Success and failure in the incorporation of gold nanoparticles inside ferri/ferrocyanide thermogalvanic cells

Passivation, phase, and morphology control of CdS nanocrystals probed using fluorinated aromatic amines and solid-state NMR spectroscopy

Nanocrystals are widely explored for a range of medical, imaging, sensing, and energy conversion applications. CdS nanocrystals have been reported as excellent photocatalysts, with thin film CdS also highly important in photovoltaic devices. To optimise properties of nanocrystals, control over phase, facet, and morphology are vital. Here, CdS nanocrystals were synthesised by the solvothermal decomposition of a Cd xanthate single source precursor. To attempt to control CdS nanocrystal surfaces and morphology, the solvent used in the nanocrystal synthesis was altered from pure trioctylphosphine oxide (TOPO) to a mixed TOPO : fluorinated aromatic amine (3-fluorobenzyl amine (3-FlBzAm) or 3-fluoroaniline (3-FlAn)), where 19F provides a sensitive NMR-active surface probe. Powder X-ray diffraction found that the CdS nanocrystals synthesised from TOPO : 3-FlAn solvent mixtures were predominantly cubic whilst the TOPO : 3-FlBzAm synthesised nanocrystals were predominantly hexagonal. Raman spectroscopy identified hexagonal CdS in all samples. Solid-state NMR of 113Cd, 19F, 13C, and 1H was employed to investigate the local Cd environments, surface ligands, and ligand interactions. This showed there was a mixture of CdS phases present in all samples and that surfaces were capped with TOPO : fluorinated aromatic amine mixtures, but also that there was a stronger binding affinity of 3-FlBzAm compared with 3-FlAn on the CdS surface, which likely impacts growth mechanisms. This work highlights that fluorinated aromatic amines can be used to probe NC surfaces and also control NC properties through their influence during NC growth.

Self-assembled monolayers for electrostatic electrocatalysis and enhanced electrode stability in thermogalvanic cells

Waste heat is ubiquitous; as such, sustainable and long-lasting devices are required to convert it into more useful forms of energy that can make use of this abundant potential resource. Thermogalvanic cells (or thermocells) can use the thermoelectrochemical properties of redox couples to achieve this; entropy-driven redox reactions allow them to act as liquid thermoelectrics. However, excellent electrocatalysis at the electrode surface is required for optimum conversion efficiency. Serendipitous observation of Nafion-based electrocatalysis prompted the exploration of electrostatically charged self-assembled monolayers (SAMs) inside a thermocell. Both electrostatic electrocatalysis and improved electrode stability were observed; in an aqueous K3[Fe(CN)6]/K4[Fe(CN)6]-based cell, modification with (3-trimethylammonium bromide)thiopropane resulted in higher electrical power, and protection against [Fe(CN)6]3-/4--induced gold passivation, relative to bare gold. Molecular-based electrostatic electrocatalysis could be an alternative to precious metal-based nanomaterial electrocatalysis, and could be integrated with (nano)carbon-based electrodes to further enhance the ability of thermogalvanic and other electrochemical energy conversion devices, e.g. redox flow batteries.

High-performance cryo-temperature ionic thermoelectric liquid cell developed through a eutectic solvent strategy

Ionic thermoelectric (i-TE) liquid cells offer an environmentally friendly, cost effective, and easy-operation route to low-grade heat recovery. However, the lowest temperature is limited by the freezing temperature of the aqueous electrolyte. Applying a eutectic solvent strategy, we fabricate a high-performance cryo-temperature i-TE liquid cell. Formamide is used as a chaotic organic solvent that destroys the hydrogen bond network between water molecules, forming a deep eutectic solvent that enables the cell to operate near cryo temperatures (down to -35 °C). After synergistic optimization of the electrode and cell structure, the as-fabricated liquid i-TE cell with cold (-35 °C) and hot (70 °C) ends achieve a high power density (17.5 W m-2) and a large two-hour energy density (27 kJ m-2). In a prototype 25-cell module, the open-circuit voltage and short-circuit current are 6.9 V and 68 mA, respectively, and the maximum power is 131 mW. The anti-freezing ability and high output performance of the as-fabricated i-TE liquid cell system are requisites for applications in frigid regions.

High-Power-Density Thermoelectrochemical Cell Based on Ni/NiO Nanostructured Microsphere Electrodes with Alkaline Electrolyte

Effective low-grade waste heat harvesting and its conversion into electric energy by the means of thermoelectrochemical cells (TECs) are a strong theme in the field of renewable energy investigation. Despite considerable scientific research, TECs have not yet been practically applied due to the high cost of electrode materials and low effectiveness levels. A large hypothetical Seebeck coefficient allow the harvest of the low-grade waste heat and, particularly, to use TECs for collecting human body heat. This paper demonstrates the investigation of estimated hypothetical Seebeck coefficient dependency on KOH electrolyte concentration for TECs with hollow nanostructured Ni/NiO microsphere electrodes. It proposes a thermoelectrochemical cell with power density of 1.72 W·m-2 and describes the chemistry of electrodes and near-electrode space. Also, the paper demonstrates a decrease in charge transfer resistance from 3.5 to 0.52 Ω and a decrease in capacitive behavior with increasing electrolyte concentration due to diffusion effects.

Direct measurement of the genuine efficiency of thermogalvanic heat-to-electricity conversion in thermocells

Harvesting wasted thermal energy could make important contributions to global energy sustainability. Thermogalvanic devices are simple, chemistry-based devices which can convert heat to electricity, through facile redox chemistry. The efficiency of this process is the ratio of electrical energy generated by the cell (in Watts) to the quantity of thermal energy that passes through the cell (also in Watts). Prior work estimated the quantity of thermal energy passed through a thermocell by applying a conductive heat transfer model to the electrolyte. Here, we employ a heat flux sensor to unambiguously quantify both heat flux and electrical power. By evaluating the effect of electrode separation, temperature difference and gelation of the electrolyte, we found significant discrepancy between the estimated model and the quantified reality. For electrode separation, the trend between estimated and measured efficiency went in opposite directions; as a function of temperature difference, they demonstrated the same trend, but estimated values were significantly higher. This was due to significant additional convection and radiation contributions to the heat flux. Conversely, gelled electrolytes were able to suppress heat flux mechanisms and achieve experimentally determined efficiency values in excess of the estimated values (at small electrode separations), with partially gelled systems being particularly effective. This study provides the ability to unambiguously benchmark and assess the absolute efficiency and Carnot efficiency of thermogalvanic electrolytes and even the whole thermocell device, allowing 'total device efficiency' to be quantified. The deviation between the routinely applied estimation methodology and actual measurement will support the rational development of novel thermal energy harvesting chemistries, materials and devices.

Harvesting Waste Thermal Energy Using a Surface-Modified Carbon Fiber-Based Thermo-Electrochemical Cell

An important direction in the development of energy saving policy is harvesting and conversion into electricity of low-grade waste heat. The present paper is devoted to the improvement of the efficiency of thermo-electrochemical cells based on carbon fiber electrodes and potassium ferri-/ferrocyanide redox electrolyte. The influence of the carbon fiber electrode surface modification (magnetron deposition of silver and titanium or infiltration implantation of nanoscale titanium oxide) on the output power and parameters of the impedance equivalent scheme of a thermo-electrochemical cell has been studied. Two kinds of cell designs (a conventional electrochemical cell with a salt bridge and a coin cell-type body) were investigated. It was found that the nature of the surface modification of electrodes can change the internal resistance of the cell by three orders of magnitude. The dependence of the equivalent scheme parameters and output power density of the thermoelectric cell on the type of electrode materials was presented. It was observed that the maximum power for carbon fiber modified with titanium metal and titanium oxide was 25.2 mW/m2 and the efficiency was 1.37%.

Competition between CO2 Reduction and Hydrogen Evolution on a Gold Electrode under Well-Defined Mass Transport Conditions

Gold is one of the most selective catalysts for the electrochemical reduction of CO₂ (CO2RR) to CO. However, the concomitant hydrogen evolution reaction (HER) remains unavoidable under aqueous conditions. In this work, a rotating ring disk electrode (RRDE) setup has been developed to study quantitatively the role of mass transport in the competition between these two reactions on the Au surface in 0.1 M bicarbonate electrolyte. Interestingly, while the faradaic selectivity for CO formation was found to increase with enhanced mass transport (from 67% to 83%), this effect is not due to an enhancement of the CO2RR rate. Remarkably, the inhibition of the competing HER from water reduction with increasing disk rotation rate is responsible for the enhanced CO2RR selectivity. This can be explained by the observation that, on the Au electrode, water reduction improves with more alkaline pH. As a result, the decrease in the local alkalinity near the electrode surface with enhanced mass transport suppresses HER due to the water reduction. Our study shows that controlling the local pH by mass transport conditions can tune the HER rate, in turn regulating the CO2RR and HER competition in the general operating potential window for CO2RR (-0.4 to -1 V vs RHE).

Structural, Thermodiffusive and Thermoelectric Properties of Maghemite Nanoparticles Dispersed in Ethylammonium Nitrate

Ethylammonium nitrate (ionic liquid) based ferrofluids with citrate-coated nanoparticles and Na + counterions were synthesized for a wide range of nanoparticle (NP) volume fractions ( Φ ) of up to 16%. Detailed structural analyses on these fluids were performed using magneto-optical birefringence and small angle X-ray scattering (SAXS) methods. Furthermore, the thermophoretic and thermodiffusive properties (Soret coefficient S T and diffusion coefficient D m ) were explored by forced Rayleigh scattering experiments as a function of T and Φ . They were compared to the thermoelectric potential (Seebeck coefficient, Se) properties induced in these fluids. The results were analyzed using a modified theoretical model on S T and Se adapted from an existing model developed for dispersions in more standard polar media which allows the determination of the Eastman entropy of transfer ( S ^ NP ) and the effective charge ( Z 0 e f f ) of the nanoparticles.

Gold nanoparticles immobilised in a superabsorbent hydrogel matrix: facile synthesis and application for the catalytic reduction of toxic compounds

AuNP easily synthesised inside bulk hydrogel spheres; stable and catalytically active, even in high ionic strength environments.