Aron Varga

Spray drying as a novel and scalable fabrication method for nanostructured CsH2PO4, Pt-thin-film composite electrodes for solid acid fuel cells

Spray drying was explored as a new CsH2PO4 nanoparticle synthesis method and a systematic parameter study was conducted to discover the set leading to optimal deposition rate and particle size distribution for applications in solid acid fuel cell electrodes. The nanoparticles were deposited directly onto either a carbon paper current collector or a dense CsH2PO4 electrolyte pellet with a deposition rate of 1 mg h−1 cm−2 measured to be the same order of magnitude as for previously employed electrospray. However, the total nanoparticle production rate is at 165 mg h−1 almost two orders of magnitude higher than the total production rate of electrospray. Novel, high performance solid acid fuel cell electrodes were fabricated by depositing CsH2PO4 nanoparticles onto a dense, uniaxially pressed CsH2PO4 electrolyte pellet, forming a three dimensional, porous, interconnected nanostructure, and thus providing a large surface area for subsequent Pt thin film deposition via magnetron sputtering. Electrochemical measurements via impedance spectroscopy in a symmetric cell configuration Pt + CsH2PO4|CsH2PO4|CsH2PO4 + Pt show good reproducibility, excellent mass normalized activity as well as stability over a 24 h period.

The next generation solid acid fuel cell electrodes: stable, high performance with minimized catalyst loading

Low electrode impedance paired with low catalyst loading in intermediate and low temperature fuel cells is extremely difficult to achieve, posing a major obstacle to commercialization. Here we demonstrate a scalable and facile route to obtain nanostructured composite solid acid fuel cell electrodes consisting of Pt decorated carbon nanotubes and CsH2PO4 microparticles as the electrolyte. Electrochemical impedance measurements in humidified hydrogen at 240 °C show very low 0.05 Ω cm2 area normalized electrode resistance, with a Pt loading of only 0.41 mgPt cm−2. This is a reduction of the Pt loading by more than one order of magnitude paired with even lower electrode impedance values compared to the current state-of-the-art in literature. Fuel-cell measurements show remarkably stable electrode performance over a 17 h period with a final degradation rate of 0.1% h−1.

In-situ determination of the electrochemically active platinum surface area: key to improvement of solid acid fuel cells

The electrochemically active surface area (ECSA) is an important parameter when investigating and developing electrodes for fuel cells. Various methods exist to determine the ECSA of polymer electrolyte membrane fuel cells both in and ex situ. However, for intermediate-temperature solid acid fuel cells (SAFC), a quantitative method to determine the ECSA has not yet been established. Here, we show that analysis of the electrochemical surface oxide reduction is a viable approach to quantify the ECSA in SAFCs. We validate the method using a Pt foil as model electrode and ex situ hydrogen underpotential deposition measurements. The applicability to SAFCs with composite powder electrodes is demonstrated in combination with an accelerated degradation test. The results reveal a surprisingly low decrease of the ECSA during operation of solid acid fuel cells. The method will enable a better understanding of novel nano-composite electrodes that are necessary for high performance devices.

Fast Degradation for High Activity: Oxygen- and Nitrogen-Functionalised Carbon Nanotubes in Solid-Acid Fuel-Cell Electrodes

Similar to polymer electrolyte membrane fuel cells, the widespread application of solid acid fuel cells (SAFCs) has been hindered partly by the necessity of the use of the precious-metal catalyst Pt in the electrodes. Here we investigate multi-walled carbon nanotubes (MWCNTs) for their potential catalytic activity by using symmetric cell measurements of solid-acid-based electrochemical cells in a cathodic environment. For all measurements, the carbon nanotubes were Pt free and subject to either nitrogen or oxygen plasma treatment. AC impedance spectroscopy of the electrochemical cells, with and without a DC bias, was performed and showed significantly lower initial impedances for oxygen-plasma-treated MWCNTs compared to those treated with a nitrogen plasma. In symmetric cell measurements with a DC bias, the current declines quickly for oxygen-plasma-treated MWCNTs and more slowly, over 12 days, for nitrogen-plasma-treated MWCNTs. To elucidate the degradation mechanisms of the oxygen-plasma-treated MWCNTs under SAFC operating conditions, theoretical calculations were performed using DFT. The results indicate that several degradation mechanisms are likely to occur in parallel through the reduction of the surface oxygen groups that were introduced by the plasma treatment. This finally leads to an inert MWCNT surface and a very low electrode performance. Nitrogen-plasma-treated MWCNTs appear to have a higher stability and may be worthwhile for future investigations.

Facile and scalable synthesis of sub-micrometer electrolyte particles for solid acid fuel cells

Nanostructuring fuel cell electrodes is a viable pathway to reach high performance with low catalyst loadings. Thus, in solid acid fuel cells, small CsH2PO4 electrolyte particles are needed for the composite powder electrodes as well as for thin electrolyte membranes. Previous efforts have resulted in significant improvements in performance when using sub-micrometer CsH2PO4 particles, but laborious methods with low throughput were employed for their synthesis. In this work, we present a simple, robust, and scalable method to synthesize CsH2PO4 particles with diameters down to below 200 nm. The method involves precipitating CsH2PO4 by mixing precursor solutions in alcohol in the presence of a dispersing additive. The influence of the concentrations, the batch size, the solvent, and the mixing process is investigated. The particle size decreases down to 119 nm with increasing amount of dispersing additive. Mixing in a microreactor leads to a narrower particle size distribution. The particle shape can be tuned by varying the solvent. The ionic conductivity under solid acid fuel cell conditions is 2.0 × 10−2 S cm−1 and thus close to that of CsH2PO4 without dispersing additive.

Towards rational electrode design: quantifying the triple-phase boundary activity of Pt in solid acid fuel cell anodes by electrochemical impedance spectroscopy

Solid acid fuel cells based on CsH2PO4 as the electrolyte and Pt as the electrocatalyst are a promising intermediate temperature energy conversion technology. However, improving the electrode microstructure to achieve an optimal area-normalized resistance, while keeping or even lowering the Pt catalyst loading is particularly challenging due to the solid nature of the electrode components. Several architectures have been empirically developed, such as CsH2PO4 micro- or nanoparticles mixed with Pt nanoparticles or covered with thin-film Pt. For an optimal electrode design, a quantitative measurement of the fundamental parameter, namely the specific triple-phase boundary activity of Pt at the fuel cell operating conditions, is needed. Geometrically simple, well-controlled electrodes are typically fabricated for this purpose via lithography techniques. This approach however is not suitable for solid acids due to the water solubility of the electrolyte. Here we present a simple, water-free fabrication scheme to create a controlled electrode geometry consisting of a hole-patterned Pt thin film that allows measurements of the specific triple-phase boundary activity of Pt in an anodic environment. Based on electrochemical impedance spectroscopy measurements in a symmetric cell configuration, the triple-phase boundary activity of Pt is determined to be on the order of 1.3 kΩ m. This information is critical for the rational design of a solid acid fuel cell electrode without tedious empirical optimization.Graphical abstract