Water dissociation (WD, H2O → H+ + OH−) is the core process in bipolar membranes (BPMs) that limits energy efficiency. Both electric-field and catalytic effects have been invoked to describe WD, but the interplay of the two and the underlying design principles for WD catalysts remain unclear. Using precise layers of metal-oxide nanoparticles, membrane-electrolyzer platforms, materials characterization, and impedance analysis, we illustrate the role of electronic conductivity in modulating the performance of WD catalysts in the BPM junction through screening and focusing the interfacial electric field and thus electrochemical potential gradients. In contrast, the ionic conductivity of the same layer is not a significant factor in limiting performance. BPM water electrolyzers, optimized via these findings, use ~30-nm-diameter anatase TiO2 as an earth-abundant WD catalyst, and generate O2 and H2 at 500 mA cm−2 with a record-low total cell voltage below 2 V. These advanced BPMs might accelerate deployment of new electrodialysis, carbon-capture, and carbon-utilization technology.

Anion‐exchange‐membrane water electrolyzers (AEMWEs) in principle operate without soluble electrolyte using earth‐abundant catalysts and cell materials and thus lower the cost of green H2. Current systems lack competitive performance and the durability needed for commercialization. One critical issue is a poor understanding of catalyst‐specific degradation processes in the electrolyzer. While non‐platinum‐group‐metal (non‐PGM) oxygen‐evolution catalysts show excellent performance and durability in strongly alkaline electrolyte, this has not transferred directly to pure‐water AEMWEs. Here, AEMWEs with five non‐PGM anode catalysts are built and the catalysts’ structural stability and interactions with the alkaline ionomer are characterized during electrolyzer operation and post‐mortem. The results show catalyst electrical conductivity is one key to obtaining high‐performing systems and that many non‐PGM catalysts restructure during operation. Dynamic Fe sites correlate with enhanced degradation rates, as does the addition of soluble Fe impurities. In contrast, electronically conductive Co3O4 nanoparticles (without Fe in the crystal structure) yield AEMWEs from simple, standard preparation methods, with performance and stability comparable to IrO2. These results reveal the fundamental dynamic catalytic processes resulting in AEMWE device failure under relevant conditions, demonstrate a viable non‐PGM catalyst for AEMWE operation, and illustrate underlying design rules for engineering anode catalyst/ionomer layers with higher performance and durability.

Assigning optical band gaps to MOFs is paramount for understanding their optical, electronic, and reactivity properties, but literature reports have produced a wide range of values for even the same MOF. Despite the molecular nature of MOF electronic structures, experimental assignments employ Tauc analysis, a method applied to semiconductors. Here, we report optical band gaps of common MOFs and demonstrate that Gaussian fitting is more appropriate for assigning accurate gap energies. We further support this claim with DFT simulation, providing a reliable method for estimating optical band gaps from ground-state hybrid-GGA. MOFs that require Tauc analysis exhibit overlapping optical transitions uncommon for typical carboxylate-based MOFs and more akin to narrow-gap semiconductors. Taken together, these results provide a simple roadmap for assigning MOF optical band gaps.

Bipolar membranes (BPMs) can generate steadystate pH gradients in electrochemical cells, enabling halfreactions to occur in different pH environments, and are thus of broad interest. Forward-bias BPMs further enable new approaches to fuel cells, redox-flow batteries, and CO 2 electrolyzers. In forward bias, the gradient in electrochemical potential drives ionic charge carriers toward the bipolar junction where they can recombine. We use a H 2 -pump electrochemical cell to study H + /OH − recombination at the bipolar junction. We discover that metal-oxide nanoparticles catalyze the recombination reaction in the bipolar junction under forward bias and find evidence that H + /OH − recombination occurs via a surface mechanism on the oxide catalyst. We propose a rate equation to describe the catalytic H + /OH − recombination mechanism, supported by numerical simulations. This work thus elucidates materials-design strategies for recombination catalysts to advance forward-bias BPM technologies.

Titanium-based metal−organic frameworks (Ti-MOFs) have attracted intense research attention because they can store charges in the form of Ti 3+ and they serve as photosensitizers to cocatalysts through heterogeneous photoredox reactions at the MOF−liquid interface. Both the charge storage and charge transfer depend on the redox potentials of the MOF and the molecular substrate, but the factors controlling these energetic aspects are not well understood. Additionally, photocatalysis involving Ti-MOFs relies on cocatalysts rather than the intrinsic Ti reactivity, in part because Ti-MOFs with open metal sites are rare. Here, we report that the class of Ti-MOFs known as MUV-10 can be synthetically modified to include a range of redox-inactive ions with flexible coordination environments that control the energies of the photoactive orbitals. Lewis acidic cations installed in the MOF cluster (Cd 2+ , Sr 2+ , and Ba 2+ ) or introduced to the pores (H + , Li + , Na + , K + ) tune the electronic structure and band gaps of the MOFs. Through the use of optical redox indicators, we report the first direct measurement of the Fermi levels (redox potentials) of photoexcited MOFs in situ. Taken together, these results explain the ability of Ti-MOFs to store charges and provide design principles for achieving heterogeneous photoredox chemistry with electrostatic control.

TiIV-containing metal-organic frameworks are known to accumulate electrons in their conduction bands, accompanied by protons, when irradiated in the presence of alcohols. The archetypal system, MIL-125, was recently shown to...

Conventional semiconductor nanocrystals exhibit wide-ranging optical behavior, whereas the size-dependent photophysical properties of metal−organic framework (MOF) nanocrystals remain an open research frontier. Here, we present sizeand temperature-dependent optical absorption spectra of common MOFs with particle sizes ranging from tens of nanometers to several micrometers. All materials exhibit optical gaps that decrease at elevated temperatures, which we attribute to the dynamic nature of MOF metal−linker bonds. Accordingly, whereas the labile titanium−carboxylate bonds of MIL-125 give rise to bandgaps that red-shift by ∼600 meV over 300 K, the more rigid zinc− imidazolate bonds of ZIF-8 produce a red-shift of only ∼10 meV. Furthermore, smaller particles induce far larger decreases to optical gaps. Taken together, these results suggest MOF bonding becomes more flexible with smaller nanocrystal sizes, offering a powerful tool for manipulating optical behavior through composition, temperature, and dimensionality.

Inclusion of a second nitrogen atom in the aromatic core of phosphorus‐nitrogen (PN) heterocycles results in unexpected tautomerization to a nonaromatic form. This tautomerization, initially observed in the solid state through X‐ray crystallography, is also explained by computational analysis. We prepared an electron deficient analogue (2 e) with a fluorine on the pyridine ring and showed that the weakly basic pyridine resisted tautomerization, providing key insights to why the transformation occurs. To study the difference in solution vs. solid‐state heterocycles, alkylated analogues that lock in the quinoidal tautomer were synthesized and their different 1H NMR and UV/Vis spectra studied. Ultimately, we determined that all heterocycles are the aromatic tautomer in solution and all but 2 e switch to the quinoidal tautomer in the solid state. Better understanding of this transformation and under what circumstances it occurs suggest future use in a switchable on/off hydrogen‐bond‐directed receptor that can be tuned for complementary hydrogen bonding.