Advanced membrane-based electrode engineering toward efficient and durable water electrolysis and cost-effective seawater electrolysis in membrane electrolyzers

Researchers have been seeking for the most technically-economical water electrolysis technology for entering the next-stage of industrial amplification for large-scale green hydrogen production. Various membrane-based electrolyzers have been developed to improve electric-efficiency, reduce the use of precious metals, enhance stability, and possibly realize direct seawater electrolysis. While electrode engineering is the key to approaching these goals by bridging the gap between catalysts design and electrolyzers development, nevertheless, as an emerging field, has not yet been systematically analyzed. Herein, this review is organized to comprehensively discuss the recent progresses of electrode engineering that have been made toward advanced membrane-based electrolyzers. For the commercialized or near-commercialized membrane electrolyzer technologies, the electrode material design principles are interpreted and the interface engineering that have been put forward to improve catalytic sites utilization and reduce precious metal loading is summarized. Given the pressing issues of electrolyzer cost reduction and efficiency improvement, the electrode structure engineering toward applying precious metal free electrocatalysts is highlighted and sufficient accessible sites within the thick catalyst layers with rational electrode architectures and effective ions/mass transport interfaces are enabled. In addition, this review also discusses the innovative ways as proposed to break the barriers of current membrane electrolyzers, including the adjustments of electrode reaction environment, and the feasible cell-voltage-breakdown strategies for durable direct seawater electrolysis. Hopefully, this review may provide insightful information of membrane-based electrode engineering and inspire the future development of advanced membrane electrolyzer technologies for cost-effective green hydrogen production

Recent Progress in Metal-Organic Frameworks for Applications in Electrocatalytic and Photocatalytic Water Splitting

The development of clean and renewable energy materials as alternatives to fossil fuels is foreseen as a potential solution to the crucial problems of environmental pollution and energy shortages. Hydrogen is an ideal energy material for the future, and water splitting using solar/electrical energy is one way to generate hydrogen. Metal-organic frameworks (MOFs) are a class of porous materials with unique properties that have received rapidly growing attention in recent years for applications in water splitting due to their remarkable design flexibility, ultra-large surface-to-volume ratios and tunable pore channels. This review focuses on recent progress in the application of MOFs in electrocatalytic and photocatalytic water splitting for hydrogen generation, including both oxygen and hydrogen evolution. It starts with the fundamentals of electrocatalytic and photocatalytic water splitting and the related factors to determine the catalytic activity. The recent progress in the exploitation of MOFs for water splitting is then summarized, and strategies for designing MOF-based catalysts for electrocatalytic and photocatalytic water splitting are presented. Finally, major challenges in the field of water splitting are highlighted, and some perspectives of MOF-based catalysts for water splitting are proposed

Modeling of proton-conducting solid oxide fuel cells fueled with syngas

Solid oxide fuel cells (SOFCs) with proton conducting electrolyte (H-SOFCs) are promising power sources for stationary applications. Compared with other types of fuel cells, one distinct feature of SOFC is their fuel flexibility. In this study, a 2D model is developed to investigate the transport and reaction in an H-SOFC fueled with syngas, which can be produced from conventional natural gas or renewable biomass. The model fully considers the fluid flow, mass transfer, heat transfer and reactions in the H-SOFC. Parametric studies are conducted to examine the physical and chemical processes in H-SOFC with a focus on how the operating parameters affect the H-SOFC performance. It is found that the presence of CO dilutes the concentration of H2, thus decreasing the H-SOFC performance. With typical syngas fuel, adding H2O cannot enhance the performance of the H-SOFC, although water gas shift reaction can facilitate H2 production

Solid Oxide Fuel Cells with both High Voltage and Power Output by Utilizing Beneficial Interfacial Reaction

An intriguing cell concept by applying proton-conducting oxide as the ionic conducting phase in the anode and taking advantage of beneficial interfacial reaction between anode and electrolyte is proposed to successfully achieve both high open circuit voltage (OCV) and power output for SOFCs with thin-film samarium doped ceria (SDC) electrolyte at temperatures higher than 600 °C. The fuel cells were fabricated by conventional route without introducing an additional processing step. A very thin and dense interfacial layer (2–3 μm) with compositional gradient was created by in situ reaction between anode and electrolyte although the anode substrate had high surface roughness (\u3e5 μm), which is, however, beneficial for increasing triple phase boundaries where electrode reactions happen. A fuel cell with Ni–BaZr0.4Ce0.4Y0.2O3 anode, thin-film SDC electrolyte and Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF) cathode has an OCV as high as 1.022 V and delivered a power density of 462 mW cm−2 at 0.7 V at 600 °C. It greatly promises an intriguing fuel cell concept for efficient power generation

Promoting Bifunctional Oxygen Catalyst Activity of Double-Perovskite-Type Cubic Nanocrystallites for Aqueous and Quasi-Solid-State Rechargeable Zinc-Air Batteries

Transition metal oxide materials are promising oxygen catalysts that are alternatives to expensive and precious metal-containing catalysts. Integration of transition metal oxides with high activity for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is an important pathway for good bifunctionality. In contrast to the conventional physical mixing and hybridization strategies, perovskite-type oxide provides an ideal structure for the integration of the transition metal element atoms on an atomic scale. Herein, B-site ordered double-perovskite-type La1.6Sr0.4MnCoO6 nanocrystallites with ultra-small cubic (20–50 nm) morphology and high specific surface areas (25 m2 g−1) were proposed. Rational designs were integrated to promote the ORR-OER catalysis, e.g., introducing oxygen vacancies via A-site cation substitution, further increasing surface oxygen vacancies via integration of a small amount of Pt/C and nanosizing of the material via a facile molten-salt method. The batteries with the La1.6Sr0.4MnCoO6 nanocrystallites and an aqueous alkaline electrolyte demonstrate decent discharge−charge voltage gaps of 0.75 and 1.10 V at 1 and 30 mA cm−2, respectively, and good cycling stability of 250 h (1500 cycles). A coin-type battery with a gel−polymer electrolyte also presents a good performance

A Thermally Self-Sustaining Miniature Solid Oxide Fuel Cell

A thermally self-sustaining miniature power generation device was developed utilizing a single-chamber solid oxide fuel cell (SOFC) placed in a controlled thermal environment provided by a spiral counterflow “Swiss roll” heat exchanger and combustor. With the single-chamber design, fuel/oxygen crossover due to cracking of seals via thermal cycling is irrelevant and coking on the anode is practically eliminated. Appropriate SOFC operating temperatures were maintained even at low Reynolds numbers (Re) via combustion of the fuel cell effluent at the center of the Swiss roll. Both propane and higher hydrocarbon fuels were examined. Extinction limits and thermal behavior of the integrated system were determined in equivalence ratio—Re parameter space and an optimal regime for SOFC operation were identified. SOFC power densities up to 420 mW/cm^2 were observed at low Re. These results suggest that single-chamber SOFCs integrated with heat-recirculating combustors may be a viable approach for small-scale power generation devices

Stability and performance of in-situ formed phosphosilicate nanoparticles in phosphoric acid-doped polybenzimidazole composite membrane fuel cells at elevated temperatures

One of the effective strategies to pursue the highly durable high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) is to introduce inorganic fillers to the phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. Among the inorganic fillers, phosphates such as phosphosilicate are effective in mitigating acid loss at elevated temperatures (200–300 °C). In this paper, the effect of in situ formed phosphosilicate on the performance and stability of SiO2/PA/PBI composite membranes is studied in detail. The mechanical properties and electrochemical performances of the in situ formed SiO2/PA/PBI membranes depend strongly on the content of in situ formed Si5P6O25 fillers and its distribution and microstructure in the membrane. Such in situ formed SiO2/PA/PBI composite membranes show a high conductivity of 53.5 mS cm−1 at 220 °C. The assembled single cell shows a maximum peak power density (PPD) of 530.6 mW cm−2 and excellent stability at elevated temperature of 220 °C for over 130 h. The exceptional stability at 220 °C is most likely due to the existence of predominant amorphous phosphosilicate phases in the in situ formed SiO2/PA/PBI composite membranes, which inhibits the evaporation and leaching of PA at elevated temperatures. The results indicate the practical application of in situ formed SiO2/PA/PBI composite membranes for HT-PEMFCs

Tuning synergy between nickel and iron in Ruddlesden–Popper perovskites through controllable crystal dimensionalities towards enhanced oxygen-evolving activity and stability

Ni–Fe-based oxides are among the most promising catalysts developed to date for the bottleneck oxygen evolution reaction (OER) in water electrolysis. However, understanding and mastering the synergy of Ni and Fe remain challenging. Herein, we report that the synergy between Ni and Fe can be tailored by crystal dimensionality of Ni, Fe-contained Ruddlesden–Popper (RP)-type perovskites (La0.125Sr0.875)n+1(Ni0.25Fe0.75)nO3n+1 (n = 1, 2, 3), where the material with n = 3 shows the best OER performance in alkaline media. Soft X-ray absorption spectroscopy spectra before and after OER reveal that the material with n = 3 shows enhanced Ni/Fe–O covalency to boost the electron transfer as compared to those with n = 1 and n = 2. Further experimental investigations demonstrate that the Fe ion is the active site and the Ni ion is the stable site in this system, where such unique synergy reaches the optimum at n = 3. Besides, as n increases, the proportion of unstable rock-salt layers accordingly decreases and the leaching of ions (especially Sr2+) into the electrolyte is suppressed, which induces a decrease in the leaching of active Fe ions, ultimately leading to enhanced stability. This work provides a new avenue for rational catalyst design through the dimensional strategy