Catalyst Ink Research Articles

Silver-Based Supportless Membrane Electrode Assemblies for Electrochemical CO2 Reduction

CO2 is one of the major contributors to the emission of greenhouse gases boosting climate change. Meanwhile, renewable energy production is fluctuating due to weather conditions, demanding for appropriate storage, e. g. by Power-to-X technology. The first step of these processes, the production of hydrogen via water electrolysis, is typically linked to a second step to obtain hydrocarbons [1]. Meanwhile, electrochemical CO2 reduction (eCO2R) is capable of generating chemical feedstocks by converting excess CO2, simultaneously using renewable energy sources during peak times. A promising pathway of eCO2R focuses on CO, as it can be produced with high selectivity at silver catalysts, concurrently generating hydrogen as the only byproduct [2]. To partly overcome the mass transport limitations resulting from the very limited solubility of CO2 in aqueous electrolytes, gas diffusion electrodes (GDEs) are typically used for eCO2R in a three-chamber setup with anolyte, catholyte and separate gas compartment [3].However, there are still considerable overpotentials in this setup, among others caused by ohmic losses such as the electrolyte resistance. Employing membrane electrode assemblies (MEAs), either both electrodes or one of them can be combined with the membrane to form a full- or half-MEA, respectively, resulting in a significant decrease in cell voltage. Although there have already been studies on the fabrication of MEAs employing silver catalysts for eCO2R [4, 5], they are still very limited in options and mostly based on carbon gas diffusion layers (GDLs).The manufacturing approach applied in this work is based on a catalyst ink recipe for sintered silver GDEs originally developed for chlor-alkali electrolysis by Moussallem et al. [6]. Instead of using Nickel mesh as a substrate, the suspension is spray-coated on a stainless steel plate to enable the required treatment at temperatures above 300 °C. Afterwards, the catalyst layer is hot-pressed on the prepared anion exchange membrane at more moderate temperatures, forming a supportless cathodic half-MEA.Resulting from variations in the manufacturing procedure, different MEAs are electrochemically characterized, examining Faradaic efficiencies as well as cell voltages, also in comparison to measurements performed in three-chamber setup. Addressing challenges in product efficiency and membrane degradation, it is shown that this type of MEA is capable of eCO2R to CO, already reducing the cell potential at elevated current densities by nearly 50 %, see fig. 1.[1] Tom Kober et al. Report: perspectives of power-to-X technologies in Switzerland: supplementary report to the white paper. en. Technical report. 2019. doi: 10.3929/ETHZ-B-000525806.[2] Yoshio Hori et al. “Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous media”. Electrochimica Acta, 39 (11-12), (1994), 1833–1839.[3] Thomas Burdyny and Wilson A. Smith. “CO2 reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially-relevant conditions”. Energy & Environmental Science, 12 (5), (2019), 1442–1453. doi: 10.1039/C8EE03134G.[4] Zengcai Liu et al. “CO2 electrolysis to CO and O2 at high selectivity, stability and efficiency using Sustainion membranes”. Journal of The Electrochemical Society, 165 (15), (2018), J3371–J3377. doi: 10.1149/2.0501815jes.[5] Jonghyeok Lee et al. “Electrochemical CO2 reduction using alkaline membrane electrode assembly on various metal electrodes”. Journal of CO2 Utilization, 31 (2019), 244–250. doi: 10.1016/j.jcou.2019.03.022.[6] Imad Moussallem et al. “Development of high-performance silver-based gas-diffusion electrodes for chlor-alkali electrolysis with oxygen depolarized cathodes”. Chemical Engineering and Processing: Process Intensification, 52 (2012), 125–131. doi: 10.1016/j.cep.2011.11.003. Figure 1

Fe and Zn Addition Effects of Ti Oxide-Based Electrocatalyst on Catalytic Activity for Oxygen Reduction Reaction

The conventional catalyst of polymer electrolyte fuel cell (PEFC) is a platinum which is categorized as precious metal with very high-cost material, and its performance is limited in principal. From this point of view, the non-precious metal electrocatalyst should be required. We have focused and studied group 4 and 5 metal oxide-based electrocatalyst as non-platinum catalysts for the oxygen reduction reaction (ORR) because of low-cost, abundant reserves, and high stability in acidic electrolytes [1-2]. We found that titanium oxide prepared from TiOTPyzPz supported on multi-walled carbon nanotubes had superior ORR activity [3]. On the other hand, it was published as an international patent that the addition of other elements such as Fe and Ni is affected to enhance the ORR activity of Ti oxide-based electrocatalyst [4]. We have also applied for the TiOTPyzPz as a starting material with Fe, Ni, and Zn addition to enhance the ORR activity of Ti oxide-based electrocatalyst [5-6]. In this study, we have investigated the addition effects of Zn and Fe for Ti oxide-based electrocatalyst on the catalytic activity for the ORR.2,3-Dicyanopyrazine, urea, and Ti isopropoxide were dissolved in quinoline and refluxed to synthesize TiOTPyzPz. Iron acetate and zinc acetate were also added to dissolve in quinoline to obtain the Fe and Zn-added TiOTPyzPz as a starting material. These starting materials were mixed with carbon nanotube by ball-milling to prepare the precursors. These precursors were heat-treated under low oxygen partial pressure at 900 oC for 3 h to obtain the oxide-based electrocatalyst powder. The catalyst powder was dispersed into 1-propanol with Nafion solution to prepare a catalyst ink. The ink was dropped on a glassy carbon rod to use as a working electrode in electrochemical measurement.Electrochemical measurements were performed in 0.5 mol dm-3 H2SO4 at 30 oC with a conventional 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as used as a reference and counter electrode, respectively. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV s-1 from 0.2 V to 1.2 V vs. RHE under O2 and N2. The ORR current (i ORR) was determined by calculating the difference between the current under O2 and N2.Figure 1 shows the ORR polarization curves Fe and/or Zn addition to the Ti oxide-based electrocatalysts. The Fe and Zn added Ti oxide-based electrocatalyst (Fe,Zn-TiOx) was obviously the highest activity for the ORR, and it was higher ORR activity than Fe added Ti oxide-based electrocatalyst (Fe-TiOx), Zn added Ti oxide-based electrocatalyst (Zn-TiOx), and Ti oxide-based electrocatalyst (TiOx) without addition. It reveals that the addition of Fe and Zn was found to be effective for enhancing the ORR activity, and Fe addition was more effective than Zn addition for enhancing the ORR activity. XRD pattern of TiOx shows several peaks identified TiO2-Rutile and TiC0.3N0.7 while the XRD pattern of Fe-TiOx, Zn-TiOx, and Fe,Zn-TiOx also shows several peaks identified similar to TiOx. According to results from ICP analysis, the contain amount of Ti and Fe in Fe,Zn-TiOx was similar to preparation amount in precursor, but that of Zn showed below detection limit. From SEM images, Zn-TiOx and Fe,Zn-TiOx have more space in its powder than Fe-TiOx and TiOx. Based on above results, it is suggested that Zn addition affected to increased electrochemical surface area of TiOx, and Fe addition affected to form active site [7] in TiOx as similar case as Zr oxide-based electrocatalyst [8].Acknowledgement: The authors thank New Energy and Industrial Technology Development Organization (NEDO) and ENEOS Tonen General Research / Development Encouragement & Scholarship Foundation for financial support.Reference A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).A. Ishihara, S. Tominaka, S. Mitsushima, H. Imai, H. Imai, O. Sugino, and K. Ota, Curr. Opin. Electrochem., 21 , 234 (2020).S. Tominaka, A. Ishihara, T. Nagai, and K. Ota, ACS Omega, 2, 5209 (2017).K. Takahashi, T. Imai, R. Monden, Y. Wakisaka, and S. Sato, Oxygen Reduction Catalyst, Process for Producing Same, and Polymer Electrolyte Membrane Fuel Cell, WO/2013/008501.K. Matsuzawa, M. Obata, Y. Takeuchi, Y. Ohgi, K. Ikegami, T. Nagai, R. Monden and A. Ishihara, ECS Trans., 108(7), 181 (2022).K. Matsuzawa, M. Obata, Y. Takeuchi, Y. Ohgi, K. Ikegami, T. Nagai, R. Monden and A. Ishihara, ECS Trans., 108(7), 189 (2022).J-H. Kim, A. Ishihara, S. Mitsushima, N. Kamiya, and K. Ota, Acta, 52, 2492 (2007).Y. Takeuchi, K. Watanabe, K. Matsuzawa, T. Nagai, K. Ikegami, R. Monden, and A. Ishihara, Chem. Lett., 51, 927 (2022). Figure 1

Catalyst Layers with Blended Ionomer Using Short- and Long-Side Chain Dispersions

Hydrogen-based energy conversion technology is based on the membrane electrode assembly (MEA) comprising of an electrolyte and a pair of catalyst layers. It should have high efficiency and allow dynamic operation at low temperature. Electrochemical reactions for hydrogen oxidation reaction and oxygen reduction reaction occur in the catalyst layers in the MEA. Catalyst layers for MEA is prepared by catalyst inks comprising of electrocatalyst, ionomer dispersion and additional solvents. Ionomers plays an important roles for the formation of porous structure of catalyst layer for transportation of gas and reactant and for the ionic conduction. The ionomers are divided that long side chain (LSC) and short side chain (SSC) depending on the length of side chain. Long side chain (LSC) ionomers shows high performance at low temperatures and high humidity, while short side chain (SSC) ionomers shows high performance at high temperatures and low humidity. It has been reported that the properties of ionomer dispersions substantially influence the performance of catalyst layers due to the different interaction between electrocatalysts and ionomer. In this study, to investigate the effect of the properties of ionomer, various contents of blended ionomer dispersions were prepared. Properties of single and blended ionomers were characterized using catalysts layers in terms of I-V polarization, cyclic voltammetry, electrochemical impedance spectroscopy and microscopic evaluation such as porosimetry. Acknowledgment This research was supported in part by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning(NRF-2019M3E6A1063677) and by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040520).

Alkaline-Stable Anion Conducting Ionomers for Anion Exchange Membrane Water Electrolyzers

Water electrolysis is a process that uses renewable energy to decompose water into oxygen and hydrogen. It is one of the most promising alternatives to produce and store new energy from renewable energy resources. In this study, pore-filled anion exchange membranes (PFAEMs) were prepared by using quaternary ammonia groups along with various cross-linker groups with different chain lengths and using porous polyethylene substrates which were pretreated from hydrophobic into hydrophilic using surfactants. The conductivity of the anion exchange membranes was measured by the in-plane cell and through-plane at room temperature or 60 ℃. In addition, the characterizations of anion exchange membranes such as water uptake, swelling ratio, mechanical strength, contact angle and TGA were carried out. As a result, the PFAEM exhibited remarkably high ion conductivity at extremely low areal swelling ratio and excellent chemical stability in a NaOH solution for 500 h. As an essential component of anion exchange membrane water electrolyzers, catalyst layers prepared by the catalyst inks comprising of ionomer binder and electrocatalysts are sandwiched between the anion exchange membrane to form a membrane electrode assembly in order to evaluate its impedance and I-V polarization curves. Acknowledgment This research was supported in part by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040520) and by 2021 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment.

(Invited) Synchrotron X-Ray Techniques for Structure-Functionality of Ionomers: Current Status and New Directions

For the desired clean energy transition, hydrogen technologies are sought to play a critical role from electrolyzers to produce clean hydrogen from water to fuel cells to decarbonize heavy-duty transportation. Centerpiece to the successful operation of these technologies is the durable performance of the membrane-electrode assembly systems, which consists of an ion-conducting membrane between the electrodes where electrochemical reactions take place. Ion-conducting polymers, ionomers, play a key function in these systems as the separator solid-electrolyte, which requires typically a well-hydrated, phase-separated morphology that can block reactant species while transporting water and ions (protons). In addition, ionomers are also present in the electrodes as a nanometer thick film with a dual function of conducting medium and catalyst binder. In such heterogeneous structures, ionomer forms many interfaces with catalyst particles (e.g., Platinum in fuel cells) and interactions with the other species thereby making it challenging, yet critically important, to characterize their multi-scale structural features.What emerges from these distinct roles of ionomers in fuel cells and electrolyzers is the need for characterizing their structure in relation to the expected properties and desired performance for the operation. For example, for membrane, while structure-property relationships have been commonly investigated via x-ray scattering techniques, with almost three decade of work on interpretation of the so-called ionomer peak associated with the water nano-domains. However, the need for understanding and mitigating membrane degradation requires an expansion to structure-property-durability relationship. Similarly, for catalyst ionomers, their confined structure interfacing metal substrates have been idealized and examined by grazing-incidence x-ray scattering or reflectometry techniques. Yet, understanding the formation of catalyst layers from the mix of catalyst inks and ionomers require a shift to characterize their structure-processing-property relationships.This presentation will provide an overview of the current state of research on ionomer characterization with synchrotron x-rays and the ongoing an expansion of modalities that aim to image not only ionomer structure, but also their evolution under various applied stimulus via in-situ and operando x-ray techniques. Selected examples will be highlighted as case studies, demonstrating, first, how small/wide-angle x-ray scattering (SAXS/WAXS) can be repurposed with various environmental controls to understand structural changes due to degradation as well as the presence of cationic particles added to mitigate degradation. We will also show how energy-tunable x-rays at targeted absorption energies can be used to enhance the scattering signal and the acquired structural information. Then, we will show how grazing-incidence (GI) SAXS/WAXS can be employed to track the real-time evolution of the ionomer film nanostructure and crystallinity on a substrate during casting from a solution. These examples will be discussed together to provide a broader summary the current status and the future directions of both ionomer research for fuel cells and electrolyzer and the required characterization needs using synchrotron x-rays.

Influence of the Electrode Deposition Method of Graphene-Based Catalyst Inks for ADEFC on Performance.

The utilization of graphene as a catalyst support has garnered significant attention due to its potential for enhancing fuel cell performance. However, a critical challenge in electrode production still lies in the electrode preparation technologies and the restacking of graphene sheets, which can greatly impact the fuel cell performance alongside with catalyst development. This study aimed to investigate the impact of different electrode deposition methods for N-rGO-based catalyst inks on catalyst layer morphology, with a specific focus on graphene sheet orientation and its influence on the performance of alkaline direct ethanol fuel cells (ADEFCs). The dispersion behavior and ink stability of the catalysts were assessed using ultraviolet-visible light (UV-vis), ζ potential, and dynamic light scattering techniques. The morphology and physical properties of the gas diffusion electrodes (GDEs) were analyzed through Brunauer-Emmett-Teller measurements, contact angle measurements and scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy. The electrochemical behavior was evaluated both ex-situ, utilizing half-cell GDE measurements, and in situ, through single-cell tests. The N-rGO-based membrane electrode assembly, comprising Pt-free catalysts and a biobased membrane, exhibited outstanding performance in ADEFCs, as evidenced by high maximum power density values and long-term durability. The N-rGO-based membrane electrode assembly has demonstrated remarkable potential for high-performance fuel cells, presenting an exciting avenue for further exploration.

The impact of different side chain ionomer on membrane electrode assembly performance and durability

The structural engineering of catalyst layers (CLs) offers a promising route to the commercialization of proton exchange membrane fuel cells (PEMFCs). It remains a challenging research topic by exploring the effect of different side chain ionomer on the structure of the catalyst layer. An in-depth analysis of the ionomer dispersion behavior in the catalyst ink is key to achieving optimal CL structure. In this work, the effect of ionomer side chain structure on cluster dispersion in catalyst ink and CL microstructure is investigated to reveal the correlation between catalyst ink, CL microstructure, and performance. When short-side chain (SSC) ionomers are used, the particle size distribution of clusters is uniform, the ionomer film on the surface of clusters is thinner, a homogeneous and continuous ionomer network is obtained, and the surface cracks on the CL are smaller and less. Therefore, the SSC membrane electrode assembly (MEA) has better proton conduction ability and lower mass transport resistance. Meanwhile, SSC-MEA has better voltage stability and less voltage degradation at high current density. For complex fuel cell structures, SSC ionomers are preferably selected as proton conductors and binders for the CL to optimize the three-phase interface structure, which can provide satisfactory fuel cell performance.

Surface enrichment of ionomer in fuel cell catalyst layer prepared using slot-die coating method

The surface enrichment process of the ionomer during the drying of catalyst ink film produced by the slot-die coating method and its effect on fuel cell performance were investigated, wherein the solvent composition of the ink was considered. The slot-die-coated catalyst layer (CL) surface was observed using helium ion microscopy, conductive atomic force microscopy, and X-ray photoelectron spectroscopy, and ionomer enrichment was observed on the slot-die-coated CL surface. Furthermore, spontaneous adsorption of the ionomer at the air–solution interface was observed in the water-rich dispersion by measuring the surface tension of water–alcohol mixed solvents with and without an ionomer. Therefore, two ionomer enrichment mechanisms were proposed based on the alcohol content of the ink: an evaporation-dominated mechanism for alcohol-rich ink and an adsorption-dominated mechanism for water-rich ink. The effect of surface ionomer enrichment on proton conduction depends on the alcohol content of the catalyst ink. In addition, ionomer enrichment is conducive to the reduction of the oxygen transport resistance because of the reduction in the ionomer distributed inside the CL. These findings provide novel insights into the structural evolution and engineering of the CL.

Independent regulation of ionomer distribution in catalyst layer for proton exchange membrane fuel cell

The ionomer distribution in the catalyst layer (CL) of proton exchange membrane fuel cell (PEMFC) has become a vital research topic, especially under the low Pt loading condition. In the drying process, the ionomers adsorbed on Pt/C particles in the catalyst ink (called adsorbed ionomers) evolve into a thin ionomer coating on catalyst particles, while free ionomers in the catalyst ink randomly deposit to form ionomer bridges (called deposited ionomers). Many studies have reported the structure and regulation of adsorbed ionomers, but little is known about deposited ionomers which are also important for the formation of CL structure. In this work, a two-pronged CL preparation method is innovatively proposed to achieve independent regulation of the deposited ionomer formed by free ionomers in the catalyst ink, and the effects of different deposited ionomer structures on CL microstructure and fuel cell performance are investigated. Furthermore, the two-pronged method is compared with the conventional method to discuss its feasibility in optimizing the CL structure. The results indicate that the characteristics of free ionomer in the catalyst ink will affect the ionomer distribution and pore structure in the CL by controlling the deposited ionomer, thus determining proton conduction and mass transport. The uniform deposited ionomer morphology and short-side-chain (SSC) ionomer structure make the ionomer distribution in the CL uniform, which promotes proton conduction but also results in high mass transport resistance. Compared with the conventional method, the proton conduction resistance of the electrode prepared by the two-pronged method is decreased by 54.7%, the thickness of ionomer film on catalyst particles surface is decreased by about 2 nm, and the power density of the single cell is increased by 40.4% at the current density of 2500 mA cm−2. These findings provide new insights into the independent regulation of each component in the catalyst ink and the structural optimization of CL.

The role of ionomers in the electrolyte management of zero-gap MEA-based CO2 electrolysers: A Fumion vs. Nafion comparison

We study the role of binders (ionomers) in determining the life-span of gas diffusion electrodes (GDEs) used for high throughput CO2-to-CO electrolysis. We compare two typical ionomer materials (Nafion and Fumion, both are widely used for the preparation of Ag nanoparticles-based catalyst inks) to show that when used in zero-gap membrane/electrode assemblies, Fumion-containing inks are superior to Nafion-based ones and can uphold a very high CO-selectivity in the electrochemical CO2 reduction reaction for longer time. This is due to the ability of Fumion containing inks to suppress precipitation inside the GDE structure. As Fumion-fixed GDEs are significantly less hydrophobic than Nafion-fixed ones, our results contradict the widely accepted opinion that it is their non-wettability what mostly protects CO2-reducing GDEs from flooding. In turn, we argue that it is more important to maintain efficient electrolyte drainage pathways in the GDE structure, and explain the superiority of Fumion-fixed GDEs on this basis.

Preparation of PtAg/C catalysts for the oxygen reduction in PEM fuel cells: comparison of ultrasonical and mechanical methods as the catalyst ink dispersion technique

Preparation of PtAg/C catalysts for the oxygen reduction in PEM fuel cells: comparison of ultrasonical and mechanical methods as the catalyst ink dispersion technique

Performance of passive direct formate fuel cells using chitosan as an anode binder

ABSTRACT In this study, passive anion-exchange membrane (AEM) direct formate fuel cells (DFFCs) were developed and tested. Passive DFFCs consist of a fuel reservoir attached to a Pd-loaded anode, a commercially available AEM, and an air-breathing Pt-loaded cathode. Nafion or chitosan content of 22, 15, 10, or 5 wt% was blended with the anode catalyst ink as electrode binders. The polarization curves of the cells and Cyclic Voltammetry (CV) curves of the anodes were measured under different analyte compositions by mixing HCOOK and KOH with the molarity ranging from 1 M to 7 M at room temperature. The results showed that the ECSA values of the anode increased with the decrease in the binder content regardless of the binder type. The 15 wt% Nafion-based and 5 wt% chitosan-based passive AEM DFFCs fed with fuel containing 3 M HCOOK in 3 M KOH produced the highest maximum power density of 24.9 mW/cm2 and 27.5 mW/cm2, respectively. Finally, the current output of discharging of the 5 wt% chitosan-based passive AEM DFFCs at 0.4 V for 10 hr showed that providing the cell with more concentrated fuel produced more stable current as the concentrations of HCOOK and KOH are identical.

Elucidating the Roles of Nafion/Solvent Formulations in Copper-Catalyzed CO2 Electrolysis.

Nafion ionomer, composed of hydrophobic perfluorocarbon backbones and hydrophilic sulfonic acid side chains, is the most widely used additive for preparing catalyst layers (CLs) for electrochemical CO2 reduction, but its impact on the performance of CO2 electrolysis remains poorly understood. Here, we systematically investigate the role of the catalyst ink formulation on CO2 electrolysis using commercial CuO nanoparticles as the model pre-catalyst. We find that the presence of Nafion is essential for achieving stable product distributions due to its ability to stabilize the catalyst morphology under reaction conditions. Moreover, the Nafion content and solvent composition (water/alcohol fraction) regulate the internal structure of Nafion coatings, as well as the catalyst morphology, thereby significantly impacting CO2 electrolysis performance, resulting in variations of C2+ product Faradaic efficiency (FE) by >3×, with C2+ FE ranging from 17 to 54% on carbon paper substrates. Using a combination of ellipsometry and in situ Raman spectroscopy during CO2 reduction, we find that such selectivity differences stem from changes to the local reaction microenvironment. In particular, the combination of high water/alcohol ratios and low Nafion fractions in the catalyst ink results in stable and favorable microenvironments, increasing the local CO2/H2O concentration ratio and promoting high CO surface coverage to facilitate C2+ production in long-term CO2 electrolysis. Therefore, this work provides insights into the critical role of Nafion binders and underlines the importance of optimizing Nafion/solvent formulations as a means of enhancing the performance of electrochemical CO2 reduction systems.

Effects of Electrostatic Force and Network Structure on the Stability of Proton-Exchange Membrane Fuel Cell Catalyst Ink

The stability of the catalyst slurry of a proton-exchange membrane fuel cell (PEMFC) is of great significance to its large-scale production and commercialization. In this study, three kinds of slurries with different stabilities were prepared using different probe ultrasonic powers. The influence of electrostatic force and network structure on slurry stability was also studied. In addition, the catalyst layer (CL) and membrane electrode assembly (MEA) were further tested to determine the relationship between slurry stability, CL, and MEA performance. The results showed that the slurry prepared with 600 W dispersion power had the least agglomeration on day 12, which is due to the clusters in the slurry having the smallest average particle size and the largest surface area, thereby allowing them to absorb the most Nafion and have the largest electrostatic force to inhibit agglomeration. However, the slurry with 1200 W dispersion power had the least sedimentation after 9.4 days because the strength of the network structure in the slurry strengthened the most, resulting in a significant increase in viscosity and inhibition of sedimentation. Electrochemical tests showed that the MEA gradually exhibited worse electrical performance and higher impedance due to the agglomeration of catalyst particles caused by the standing process. Altogether, this study provides insights to better understand and regulate the stability of catalyst slurries.

Fine adjustment of catalyst agglomerate for the controllable construction of Co/Fe–N–C catalyst layers

Adjusting catalyst/ionomer agglomerate is crucial to a high-performance catalyst layer (CL), particularly for non-precious metal catalysts. As a sacrificial component, the solvent plays a vital role during the CL formation process and resulting microstructure, determining the performance of the anion exchange membrane fuel cells (AEMFCs). Here, solvent mixtures of isopropanol (IPA) and tetrahydrofuran (THF) are studied for the controllable construction of the Co/Fe–N–C CL. The results indicate that the aggregate state of the ionomers and catalysts and the final CL microstructure strongly depend on the ratio of IPA and THF. The CL prepared with IPA: THF = 1:2 (v/v) shows a homogeneous distribution of Co/Fe–N–C catalysts, embedded in the ionomer matrix, due to the fast volatilization and low viscosity of the catalyst ink. The Co/Fe–N–C CL with high moisture adsorption behavior and retention ability results in a high performance AEMFC with peak power density of 545.3 mW cm−2 at 80 °C. The obtained results can guide the rational design of microstructure in non-precious metal CLs for high-performance AEMFCs.

Scattering investigations into the structures of polymer-electrolyte-fuel-cell catalyst layers exhibiting robust performance against varying water fractions of catalyst ink solvents

Catalyst layers in polymer electrolyte fuel cells have a performance-influencing hierarchical porous structure comprising Pt-loaded carbon (Pt/C) particles and ionomer molecules. Ionomer thin films are formed on Pt/C particle aggregates by wet-coating solvent-based dispersions of Pt/C and an ionomer. Although the solvent composition critically influences the coatability of the dispersion, its effects on the catalyst layer performance remain unknown owing to hurdles in the evaluation of the ionomer distribution on the particles. We investigate the catalyst layers formed using dispersions of a water–ethanol solvent with varying water fractions. Electrochemical analyses indicate that the different ionomer structures in the dispersion minimally affect the catalyst layer performance after solvent drying. Small-angle scattering and neutron reflectivity measurements reveal the origin of the robust performance of the catalyst layers. The solvent-related difference in the catalyst inks marginally changes the hierarchical structure of the ionomer, resulting in a robust performance with tunable dispersion structures.

How Acid Washing Nickel Foam Substrates Improves the Efficiency of the Alkaline Hydrogen Evolution Reaction

Nickel foam substrates are frequently utilised as porous 3D substrates for renewable energy applications. The preparation of these substrates usually includes an acid-washing step, but the degree to which this step affects the final electrochemical performance after spray-coating a catalyst ink is unreported. Herein, we report the effects of acid washing through physicochemical and electrochemical characterisation. The electrochemical performance was determined through repeated measurements of catalyst-coated nickel foam substrates both with and without the initial step of acid washing. It was found that acid washing increased the current density by 17.9% for the acid-treated MoS2-coated nickel foam electrode. This increment was affiliated with an electrochemically active surface area that increased by 11.2%, and a Tafel analysis indicated that the acid-treated MoS2-coated electrodes facilitated the initial water dissociation step of the hydrogen evolution reaction with greater ease. Similar effects were also discovered for acid-treated PtIr(1:3)/C-coated nickel foam substrates. The stability was also improved; the degradation rate was reduced by 18.9% for the acid-treated MoS2-coated electrodes. This demonstrates the utility of acid washing nickel foam electrodes.

Toward Understanding Catalyst Layer Deposition Processes and Distribution in Anodic Porous Transport Electrodes in Proton Exchange Membrane Water Electrolyzers

AbstractFinding the optimum structure in porous transport electrodes (PTEs) for proton exchange membrane water electrolyzer anodes is one of the central current technological challenges. Both the structure of the porous transport layer (PTL) and its interaction with the catalyst layer are crucial in finding this optimum structure. In this regard, manufacturing the catalyst layer on top of a PTL as a structure‐building process must be understood to find improved transport electrode structures. This work presents a PTE tomography where the catalyst ink is directly processed on a PTL. The catalyst distribution of anodic PTEs is analyzed and compared via X‐ray microtomography and cross‐sectional imaging of embedded PTE samples. The majority of the catalyst lies within the first 100 µm of the PTE. Considering the penetration depth of the membrane, a maximum of 60% of the catalyst is effectively used. For the first time, a voxel‐based catalyst layer deposition model is created and analyzed that is based on simple assumptions in the deposition process. This deposition model fits very well with the previous tomographic analysis. In the future, this model will allow more profound insight into the manufacturing process and is an important prerequisite for a future optimum design of PTEs.

Tailoring a low-energy ball milled MnCo2O4 spinel catalyst to boost oxygen evolution reaction performance

The development of cost-efficient oxygen evolution reaction (OER) catalysts is one of the most important tasks facing modern techniques for hydrogen production. In this work, for the first time, a low-energy ball milling process of MnCo2O4 (MCO) spinel powders, with a mechanical modification time exceeding 1 day was used. After 6 days of ball-milling, the obtained overpotential of the electrocatalyst reached the value of 375 mV at 10 mA cm−2, which is a relatively low value obtained for this type of compound. The studies showed how the mechanical (low-energy long-term milling process) and chemical modification of the fragmented spinel powder nanoparticle surfaces affects the increase of the electrocatalytic properties. The addition of the appropriate amount of conductive carbon black (cCB) and Nafion ionomer to the ink of the MCO spinel also has a significant effect on the improvement of the catalytic performance of the manganese-cobalt oxide during the milling process. By reducing the amount of Nafion to 10 % of its initial value, the overpotential dropped to 352 mV at 10 mA cm−2 after 30 days of ball-milling. This shows that catalyst ink and layer composition are important factors influencing the catalyst’s efficiency in the OER.

Correlating catalyst ink design and catalyst layer fabrication with electrochemical CO2 reduction performance

The controllable fabrication of catalyst layers (CL) by tuning the multiscale structure formation is complex but vital to achieving optimum carbon dioxide reduction (CO2R) performance. An in-depth understanding on the role of each catalyst ink component and how multi-component interactions affect ink status, catalyst layer structure, and CO2R performance is crucial. In this work, the roles of different ingredients of catalyst ink were systematically investigated from simple binary inks to complete catalyst inks. Silver (Ag) particles-Nafion interactions were found to play a decisive role in stabilizing catalyst ink, mitigating agglomeration and particle sintering. The catalyst ink was comprehensively characterized and reported by static multiple light scattering (SMLS) for the first time in this paper. The evolution of catalyst ink was identified in three stages: stable, flocculation and sedimentation. Isopropanol (IPA)-rich solvents were found to be more effective in stabilizing catalyst ink due to better dispersed Nafion aggregates and further enhanced Ag particle-Nafion interactions. Subsequently, catalyst layer structure and CO2R performance were correlated with multi-component interactions in catalyst ink. Strong Ag particle-Nafion interactions were proven to promote not only ink stability, but also catalyst layer homogeneity and reaction site distribution. The carbon monoxide (CO) selectivity was boosted from 80.5 % to 94 % at an industrial meaningful current density of 200 mA/cm2 using commercial Ag nanoparticles by rational design of ink formulation, dispersing and fabrication processes. Simultaneously, a scalable manufacturing methodology of robust gas diffusion electrodes (GDEs) to achieve optimal CO2R performance was developed and validated.