Ongoing efforts to stabilize the operation of photoelectrochemical (PEC) devices remain critical for achieving economically viable solar fuel production, as devices with lifetimes on the order of 10 to 30 years are projected requirements for utility-scale, PEC device implementation. However, insight into the causes of device degradation and activity losses is generally provided by monitoring the device current, a quantity which masks the relative contributions of photovoltaic component degradation and electrocatalyst activity drops to overall performance losses. In this study, an approach for deconvoluting the various contributors to PEC device losses is described. In particular, the causes for observed fluctuations in device performance are determined through the collection of real-time, current-voltage data, paired with an analytical method that enables the decomposition of drops in device current into its constituent photovoltaic- and catalyst-driven performance losses. We test the validity of this approach by applying it to the data collected for a PEC hydrogen evolution test-bed.

The Tafel equation, which serves as a linearized representation of the Butler-Volmer expression, permits the extraction of fundamental parameters of electrochemical systems, such as exchange current densities and Tafel slopes. However, the application of these linear fitting procedures become problematic in the cases of experimental data featuring nonlinearities in the Tafel (log(J) - V) response. This is especially troubling in low-overpotential regimes, where charge transfer kinetics generally dominate electrode behavior (as opposed to diffusion-limited currents at higher overpotentials) and linearization is expected to be most applicable. As a response to this complication and the ad-hoc forms of fitting that may arise from it, this report outlines a systematic method for the determination of Tafel slopes through a rigorous, quantitative framework. The algorithm's performance is benchmarked using electrochemical measurements of well-characterized reactions for water splitting, hydrogen evolution, and dioxygen reduction at iridium and platinum electrodes. As a final validation test, we show that the exchange currents extracted through this automated fitting procedure, used in conjunction with the Butler-Volmer model, are capable of reproducing the original electrochemical data in kinetically controlled regimes.

To date, the breadth of scientific research that has been devoted to investigating the photochemical and photophysical behavior of the lanthanide elements has generally fallen into one of two camps: solution studies of luminescent lanthanide metal-ligand complexes or investigations of solid-state nanoparticles, composed primarily of, or doped with, lanthanide lumiphores. In the latter case, most research of lanthanide nanocolloids has precluded any investigations regarding the use of organic ligands to overcome the difficulties associated with f-f excitation of lanthanides. Instead, most work on condensed-phase lanthanide luminescence has centered on strategies such as d-f charge separation in divalent lanthanides and the sensitization of lanthanide excited states using quantum dots. Current work now aims at bridging the camps of condensed-phase lanthanide photophysics and the solution chemistry of ligand-lanthanide molecular complexes. Recent efforts have partly focused on the fundamental characterization of NaGd_1-xLn_xF₄ nanoparticles featuring surface display of the sensitizer ligand 3,4,3-LI(1,2-HOPO), showing these structures to be capable of converting absorbed UV light into luminescence from Eu³⁺ and Tb³⁺ ions. These results suggest such a use of the ligand sensitization as a tool of choice to overcome the constraints of UV solar spectrum/semiconductor band-gap mismatch and low absorption cross sections in solid-state lanthanide systems.

We report the development of a fully-integrated, photoelectrochemical (PEC) device coupling water oxidation to hydrogen evolution using a III-V triple-junction photovoltaic (PV) embedded in a Nafion membrane. This architecture is genuinely monolithic, with wireless catalyst integration being achieved via compression of metal sputter-coated carbon electrodes against the front and back PV contacts. The resulting MEA-type, sandwich structure minimizes the path length for proton conduction through the membrane ionomer, while simultaneously preventing PV light attenuation by the catalyst layer, a common issue for monolithic PEC structures. Simulated, solar illumination of this construct, when operating in neutral-pH water, yields a peak solar-to-hydrogen efficiency of 12.6% during a four-day trial. While the wireless nature of monolithic PEC devices typically prevents the measurement of current flow and faradaic efficiencies, we circumvent this complication through the placement of an electrical shunt between the PV and the cathode catalyst layer, rerouting charge generated at the PV through a potentiostat prior to cathodic proton reduction. Using this configuration, we also show evidence of a corrosion current competing with anodic oxygen evolution under acidic conditions, highlighting the importance of quantifying product generation in monolithic devices.

This report builds on our recent disclosure of a fully-integrated, photoelectrochemical (PEC) device for hydrogen evolution using a structure incorporating a III-V triple-junction photovoltaic (PV) embedded in a Nafion membrane. Catalyst integration is realized by compression of catalyst-modified, carbon electrodes against the front and back PV contacts, resulting in a wireless, monolithic PEC assembly. Using this device architecture, we demonstrate significant enhancements in device stability and longevity, by transitioning from a liquid-water to water-vapor anode. Our use of a gas-fed anode enables 1000 h of cumulative device operation at a peak solar-to-hydrogen efficiency of 14%, during simulated, solar illumination at 1 sun and outdoor, diurnal cycling. Vapor-fed water oxidation is shown to reduce drops in device performance by mitigating the corrosion effects that are commonly associated with full-aqueous immersion of the electrochemical and photovoltaic elements in PEC devices.

Electrochemical carbon dioxide reduction represents a promising path to utilize CO2 as a feedstock for generating valuable products such as fuels and chemicals. Faradaic efficiencies near 100% have been achieved for certain CO2 reduction products such as CO, but the electrolyzer outlet streams usually contain large fractions of unreacted CO2, dropping the product concentrations below 1% in many cases. The system disclosed here recycles the unreacted CO2 together with the products and flows them back into the CO2 reduction reactor, enabling much higher CO2 conversion rates without dropping the gas flow rate. However, simple recirculation is shown to accumulate significant amounts of hydrogen, impeding effective CO2 reduction. In this looped system, an electrochemical H2 pump is placed in series with the CO2 reactor, which effectively removes all the H2 from the recycled gas stream, increasing the concentrations of carbon-containing products. The system was initially tested with a CO-generating catalyst and CO concentrations above 70% were achieved in the recycled gas stream, compared to a maximum CO concentration of 8% in single-pass configuration. Results with a CO2 reactor targeting ethylene as the main product show that ethylene concentrations of at least 10% can be achieved, which is roughly 20 times higher compared to a single-pass system.

This report details spectroscopic characterizations of rare-earth, core-shell nanoparticles decorated with the f-element chelator 3,4,3-LI(1,2-HOPO). Evidence of photon downconversion is corroborated through detailed power dependence measurements, which suggest two-photon decay paths are active in these materials, albeit only representing a minority contribution of the sum luminescence, with emission being dominated by normal, Stokes' shifted fluorescence. Specifically, ultraviolet ligand photosensitization of Nd³⁺ ions in a NaGdF₄ host shell results in energy transfer to a Nd³⁺/Yb³⁺-doped NaGdF₄ nanoparticle core. The population and subsequent decay of core, Yb^{3+ 2} F _5/2 states result in a spectral shift of 620 nm, manifested in a NIR emission displaying luminescence profiles diagnostic of Yb³⁺ and Nd³⁺ excited state decays. Emphasis is placed on the generality of this material architecture for realizing ligand-pumped, multi-photon downconversion, with the Nd³⁺/Yb³⁺ system presented here functioning as a working prototype for a design principle that may be readily extended to other lanthanide pairs.

The small absorption cross sections (ϵ < 10 M-1 cm-1) characteristic of Laporte-forbidden transitions in the f-elements have limited the practical implementation of lanthanide nanoparticles in solar capture devices. While various strategies designed to circumvent the problems of low f-f oscillator strengths have been investigated, comparatively little work has explored the utility of organic ligands with high absorption coefficients (ϵ ≈ 103-105 M-1 cm-1) in sensitizing excited states in lanthanide nanocrystals. Here, we detail the photophysics of NaGd1-xEuxF4 nanoparticles featuring surface display of the ligand 3,4,3-LI(1,2-HOPO), an aromatic antenna functioning as the terminal light absorber in this system. The result is a ligand-nanocrystal hybrid that converts UV (250-360 nm) light into red Eu(III) luminescence with an external quantum yield of 3.3%. We analyze this sensitization process, responsible for a 104-fold increase in luminescence relative to metal-centered excitation, through a quantitative treatment of energy transfer between ligand and metal states.

Photoelectrochemical (PEC) conversion of carbon dioxide into valuable chemicals and fuels represents a promising path towards combating anthropogenic CO2 emissions. However, the limited conversion efficiencies, operation lifetimes and CO2 utilization efficiencies of PEC devices currently prohibit their application beyond the laboratory scale. Here, a wireless device converting CO2 and water into carbon monoxide and hydrogen at a peak solar conversion efficiency exceeding 16% under an illumination intensity of 5 suns is demonstrated. A CO/H2 product ratio between 10-20 is measured during a 17 h stability test. Fluctuations in device performance are rigorously analyzed via deconvolution of electrochemical and photoabsorber contributions. It is demonstrated that beneficial heat dissipation is enabled by wireless integration of the photoabsorber and electrocatalyst components, accounting for roughly 10% of the achieved conversion efficiency, an achievement unattainable with physically separated photoabsorber and electrolyzer components.

We present the first report of ligand-sensitized, actinide luminescence in a lanthanide nanoparticle host. Amplified luminescence of 248Cm3+ doped in a NaGdF4 lattice is achieved through optical pumping of a surface-localized metal chelator, 3,4,3-LI(1,2-HOPO), capable of sensitizing Cm3+ excited states. The data suggest the possibility of using such materials in theranostic applications, with a ligand-sensitized actinide or radio-lanthanide serving the dual roles of a nuclear decay source for radiotherapeutics, and as a luminescent center or energy transfer conduit to another emissive metal ion, for biological imaging.