Directed ion transport in liquid electrolyte solutions underlies many phenomena in natural and industrial settings. While nature has evolved structures that drive continuous ion flow without Faradaic redox reactions, establishing this process in synthetic systems has been challenging. Here we report an ion pump that drives aqueous ions against a force using a capacitive ratchet mechanism independent of redox reactions. Modulation of an electric potential between thin metallic layers on either face of a nanoporous alumina wafer immersed in solution results in persistent voltages and ionic currents. This occurs due to the nonlinear capacitive nature of electric double layers, whose repeated charging and discharging sustains a continuous ion flux. Using this approach, we demonstrate ratchet-driven electrodialysis that reaches a 50% decrease in the conductivity of the solution in a dilution cell. These ratchet-based ion pumps can enable continuous desalination and selective ion separation using an electrically powered device with no moving parts.

Interfacial proton-coupled electron transfer (I-PCET) is typically viewed as a single elementary reaction despite the accepted recognition that analogous solution-phase reactivity requires proton donor/acceptor pre association. Herein, we examine the role of pre-association in I-PCET to a molecularly well-defined graphite-conjugated carboxylic acid (GC-COOH) surface site. We quantify electrolyte proton activity and I PCET kinetics in acidic, acetate-buffered, and alkaline electrolytes as a function of NaClO4 concentration ranging from 1 mole kg−1 to 17 mole kg−1. Measured I-PCET rates at GC COOH are attenuated by a factor of 4.3 in acidic and 4.6 in acetate-buffered media relative to the rates expected based on the solution pH across this ionic strength range. In contrast, in alkaline electrolytes the apparent rate of I-PCET is far less diminished upon increasing NaClO4 concentration. To account for these findings, we propose a multiple-step model for I-PCET in acidic and acetate-buffered media that invokes quasi-equilibrated exchange of interfacial Na+ for H3O+ to form a hydrogen-bonded pre association complex prior to rate-limiting concerted proton-electron transfer. In this model, a decrease in the ratio of interfacial vs bulk Na+ activity with added electrolyte inhibits H3O+ pre-association, a phenomenon that is also captured in our molecular dynamics simulations. These studies emphasize the non-innocence of the supporting electrolyte species and exposes the key role pre association equilibria play in I-PCET mechanisms. The work suggests that control over pre-association equilibria could be used as an effective handle for tailoring interfacial ion transfer reaction kinetics.

The Ensembles of Photosynthetic Nanoreactors (EPN) Energy Frontier Research Center takes a multifaceted approach to gain new knowledge that will help to bridge the gap in efficiency of photocatalytic solar water splitting between what is observed, i.e. ~1%, and necessary, i.e. >10%, for substantial positive global impact. Our distinctive approach couples correlative microscopic, spectroscopic, electrochemical, and photochemical measurements with numerical models and simulations that are revealing a more complete picture for how state-of-the-art photocatalytic particles are so effective. Notably, we have shown that state-of-the-art doped SrTiO 3 and BiVO 4 nanoparticles are highly heterogeneous with small inherent electrostatic driving forces for photoinduced charge separation. This means that if photoinduced charge separation is driven by electric fields, resulting photovoltages are insufficient to explain observations of overall water splitting. Instead, our numerical models support that photogenerated free energy – from even a single electron/hole pair – greatly exceeds electric potential differences. Coupled to microscopic measurements, our results suggest that a stochastic photoexcitation process can result in exceptionally long-lived carriers that can impart overall water splitting chemistries. Just as John Turner had preached: it is undeniable that the number of minority carriers is too small to describe their ensemble free energy as a quasi-Fermi level. Collectively, our interpretations of experimental and computational data support new approaches, and motivate additional research pathways, that will aid in the development of technoeconomically promising approaches, devices, and reactors for the generation of fuels, driven by light, as artificial photosynthesis.

Defects trap photocarriers and hinder solar water splitting. The nanoscale photocarrier transport, trapping, and recombination mechanisms are usually inferred from ensemble-averaged measurements and remain elusive. Because an individual high-performing nanoparticle photocatalyst may outperform the ensemble average, design rules that would otherwise enhance catalytic efficiency remain unclear. Here, we introduce photomodulated electron energy-loss spectroscopy (EELS) in an optically coupled scanning transmission electron microscope (STEM) to map photocarrier localization. Using rhodium-doped strontium titanate (SrTiO3:Rh) solar water-splitting nanoparticles, we directly image the carrier densities concentrated at oxygen-vacancy surface trap states. This is achieved by separating photothermal heating from photocarrier populations through experimental and computational analyses of low-loss spectra. Photomodulated STEM-EELS enables angstrom-scale imaging of defect-induced photocarrier traps and their impact on photocatalytic efficiency.

Solar-based water splitting using suspensions of photocatalyst nanoparticles could be an attractive means for the production of hydrogen but requires similar to 10% solar-to-hydrogen efficiency for economic viability. Here we investigate the effects of increasing temperature on the rate of photoelectrochemical (PEC) hydrogen evolution reaction (HER) for photocathodes based on visible light absorbing SrTiO3 nanoparticles doped with Rh (SrTiO3:Rh) and a p-Si/SiO2/Cr/Pt Schottky metal-insulator semiconductor (Si-MIS) diode. We construct our PEC cells using Aquivion, a perfluorosulfonic acid ionomer, which facilitates temperature control. Under visible light illumination, we observe that the HER rate for Si-MIS decreases by approximately 6 & times; when temperature is increased from 25 degrees C to similar to 40 degrees C, while the rate for SrTiO3:Rh increases by a factor of 2 & times; over the same temperature range. We also observe that under UV (365 nm) illumination the PEC rates for the SrTiO3:Rh nanoparticle photocathodes increase by similar to 10 & times;. The increase in PEC efficiency is consistent with our previous report of improved photocarrier transport in SrTiO3:Rh nanoparticles with increasing temperature, and points to the significance of using heat both as a tool for gaining mechanistic insights into the water splitting process and to improve its efficiency.

The Ensembles of Photosynthetic Nanoreactors (EPN) Energy Frontier Research Center is gaining new knowledge that will help bridge the gap in solar-to-hydrogen energy conversion efficiency between what is observed, i.e. <1%, and necessary, i.e. >10%, to substantially mitigate the effects of global climate change. A major focus is to couple correlative microscopic and spectroscopic measurements with numerical simulations. By doing so, we are overturning conventional wisdom in the understanding of the basic science and engineering that dictate several observations in the field of photocatalytic solar water splitting. Notably, charge separation in state-of-the-art Rh-doped SrTiO 3 and BiVO 4 nanoparticles is not driven by electric fields due to band bending, but instead by differences in mobility and/or lifetime of mobile electronic carriers. Moreover, we have observed that dopants in Rh,La-codoped SrTiO 3 nanoparticles sometimes reside in unexpected crystallographic locations. We have also observed extensive incorporation of Pt cocatalysts into the bulk of Rh-doped SrTiO 3 nanoparticles during Pt photodeposition, which coincides with the induction period for observation of H 2 . Also, using atomic layer deposition to deposit ultrathin permeable oxide coatings on Rh-doped SrTiO 3 nanoparticles, we have observed increased selectivity for photocatalytic H 2 evolution. Lastly, using thermodynamically rigorous detailed balance models, which support observations from experiments, we have shown that the solar-to-hydrogen energy conversion efficiency of an ensemble of optically thin light absorbers can exceed that of optically thick materials, providing new motivation for the study and advancement of photocatalytic, over photoelectrochemical, solar water splitting. Collectively, our discoveries support new approaches, and motivate additional research pathways, toward the development of technoeconomically promising artificial photosynthetic devices.

This seminar session critically examines advances and challenges in photoelectrochemical (PEC) and photocatalytic (PC) water splitting for green hydrogen production, emphasizing material development, system design, and techno-economic considerations. The discussion highlights the fundamental trade-offs in photocatalyst design, particularly balancing band gap width for visible-light absorption with sufficient driving force for water redox reactions. Innovative approaches include the development of narrow band gap oxysulfide and oxynitride photocatalysts, selective dual co-catalyst deposition to enhance charge separation, and the fabrication of scalable photocatalyst sheets integrating solid mediators to improve electron transfer and stability. Z-scheme systems, both in suspension and with spatially separated reaction chambers, demonstrate improved solar-to-hydrogen (STH) efficiencies up to 2.5%, with outdoor panel reactors achieving around 1.2% STH. Techno-economic analyses underscore the necessity of increasing efficiency and reducing system costs to approach the target hydrogen production cost of approximately $1–3.5 per kilogram, with system designs evolving toward simplified, scalable reactors minimizing ancillary costs. The session also addresses the critical need for standardized testing protocols and international collaboration to ensure reproducibility and accelerate industrial adoption. While current efficiencies remain below the ideal 20–30% STH, incremental improvements in material quality, charge separation, and device engineering are projected to enable practical applications, particularly for niche uses where hydrogen storage and intermittent production are acceptable. The integration of advanced characterization techniques and interdisciplinary efforts is emphasized as essential for overcoming existing “valleys of death” in scaling solar-driven hydrogen technologies. Welcome from the Chair Photocatalysts for Solar-Driven Water Splitting toward Green Hydrogen Production Suspended particulate photocatalysts for Redox-driven Z-scheme overall water splitting to produce hydrogen Scalable Solar Water Splitting using Nano-to-Giga-Scale Reactors Discussion

Even though highly selective ion pumps are found in the membrane of every living cell, artificial ion selective separation is a longstanding unmet challenge in science and engineering. The development of a membrane-based ion separation technology can drive a dramatic progress in a wide range of applications such as: water treatment, bio-medical devices, extraction of precious metals from sea water, chemical sensors, solar fuels and more. In this contribution we report on the experimental demonstration of ion pumps based on an electronic flashing ratchet mechanism and their theoretical ion sorting performance. Electronic flashing ratchets are devices that utilize a temporal modulation of a spatially asymmetric electric field to drive steady state current. Like peristaltic pumps, where the pump mechanism is not in direct contact with the pumped fluid, electronic ratchets induce a net current with no direct charge transport between the power source and the pumped charge carriers. Thus, electronic ratchets can be used to pump ions in steady state with no electrochemical reactions between the power source and the pumped ions resulting in an 'all-electric' ion pump. Ratchet-based ion pumps (RBIPs) were fabricated by coating the two surfaces of nano-porous alumina wafers with metal, thus forming nano-porous capacitor-like devices. The electric field within the nano-pores is modulated by oscillating the capacitor voltage. Thus, when immersed in a solution, ions within the pores experience a modulating electric field resulting in ratchet-based ion pumping. The RBIPs performance was studied for various input signals, geometries, and solutions. RBIPs were shown to drive ionic current densities of several uA/cm^2 even when opposed by an electrostatic force. A significant ratchet action was observed with input signal amplitudes as low as 0.1V thus demonstrating that RBIPs can drive an ionic current with no associated redox reactions. An important hallmark of ratchets is the ability to invert the direction of particle flow with a change in the input signal frequency. The stopping frequency, which is the frequency at which the particle flux changes its direction, is determined by the potential distribution and particles transport properties. As a result, for a given ratchet, there can be a frequency at which particles with the same charge, but different diffusion coefficients, are transported in opposite directions. This concept, that was never applied to ion separations, can enable the extraction of ions with extremely low relative concentrations if their diffusion coefficient is even slightly different from the diffusion coefficient of other ions in the solution. We show by simulation, that for the prevalent ions in water, ions with a relative diffusion coefficient difference as small as 1% can be driven to opposite directions with a velocity difference as high as 1.2 mm/s. Since the direction of ion transport is determined by the input signal frequency, the sorting properties can be tuned in real time providing a simple fit-to-purpose solution for a variety of ion separations applications. Figure 1

Molecular catalysts, such as metalated porphyrins, are attractive cocatalysts for photocatalytic water splitting owing to their potential to simultaneously catalyze target reactions at their metal center, extend charge-separated-state lifetimes, and accumulate the requisite charge for product formation. However, porphyrin catalysts, like most molecular catalysts, are often limited by poor stability associated with demetalation, inactivation by undesired bonding (e.g., O2 coordination/redox/dimerization), and detachment from electrode supports or semiconducting photoabsorbers. In this study, nanoscopic titanium dioxide (TiO2) overlayers, deposited by atomic layer deposition (ALD), are demonstrated to encapsulate cobalt(III) meso-tetra(4-carboxyphenyl) porphyrin chloride (CoTCPP) molecular catalysts and thereby improve their adhesion to electrode surfaces over a wide range of electrode potentials spanning from -1.0 V vs RHE to +1.8 V vs RHE. Through analysis of Raman and ultraviolet-visible spectroscopy, it was confirmed that the metalloporphyrin structure was maintained when the surface-bound CoTCPP was encapsulated by 10 - 250 ALD cycles (≈2 - 18 nm thick) of TiO2. Additional characterization of CoTCPP catalysts before and after electrochemical measurements reveals that up to 97% of the encapsulated CoTCPP remains tethered to the electrode surface after chronoamperometry tests under hydrogen evolution reaction (HER) conditions, compared to <36% for unencapsulated CoTCPP. This study also shows that encapsulated CoTCPP molecules remain partially redox active for overlayers up to 8 nm, which can also attenuate undesired redox mediator back reactions like ferricyanide reduction.

Nanowire arrays present many unique advantages for solar-to-chemical energy conversion and are good model systems to investigate how the performance of one nanowire can influence others in an array. Spatially resolved photoluminescence is a powerful experimental characterization tool to quantify optical and electronic coupling between nanowires in an array. However, three underlying mechanisms of incident photon scattering, photon recycling, and charge-carrier diffusion dictate this coupling. In this study, we present a comprehensive analysis of light absorption and emission of a single nanowire at open circuit, and subsequent re-absorption and re-emission by a neighboring nanowire. We developed a novel correlated single nanowire micro-spectroscopy and widefield imaging methodology to spatially resolve photon communication pathways between neighboring nanowires and selectively image re-emitted and reflected photons. Unique multiphysics models have been developed to couple wave optics and semiconductor photophysics to especially isolate contributions from photon recycling and electronic transport to photon emission from neighboring nanowires. By systematically varying the morphologies of the nanowires modeled, we identify pathways to maximize photon recycling between neighboring nanowires. We conclude that the measured photoluminescence is more strongly influenced by the diffusion of charge-carriers as compared to photon recycling in materials with moderate-to-large charge-carrier mobilities (> 10 cm2 V-1 s-1), and that photon recycling dictates photoluminescence intensity only when the charge-carrier mobility is low (< 1 cm2 V-1 s-1). The experimental and simulation platforms developed herein for photon management strategies can be leveraged by the semiconductor photocatalysis community to enhance solar-to-chemical conversion efficiencies in semiconductor nanowire arrays.

O-phenylenediamines have emerged as powerful synthons for the installation of molecularly well-defined active sites conjugated to graphitic carbon electrode surfaces. These graphite-conjugated actives sites can serve as rich platforms for the electrochemical investigation of interfacial ion transfer reactions at the molecular level. But widespread utilization of this platform is restricted by the limited synthetic access to o-phenylenediamines bearing an array of additional functional groups. Herein, we present three distinct and modular synthetic strategies to symmetric and asymmetric 4,5-o-phenylenediamines. We demonstrate the utility of 4,5-o-dinitrobenzenes as relatively stable precursors to this class of compounds, as well as a modular route to 4,5-o-phenylenediamines in as little as 2 steps from commercial starting materials. We then show, using cyclic voltammetry, that graphitic electrodes modified with molecules obtained from our syntheses exhibit expected electrochemical responses.

Junctions formed at semiconductor interfaces generate space–charge regions. This can result in diode behavior, a prerequisite for efficient electronic charge separation and photoconversion. Space–charge regions also form at ion-exchange-membrane interfaces, which we are leveraging to demonstrate protonic charge separation from our new light-driven proton-pump platform. Recently, we discovered that an ion-exchange membrane can be diffusion doped to form covalent bonds with dopants of opposite charge to that of the native backbone, thus converting it into a monolithic bipolar membrane. We confirmed this modification using our newly developed membrane-electric-potential sensing procedure. These results are important because diffusion doping is the state-of-the-art means to generate an abrupt buried junction in semiconductors, resulting in high-quality diodes and efficient charge separation. By fabricating our buried-junction bipolar membranes into membrane–electrode assemblies, analogous to fuel cells but instead driving reversible H 2 redox, we were able to measure their electrochemical properties. Results suggest that these junctions are more abrupt than those formed by drop-casting or hot-pressing bipolar membranes. Mott–Schottky analysis of potential-dependent capacitance data provides additional insight into junction properties, including potential of zero net charge, electroactive dopant density, and permittivity. Collectively, our discoveries form the foundational framework for new devices and functions that benefit from purely protonic transport and reactivity, such as direct oceanic capture of CO 2 from HCO 3 – . We are hopeful that they motivate researchers to help us expand our platform to protonic versions of other condensed matter physics phenomena, such as 2D gases, spintronic devices, topological insulators, and chiral semiconductors.

Immersing polymer solar cells in aqueous electrolyte for photoelectrochemical (PEC) hydrogen production is likely to cause photophysical changes that could present both challenges and opportunities for engineering functional and durable devices. Herein we study the bulk heterojunction blend poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b ']dithiophene-2,6-diyl)-alt-(2-(((2-ethylhexyl)oxy)carbonyl)-3-fluorothieno[3,4-b]thiophene-4,6-diyl):poly(N,N '-di(2-octyldodecyl)naphthalene-1,8:4,5-bis(dicarboximide)-2,6-diyl)-alt-(2,2-bithiophene-5,5 '-diyl) (PTB7-Th:N2200) excited-state dynamics in electrolyte from femtosecond to millisecond time scales using pump-probe microwave conductivity and absorption spectroscopy. While the blend swells very little, electrolyte exposure increases the microwave-frequency mobility and possibly the yield of photogenerated charges while also decreasing crystallinity. These results indicate an enhancement in key performance metrics, implying that any limitations on the performance of PEC test devices do not arise from active layer-electrolyte interactions. For the PTB7-Th:N2200 blend or similar photocathode systems, our results indicate that improving the interfacial kinetics and/or the carrier lifetime should be prioritized, not protecting the active layer from the electrolyte. Since this observation may not be universal to all polymer systems, future research should focus on identifying their limiting photophysical processes.

Generating hydrogen from renewable resources would unlock a low-carbon energy carrier that could be used to reduce greenhouse gas emissions in sectors such as industry and transportation. Yet, the allocation...

Proton transfer at electrochemical interfaces is fundamentally important across science and technology, yet kinetic measurements of this elementary step at electrode|electrolyte interfaces are convoluted with other electron-transfer steps and by inhomogeneous electrode surfaces. We use facilitated proton transfer at the interface between two immiscible electrolyte solutions (ITIES) as a platform to study proton-transfer kinetics in the absence of interfacial electron transfer and without the defects at solid|electrolyte interfaces. Diffusion-controlled micropipette voltammetry revealed that 2,6-diphenylpyridine (DPP) facilitates proton transfer across the HCl(aq)|trifluorotoluene interface, while voltammetry at nanopipette-supported interfaces yielded activation-controlled ion-transfer currents. We extract kinetic parameters kapp0 and αapp, 3.0 ± 1.8 cm/s and 0.3 ± 0.2, respectively, for DPP-facilitated proton transfer by fitting quasi-reversible voltammograms to a mixed diffusive-kinetic model. Finite-element simulations highlighted regimes of direct proton transfer and sequential proton transfer, where the current divided between these two possible pathways was shown to favor direct proton transfer when the neutral partitioning step DPP(org) → DPP(aq) was rate-determining. Atomistic molecular-dynamics simulations were used to compute the free energy change to move DPP and its protonated analogue within, and across, the liquid|liquid interface. The most-likely location for proton transfer is predicted to be in the surface region where significant interpenetration of the two liquids occurs. Understanding the kinetics of ion transfer at the ITIES illustrated here is important in the development of general theories of ion transfer in electrochemical science and technology.

Electrocatalysts encapsulated by nanoscopic overlayers can catalyze redox reactions at the outer surface of the overlayer or at the buried interface between the overlayer and the active catalyst, leading to complex behavior in the presence of two competing electrochemical reactions. This study investigated oxide encapsulated electrocatalysts (OECs) comprised of iridium (Ir) thin films coated with an ultrathin (2-10 nm thick) silicon oxide (SiOx) or titanium oxide (TiOx) overlayer. The performance of SiOx|Ir and TiOx|Ir thin film electrodes towards the oxygen evolution reaction (OER) and Fe(II)/Fe(III) redox reactions were evaluated. An improvement in selectivity towards the OER was observed for all OECs. Overlayer properties, namely ionic and electronic conductivity, were assessed using a combination of electroanalytical methods and molecular dynamics simulations. SiOx and TiO¬x overlayers were found to be permeable to H2O and O2 such that the OER can occur at the MOx|Ir (M = Ti, Si) buried interface, which was further supported with molecular dynamics simulations. In contrast, Fe(II)/Fe(III) redox reactions occur to the same degree irrespective of whether electrocatalysts are bare, have TiOx overlayers with thicknesses less than 4 nm, or have SiOx overlayers with thicknesses less than 2 nm. This observation is attributed to facile electronic transport between the buried interface and outer surface of the overlayer, as measured with through-plane conductivity and ionic permeability measurements of wetted overlayer materials. These findings reveal the influence of oxide overlayer properties on the activity and selectivity of OECs and suggest opportunities to tune these properties for a wide range of electrochemical reactions.

Doped SrTiO3 nanoparticles constitute a leading materials platform for solar hydrogen production. One of the most ubiquitous and efficient SrTiO3 materials utilizes Rh and La as dopants, designed to improve both visible-light absorption and charge separation ability. Typically, dopant positions in the SrTiO3 lattice are assigned using ionic size and charge or ensemble-averaged techniques such as power X-ray diffraction or Raman spectroscopy. Direct observations of dopant locations in the SrTiO3 lattice have not been previously reported. Here, we apply atomic-scale scanning transmission electron microscopy (STEM) coupled with image processing to directly elucidate dopant locations in 2% Rh-doped SrTiO3 (Rh:SrTiO3) and 2% La, 2% Rh codoped SrTiO3 (La,Rh:SrTiO3). Ensemble measurements were first performed to quantify aggregate properties across many particles of each type. Measurements of H2 using in-line mass spectrometry suggest that the materials have different quantum yields for photocatalytic H2 evolution, and Raman spectroscopy suggest that the concentration of dopants in the B-site (Ti site) differs. Simultaneous atomic-scale STEM, energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) were performed on individual nanoparticles to directly determine dopant locations. Results indicate that in Rh:SrTiO3, Rh occupies both the A-site and the B-site, which is unexpected because ionic radii argument suggests Rh will occupy only the B-site. In La,Rh:SrTiO3, La primarily occupies the A-site and Rh primarily occupies the B-site. Precise knowledge of dopant positions is used to inform density functional theory (DFT) simulations for each doped lattice's electronic structure. These combined results suggest that La codoping can hinder photocatalytic H2 evolution activity when Rh dopants exist in B-site recombination centers. The methods presented here demonstrate the effectiveness of correlating ensemble measurements, atomic-scale STEM imaging, and DFT simulations to establish structure-performance relationships for doped SrTiO3 nanoparticles.

Anchoring of molecules to the surfaces of semiconductor nanocrystals (NCs) presents an opportunity to leverage the precise synthetic tunability of molecular function and the remarkable light harvesting properties of NCs to drive photochemical reactions. However, charge transfer between the two species depends not only on the energy level alignments but also on the details of their binding interactions, which are difficult to probe. Here, we characterize the binding between CdSe quantum dots (QDs) and a new phosphonated derivative of the electron acceptor methyl viologen, designed to attach to the QD surface via the phosphonate group. We use isothermal titration calorimetry to probe the thermodynamics of the QD-molecule interaction and use the parameters determined therein to analyze transient absorption spectroscopy measurements of forward and back electron transfer from QDs to the viologen. We find that the ligand-like phosphonate binding leads to an electron-transfer rate constant that is 3 orders of magnitude smaller than that for the face-on binding of the bipyridine ring of methyl viologen. Back electron transfer is also significantly slower in the derivative. Interestingly, a minor fraction of the phosphonated derivative also binds in the face-on configuration, with similar forward and back electron transfer kinetics as methyl viologen. Numerical simulations show that the ligand-like binding will lead to significantly improved quantum yields of photocatalysis over a wide range of reaction rates. By independently characterizing binding thermodynamics and charge transfer kinetics, this work reveals how the complexities underlying electron transfer at the NC-molecule interface determine photocatalytic outcomes. This work also represents a step toward controlling forward and back electron transfer kinetics via rational molecular design.

Semiconductor photoelectrochemistry is a dynamic and interdisciplinary field at the forefront of research in solar fuels, energy conversion, and catalysis. This Perspective captures the collective insights from the second Gerischer Electrochemistry Today Symposium, held at Colorado State University in Fort Collins, CO, in August 2024, which convened leading researchers, early-career scientists, and industry partners to define the critical next steps for the field. Through interactive sessions, technical talks, panel discussions, and training initiatives-including a Semiconductor Electrochemistry Bootcamp-the symposium emphasized three pillars of advancement: (i) facilitating the exchange of new ideas in semiconductor electrochemistry and charge separation; (ii) fostering the development of future researchers, research topics, and participation in the semiconductor workforce; and (iii) building community. This Energy Focus distills key themes from the meeting and identifies major knowledge gaps in the following areas: mechanisms of charge separation and recombination, role of defects and disorder, dynamic and operando characterization methods, interfacial chemistry and surface passivation, theoretical and modeling limitations, and standardization and benchmarking. The inclusive and collaborative structure of the symposium enabled the generation of this comprehensive report that will serve as a roadmap for fundamental and applied research in the rapidly evolving field of semiconductor electrochemistry over the next decade.