importance of covalently attached hydrophobic coatings for long-term operation. These findings demonstrate the potential of PDMS-based coatings as sustainable fluorine-free alternatives for hydrophobizing carbonaceous porous media and provide practical guidelines for engineering the wetting behavior and coating stability.

Polymer electrolyte fuel cells (PEFCs) offer high efficiency, rapid refueling, and zero-carbon operation, but commercialization is constrained by the cost of platinum-based oxygen reduction catalysts. Transition metal-nitrogen-carbon materials, particularly Fe-N-C, are promising platinum-free alternatives, although their activity and durability still require improvement. Ammonia is often introduced during synthesis to enhance nitrogen incorporation, but safer nitrogen sources are desirable to simplify processing and reduce associated hazards. Here, we investigate the use of urea, melamine, cyanoguanidine, and nicarbazin as nitrogen precursors during the thermal activation of ZIF-8-derived Fe-N-C catalysts, aiming to promote nitrogen incorporation and active-site formation without ammonia use. Structural analysis shows that urea, melamine, and cyanoguanidine largely preserve the ZIF-8 morphology, while nicarbazin yields carbonaceous flakes. The high surface area of ZIF-8 (~1600 m² g⁻¹) is partially maintained after pyrolysis (~1200 m² g⁻¹). X-ray photoelectron spectroscopy reveals enhanced nitrogen content and increased Fe-Nₓ species, most notably in urea-activated samples. Electrochemical testing confirms higher onset potential and mass activity for urea- and melamine-activated catalysts compared to the control, with consistent trends in rotating disk electrode and single-cell PEFC measurements. Despite their lower intrinsic activity compared to Pt/C, Fe-N-C catalysts exhibit enhanced ORR kinetics when secondary nitrogen precursors are used during synthesis. Despite elevated peroxide yields predicted from RDE, ion chromatography indicates a modest increase in ionomer degradation compared to Pt/C during fuel cell tests. Overall, nitrogen-rich molecular precursors enhance Fe-N-C activity while providing a safer and scalable pathway for nitrogen doping, advancing the development of cost-effective non-PGM catalysts for fuel cells

Carbonaceous porous diffusion media play a critical role in electrochemical technologies sustaining multiphase (gas and liquid) flows, including low-temperature fuel cells and CO₂ electrolyzers. To resist liquid intrusion and preserve gas pathways, these materials are typically hydrophobized with polytetrafluoroethylene dispersions. However, the application of dispersion-based coatings limits uniformity and performance, while the persistence and toxicity associated to per- and polyfluoroalkyl compounds have prompted regulatory scrutiny. Here, we investigate polydimethylsiloxane (PDMS), a fluorine-free, low-surface-energy, and environmentally benign polymer as a viable alternative. We assess four PDMS coating application strategies: (1) dip-coating, vapor deposition in (2) oxygen and (3) nitrogen atmospheres, and (4) electrografting of an amine-functionalized derivative. We perform spectroscopic, microscopic, wetting, and electrochemical double-layer measurements to correlate surface chemical composition and morphology with the resulting wettability. Vapor deposition in an oxygen environment produces a superhydrophobic rough microstructure; however, weak adhesion of the coating and substrate oxidation result in severe flooding when the substrate is exposed to an alkaline liquid flow. Notably, electrografting and vapor deposition in nitrogen yield thin, uniform coatings with ethanol–potassium hydroxide solution mixture repellency in flow on par with the polytetrafluoroethylene baseline. On the contrary, dip-coated samples feature thick, unevenly distributed layers exhibiting poor liquid infiltration resistance. Under prolonged flow in the presence of 10 wt% ethanol, desorption of the physisorbed layer vapor deposited in nitrogen led to a continuous increase in wetting, resulting in complete electrode flooding within 65 hours. In contrast, the electrografted substrates showed enhanced durability under liquid flows with various compositions, retaining repellency for the same timescale comparably with PTFE baseline, highlighting the importance of covalently attached hydrophobic coatings for long-term operation. These findings demonstrate the potential of PDMS-based coatings as sustainable, fluorine-free alternatives for hydrophobizing carbonaceous porous media, and provides practical guidelines for engineering the wetting behavior and coating stability.

This study uses operando neutron imaging and electrochemical data to reveal how flow field geometry and electrode structure shape concentration profiles and transport dynamics in organic redox flow cells.

Understanding reactive mass transport in redox flow reactors is key to improving performance, yet conventional characterization techniques often rely on cell-averaged metrics and fail to resolve local transport phenomena. In this study, we employ operando neutron radiography to visualize concentration distributions in redox flow cells with non-aqueous electrolytes, leveraging the high attenuation of hydrogen-containing organic molecules and boron-containing supporting ions. Symmetric flow cell experiments were conducted with three electrode types (paper, cloth, and a hierarchical porous electrode fabricated via non-solvent induced phase separation), and two flow field designs (parallel and interdigitated). We find that for kinetically facile electrolytes with low ionic conductivity and with parallel flow fields, electrodes with large pores in the though-plane direction (i.e., carbon cloth) augment the current output. Additionally, interdigitated flow fields sustain higher currents than parallel flow fields at a fixed potential and flow rate due to enhanced convective transport. Despite significant differences in macroscopic performance among the studied materials, the concentration profiles within the cell showed only minor variations within the studied operating conditions and imaging configuration. The cloth electrode and interdigitated flow field exhibited slightly more uniform concentration profiles across the electrode thickness compared to the paper electrode with the parallel flow field. In contrast, the phase-separation electrode displayed more steep concentration profiles and a stronger dependency on polarity reversal. Neutron radiography further uncovered critical secondary effects, including salt precipitation and flow field underutilization. These findings highlight the potential of operando imaging to inform the design and operation of electrochemical reactors for a range of technologies.

Aqueous all‐iron redox flow batteries are an attractive and economic technology for grid‐scale energy storage owing to their use of abundant and environmentally benign iron as the redox active material and water as solvent. However, the battery operation is challenged by the plating/stripping reactions of iron and the competing hydrogen evolution reaction at the negative electrode, which hinder performance and durability. Here, the reaction selectivity of the negative electrode is tailored by introducing conductive polymer coatings onto porous carbonaceous electrodes. Two conductive polymers, poly(3,4‐ethylenedioxythiophene) (PEDOT) and poly(pyrrole) (PPy) are conformally coated with the dopant poly(4‐styrenesulfonate) (PSS) and the resulting electrochemistry is studied on model electroanalytical platforms and redox flow batteries. Both polymers decrease the hydrogen evolution current on rotating disc electrodes, with PPy/PSS strongly inhibiting the reaction at high overpotentials. In full all‐iron redox flow cells, PPy/PSS coating extends cyclability and significantly reduces hydrogen evolution, while PEDOT/PSS coating improves the round‐trip efficiency, possibly acting as a redox shuttle for the iron stripping reaction. These findings motivate broader investigation and implementation of conductive polymers to engineer reaction selectivity for flow batteries and other electrochemical technologies.

Aqueous all-iron redox flow batteries are an attractive and economic technology for grid-scale energy storage owing to their use of abundant and environmentally benign iron as redox active material and water as solvent. However, the battery operation is challenged by the plating/stripping reactions of iron and the competing hydrogen evolution reaction at the negative electrode, which hinder performance and durability. Here we tailor the reaction selectivity of the negative electrode by introducing conductive polymer coatings onto porous carbonaceous electrodes. We conformally coated two conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(pyrrole) (PPy) with the dopant poly(4-styrenesulfonate) (PSS), and studied the resulting electrochemistry on model electroanalytical platforms and redox flow batteries. Both polymers decrease the hydrogen evolution current on rotating disc electrodes, with PPy/PSS strongly inhibiting the reaction at high overpotentials. In full all-iron redox flow cells, we find that PPy/PSS coating extends cyclability and significantly reduces hydrogen evolution, while PEDOT/PSS coating improves the round-trip efficiency, possibly acting as a redox shuttle for iron stripping reactions. These findings motivate the broader investigation and implementation of conductive polymers to engineer reaction selectivity for flow batteries and other electrochemical technologies.