Catalyst-coated Electrodes Research Articles

Impact of cell design and conditioning on polymer electrolyte membrane water electrolyzer operation

Integration of polymer electrolyte membrane water electrolyzers (PEMWEs) for clean hydrogen generation requires a robust understanding of the impact cell designs and conditioning protocols have on operation and stability. Here, catalyst-coated electrode and catalyst-coated membrane cells employing Pt/C cathode catalyst layer, an IrO2 anode catalyst layer, with a platinized titanium mesh or a carbon paper with a microporous layer as the porous transport layer were developed. The impact of cell conditioning above and below 0.25 A cm−2 was investigated using advanced electrochemical impedance spectroscopy analyses and microscopic imaging, with the electrochemical response related to physicochemical processes. Operation below 0.25 A cm−2 prior to operation above 0.25 A cm−2 resulted in anode corrosion and titanium cation contamination, increasing the cell voltage at 1 A cm−2 by 200 mV compared to uncontaminated cells. Conditioning above 0.25 A cm−2 led to non-negligible hydrogen transport resistances due to cathode flooding that resulted in a ca. 50 mV contribution at 1 A cm−2 and convoluted with the anode impedance response. The presence of a microporous layer increased catalyst utilization but increased the cell voltage by 300 mV at 1 A cm−2 due to increased anodic mass transport resistances. These results yield critical insights into the impact of PEMWE cell design and operation on corresponding cell performance and stability while highlighting the need for application dependent standardized operating protocols and operational windows.

(Digital Presentation) Integrated Membrane Electrode Assembly in Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs), an electrical device with high energy conversion efficiency and zero-carbon emission, has attracted more attention. The membrane electrode assembly (MEA) is the core component of PEMFCs, mainly including proton exchange membrane (PEM), catalyst layer (CL), microporous layer (MPL) and carbon fiber gas diffusion layer (GDL), which has different structures and significantly affects its power density and durability [1].Generally, MEA is fabricated by catalyst coated electrodes (CCE) and catalyst coated membrane (CCM) methods, which often results in a huge interface resistance between electrode and membrane, the weak adhesion and poor proton and electron transfer, limiting the electrochemical surface area (ECSA) and the power density of PEMFCs [2].To solve this problem, many approaches have been adopted, such as improving sintering temperature for MEA, incorporating a guided cracked layer between cathode and membrane, double-layered catalyst and direct deposition method. Zhao et al prepared an integrated membrane electrode assembly structure to reduce the contact resistance between proton exchange membrane and catalyst layer, which significantly improves the efficiency and stability of water electrolysis [4]. At the same time, Klingele and Breitwieser developed a direct deposition of proton exchange membrane technology to improve high performance and long-term stability of PEMFCs. However, these methods are complex and almost impossible to industrialize [5].In this study, integrated membrane electrode assembly (I-MEA) was prepared by combining casting and doctor blade method techniques. The results show that the casting method is more suitable for fabricating I-MEALT in low-temperature proton exchange membrane fuel cells. The power density of the I-MEALT cell is increased by 18% compared with that of the conventional MEALT (Fig. 1). The doctor blade method is more suitable for preparing I-MEAHT in high-temperature proton exchange membrane fuel cells. The maximum power density of I-MEAHT is increased by about 57% compared with that of the conventional MEAHT. The electrochemical impedance shows that this is mainly attributed to the reduced interface resistance between the electrode and the membrane, which facilitates the proton transfer.

(Invited) Comparison of Catalyst-Coated Electrodes vs. Catalyst-Coated Membranes in PEM Water Electrolysis Cells

The PEM (Proton-Exchange Membrane or Polymer Electrolyte Membrane) cell concept [1] is used in fuel cells and water electrolysis cells. PEM water electrolysis is a compact, efficient and flexible technology for the production of “green-hydrogen” from renewable (intermittent) energy sources. The unit cell (Figure 1) is made of several adjacent functional layers: (i) the proton-conducting polymer membrane (1); (ii) two catalysts layers (CLs) for hydrogen/oxygen evolution containing platinum group metals or PGMs (2;2’); (iii) a porous transport layer used for mass transport of reactants/products and electricity distribution (3;3’); (iv) water flow fields for water circulation (4;4’); bipolar plates (5;5’). From a practical viewpoint, the CLs can be deposited on either side of the interfaces formed between the polymer membrane and the porous transport layer: (i) on the supporting porous transport layers to form catalyst-coated electrodes which are then hot pressed against the bare membrane to form membrane – electrode assemblies; (ii) directly onto the membrane to form catalyst-coated membranes (CCM).The aim of this paper is to evaluate the efficiency and durability of both concepts, drawing on recent advances in manufacturing techniques to reduce the loading of PGM catalysts.[1] W.T. Grubb Jr., Batteries with Solid Ion-Exchange Electrolytes. Journal of the Electrochemical Society. 106 (1959) 275-279.[2] C. Rozain, P. Millet, Electrochemical characterization of Polymer Electrolyte Membrane Water Electrolysis Cells, Electrochimica Acta, 131 (2014) 160-167. Figure 1. Schematic diagram showing the cross-section of a PEM water electrolysis cell [2]. Figure 1

The fabrication of integrated proton exchange membrane fuel cells: Excellent mechanical strength and electrochemical performance

Traditional membrane electrode assembly, prepared by catalyst coated electrodes (CCE) and catalyst coated membrane (CCM), has a huge interface between electrodes and membranes, which affects the mechanical adhesion, proton/electron transfer and gas/liquid transport in proton exchange membrane fuel cells (PEMFCs). In this study, an integrated membrane electrode assembly (I-MEA) was fabricated by the casting and doctor blade methods for low- and high-temperature PEMFCs, respectively. The results show that the maximum power density of I-MEALT is improved around 18% and its stripping force increased from 6.091 N to 7.387 N. I-MEAHT reached 0.418 W/cm2, which is 1.5 times higher than that of traditional MEAHT. Meanwhile, it achieved the maximum stripping force of 9.374 N.

Advanced Porous Transport Layers for PEM Water Electrolyzers: Impact of the Interfacial and Bulk Properties of the PTLs on the Electrolyzers Performance

Proton exchange membrane water electrolyzers (PEMWEs) are widely seen as a crucial technology for integration with renewable energy sources to convert the generated electricity to green hydrogen, which is a clean and sustainable energy carrier.1 However, their high capital cost and operational expenditures increase the production cost of green H2.2 To make the technology economically competitive and boost its market penetration, DOE implemented targets of $2 kg-1 by 2025 and $1 kg-1 hydrogen by 2030.3 The interfacial and bulk properties of the porous transport layer (PTL) are vital to the PEMWEs performance.4–6 The optimal bulk properties of PTL improve cell performance mainly by improving the mass transport, while the reduced interfacial contact resistance between the anode catalyst layer and PTL, also enhances the catalyst utilization, allowing for a reduction of the precious metal loading. Further reduction of the interfacial contact resistance between the PTL and the catalyst layer is of crucial importance for increasing the efficiency of the PEMWEs, as well as for decreasing the production cost of green H2.In this study, an innovative reactive spray deposition technology (RSDT) is used to fabricate catalyst-coated membranes (CCMs) and catalyst-coated electrodes (CCEs) with ultra-low PGM loading (0.2 - 0.3 mgPGM cm-2) in both catalyst layers. The RSDT is a flame-based method that combines the synthesis and deposition of the catalyst in a single step, which results in a significant reduction of the MEA fabrication time and cost, respectively.7–9 A set of commercially available Ti PTLs with various thicknesses and porosities have been used to assemble single cells as fabricated MEAs, and their performance has been assessed and compared to the state-of-the-art MEAs for PEMWEs. In addition, the impact of the thickness and porosity of the PTL, as well as the interfacial contact resistance between the PTL and catalyst layer for both single-cell PEMWE configurations (CCMs and CCEs) have been investigated. The performance loss in each cell configuration has been identified and discussed in detail. Furthermore, a standard accelerated stress test (AST) protocol has been applied to assess the durability of the RSDT-fabricated MEAs, with one order of magnitude lower PGM loading in their catalyst layers in comparison to the best reported in the literature MEAs for PEMWEs.Reference1. Carmo, M. et al. A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 38, 4901–4934 (2013).2. Babic, U. et al. Critical Review — Identifying Critical Gaps for Polymer Electrolyte Water Electrolysis Development Review — Identifying Critical Gaps for Polymer Electrolyte Water. (2017) doi:10.1149/2.1441704jes.3. https://www.energy.gov/eere/fuelcells/hydrogen-shot4. Peng, X. et al. Insights into Interfacial and Bulk Transport Phenomena Affecting Proton Exchange Membrane Water Electrolyzer Performance at Ultra-Low Iridium Loadings. Adv. Sci. 8, 1–9 (2021).5. Bühler, M. et al. Optimization of anodic porous transport electrodes for proton exchange membrane water electrolyzers. J. Mater. Chem. A 7, 26984–26995 (2019).6. Kulkarni, D. et al. Elucidating effects of catalyst loadings and porous transport layer morphologies on the operation of proton exchange membrane water electrolyzers. Appl. Catal. B Environ. 308, 121213 (2022).7. Zeng, Z. et al. Degradation Mechanisms in Advanced MEAs for PEM Water Electrolyzers Fabricated by Reactive Spray Deposition Technology. J. Electrochem. Soc. 169, 054536 (2022).8. Mirshekari, G. et al. High-performance and cost-effective membrane electrode assemblies for advanced proton exchange membrane water electrolyzers: Long-term durability assessment. Int. J. Hydrogen Energy 46, 1526–1539 (2021).9. Yu, H. et al. Nano-size IrOx catalyst of high activity and stability in PEM water electrolyzer with ultra-low iridium loading. Appl. Catal. B Environ. 239, 133–146 (2018).

A brief introduction of electrode fabrication for proton exchange membrane water electrolyzers

Proton exchange membrane water electrolyzer (PEMWE) is a major enabler of green hydrogen production. The development of water electrolyzers is a vital step in driving the progress of a hydrogen-based economy. The system inside the electrolyzer is a zero-gap cell featuring low ohmic resistance and boosted mass transport, leading to higher energy efficiency and minimized capital cost. Besides, utilizing PEM in the electrolyzer for sustainable hydrogen production enables the system to perform with many advantages, including superior energy efficiency, higher hydrogen purity, and high flexibility. Therefore, as PEM electrolyzers continue to evolve, sustainable hydrogen production on a larger scale will be realized in the near future. This review summarizes the status quo of PEM water electrolyzers in the past four years. We will start with a brief introduction of the core of a water electrolyzer, namely the membrane electrode assembly (MEA), which will be followed by an introduction of fabrication methods of MEA, including CCM methods, catalyst-coated electrode methods, and other innovative fabrication methods. Next, we will summarize recent attempts to modify electrodes and membranes in MEAs to promote the performance of PEMWE. Subsequently, catalyst development for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in MEA is discussed, highlighting novel HER/OER catalysts and strategies to reduce the content of noble metals. Lastly, conclusion and perspectives are provided to present a blueprint to inspire the future development of PEMWE.

Design and Scale-Up of Zero-Gap AEM Water Electrolysers for Hydrogen Production

The design of a 10 cm2 (3.4 cm by 3.4 cm) and a 100 cm2 (10 cm by 10 cm) anion exchange membrane (AEM) water electrolyser cell for hydrogen production are described. The AEM cells are based on a zero-gap configuration where the AEM is sandwiched between the anode and cathode so as to minimise voltage drop between the electrodes. Nonprecious nickel-based metal alloy and metal oxide catalysts were employed. Various experiments were carried out to understand the effects of operating parameters such as current densities, electrolyte concentrations, and testing regimes on the performance of both 10 cm2 and 100 cm2 AEM electrolyser cells. Increasing electrolyte concentration was seen to result in reductions in overpotentials which were proportional to current applied, whilst the use of catalysts improved performance consistently over the range of current densities tested. Extended galvanostatic and intermittent tests were demonstrated on both 10 cm2 and 100 cm2 cells, with higher voltage efficiencies achieved with the use of electrocatalysts. Stability tests in the 100 cm2 AEM electrolyser cell assembled with catalyst-coated electrodes demonstrated that the cell voltages remained stable at 2.03 V and 2.17 V during 72 h operation in 4 M KOH and 1 M KOH electrolyte, respectively, at a current density of 0.3 A cm−2 at 323 K. The inclusion of cycling load tests in testing protocols is emphasized for rational evaluation of cell performance as this was observed to speed up the rate of degradation mechanisms such as membrane degradation.

Investigation of Wet-Preparation Methods of Nickel Foam For Alkaline Water Electrolysis

Water electrolysers are multi-component systems whose performance relies on each part performing its task. A great emphasis has been placed on the development of efficient catalyst-coated electrodes, however the efficacy of the underlying substrate itself has been overlooked. This paper investigates the resulting performance of nickel foam electrodes in 1.0 M KOH after being treated in various concentrations of hydrochloric acid and sulphuric acid. The greatest performance was achieved utilising 0.50 M H2SO4 as measured by LSV, EIS and CV and ECSA, resulting in a 27% decline in series resistance relative to untreated nickel foam. The series resistance decreased continuously with acid concentration until a plateau was reached at the concentration of 0.5 M, where this trend was seen for both types of acid. Utilising these preparation methods for nickel foam electrodes can notably enhance electrode performance.

Long-term durability test of highly efficient membrane electrode assemblies for anion exchange membrane seawater electrolyzers

Direct seawater electrolysis faces fundamental electrochemical challenges, such as suppression of chlorine adsorption and/or evolution reactions that poison the current state-of-the-art anode catalysts and accelerate the degradation of the anion exchange membrane (AEM). The technically favored solution path for addressing these issues includes development of novel selective catalysts and implementation of limited operating conditions. Therefore, development of highly active, selective, cost-effective, and stable seawater-splitting catalysts is required for a successful commercialization of the AEM water electrolyzers (AEMWEs). Herein, we report for the first-time an extended operation for over 1000 h of advanced AEMWEs, operating directly with seawater. To fabricate the membrane electrode assemblies (MEAs), catalyst coated electrodes (CCEs) are assembled with Sustainion® X37-50 grade T anion exchange membranes. The anode CCEs are fabricated by spraying an ink prepared from nanostructured NiFe-layered double hydroxide (LDH) catalyst that are synthesized by the solvothermal method and deposited directly onto platinized titanium porous transport layers (PTLs). The cathode CCEs are prepared by spraying an ink prepared from commercial Renay-Nickel catalyst onto a nickel fiber felt. As prepared AEM electrolyzer cells operated with seawater and demonstrated better than previously reported in the literature performance of over 1000 h of operation at a constant current density of 300 mA cm−2.

One-Step Synthesis of Selective Nife-Layered Double Hydroxide Anode Catalyst for Highly Efficient and Stable Anion Exchange Membrane Water Electrolyzers, Operating with Seawater

Seawater electrolysis faces unique and fundamental chemical challenges, such as the suppression of highly detrimental halogen chemistries [1], that poison the current state-of-the-art anode catalyst and accelerate the degradation of the membrane [2]. The technically favored solution path is to address these issues by developing a new selective catalyst and possibly by limiting operating conditions [3]. Therefore, developing a facile and cost-effective way to design highly active, selective, and and stable seawater-splitting catalysts is of great importance for research and industry [4]. In this work, hexagonal NiFe-layered double hydroxide (LDH) nanosheets were synthesized by one-step solvothermal reactions. As synthesized NiFe-LDH catalyst exhibits high activity, selectivity, and stability towards the oxygen evolution reaction (OER) in alkaline electrolyte, delivering current densities of 10 mA/cm2 at low overpotentials of 359 mV, with no significant degradation observed during RDE testing at constant voltage of 1.57 V for 72 hours in 1.0 M KOH electrolyte. In addition, in order to fabricate catalyst coated electrode (CCE), the nanostructured NiFe-LDH catalyst was homogeneously sprayed onto a platinized titanium porous transport layer (PTL) using an ultrasonic spray coating system. As prepared CCE is used as an anode and commercially available Raney nickel catalyst deposited on a nickel fiber frit is used as a cathode to assemble a MEA with Sustainion® X37-50 grade T anion exchange membrane (AEM) that was tested in alkaline seawater. Employing local seawater, the AEM electrolyzer cell demonstrated stable performance for over 1000 hours at a constant current density of 300 mA cm -2. Reference: [1] S.-C. Ke, R. Chen, G.-H. Chen, X.-L. Ma, Energy Fuels 2021, 35, 12948.[2] S. Dresp, F. Luo, R. Schmack, S. Kühl, M. Gliech, P. Strasser, Energy Environ. Sci. 2016, 9, 2020.[3] S. Dresp, F. Dionigi, S. Loos, J. Ferreira de Araujo, C. Spöri, M. Gliech, H. Dau, P. Strasser, Advanced Energy Materials 2018, 8, 1800338.[4] H. Koshikawa, H. Murase, T. Hayashi, K. Nakajima, H. Mashiko, S. Shiraishi, Y. Tsuji, ACS Catal. 2020, 10, 1886.

Operando X-Ray Absorption Spectroscopic Study of Pt-Based Nanowire Catalysts Under Oxygen Reduction Reaction

Polymer electrolyte fuel cells (PEFCs) are attracting attention as an environmentally friendly electrochemical device that uses hydrogen as a fuel. The application of PEFC has been limited by the consumption of platinum due to the sluggish kinetics of the cathode reaction and limiting platinum reserves. It has been noted that the interaction between the cathode catalyst and the oxygen intermediate species results in an overpotential in the ORR process. Therefore, it is very important to evaluate the interaction between the oxygen species and the catalyst based on the electronic structure. Pt nanowires are known to be highly active as ORR catalysts because they tend to expose more active planes for ORR, such as (111) planes1. Pt oxides formed on Pt nanowires with such 1D anisotropy are different from conventional nanoparticle catalysts in terms of structure and electronic structure2. However, this has not been fully clarified yet under oxygen reduction conditions. In this study, we analyzed the oxidation state of Pt nanowire catalysts in the ORR by operando analysis and analyzed the electronic and local structures.Synthesis of Pt nanowire catalyst : Oleylamine (5 mL) was used as a solvent and Pt(acac)2, Ni(acac)2, W(CO)6 and CTAC (hexadecyltrimethylammonium chloride) were added and sonicated for 1 hour. The product was dissolved and allowed to react in an oil bath at 160°C for 2 hours. The product particles were collected by centrifugation, and the washing and centrifugation were repeated three times with a mixture of cyclohexane: ethanol = 1:9. The nanowires were then loaded on Vulcan XC-72. ORR activity evaluation and XAS measurement: The rotating disk electrode (RDE) method was used for electrochemical measurements. A tripolar cell was used for the measurements. catalyst ink containing Pt nanowires was prepared and applied to a glassy carbon electrode for the RDE to prepare the electrode. A catalyst-coated electrode was used as the working electrode, a platinum mesh as the counter electrode, and a reversible hydrogen electrode (RHE) as the reference electrode. The electrolyte was 0.1 M HClO4. The cell temperature was controlled at 25 oC. XAS measurements were performed using a home-made measuring cell and the same electrochemical measurement conditions as for the RDE.The morphology and structure of the prepared Pt nanowires were observed by electron microscopy. As shown in Figure 1, the nanowires were uniformly distributed on the carbon and no significant aggregation was observed. The average diameter of the nanowires was 2.0 nm.To analyze the local structure of the Pt nanowire catalyst, the Pt-Pt bond length was estimated from EXAFS. Figure 2 shows the fitting results of the Pt-Pt bond lengths of Pt nanowire and Pt nanoparticle (TEC10V30E, TKK) catalysts in the range 0.5-1.1 V vs. RHE. With increasing potential, the Pt-Pt bond lengths of both catalysts gradually increased, but the Pt nanowire catalyst maintained a short Pt-Pt bond, and this nanowire characteristic structure may be the reason for its higher ORR activity than the nanoparticle. Acknowledgements: This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Β-Feooh Nanorods As a Highly Active Self-Repairing Anode Catalyst for Alkaline Water Electrolysis Powered By Renewable Energy

The use of renewable energy as a primary energy source is urged because of the rapid progress of global warming. Because the renewable energy is unevenly distributed in terms of place and time, the production of hydrogen from renewable energy is crucial. Alkaline water electrolysis is a cost-effective technology for this purpose, though the degradation of electrodes under fluctuating power of renewable energy is recently regarded as a crucial problem. We have recently reported that a in-situ repair of anode catalyst (self-repairing catalyst), supplying fresh catalyst from electrolyte is quite useful to improve the lifetime under unsteady operation [1]. A hybrid cobalt hydroxide nanosheet (Co-ns) was used as the self-repairing catalyst, whereas its electrocatalytic activity is moderate. In this study, we demonstrate the use of β-FeOOH nanorods (β-FeOOH-nr) as a highly active and durable self-repairing catalyst. Because β-FeOOH-nr consists of iron as a metallic element, it is promising for its low cost and resource abundance. β-FeOOH-nr was synthesized by the coprecipitation of Fe3+ with tris(hydroxymethyl)aminomethane, followed by heating at 80 °C. The width and length of β-FeOOH-nr were 3–6 nm and 4–200 nm, respectively. The Fe K-edge XANES analysis showed that β-FeOOH-nr consists of Fe3+ species. Electrochemical tests were performed in a three-electrode PFA cell, using a nickel wire, a nickel coil, and a reversible hydrogen electrode, as working, counter, and reference electrodes, respectively. The β-FeOOH-nr was dispersed in 1 M KOH as an electrolyte. β-FeOOH-nr was first deposited on the nickel wire electrode by constant current electrolysis at 1 A·cm–2 for 240 min (30 min × 8 cycles). β-FeOOH-nr formed a thin layer of bundled nanorods and exhibited the oxygen evolution reaction (OER) overpotential of 320 mV at 100 mA cm–2, while previously reported Co-ns exhibited 342 mV. In addition, the activation of β-FeOOH-nr-coated electrode is almost finished at the first 30 min of the electrolysis, whereas Co-ns required 240 min for the activation. Because β-FeOOH-nr-coated electrode exhibited high OER activity with a small amount of β-FeOOH-nr deposited on the electrode, β-FeOOH-nr could activate electrode with shorted electrolysis time. The β-FeOOH-nr-coated electrode indicated the redox peak due to Ni2+/Ni3+ at 1.44 V vs. RHE which is at higher potential than that of a bare nickel electrode, implying that β-FeOOH-nr formed composite with active nickel species, such as Ni(OH)2 as highly active materials. The durability of the catalyst-coated electrode was examined in the presence of β-FeOOH-nr in the electrolyte by the newly reported shutdown-based accelerated durability test (SD-ADT) [2]. SD-ADT consists of multiple cycles of constant current electrolysis at 600 mA·cm–2 for 1 min and potential retainment at low potential of 0.5 V vs. RHE for 1 min. A bare nickel electrode is degraded only within 50 cycles by the SD-ADT (OER overpotential reached 640 mV at the 200th cycle), whereas β-FeOOH-nr-coated nickel electrode retained its low OER overpotential of 285 mV in at least 4000 cycles. Comparing with the SD-ADT in the absence of β-FeOOH-nr in the electrolyte, it was shown that β-FeOOH-nr was deposited during the SD-ADT to repair degraded electrode. In conclusion, β-FeOOH-nr was shown to be a useful anode catalyst for alkaline water electrolysis powered by fluctuating renewable energy. By dispersing β-FeOOH-nr in an electrolyte, the alkaline water electrolyzer can establish self-repairing system with very long lifetime under frequent potential change. The synchrotron radiation experiments were performed at the BL16B2 of SPring-8 with approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2021A5310). This work was supported by the JSPS KAKENHI (grant number 20H02821) from Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan. Part of this study uses outcomes of the development of fundamental technology for the advancement of water electrolysis hydrogen production in the advancement of hydrogen technologies and utilization projects (grant number JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) in Japan.

Single Co3O4 Nanocubes Electrocatalyzing the Oxygen Evolution Reaction: Nano-Impact Insights into Intrinsic Activity and Support Effects.

Single-entity electrochemistry allows for assessing electrocatalytic activities of individual material entities such as nanoparticles (NPs). Thus, it becomes possible to consider intrinsic electrochemical properties of nanocatalysts when researching how activity relates to physical and structural material properties. Conversely, conventional electrochemical techniques provide a normalized sum current referring to a huge ensemble of NPs constituting, along with additives (e.g., binders), a complete catalyst-coated electrode. Accordingly, recording electrocatalytic responses of single NPs avoids interferences of ensemble effects and reduces the complexity of electrocatalytic processes, thus enabling detailed description and modelling. Herein, we present insights into the oxygen evolution catalysis at individual cubic Co3O4 NPs impacting microelectrodes of different support materials. Simulating diffusion at supported nanocubes, measured step current signals can be analyzed, providing edge lengths, corresponding size distributions, and interference-free turnover frequencies. The provided nano-impact investigation of (electro-)catalyst-support effects contradicts assumptions on a low number of highly active sites.

Pd nanocluster-decorated CoFe composite supported on nitrogen carbon nanotubes as a high-performance trifunctional electrocatalyst

Rational design and synthesis of low-cost trifunctional electrocatalysts with improved stability and superior electrocatalytic activity for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are highly desirable but remain as the bottlenecks at the current state of technology. In this paper, the cobalt-iron (Co–Fe) composite supported on nitrogen-doped carbon nanotubes (CoFe composite/NCNTs) is synthesized. The intrinsic OER and HER catalytic activities of this CoFe composite/NCNTs composite are significantly improved with palladium (Pd) nanocluster decoration [Pd-coated (CoFe composite/NCNTs)]. The as-prepared Pd-coated (CoFe composite/NCNTs) catalyst exhibits excellent trifunctional electrocatalytic activity and stability due to the interfacial coupling between Pd and (CoFe composite/NCNTs). This catalyst is successfully employed in the water electrolysis cell as both OER and HER electrode catalysts, flexible rechargeable Zn-air battery as the bifunctional ORR and OER electrode catalyst. The cell voltage of this catalyst-coated electrodes requires only 1.60 V to achieve 10 mA cm−2 current density for water electrolysis cell, which is comparable to and even better than that of Pt/C and Ir/C based cell. The primary Zn-air battery using this catalyst shows a constant high open-circuit voltage (OCV) of 1.47 V and a maximum power density of 261 mW cm−2 in the flooded mode configuration. Most importantly, a flexible Zn-air battery with this catalyst runs very smoothly without a change in voltage gap during flat, bending, and twisting positions.

Stainless Steel as A Bi-Functional Electrocatalyst-A Top-Down Approach.

For a hydrogen economy to be viable, clean and economical hydrogen production methods are vital. Electrolysis of water is a promising hydrogen production technique with zero emissions, but suffer from relatively high production costs. In order to make electrolysis of water sustainable, abundant, and efficient materials has to replace expensive and scarce noble metals as electrocatalysts in the reaction cells. Herein, we study activated stainless steel as a bi-functional electrocatalyst for the full water splitting reaction by taking advantage of nickel and iron suppressed within the bulk. The final electrocatalyst consists of a stainless steel mesh with a modified surface of layered NiFe nanosheets. By using a top down approach, the nanosheets stay well anchored to the surface and maintain an excellent electrical connection to the bulk structure. At ambient temperature, the activated stainless steel electrodes produce 10 mA/cm2 at a cell voltage of 1.78 V and display an onset for water splitting at 1.68 V in 1M KOH, which is close to benchmarking nanosized catalysts. Furthermore, we use a scalable activation method using no externally added electrocatalyst, which could be a practical and cheap alternative to traditionally catalyst-coated electrodes.

Effect of the MEA design on the performance of PEMWE single cells with different sizes

In the field of polymer electrolyte membrane water electrolysis (PEMWE), a significant amount of excellent scientific results has been generated during the past decades. However, the comparability and reproducibility of these results between different cell types and different laboratories is not always straightforward. In this contribution, an exemplary ring experiment on the single-cell level compares the performances of three cell types: the differential cell ($${4}{\text { cm}^{2}}$$) and two integral cells: an elongated cell ($${50.4}{\text { cm}}\times {0.45}{\text { cm}}$$) and a circular cell ($${63.5}{\text { cm}^{2}}$$). Therefore bi- and trilateral experiments were carried out with differently prepared catalyst-coated membranes (CCMs) and porous transport layers (PTLs) as well as with an alternative catalyst-coated electrode (CCE) concept in three laboratories. This contribution aims to evaluate the grade of systemic inequality, which still permits a comparison of individual parameters. The comparison of CCM preparation methods showed no significant influence on the initial electrochemical characteristics. An HCl etching of the anode PTLs in two different cells confirmed to be a useful treatment for the reduction of Ohmic losses in PEMWE cells. Self-made CCEs could not serve as an alternative concept, owing to their inadequate contact between the electrode and the membrane, which was observed in three laboratories as well. The general compatibility between the different cells was proven by the observation of a phenomenon in one laboratory that could be reproduced in one or two other laboratories. In this context, the size and geometry of the single cells did not influence the performance, indicating that up to the present measuring range and with sufficient water feed rates, the different single cells were functioning comparably.

Disentangling interfacial energetics

Understanding interfacial charge transfer in water-splitting photoelectrodes is complicated by the delicate interplay between catalyst and light absorber. Now, an approach based on atomic force microscopy is exploited to measure the surface electrochemical potential of nanostructured catalyst-coated electrodes in operando.

Potential-sensing electrochemical atomic force microscopy for in operando analysis of water-splitting catalysts and interfaces

Heterogeneous electrochemical phenomena, such as (photo)electrochemical water splitting to generate hydrogen using semiconductors and/or electrocatalysts, are driven by the accumulated charge carriers and thus the interfacial electrochemical potential gradients that promote charge transfer. However, measurements of the “surface” electrochemical potential during operation are not generally possible using conventional electrochemical techniques, which measure/control the potential of a conducting electrode substrate. Here we show that the nanoscale conducting tip of an atomic force microscope cantilever can sense the surface electrochemical potential of electrocatalysts in operando. To demonstrate utility, we measure the potential-dependent and thickness-dependent electronic properties of cobalt (oxy)hydroxide phosphate (CoPi). We then show that CoPi, when deposited on illuminated haematite (α-Fe2O3) photoelectrodes, acts as both a hole collector and an oxygen evolution catalyst. We demonstrate the versatility of the technique by comparing surface potentials of CoPi-decorated planar and mesoporous haematite and discuss viability for broader application in the study of electrochemical phenomena. Photoelectrochemical water splitting to produce H2 is dependent on the electrochemical potentials between photoelectrodes and the electrolyte, which are difficult to measure. Here the authors use an electrochemical atomic force microscope technique to sense the potential of catalyst-coated electrode surfaces in operando.

(Invited) Robust and Active Mixed-Conducting Electrodes for Intermediate-Temperature Fuel Cells

Intermediate-temperature solid oxide fuel cells are ideally suited for distributed generation (which may be integrated with smart grids). To make these fuel cells economically competitive and commercially viable, however, several materials challenges must be overcome. One of them is the creation of durable, low-cost cathode materials and nanostructures of high electro-catalytic activity for oxygen reduction reaction at intermediate temperatures. Here we report our findings in the development of an effective approach to fabrication of high-performance catalyst-coated electrodes of dramatically enhanced electrocatalytic activity and durability. The concept of modifying electrode surface through solution infiltration of a catalyst to create unique hybrid electrode structures (e.g., exsoluted nanoparticles on a conformal coating) are readily applicable to other electrochemical energy storage and conversion systems, including metal-air batteries, supercapacitors, and electrolyzers.

Electrocatalytic activity of a graphite electrode coated with hexadecachlorophthalocyanatoiron(II) toward sulfide oxidation, and its possible application in electroanalysis

A graphite electrode coated with hexadecachlorophthalocyanatoiron(II), FeII[Cl16Pc(2-)], serves as an electrocatalyst for the oxidation of sulfide ion in basic solutions (pH range: 8–14). The surface electrochemical responses of this catalyst and corresponding electrocatalytic activity are discussed. The sulfide ion concentration dependences of the peak current in cyclic voltammograms are examined to show the possible application of this catalyst-coated electrode in amperometric sulfide sensors. Keywords: phthalocyanine, modified graphite electrode, sulfide oxidation, electroanalysis, electrocatalysis.