Study of failure mechanisms of the reversal tolerant fuel cell anode via novel in-situ measurements

Cell voltage reversal resulting from hydrogen starvation at anode is one of the factors that exacerbate the overall degradation of polymer electrolyte fuel cells (PEFCs). An effective material-based mitigation strategy against cell reversal is to add oxygen evolution reaction (OER) catalysts into the anode to make reversal-tolerant-anodes (RTAs). However, RTAs still suffer from an eventual sudden death, and the failure mechanisms of this sudden death have not been well studied thus far. Here we show a novel in-situ measurement technique with a distinctive partition membrane electrode assembly (MEA) to research the failure mechanism of RTAs. It is observed for the first time that the failure of RTAs is mainly attributed to the destruction of electron conducting paths caused by carbon corrosion from catalyst layers (CLs), gas diffusion layers (GDLs) and bipolar plates (BPs), rather than deactivation of the OER catalyst. As a verification, the application of additional OER catalysts on the GDL is found to effectively prolong the reversal tolerant time. These results add significant new insights into the failure mechanism of the RTA MEA and will be of practical importance in directing to design advanced MEAs and BPs that can withstand cell voltage reversal.     

Ir-Pt/C composite with high metal loading as a high-performance anti-reversal anode catalyst for proton exchange membrane fuel cells

Voltage reversal induced by hydrogen starvation can severely corrode the anode catalyst support and deteriorate the performance of proton exchange membrane fuel cells. A material-based strategy is the inclusion of an oxygen evolution reaction catalyst (e.g., IrO₂) in the anode to promote water electrolysis over harmful carbon corrosion. In this work, an Ir-Pt/C composite catalyst with high metal loading is prepared. The membrane-electrode-assembly (MEA) with 80 wt% Ir-Pt(1:2)/C shows a first reversal time (FRT) of up to 20 hours, which is about ten times that of MEA with 50 wt% Ir-Pt(1:2)/C does. Furthermore, the MEA with 80 wt% Ir-Pt(1:2)/C exhibits a minimum cell voltage loss of 6 mV@1 A/cm² when the FRT is terminated in 2 hours, in which the MEA with 50 wt% Ir-Pt(1:2)/C exhibits a voltage loss of 105 mV@1 A/cm². Further physicochemical and electrochemical characterizations demonstrate that the destruction of anode catalyst layer caused by the voltage reversal process is alleviated by the use of the composite catalyst with high metal loading. Hence, our results reveal that the combination of OER catalyst on the Pt/C with high metal loading is a promising approach to alleviate the degradation of anode catalyst layer during the voltage reversal process for PEMFCs.     

The effects of ionomer content on PEM water electrolyser membrane electrode assembly performance

In this study, the effects of Nafion^® ionomer content in membrane electrode assemblies (MEAs) of polymer electrolyte membrane (PEM) water electrolyser were discussed. The MEAs were prepared with a catalyst coated membrane (CCM) method. The catalysts inks with Nafion ionomer could form uniform coatings deposited on the membrane surfaces. SEM and area EDX mapping demonstrated that anode catalyst coating was uniformly distributed, with a microporous structure. The contents of Nafion ionomer were optimized to 25% for the anode and 20% for cathode. A current density of 1 A cm⁻² was achieved at terminal voltage 1.586 V at 80 °C in a PEMWE single cell, with Nafion 117, Pt/C as cathode, and Ru_0.7Ir_0.3O₂ as anode.     

Membrane electrode assemblies with low noble metal loadings for hydrogen production from solid polymer electrolyte water electrolysis

High performance membrane electrode assemblies (MEAs) with low noble metal loadings (NMLs) were developed for solid polymer electrolyte (SPE) water electrolysis. The electrochemical and physical characterization of the MEAs was performed by I–V curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). Even though the total NML was lowered to 0.38 mg cm⁻², it still reached a high performance of 1.633 V at 2 A cm⁻² and 80 °C, with IrO₂ as anode catalyst. The influences of the ionomer content in the anode catalyst layer (CL) and the cell temperature were investigated with the purpose of optimizing the performance. SEM and EIS measurements revealed that the MEA with low NML has very thin porous cathode and anode CLs that get intimate contact with the electrolyte membrane, which makes a reduced mass transport limitation and lower ohmic resistance of the MEA. A short-term water electrolysis operation at 1 A cm⁻² showed that the MEA has good stability: the cell voltage maintained at ∼1.60 V without distinct degradation after 122 h operation at 80 °C and atmospheric pressure.     

Synthesis and electrochemical properties of nano-composite IrO2/TiO2 anode catalyst for SPE electrolysis cell

A composite catalyst of nano-grade IrO₂/TiO₂ powder is synthesized by Adams’ fusion method for reducing overvoltage of solid polymer electrolyte (SPE) cell and cost-down of noble metal catalyst, simultaneously. The IrO₂/TiO₂ catalysts, which has a porous composite nanostructure, are prepared according to molar ratio of Ir and Ti element with a specific surface area of 34.1–55.3 m² g^?1. It is found that crystal structure of TiO₂ is more dominated by the rutile phase than by Anatase. For a SPE system, total catalyst loading of anode which made of TiO₂ and IrO₂ is prepared as low as 0.77 mg cm^?2 or less, in which the loading amount of the IrO₂ only is set to 0.6 mg cm^?2 or less. The anode catalyst layer of about 10 ? thickness is coated on the membrane (Nafion 212) for the membrane electrode assembly (MEA) by the decal method. The strong adhesion between the catalyst electrode the membrane is observed by Scanning electron microscopy (SEM). Linear sweep voltammetry (LSV) results shows that the nano-composite IrO₂/TiO₂ catalysts has better oxygen evolution reaction (OER) than that of the synthesis IrO₂ only. Finally, the IrO₂/TiO₂ catalysts is applied as anode electrode for SPE cells and it is observed that in spite of the lower loading amount of the IrO₂ less than 0.5 mg cm^?2, working voltage of 1.68 V is observed at a current density of 1 A cm^?2 and operating temperature of 80 °C.     

Effect of anode iridium oxide content on the electrochemical performance and resistance to cell reversal potential of polymer electrolyte membrane fuel cells

An ideal polymer electrolyte membrane fuel cell (PEMFC) is one that continuously generates electricity as long as hydrogen and oxygen (or air) are supplied to its anode and cathode, respectively. However, internal and/or external conditions could bring about the degradation of its electrodes, which are composed of nanoparticle catalysts. Particularly, when the hydrogen supply to the anode is disrupted, a reverse voltage is generated. This phenomenon, which seriously degrades the anode catalyst, is referred to as cell reversal. To prevent its occurrence, iridium oxide (IrO₂) particles were added to the anode in the membrane-electrode assembly of the PEMFC single-cells. After 100 cell reversal cycles, the single-cell voltage profiles of the anode with Pt/C only and the anodes with Pt/C and various IrO₂ contents were obtained. Additionally, the cell reversal-induced degradation phenomenon was also confirmed electrochemically and physically, and the use of anodes with various IrO₂ contents was also discussed.     

Highly efficient,cell reversal resistant PEMFC based on PtNi/C octahedral and OER composite catalyst

In order to obtain a fuel cell with both enhanced power generation performance and cell reversal resistance, the composite catalyst consisting of the self-made PtNi/C octahedral and the oxygen evolution reaction (OER) catalyst IrO₂ and RuO₂ is mixed and applied in the anode, and the only octahedral catalyst is employed as the cathode to prepare the membrane electrode assembly (MEA). The electrochemical activity of the composite catalyst decreases slightly, but its performance retention after the accelerated durability test (ADT) is higher. In the single cell test, the MEA fabricated using the composite catalyst maintains good single cell power generation performance. Compared with the control fabricated with Pt/C (JM), the cell voltage at 1 A cm⁻² and the maximum power density are increased by 23 mV and 119 mW cm⁻², respectively. Especially, its durability under continuous cell reversal condition is also improved significantly, and the holding time is prolonged by 1 h. This work realizes the transformation of the octahedral catalyst from the laboratory research to the actual application, and solves the difficulties in fuel cell application, and promotes its commercialization.     

The Fe3+ role in decreasing the activity of Nafion-bonded IrO2 catalyst for proton exchange membrane water electrolyser

Fe³⁺ is a common ion contaminant for the proton exchange membrane water electrolyser (PEMWE). In this work, three-electrode-system was employed to study the effect of Fe³⁺ on Nafion-bonded IrO₂ catalyst which is conventional anode catalyst for PEMWE. Study results showed that Fe³⁺ contamination decreased IrO₂ catalytic activity significantly only when the following two conditions were both satisfied: 1) Nafion resin exists in working electrode; 2) working electrode potential was over 1.471 V (vs. NHE) which is around the initial voltage of oxygen evolution reaction (OER). Besides, the contaminated working electrode activity was recovered to about 16% by being immersed into 3 M H₂SO₄ solution, but it was recovered to about 59% by ethanol washing method. These study results revealed that Fe³⁺ plays a role of catalyst for H₂O₂ production during OER process, which leads to Nafion resin decomposition. The degradation products covered working electrode surface, and thus decreased effective active sites of IrO₂. Nafion degradation was further confirmed by analyzing 1) F⁻ content in anode water and 2) FTIR of contaminated Nafion membrane.     

Influence of anion ionomer content and silver cathode catalyst on the performance of alkaline membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs)

Alkaline membrane electrode assemblies (MEAs) were fabricated by a dry spraying method in order to evaluate and improve their performance. I–V tests indicated that the performance of alkaline direct methanol fuel cells (DMFCs) deeply depends on the ionomer contents of MEAs. MEA with 45.4% mass ionomer content showed the highest performance when non-alkaline (MeOH (1 M)) and alkaline (MeOH (1 M), NaOH (0.5 M)) fuels were used. When alkaline fuel was used, the anode and cathode performances of MEAs were also measured. The ionomer content has been shown to contribute ohmic polarization of the anode and diffusion polarization of the cathode. Furthermore, the performance of MEA with an Ag cathode catalyst was characterized. The Ag cathode catalyst was demonstrated to be a promising alternative to a Pt cathode catalyst because of its tolerance for methanol crossover.     

Effect of iridium oxide as an additive on catalysts with different Pt contents in cell reversal conditions of polymer electrolyte membrane fuel cells

Cell reversal is observed when a current load is applied to the polymer electrolyte membrane fuel cell under fuel starvation conditions. Cell reversal causes severe corrosion (or oxidation) of the carbon support in the anode, which leads to a decrease in overall fuel cell performance. To suppress the corrosion reaction of carbon under cell reversal conditions and to increase the durability of fuel cells, studies on anode additives are being conducted. However, studies on the effect of additives on catalysts with different platinum contents have not been conducted. In this study, 20 wt%, 40 wt%, 60 wt% commercial Pt/C catalyst was applied to the anode, and 50 cycles of cell reversal were performed. Furthermore, the performance change with and without IrO₂ as an additive was observed and its effect was assessed. Changes in the morphologies of the electrodes before and after cell reversal tests were also observed using a transmission electron microscope and a scanning electron microscope. The higher the platinum content of the catalyst, the more resistant to cell reversal. In addition, the addition of IrO₂ to the anode effectively prevents performance degradation due to cell reversal.     

Low precious metal loading porous transport layer coating and anode catalyst layer for proton exchange membrane water electrolysis

A mixed metal oxide-coated Ti felt was constructed for use as a porous transport layer (PTL) in a proton exchange membrane (PEM) water electrolyzer. The PTL was fabricated by coating Ti felt with 0.43 mg_TaOx cm^?2 and 1 mg_Ir + Ru cm^?2 IrO₂–RuO₂-TaO_x using a thermal decomposition method. The coated Ti felts exhibited high conductivity, mass transport performance, stability and oxygen evolution reaction (OER) catalytic activities. The stability of IrO₂–RuO₂-TaO_x coating obviously improved than traditional electroplated Pt coating. Using the PTL, a single cell performance of 1.836 V @ 2000 mA cm^?2 was achieved at 80 °C under ambient pressure with 1 mg cm^?2 of precious metal in anode CL. However, the precious metal loading is about 2 mg cm^?2 in common PEM electrolyzer anode catalyst layer (CL). The IrO₂–RuO₂-TaO_x-coated Ti felt proved to be a promising low-cost PTL for PEM water electrolysis with high performance.     

Ternary NiCoFe nanosheets for oxygen evolution in anion exchange membrane water electrolysis

Tuning nickel-based catalyst activity and understanding electrolyte and ionomer interaction for oxygen evolution reaction (OER) is crucial to improve anion exchange membrane (AEM) water electrolyzers. Herein, an investigation of multimetallic Ni_0.6Co_0.2Fe_0.2 OER activity, coupled with in-situ Raman spectroscopy to track dynamic structure changes, was carried out and compared to other Ni catalysts. The effect of KOH concentration, KOH purity, ionomer type, and electrolyte with organic cations was evaluated. The Ni_0.6Co_0.2Fe_0.2 catalyst achieved 10 mA/cm² at 260 mV overpotential with stability over 50 h and 5000 cycles in 1 M KOH. In-situ Raman spectroscopy showed that Ni_0.6Co_0.2Fe_0.2 activity originates from promoting Ni(OH)₂/NiOOH transformation at low potentials compared to bi- and mono-metallic nickel-based catalysts. Fumion anion ionomer in the catalyst inks led to a lower OER activity than catalysts with inks containing Nafion ionomer. The OER activity of Ni_0.6Co_0.2Fe_0.2 is adversely influenced in the presence of fumion anion ionomer and benzyltrimethylammonium hydroxide (BTMAOH) with possible phenyl oxidation under applied high anodic potentials. The alkaline AEM water electrolyzer circulating 1 M KOH electrolyte, with a Pt/C cathode and a Ni_0.6Co_0.2Fe_0.2 anode, achieved 1.5 A/cm² at 2 V.     

The effect of electrode parameters on the performance of anion exchange polymer membrane fuel cells

An investigation of several electrode parameters on performance of an alkaline membrane fuel cell is described. The studied parameters were: ionomer content, anode and cathode catalyst layer thickness, electrode aminating agent and membrane thickness.It was found that an optimum ionomer content depended on a balance between the OH⁻ ion/water mobility and the oxygen solubility/diffusivity through it and which varied with temperature. Thick catalyst layers were necessary for the anode as thin anode catalyst layers suffered from flooding. 40%Pt/C provided the best thickness (with loading of 0.4 mg_Pt cm⁻²) for cathodes operating with air.An aminated low density poly(ethylene-co-vinyl benzyl chloride) (LDPE-VBC) membrane was shown to be a good membrane for an alkaline membrane fuel cell, giving conductivities up to 0.13 S cm⁻¹ at 80 °C. A Membrane Electrode Assembly (MEA) utilizing this membrane with fully hydrated thickness of 57 μm produced good peak power density, at a high potential of 500 mV, of 337 mW cm⁻² with air (1 bar gauge) at 60 °C.     

Quantifying potential losses of non-Pt MEAs for gas-phase HBr PEM electrolyzer to produce hydrogen

Gas-phase HBr can be converted to hydrogen and bromine in a proton exchange membrane (PEM) electrolyzer. However, due to high cost and the poisoning of bromine and bromide ions on Pt electrodes, non-Pt MEAs (membrane electrode assembly) need to be developed and evaluated. In this paper, RuO₂, carbon (Vulcan XC 72R) and TiO₂–Nb (10% wt.) are prepared as anodes, and IrO₂/C and MoS₂ are prepared as cathodes for incorporation into MEAs. The individual electrodes in these MEAs are then evaluated by de-convoluting the individual voltage losses in-situ from the total electrolyzer voltage. On the anode, Pt, Vulcan XC 72R, TiO₂–Nb (10% wt.) and RuO₂ are all found to have comparable activity toward bromine evolution. On the cathode, Pt was more active toward the hydrogen evolution reaction (HER) compared to IrO₂/C, and both were far superior to MoS₂.     

Three-dimensional hierarchically porous iridium oxide-nitrogen doped carbon hybrid: An efficient bifunctional catalyst for oxygen evolution and hydrogen evolution reaction in acid

Active and durable acid medium electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are of critical importance for the development of proton exchange membrane (PEM) water electrolyser or Fuel cells. Herein, we report a facile method for the synthesis of 3D-hierarchical porous iridium oxide/N-doped carbon hybrid (3D-IrO₂/N@C) and its superior OER and HER activity in acid. In 0.5 M HClO₄, this catalyst exhibited remarkable activity towards OER with a low overpotential of 280 mV at 10 mA/cm² current density, a low Tafel slope of 45 mV/dec and ∼98% faradaic efficiency. The mass activity (MA) and turnover frequency (TOF) are found to be 833 mA/mg and 0.432 s⁻¹ at overpotential of 350 mV which are ∼32 times higher than commercial (comm.) IrO₂. The HER performance of this 3D-IrO₂/N@C is comparable with comm. Pt/C catalyst in acid. This 3D-IrO₂/N@C catalyst requires only 35 mV overpotential to reach a current density 10 mA/cm² with Tafel slope 31 mV/dec. Most importantly, chronoamperometric stability test confirmed superior stability of this catalyst towards HER and OER in acid. This 3D-IrO₂/N@C catalyst was applied as both cathode and anode for over-all water splitting and required only 1.55 V overpotential to achieve a current density of 10 mA/cm² in acid. The outstanding activity of the 3D-IrO₂/N@C catalyst can be attributed to a unique hierarchical porous network, high surface area, higher electron and mass transportation, synergistic interaction between IrO₂ and carbon support.     

N/C doped nano-size IrO2 catalyst of high activity and stability in proton exchange membrane water electrolysis

It is highly desirable to synthesize and deploy low-cost and highly efficient catalysts for the oxygen evolution reaction (OER) to catalyze water splitting. We show that N/C doped amorphous iridium oxide combines the benefits of nano-size (approximately 2 nm), which results in exposure to large active surface areas and features of oxygen defects, which make for an electronic structure suitable for the OER. Systematic studies indicate that the OER activity of the iridium oxide catalyst is accelerated by the effect of the structure and chemical state of the iridium element. Remarkably, the N/C doped amorphous iridium oxide catalyst shows a lower cell voltage of 1.774 V at 1.5 A cm⁻², compared with IrO₂ (1.847 V at 1.5 A cm⁻²), and it can maintain such a high current density for over 200 h without noticeable performance deterioration. This work provides a promising method for the improving OER electrocatalysts and the construction of an efficient and stable PEM water cracking system.     

A novel proton exchange membrane fuel cell anode for enhancing CO tolerance

A novel proton exchange membrane fuel cell (PEMFC) anode which can facilitate the CO oxidation by air bleeding and reduce the direct combustion of hydrogen with oxygen within the electrode is described. This novel anode consists of placing Pt or Au particles in the diffusion layer which is called Pt- or Au-refined diffusion layer. Thus, the chemical oxidation of CO occurs at Pt or Au particles before it reaches the electrochemical catalyst layer when trace amount of oxygen is injected into the anode. All membrane electrode assemblies (MEAs) composed of Pt- or Au-refined diffusion layer do perform better than the traditionary MEA when 100 ppm CO/H₂ and 2% air are fed and have the performance as excellent as the traditionary MEA with neat hydrogen. Furthermore, CO tolerance of the MEAs composed of Au-refined diffusion layer was also assessed without oxygen injection. When 100 ppm CO/H₂ is fed, MEAs composed of Au-refined diffusion layer have the slightly better performance than traditionary MEA do because Au particles in the diffusion layer have activity in the water gas shift (WGS) reaction at low temperature.     

Multilayer additive manufacturing of catalyst-coated membranes for polymer electrolyte membrane fuel cells by inkjet printing

Inkjet printing is a versatile, contactless and accurate material deposition technology. The present work is focused on developing innovative strategies for inkjet printing of Catalyst-Coated Membranes (CCM) by performing Additive Manufacturing (AM) applied to Polymer Electrolyte Membrane Fuel Cells (PEMFC), without resorting to intermediate substrates. Three different approaches for AM are presented and discussed: a) inkjet-printing of the membrane ionomer layer and the top catalyst layer; b) inkjet-printing of both catalyst layers onto a membrane; c) inkjet-printing of the ionomer layer as well as the catalyst layers onto the reinforcement layer of the membrane. The produced catalyst and membrane layers were characterized and proved uniform in terms of catalyst loading (0.2–0.4 and 0.08 mg_Pt cm^?2 for cathode and anode, respectively), ionomer distribution and thickness homogeneity (4 μm for catalyst layers). The fully inkjet-printed CCM outperformed conventionally made assemblies in electrochemical-performance testing, even reaching 15% higher power density.     

Synthesis of non-noble NiMoO4–Ni(OH)2/NF bifunctional electrocatalyst and its application in water-urea electrolysis

Electrochemical water splitting to produce hydrogen is one of the most important technologies for energy storage and conversion. Urea oxidation reaction (UOR) with a lower electrode potential instead of oxygen evolution reaction (OER, water-splitting anode) in the water-urea electrolysis is an energy-saving approach. In this paper, NiMoO₄–Ni(OH)₂/NF is synthesized by hydrothermal reactions and explored as both hydrogen evolution reaction (HER) and UOR catalyst electrodes. This composite catalyst shows high catalytic bifunctional activities towards both HER and UOR. To validate both catalytic UOR and HER activities and durability, a two-electrode water-urea electrolyzer composed of NiMoO₄–Ni(OH)₂/NF as both anode and cathode materials is constructed (NiMoO₄–Ni(OH)₂/NF||NiMoO₄–Ni(OH)₂). Experiments show that a voltage of 1.341 V with a high stability (over 3000 CV cycles) can be achieved at 10 mA cm⁻², which are much better than those obtained using a Pt/C||IrO₂.     

Fabrication and evaluation of membrane electrode assemblies by low-temperature decal methods for direct methanol fuel cells

In this study, a low-temperature decal transfer method is used to fabricate membrane electrode assemblies (MEAs) and the MEAs are tested for application in a direct methanol fuel cell (DMFC). The low-temperature decal transfer uses a carbon-layered decal substrate with a structure of ionomer/catalyst/carbon/substrate to facilitate the transfer of catalyst layers from the decal substrates to the membranes at a temperature as low as 140 °C, and also to prevent the formation of ionomer skin layer that is known to be formed on the surface of the transferred catalyst layer. The DMFC performance of the MEA (with carbon layer) fabricated by the low-temperature decal transfer method is higher than those of MEAs fabricated by the same method without a carbon layer, a conventional high-temperature decal method, and a direct spray-coating method. The improved DMFC performance of the MEA fabricated with carbon layer by the low-temperature decal transfer method can be attributed to the absence of an ionomer skin on the catalyst layer, which can streamline the diffusion of reactants. Furthermore, the intrinsic properties of the MEA fabricated by the low-temperature decal transfer method are elucidated by field-emission scanning electron microscopy (FESEM), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) techniques, and cathode CO₂ analysis.