Bipolar‐Interface Hydrogen Fuel Cells ‐ A Review and Perspective on Future High Performance, Low Platinum Group Metal Content Designs

This Review elucidates the state-of-the-art, challenges, andfuture prospects for bipolar-interface-based fuel cells (BPIFCs),focusing on hydrogen-based systems. On the one side, itprovides a summary of state-of-the-art design strategies forBPIFCs from different fields of application. On the other hand, itidentifies the two most pivotal areas for a future understandingthat particularly refer to the changed mass transport situationsintroduced by the bipolar fuel cell design, that is, the watermanagement and the bipolar interface layer itself. All operation-relevant components such as the gas diffusion layers,catalyst layers, and membrane designs are discussed within theframework of these two main areas. As non-platinum-groupmetal (PGM) oxygen reduction is one of the key benefits ofbipolar hydrogen fuel cells, a particular focus is put on thisconfiguration. Several additional challenges that exclusivelyoriginate from non-PGM-based catalyst layers and possiblemitigation approaches are discussed. One key insight is thatthese thick layers could take over the role of microporous layersin the future. Finally, we emphasize that there is a strong lack inboth theoretical and experimental approaches that improve ourunderstanding of the underlying processes in bipolar fuel celldesigns

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

Finding 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

Digital Twin of a Hierarchical CO 2 Electrolyzer Gas Diffusion Electrode

The electrochemical reduction of CO2 provides a pathway to a sustainable carbon cycle, allowing for the production of hydrocarbons critical for both chemical industry and mobility. Major engineering challenges need to be met in order to achieve efficient and large-scale CO2 electrolysis. One central challenge is to find the optimum structure for the gas diffusion electrode at the heart of the electrolyzer. An optimum structure would achieve higher conversion efficiencies, lifetime and product selectivity at lower cost. Critical structural properties of the electrode span the scale from nanometers to millimeters. To rationalize the optimization process, it behooves us to obtain a fully resolved multi-scale model of the electrode. This digital twin, produced by bridging scale and employing multiple imaging methods, enables our digital study, simulation and modification of the structure of the electrode. We use the model to simulate the transport processes vital to the functioning of the electrode thus further advancing the digital twin. Subsequently we explore how changes to the structure affect the predicted transport properties. The digital twin presented is only supposed to be the kernel and will be complemented by numerous future works

Bipolar Membrane Electrode Assemblies for Water Electrolysis

We present the first analysis of a zero-gap bipolar membrane water electrolyzer fed with liquid water. Our electrolyzers feature a high-pH environment for the oxygen evolution reaction and a low-pH environment for the hydrogen evolution reaction. The advantages of proton exchange membrane water electrolysis can be combined with those of anion exchange membrane water electrolysis by including a water splitting bipolar interface. First, we develop a KOH-free anion exchange membrane electrolysis cell. The cell’s alkaline anode serves as an integral building block on the path to a bipolar system. In a second step, we use this building block to investigate the cell operation characteristics of various cell configurations. We study the cell performance as the bipolar interface is shifted progressively toward the anode. A bipolar membrane with and without a water splitting catalyst resulted in cell current densities of 450 and 5 mA cm$^{–2}$ at cell voltages of 2.2 V, respectively. Upon moving the bipolar interface directly between the acidic membrane and the high-pH anode, we achieved current densities of 9000 mA cm$^{–2}$ at cell voltages of 2.2 V. Our study demonstrates the potential of this water electrolysis configuration, which should be adopted for further scientific studies and may show promise for future commercial water electrolysis systems

Bipolar Membrane Electrode Assemblies for Water Electrolysis

We present the first analysis of a zero-gap bipolar membrane water electrolyzer fed with liquid water. Our electrolyzers feature a high-pH environment for the oxygen evolution reaction and a low-pH environment for the hydrogen evolution reaction. The advantages of proton exchange membrane water electrolysis can be combined with those of anion exchange membrane water electrolysis by including a water splitting bipolar interface. First, we develop a KOH-free anion exchange membrane electrolysis cell. The cell’s alkaline anode serves as an integral building block on the path to a bipolar system. In a second step, we use this building block to investigate the cell operation characteristics of various cell configurations. We study the cell performance as the bipolar interface is shifted progressively toward the anode. A bipolar membrane with and without a water splitting catalyst resulted in cell current densities of 450 and 5 mA cm$^{–2}$ at cell voltages of 2.2 V, respectively. Upon moving the bipolar interface directly between the acidic membrane and the high-pH anode, we achieved current densities of 9000 mA cm$^{–2}$ at cell voltages of 2.2 V. Our study demonstrates the potential of this water electrolysis configuration, which should be adopted for further scientific studies and may show promise for future commercial water electrolysis systems

Photocorrosion of WO 3 Photoanodes in Different Electrolytes

Photocorrosion of an n-type semiconductor is anticipated to be unfavorable if its decomposition potential is situated below its valence band-edge position. Tungsten trioxide (WO3) is generally considered as a stable photoanode for different photoelectrochemical (PEC) applications. Such oversimplified considerations ignore reactions with electrolytes added to the solvent. Moreover, kinetic effects are neglected. The fallacy of such approaches has been demonstrated in our previous study dealing with WO3 instability in H2SO4. In this work, in order to understand parameters influencing WO3 photocorrosion and to identify more suitable reaction environments, H2SO4, HClO4, HNO3, CH3O3SH, as electrolytes are considered. Model WO3 thin films are fabricated with a spray-coating process. Photoactivity of the samples is determined with a photoelectrochemical scanning flow cell. Photostability is measured in real time by coupling an inductively coupled plasma mass spectrometer to the scanning flow cell to determine the photoanode dissolution products. It is found that the photoactivity of the WO3 films increases as HNO3 < HClO4 ≈ H2SO4 < CH3O3SH, whereas the photostability exhibits the opposite trend. The differences observed in photocorrosion are explained considering stability of the electrolytes toward decomposition. This work demonstrates that electrolytes and their reactive intermediates clearly influence the photostability of photoelectrodes. Thus, the careful selection of the photoelectrode/electrolyte combination is of crucial importance in the design of a stable photoelectrochemical water-splitting device

H+-conducting aromatic multiblock copolymer and blend membranesand their application in PEM electrolysis

As an alternative to common perfluorosulfonic acid-based polyelectrolytes, we present the synthesis and characterization of proton exchange membranes based on two different concepts: (i) Covalently bound multiblock-co-ionomers with a nanophase-separated structure exhibit tunable properties depending on hydrophilic and hydrophobic components’ ratios. Here, the blocks were synthesized individually via step-growth polycondensation from either partially fluorinated or sulfonated aromatic monomers. (ii) Ionically crosslinked blend membranes of partially fluorinated polybenzimidazole and pyridine side-chain-modified polysulfones combine the hydrophilic component’s high proton conductivities with high mechanical stability established by the hydrophobic components. In addition to the polymer synthesis, membrane preparation, and thorough characterization of the obtained materials, hydrogen permeability is determined using linear sweep voltammetry. Furthermore, initial in situ tests in a PEM electrolysis cell show promising cell performance, which can be increased by optimizing electrodes with regard to binders for the respective membrane material

Platinum Dissolution in Realistic Fuel Cell Catalyst Layers

Pt dissolution has already been intensively studied in aqueous model systems and many mechanistic insights were gained. Nevertheless, transfer of new knowledge to real‐world fuel cell systems is still a significant challenge. To close this gap, we present a novel in‐situ method combining a gas diffusion electrode (GDE) half‐cell with inductively coupled plasma mass spectrometry (ICP‐MS). With this setup, Pt dissolution in realistic catalyst layers and the transport of dissolved Pt species through Nafion membranes are evaluated directly. We observe that (i) specific Pt dissolution is increasing significantly with decreasing Pt loading, (ii) in comparison to experiments on aqueous model systems with flow cells, the measured dissolution in GDE experiments is considerably lower and, (iii) by adding a membrane onto the catalyst layer, Pt dissolution is reduced even further. All these phenomena are attributed to the varying mass transport conditions of dissolved Pt species, influencing re‐deposition and equilibrium potential