Development of novel technologies for catalyst synthesis and membrane electrode assembly (MEA) fabrication is of primary importance for further improvement of the performance and economics of proton exchange membrane fuel cells (PEMFCs) and proton exchange membrane water electrolyzers (PEMWEs). While the traditional manufacturing methods are time-consuming, energy intensive, and require many processing steps, newer vapor-based methods provide many benefits including the development of improved catalysts and catalyst supports, deposition of uniform thin films, reduction of catalyst loading, and minimizing the number of manufacturing steps. Recent publications in the field identified spray pyrolysis, reactive spray deposition technology, chemical vapor deposition, and atomic layer deposition as advanced vapor-based catalyst synthesis and deposition methods used for fabrication of MEAs for PEMFCs and PEMWEs. The MEAs fabricated via vapor-based processes have shown significant performance improvements in comparison to the state-of-the-art MEAs, which are attributed to better catalyst distribution, improved catalyst supports, and controlled, uniform catalyst layer microstructures. This review provides an overview of the vapor-based synthesis and deposition methods currently being used for the development of PEM-based devices. The advantages and disadvantages of these methods are critically compared and discussed while the outlook for future development is provided.

Capacity loss induced by the undesired transport of vanadium ions across the ion-exchange membrane (i.e. crossover) is one of the most critical issues associated with vanadium redox flow batteries. This work reports on the manufacturing and testing of an innovative barrier layer to mitigate crossover. The barrier layer conceptual design is described in detail in the patent application WO 2019/197917. The barrier was deposited directly onto Nafion® 212 using the Reactive Spray Deposition Technology, in which carbon-rich particles (∼4–10 nm in diameter) formed in the flame were deposited simultaneously with a mixture of 1100EW Nafion® and Vulcan® XC-72R (∼40 nm diameter) that was sprayed from air-assisted secondary nozzles. During cycles at fixed capacity, the presence of the barrier layer significantly reduced battery self-discharge; the average variation of battery state of charge compared to a reference cell with Nafion® 115 was reduced from 21% to 7%. Moreover, battery energy efficiency was increased by nearly 5%, indicating that the barrier layer does not significantly hinder proton transport. During cycles at 50 mA cm−2 with fixed cut-off voltages, the barrier layer exhibited stable operation, maintaining a coulombic efficiency around 99.4%. Additionally, the use of the barrier layer projects to a 30% reduction of stack-specific cost.

Development of novel and improved catalysts and membrane electrode assemblies (MEAs) for proton exchange membrane (PEM) based energy conversion devices is of crucial importance for widespread application of the PEM fuel cells and electrolyzers. These PEM based devices are critical for the Hydrogen Economy (HE) implementation. The HE is the economy of the near future and is the only viable alternative to the current fossil fuel-based economy. This future green economy will eliminate the greenhouse gas emissions and stop the imminent global warming and climate change. Currently, the main challenges that the state-of-the-art MEAs are facing are: (i) high cost because of the high platinum group metals (PGM) loadings in their catalysts layers and time consuming and expensive multi-step fabrication processes; and (ii) poor durability caused by the instability of the catalysts and the materials [1, 2]. Herein, we demonstrate the capabilities of the unique Reactive Spray Deposition Technology (RSDT) for fabrication of advanced catalysts for the oxygen evolution reaction (OER) and MEAs that could overcome these challenges. The RSDT is a flame assisted method [3, 4] that combines the catalysts synthesis and deposition directly on the PEM membrane in one-step, which results in fast and facile fabrication of large (up to 1000 cm2) MEAs for application in PEM fuel cells and electrolyzers [4]. In addition, this technology allows precise control of the composition, morphology, and particle size distribution of wide range of nanoparticles supported and unsupported on carbon, and thus ensures fine tuning of the catalysts’ activity and durability. Ir/IrOx unsupported catalysts fabricated by RSDT have been studied in half cell configuration by rotating disc electrode (RDE) technique in 0.1 M HClO4 and showed improved activity towards the OER in comparison to the state-of-the-art commercial IrO2 catalyst. Furthermore, MEAs with geometric area of 86 cm2 and one order of magnitude lower Ir loading in the anode catalyst layer in comparison to the state-of-the-art MEAs for PEM water electrolyzers [5], were fabricated by the RSDT and tested for 5000 hours at current density of 1.8 A cm-2, 50 oC, and 400 psi differential hydrogen pressure. Diagnostic tests that include polarization curves, electrochemical impedance spectroscopy, linear sweep voltammetry, and hydrogen crossover measurements were performed periodically in order to evaluate the cell performance change during the long-term durability test. After the test, the MEAs were disassembled and subjected to comprehensive post test analysis. A wide range of techniques including high-resolution TEM, STEM, EDS, SEM, ICP, XCT, XPS, and digital optical microscopy, have been used to study the degradation mechanisms governing the performance loss in the MEAs during the long-term steady state operation, and the results will be presented and discussed in detail in this talk. References https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf https://www.energy.gov/sites/prod/files/2015/06/f23/fcto_myrdd_production.pdf Kim, S., Myles, Maric, R., et al. Electrochimica Acta, 177, 190-200 (2015). Yu, H., Baricci, A., Bisello, A., Bonville, L., Maric, R., et al. Electrochimica Acta, 247, 1155-1168 (2017). Ayers, K. Current Opinion in Electrochemistry, 18, 9–15 (2019).

The reactive spray deposition technology (RSDT) that has been developed at UConn, is one of the most promising new methodologies for direct fabrication of large scale MEAs for proton exchange membrane water electrolyzers (PEMWEs) with ultra-low platinum group metals (PGM) loadings in their catalyst layers [1,2]. The RSDT is an open to air flame assisted method for fabrication of MEAs that combines the catalysts synthesis and direct deposition on the Nafion® membrane in one step [2,3]. Thus, this method eliminates multiple time consuming and expensive steps in the MEA manufacturing process and reduces the fabrication time from days and weeks to hours. As a dry spray methodology with integrated system for in-situ quality control of the catalysts and catalyst layers (CLs),[1] the RSDT allows precise control of the catalysts composition, loading, porosity, thickness, and ionomer content. Furthermore, it has been demonstrated that by controlling the precursor solutions flow rates, and the flame deposition parameters, catalyst layers with gradient distribution in the nanoparticle size, as well as in the PGM loading in the CLs can be achieved, which ensures fine tuning of the activity and durability performance of the MEAs of interest [4].Herein, we will demonstrate the capabilities of the RSDT methodology to fabricate large scale MEAs for PEMWEs, that have one order of magnitude lower PGM loadings in the CLs and activity and durability performance comparable to the state-of-the-art commercial MEAs. RSDT fabricated MEAs with geometric area of the electrodes of 86 cm2 and 680 cm2, and loadings of 0.2 mgPt/cm2 in the cathode and 0.3 mgIr/cm2 in the anode have been tested at conditions typical for industrial electrolyzers. The performed long term steady-state test for over 5000 hours, as well as the measured polarization curves and EIS spectra clearly show excellent activity and durability performance of these MEAs. In addition, the RSDT fabricated MEAs have integrated recombination layers that effectively reduce the hydrogen crossover to below 10 %LFL. Furthermore, comprehensive post-test analysis of the MEAs after 5000 hours of operation has been performed and the degradation mechanisms governing their performance loss were identified and will be discussed in detail.References Mirshekari, G., Ouimet, R., Zeng, Z, Yu, H., Bliznakov, S., Bonville, L., Niedzwiecki, A., Errico, S., Capuano, C., Mani, P., Ayers, K., Maric, R., 2021.“High-Performance and Cost-Effective Membrane Electrode Assemblies for Advanced Proton Exchange Membrane Electrolyzers: Long-Term Durability Assessment”, International Journal for Hydrogen Energy, 46(2), 2021, pp. 1526-1539.Roller, J., Maric, R., 2015. A Study on Reactive Spray Deposition Technology Processing Parameters in the Context of Pt Nanoparticle Formation, Journal of Thermal Spray Technology, 24(8) December 2015 pp. 1529-1541.Yu, H., Baricci, A., Casalegno, A., Guetaz, L., Bonville, L., Maric, R., 2017. Strategies to mitigate Pt dissolution in low Pt loading proton exchange membrane f...

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Chemi-resistive ammonia sensors based on α-MoO3 are a simple and selective technology for measuring the concentration of gaseous ammonia [1-4]. Typically, the sensor consists of an interdigitated electrode substrate covered with the α-MoO3 film [2-4]. The measured electrical resistance of the α-MoO3 film on the substrate changes as a function of gaseous ammonia concentration to which the film is exposed. α-MoO3 is a n-type semiconductor [4], meaning that the electrical conductivity of α-MoO3 is dominated by free electrons as opposed to holes. The mechanism of ammonia sensing by n-type semiconducting metal oxides is commonly attributed to the alteration of the charge depletion layer (Λ) of the metal oxide particles through the reaction of ammonia with adsorbed oxygen species [4-10]. For n-type semiconducting metal oxides, the charge depletion layer model predicts that the resistance of the sensing film (α-MoO3) decreases with increasing concentration of reducing gas, such as ammonia, to which the sensor is exposed. In addition to the charge depletion layer model, other sensing mechanisms have been suggested, such as the partial reduction by ammonia of the surface of the MoO3 film and the formation of oxygen vacancies in the MoO3 lattice [1, 2, 11]. It is generally understood that the sensitivity of the sensor can be tailored based on the particle size and morphology of the metal oxide film [5-10]. Therefore, as initial work, sensors with α-MoO3 films of varying morphology on gold-interdigitated electrodes with alumina substrate (DropSens P/N: IDEAU200) were fabricated by Reactive Spray Deposition Technology (RSDT) and tested for sensitivity to ammonia in synthetic air at 400ºC. This work was presented at the 236th ECS meeting as an oral presentation, “Ammonia-Sensing Properties of α-MoO3 Fabricated by Reactive Spray Deposition Technology (RSDT).” From these sensors, the sensor with the strongest response to ammonia was selected for further testing in ammonia and synthetic air at 400ºC with constant 1V DC bias across the sensor. This additional testing consisted of three 12-hour cycles, in which six ammonia concentrations were tested: 0.1, 0.3, 0.5, 1, 3, and 4.9ppm. Between each concentration, the sensor was flushed with synthetic air. Between each 12-hour cycle, the sensor was held at 400ºC in synthetic air for approximately 12 hours. For each ammonia concentration in each cycle, the sensing response (S) was calculated as S=Rgas/Rair where Rgas is the resistance of the sensor for a particular concentration of ammonia and Rair is the resistance of the sensor for the air flush immediately before the particular concentration of ammonia. Sensing results are shown in Figure 1. The results from cycle to cycle are quite repeatable; however, the results consistently depart from the prediction of the charge depletion layer model between 3ppm and 4.9ppm. In an attempt to assess whether these unexpected results could be attributed to morphological or crystallographic changes in the MoO3 film, SEM images and XRD plots were compared before and after the three 12-hour test cycles. SEM images were taken on both the gold and alumina portions of the electrode. Figures 2 and 3 show the SEM images and XRD plots, respectively. Neither SEM nor XRD results suggest that the unexpected sensing behavior may be attributed to morphological and/or crystallographic changes of the MoO3 film. To better explain the sensing behavior, additional testing, including electrochemical impedance spectroscopy (EIS) and X-Ray photoelectron spectroscopy (XPS), will be performed on the sensor. XPS has been reported to provide information regarding the partial reduction of the surface of the MoO3 film [1]. EIS can be used to develop equivalent circuits that are representative of the sensor. It has been shown that different properties of the sensing film, such as grain size and particle contact, correspond to different equivalent circuits [12]. Therefore, EIS data at varying ammonia concentrations can help to develop an understanding of the film properties involved in the sensing mechanism. Results from EIS, XPS, SEM, and XRD, along with an understanding of the charge depletion layer model, will be used to propose a thorough ammonia sensing mechanism for α-MoO3. References 1. A. Prasad, etal. Thin Solid Films. 436 2003 pp. 46-51. 2. P. Gouma, etal. IEEE Sensors Journal. 18 (1) 2010 pp. 49-53 3. A. Güntner, etal. Sensors and Actuators B. 223 2016 pp. 266-273. 4. D. Kwak, etal. ACS Appl. Mater. Interfaces. 11 2019 pp. 10697-10706 5. C. Rout, etal. Nanotechnology. 18 2007. 6. M. Franke, etal. Small. 2 (1) 2006 pp. 36-50. 7. P. Feng, etal. Appl. Phys. Letters. 87 (213111) 2005 8. Y. Chen, etal. Nanotechnology. 17 2006 pp. 4537-4541. 9. V. Sysoev, etal. Nano Letters. 6 (8) 2006 pp. 1584-1588. 10. H. Ogawa, etal. Journal of Applied Physics. 53 1982 pp. 4448-4455. 11. A. Prasad, etal. Sensors and Actuators B. 93 2003 pp. 25-30. 12. N. Barsan, etal. Fresenius Journal of Analytical Chemistry. 365 1999 pp. 287-304. Figure 1

Vanadium redox flow battery (VRFB) is a promising technology for energy storage because of its independent energy to power ratio and long cycle life. However, VRFB commercialization is still hindered by some technological issues, among which the capacity loss induced by the undesired transport of ions across the ion-exchange membrane. Depending on the nominal operating condition, the choice of a suitable membrane for VRFB application results from the trade-off between low vanadium ions permeability and high proton conductivity. Usually, in order to reduce undesired ion fluxes between the two half-cell electrolytes, the membrane thickness is relatively high, implying increased ohmic losses (i.e., reduced energy efficiency) and system capital cost. In fact, state of the art membranes can represent up to 50% of stack capital cost [1]. Nafion® is widely used due to its high conductivity, but it is not ideally selective towards vanadium ions, leading to the adoption of thicker membrane to limit capacity loss. Alternative cation exchange membranes like SPEEK or SPI are promising because of their reduced permeability, but the low conductivity limits system power density. Instead, anion exchange membranes are still limited by the poor chemical stability and low conductivity [2]. In a recent work [3], the authors demonstrated the proof of concept of an additional selective layer to mitigate vanadium crossover. The selective layer, termed as barrier, is a porous component in which pores size, tortuous path, thickness and composition are designed to improve ion/proton selectivity. The proof of concept was manufactured with reactive spray deposition technology (RSDT), which is a flame-based synthesis process unique to Dr. Radenka Maric’s research group. For the fabrication of the proof of concept, carbon-rich particles ∼4-10 nm in diameter were formed in the RSDT flame and were deposited directly onto Nafion® 212 (50 μm thick) simultaneously with a mixture of 1100EW Nafion® and Vulcan® XC-72R (∼40 nm diameter) that was sprayed from air-assisted secondary nozzles. The presence of the barrier layer significantly reduced battery self-discharge, as reported in [3]. In this work, different compositions and morphologies of the barrier layers were analysed in order to improve ion/proton selectivity. In particular, the influence of ionomer to carbon ratio (I/C), the amount of carbon-rich particles and the introduction of silica were investigated. Moreover, the effect of Nafion® 211 (25 μm thick) as a support for barrier deposition was also evaluated. The barrier layers were characterized in a 25 cm2 cell [4] equipped with reference electrodes at both positive and negative electrode in order to monitor the corresponding electrolyte potential and get an insight into battery state of charge (SoC). In addition to electrochemical testing, the structure of the barrier layer was characterized using TEM and SEM. The most suitable I/C ratio was found to be included between 1 and 2, while the presence of carbon-rich particles significantly contributed to crossover reduction, with a minor impact on proton conductivity. Also the introduction of silica nanoparticles from primary nozzle was effective for vanadium ion selectivity, in particular when the amount of carbon from the secondary nozzle is reduced. In fact, the most promising barrier layer resulted the one composed by only silica and ionomer. This layer was also deposited on Nafion® 211 (Figure 1), exhibiting an excellent trade-off between ion selectivity and proton conductivity. This barrier was proved to be stable over 1,000 cycles, presenting a stable coulombic efficiency of 99.5%, with an average capacity decay of 0.08%/cycle at 100 mA cm-2. Figure 1 – SEM image of only Silica barrier deposited on Nafion® 211. References: [1] C. Minke et al., Journal of Power Sources 376 (2018) 66-81. [2] Y. Shi et al., Applied Energy 238 (2019) 202-224. [3] M. Cecchetti et al., Journal of the Electrochemical Society 167 (2020) 130535. [4] M. Cecchetti et al., Journal of Power Sources 400 (2018) 218-224. Figure 1

Chemi-resistive ammonia sensors based on alpha-phase molybdenum trioxide (α-MoO3) are a simple and selective technology for measuring the concentration of gaseous ammonia for applications such as human breath sensing and monitoring selective catalytic reduction (SCR) systems [1-7]. The basic operating principle of these sensors is the correlation between the electrical resistance of the sensor and the concentration of ammonia to which the sensor is exposed. Typically, the sensor consists of an interdigitated electrode substrate covered with the α-MoO3 film [1-7]. The electrical resistance of the α-MoO3 film changes as a function of ammonia concentration. This change in resistance of the α-MoO3 film is commonly attributed to the alteration of the charge depletion layer (Λ) of the α-MoO3 particles through the reaction of ammonia with adsorbed oxygen species [5-9]. For α-MoO3, the charge depletion layer model predicts that the resistance of the film should decrease with increasing ammonia concentration. In addition to the charge depletion layer model, other sensing mechanisms have been suggested, such as the partial reduction by ammonia of the surface of the α-MoO3 film [1,2] and the formation of oxygen vacancies in the α-MoO3 lattice [3]. The α-MoO3 film can be fabricated by a variety of methods, including sol-gel synthesis, ion beam deposition, hydrothermal synthesis, and flame spray pyrolysis [1-6]. In this work, the α-MoO3 film is fabricated using Reactive Spray Deposition Technology (RSDT) followed by annealing to develop crystallinity. The RSDT is a flame-based process that can be used for the fabrication of α-MoO3 films at a commercial scale. The initial development of ammonia sensors with RSDT-fabricated α-MoO3 films of varying morphology was presented at the 236th ECS Meeting as an oral presentation, “Ammonia-Sensing Properties of α-MoO3 Fabricated by Reactive Spray Deposition Technology (RSDT).” From these initial sensors, the sensor with the strongest response to ammonia was selected for further testing at concentrations between 0.1 ppm and 5 ppm ammonia in dry air at 400°C. These sensing results and the materials characterization of the sensor by SEM and XRD before and after 80 hours of continuous testing are reported in Ebaugh et al. [7] and are shown in Figures 1-3. In Figure 1, the sensing results among the three test cycles comprising the 80 hours of testing are quite repeatable; however, the results consistently depart from the prediction of the charge depletion layer model between 3 ppm and 5 ppm. To better explain this unexpected sensing behavior, additional electrochemical testing and materials characterization is needed. Specifically, normal pulse voltammetry (NPV) can be used to provide data pertaining to the electronic path through the α-MoO3 film and the reaction and transport of chemical species adsorbed on the α-MoO3 film. TEM analysis of the α-MoO3 can show the nanoscale morphology of the sensing material. It is well established that the nanoscale morphology of the metal-oxide film impacts its sensing behavior [8-10]. XPS can also be used to investigate the possible reduction of the surface of the α-MoO3 particles and the formation of oxygen vacancies in the α-MoO3 lattice. Data from the materials characterization of the sensing film (XRD, SEM, TEM, and XPS) will be used in conjunction with sensing results and NPV data to develop an explanation for the sensor behavior observed in [7]. An understanding of the ammonia-sensing mechanism will help to guide the optimization of the RSDT-fabricated α-MoO3 film to improve the performance of the sensor for a wide range of applications. References A. K. Prasad, et al. Thin Solid Films. 436 2003 pp. 46-51. P. Gouma, et al. IEEE Sensors Journal. 10 (1) 2010 pp. 49-53. A. K. Prasad, et al. Sensors and Actuators B: Chemical. 93 2003 pp. 25-30. P Gouma, et al. Journal of Breath Research. 5 2011 p. 037110. A. T. Güntner, et al. Sensors and Actuators B: Chemical. 223 2016 pp. 266-273. D. Kwak, et al. ACS Applied Materials & Interfaces. 11 2019 pp. 10697-10706. T. A. Ebaugh, et al. ACS Applied Materials & Interfaces. Under review. C. S. Rout, et al. Nanotechnology. 18 2007 p. 205504. M. E. Franke, et al. Small. 2 (1) 2006 pp. 36-50. N. Barsan, et al. Fresenius Journal of Analytical Chemistry. 365 1999 pp. 287-304. Figure 1