The rapidly developing world poses a high energy demand. Hydrogen is recognized as one of the most important renewable energy sources with some great features including cleanness, and high gravimetric energy density. Hydrogen is predominantly produced by steam-methane reforming process at present, which is an energy intensive and CO2 emission intensive process. Alternatively, the hydrogen evolution reaction (HER) by electrochemical water splitting is part of a sustainable and scalable process to produce H2. Currently, precious metal platinum is required to catalyze this reaction which limits its industrial application. Hence, the design of a highly efficient and low-cost electrocatalyst is in demand. Recently, metal phosphides were developed as a good substitute of platinum for HER catalysis. Some examples are nickel phosphide, cobalt phosphide, iron phosphide. All of them have shown excellent HER activity and stability. Among them, iron phosphide is particularly attractive because of the abundance of iron on earth. Traditionally, the synthesis of iron phosphide is based on high temperature process that generates highly corrosive and flammable phosphine byproduct. Thus, greener and more straightforward synthetic method is desired. Herein, an easy and cost-effective method is developed to electrodeposit iron phosphide (FeP) film on a copper electrode at room temperature. The pulsed electrodeposition method was used to electrochemically etch co-deposited metallic iron which improved the film’s stability. In addition, formic acid and pH are crucial to promoting FeP deposition and it was well explained by experiments of cyclic voltammetry, capacitance studies, electron microscopy, and inductively coupled plasma mass spectrometry.

The as-deposited FeP film was shown to be a highly active HER catalyst in all-pH conditions. The FeP modified electrode produced current densities of-10 mA/cm2 at a low overpotential of 66 mV in 0.5 M sulfuric acid (H2SO4, pH=0.3), 131 mV in 1.0 M phosphate buffer (PB, pH=6.4), and 110 mV in 1.0 M potassium hydroxide (KOH, pH=14.0) solution, as well as a Tafel slope of 55 mV/dec, 108 mV/dec, 60 mV/dec, respectively. Electrochemical impedance studies revealed the low charge transfer resistance and fast HER kinetics of FeP electrocatalyst. More importantly, the film has excellent stability during HER catalysis under acidic and neutral conditions, as shown by long-term chronoamperometry, supported further with electron microscopy, and X-ray photoelectron spectroscopy. Specifically, the overpotentials increased diminutively 7 mV after 24 hours of electrocatalysis at a current density of -10 mA/cm2 in acidic solution. Similarly, only a 10 mV increase in overpotential was observed under neutral conditions. Overall, in this work, I was able to successfully demonstrate that a carefully controlled electrodeposition process provided a simple yet powerful method to synthesize heterogeneous iron phosphide catalyst that is highly efficient towards hydrogen evolution reaction. Via potentiostatic tuning of the deposition rate proceeding by electrochemical etching of phosphide reach inactive alloy, we were able to achieve a 2-D film that could find applications in large scale hydrogen production under wide range of industrial conditions.

Electrochemistry is the study of the interface between chemical species and electrified surfaces. These interfaces, which use electrical current to drive chemical transformations or derive electrical energy from chemical reactions, are critically important in the context of climate change. Using electrochemistry, renewably-generated electricity can be stored in the form of chemical potential in the chemical bonds of a fuel (such as hydrogen) or in the energy of electrons in a battery. These reactions are often catalyzed by nanoparticles of various materials, chosen due to their high surface areas and ostensibly tunable proparties. However, a significant gap in our understanding of these nanoparticles remains: as nanoparticles are not atomically precise, each individual particle has different properties (e.g., size, shape, or catalytic performance). However, conventional analysis methods study ensembles of many particles and cannot deconvolute each individual’s contribution. This means that the best-performing particles (compared to those which contributed little to the reaction) cannot be readily identified, and future syntheses cannot be tailored to target these particles.

The first three chapters of this dissertation describe my work to develop new methods and tools to study individual particles at the nanoscale, one particle at a time. First, I describe our efforts to understand the reactivity of platinum nanoparticles which catalyze the hydrogen evolution and oxygen reduction reactions. The broad distribution of catalytic activities we measured could not be explained simply by particle size, and our insights let us identify a possible mechanism degraded the particles' activity. In order to glean more information from each particle, I next developed a method to measure the electrochemical impedance of single nanoparticles. Electrochemical impedance spectroscopy has the ability to resolve different interfacial processes based on their timescales and was used to detect individual, insulating microparticles as they collided with an electrode; the time-resolved impedance spectra enabled accurate measurement of the particle-electrode contact areas. Finally, the single-particle impedance technique was applied to individual Prussian blue nanocubes, which reversibly intercalate sodium ions. In this case, spatially-resolved impedance spectra allowed us to measure the ionic and electronic conductivities of the particles as they stored sodium ions and electrons and revealed that these conductivities varied by up to an order of magnitude particle-to-particle.

The final chapter of this work extends the time-resolved impedance techniques developed for single particle analysis to the field of electrochemical biosensing. Biosensors aim to measure the presence or concentration of a particular species, and electrochemistry provides a natural way to translate between a chemical species and an electrical signal. Specifically, I describe the interrogation of a class of devices known as electrochemical aptamer-based sensors using electrochemical impedance spectroscopy. This technique enabled us to measure the state of the sensors more precisely and more rapidly than comparable methods, allowing real-time measurements of an antibiotic and an amino acid, even in the blood stream of living animals.

Cold molecules (1-7K) have been a powerful probe of cold chemistry, chemical analysis and fundamental physics beyond the standard model. By far the most widely used source of cold molecules have been the pulsed supersonic jet. In the past decade another cooling method known as buffer gas cooling has emerged for the production of cold diatomic and polyatomic molecules in a continuous manner. Buffer gas cooled molecules are brought to low temperatures via collisions with cryogenic gas, which enables faster acquisition and assignment of molecular spectra. Buffer gas cooling coupled with microwave spectroscopy has enabled novel applications in chiral detection and mixture analysis due to its high sensitivity. In this work, such capabilities are further expanded with a focus on chiral systems. I describe the experimental advances in buffer gas cell design, and then discuss three key results: the chiral detection of unknown multi-component mixtures, the first microwave differential precision measurement for probing parity violation effects in chiral molecules, and the study of low temperature formation kinetics of ethanol-methanol and water dimers. Finally, I discuss the source and effects of stray electric fields in the buffer gas cell and present preliminary efforts towards studying chiral imprinting and buffer gas cooled reactive species.

Electrochemistry serves as a sensitive and rapid signal transduction platform for sensing applications. In contrast to commonly used diagnostic platforms which rely on polymerase-chain reaction for target amplification or immunological recognition, electrochemical-based systems are generalizable to many targets at various length-scales. At the ensemble scale, electrochemical aptamer-based (EAB) sensors convert target-recognition into an electrochemical signal via the binding-induced change in electron transfer from a redox reporter. Because aptamers are artificially selected for the detection of a wide range of small molecule, metabolite, and protein targets, EAB sensors are not restricted to chemically specific ligand-binding events and represent a generalizable platform technology that enable real-time, in vivo measurements. On a single particle scale, single-entity electrochemistry (SEE) offers unparalleled insights into the fundamental electron-transfer events that enable life. With this technique, we can parse individual differences, typically masked by some average thermodynamic response, by studying biological molecules one at a time. Doing so, we can size molecules, determine individual catalytic activity, and more broadly, characterize heterogeneous behaviors.

In this dissertation, I describe my work focused on utilizing electrochemistry to poke, prod, and improve bioanalytical sensing systems. I first address the practical needs of transitioning EAB sensors from the laboratory to the clinic; namely, I use electrochemistry to characterize disinfected or stored EAB sensors to ensure optimal device performance. Second, I tackle a longstanding issue within the electrochemistry literature that has resulted in imprecise SEE measurements. I discovered that by introducing a homogeneous chemical reaction in solution, I was able to shift the rate limiting step in current generation at an electrode from mass transport of a redox mediator to its local regeneration by a substrate. Doing so, I can achieve a ten-fold gain in measurement precision when sizing insulating particles using nanoimpact electrochemistry. Finally, with the imprecision associated with SEE greatly reduced, I lay out a foundation to explore biological interactions at a nano-scale electrode. In closing, I hope this work adds to the current body of literature establishing electrochemistry as a powerful tool for nanoscale bioanalytical systems.

Since the 1990s, the advent of lithium-ion batteries has revolutionized energy storage in compact, high-density applications ranging from personal devices to passenger automobiles.

Over the past 5 years, they have become crucial to making electrical grids resilient to weather anomalies. In this context, we will discuss nascent chemistry and system design around two energy storage/conversion systems: direct ammonia fuel cells (DAFCs), and membrane-less redox-flow batteries (RFBs). Both systems share a common layout and utilize the membrane-electrode assembly (MEA), which sandwiches the anode and cathode electrodes with aselective ion-exchange membrane.

DAFCs operate as an open system, where ammonia and oxygen streams are converted into dinitrogen and water within the MEA while generating electricity for external loads. While ammonia offers logistical advantages over hydrogen, it presents a challenge in terms of the scarcity of efficient electrocatalysts for the sluggish 6e- NH3 oxidation. Our work emerges from the fundamental necessity for developing these electrocatalysts for protic conditions. We have found a molecular Ru complex that catalyzes rapid NH3 oxidation in aqueous conditions, demonstrating high selectivity for N2 – hitherto unprecedented among molecular electrocatalysts under aqueous conditions.

Shifting focus to the membrane component, we will describe our efforts to improve membrane-less RFBs. RFBs employ the MEAs in a closed system, with anolyte and catholyte solutions containing redox active compounds circulating across the anode and cathode during operation. The ion-selective membrane in typical RFBs prevents self-discharge by separating the anolyte and catholyte, constituting a significant portion of the MEA cost. We will explore the shortcomings on previous attempts at biphasic membrane-less RFBs and explain a straightforward modification to the cell layout that overcomes interfacial self-discharge in these biphasic cells, elevating their Coulombic efficiency to >99%. In scaling up these cells,we will peek at ion transfer kinetics across liquid-liquid interfaces.