Electrochemical water splitting is one of the most sustainable approaches for generating hydrogen. Because of the inherent constraints associated with the architecture and materials, the conventional alkaline water electrolyzer and the emerging proton exchange membrane electrolyzer are suffering from low efficiency and high materials/operation costs, respectively. Herein, we design a membrane-free flow electrolyzer, featuring a sandwich-like architecture and a cyclic operation mode, for decoupled overall water splitting. Comprised of two physically-separated compartments with flowing H₂-rich catholyte and O₂-rich anolyte, the cell delivers H₂ with a purity >99.1%. Its low internal ohmic resistance, highly active yet affordable bifunctional catalysts and efficient mass transport enable the water splitting at current density of 750 mA cm⁻² biased at 2.1 V. The eletrolyzer works equally well both in deionized water and in regular tap water. This work demonstrates the opportunity of combining the advantages of different electrolyzer concepts for water splitting via cell architecture and materials design, opening pathways for sustainable hydrogen generation.

We discuss the latest developments in alternative battery systems based on sodium, magnesium, zinc and aluminum. In each case, we categorize the individual metals by the overarching cathode material type, focusing on the energy storage mechanism. Specifically, sodium-ion batteries are the closest in technology and chemistry to today’s lithium-ion batteries. This lowers the technology transition barrier in the short term, but their low specific capacity creates a long-term problem. The lower reactivity of magnesium makes pure Mg metal anodes much safer than alkali ones. However, these are still reactive enough to be deactivated over time. Alloying magnesium with different metals can solve this problem. Combining this with different cathodes gives good specific capacities, but with a lower voltage (<1.3 V, compared with 3.8 V for Li-ion batteries). Zinc has the lowest theoretical specific capacity, but zinc metal anodes are so stable that they can be used without alterations. This results in comparable capacities to the other materials and can be immediately used in systems where weight is not a problem. Theoretically, aluminum is the most promising alternative, with its high specific capacity thanks to its three-electron redox reaction. However, the trade-off between stability and specific capacity is a problem. After analyzing each option separately, we compare them all via a political, economic, socio-cultural and technological (PEST) analysis. The review concludes with recommendations for future applications in the mobile and stationary power sectors.

Herein, a unique anion‐exchange membrane fuel cell (AEMFC) containing only affordable and abundant materials is presented: NiFe hydrogen oxidation reaction (HOR) and nitrogen‐doped carbon oxygen reduction reaction (ORR) electrocatalysts. AEMFCs are an attractive alternative to proton‐exchange membrane fuel cells. They can run under alkaline conditions, allowing the use of platinum group metal (PGM)‐free electrocatalysts. Yet, the same alkaline conditions incur an overpotential loss in ORR and also slow the HOR. This can be solved by using PGM electrodes, but then the original advantage disappears. In contrast, the fuel cell is free of both PGMs and critical raw materials (CRMs). Electrochemical studies confirmed that the catalysts are highly active in both HOR and ORR in an alkaline electrolyte. The morphology, composition, and chemical states of the electrocatalysts are characterized by different techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), and electron energy loss spectroscopy (EELS). Then, the electrocatalysts’ performance is tested in a fuel cell device. The cell gives a maximum power density of 56 mW cm⁻² and a limiting current density of 220 mA cm⁻². These results are among the best CRM‐free anion‐exchange membrane fuel cells reported to date.

The specific capacitance of a highly porous, nitrogen‐doped carbon is nearly tripled by orthogonal optimization of the microstructure and surface chemistry. First, the carbons’ hierarchical pore structure and specific surface area were tweaked by controlling the temperature and sequence of the thermal treatments. The best process (pyrolysis at 900 °C, washing, and subsequent annealing at 1000 °C) yielded a carbon with a specific capacitance of 117 F g⁻¹—nearly double that of a carbon made by a typical single‐step synthesis at 700 °C. Following the structural optimization, the surface chemistry of the carbons was enriched by applying an oxidation routine based on a mixture of nitric and sulfuric acid in a 1:4 ratio at two different treatment temperatures (0 and 20 °C) and different treatment times. The optimal treatment times were 4 h at 0 °C and only 1 h at 20 °C. Overall, the specific capacitance nearly tripled relative to the original carbon, reaching 168 F g⁻¹. The inherent nitrogen doping of the carbon comes into interplay with the acid‐induced surface functionalization, creating a mixture of oxygen‐ and nitrogen‐oxygen functionalities. The evolution of the surface chemistry was carefully followed by X‐ray photoelectron spectroscopy and by N₂ sorption porosimetry, revealing stepwise surface functionalization and simultaneous carbon etching. Overall, these processes are responsible for the peak‐shaped capacitance trends in the carbons.

Bipolar membranes operated under reverse-bias (r-BPM) provide the only potential route to use anodes free of platinum group metals in CO ₂ electrolyzers when paired with the oxygen evolution reaction. Under 100% water dissociation efficiency (WDE) conditions, the OH ⁻ generated by a r-BPM fully replenishes the OH ⁻ consumed by the oxygen evolution reaction, maintaining an alkaline anolyte. However, unwanted co-ion crossover leads to 99.8% are required to operate for >10,000 h with reasonable anolyte volumes. Our results show that r-BPMs CO ₂ electrolyzers require additional strategies, such as reverting to platinum group metal anodes or regenerating the anolyte, to operate stably at an industrial scale.