Hydrovoltaic cells. Part II: Thermogalvanic cells and numerical simulations of thermal diffusion potentials

Efficient Photo-Thermo-Electric Conversion Using Polyoxovanadate in Ionic Liquid for Low-Grade Heat Utilization

A large fraction of energy, including solar energy, is dissipated into ambient atmosphere as low-grade waste heat. Efficient utilization of such energy is critical to address the current energy crisis and global warming issue. Herein, the efficient near-IR (NIR)-photothermal, thermoelectric, and thus photo-thermo-electric conversion of polyoxovanadate compound {[Ni(1,10-phenanthroline)₃ ][V₁₄ O₃₄ Cl]Cl, NiV₁₄ } in ionic liquid was achieved. The solution displayed a NIR-photothermal efficiency of 16.04 and 23.43 % at 808 and 1064 nm, respectively. Taking advantage of the synergetic thermodiffusive and thermogalvanic effects of various ion species in NiV₁₄ solution, an open circuit voltage of approximately 0.45 V was obtained at ΔT=70 K generated by physical heating or NIR irradiation, indicating a large Seebeck coefficient of 6.38 mV K^-1 and an optimized thermal power at 1.2 W m^-2 . The polyoxovanadate-ionic liquid system offers a new platform for efficiently utilizing not only low-grade thermal energy but also solar energy for electricity generation.

Redox Targeting-Based Thermally Regenerative Electrochemical Cycle Flow Cell for Enhanced Low-Grade Heat Harnessing

A large amount of low-grade heat (<100 °C) is produced in electrical devices and mostly wasted. This type of heat without effective dissipation also causes compromised device performance, reliability, and lifespan. To tackle these issues, a redox targeting (RT)-based flow cell with judiciously designed thermoelectrically active redox materials is demonstrated for the first time for efficient heat-to-electricity conversion through a thermally regenerative electrochemical cycle (TREC). Compared with the conventional TREC systems, the RT-based flow cell not only reveals considerably enhanced thermoelectric efficiency, but the flow of redox fluids also provides a cooling function to the system. In this work, solid material Ni_0.2 Co_0.8 (OH)₂ and redox mediator [Fe(CN)₆ ]^4-/3- , both of which have negative temperature coefficient and share identical redox potential, are paired via RT-reactions to boost the capacity and meanwhile thermoelectric efficiency of a [Fe(CN)₆ ]^4-/3- /Zn^0/2+ -based flow cell. Upon operating over the TREC cycle, the RT-based flow cell converts heat to electricity at an unprecedented absolute thermoelectric efficiency of 3.61% in the temperature range of 25-55 °C.

Potentiometric investigation of protonation reactions at aqueous-aqueous boundaries within a dual-stream microfluidic structure

The laminar flow regime prevailing in pressure-driven flow through a Y-shaped microfluidic channel was utilized to create a stable boundary between two aqueous liquids. Transverse transport of ions between these two liquids gave rise to a diffusion potential, which was monitored by measurement of the open circuit potential. In this report, the influence on the cross-channel potential distribution of protonation reactions occurring in the boundary zone between the two co-flowing liquids is presented. The proton source was present in one of the co-flowing streams, and an uncharged proton acceptor was present in the other aqueous stream. The time-dependent transport equation for diffusion and migration was augmented by chemical reaction terms and was solved for all species present in both streams as a theoretical basis for the analysis. Within this model, the system was assumed to be homogeneous along the channel height, and effects of nonuniform velocity profiles were neglected. A reduction in potential by several millivolts was predicted for a protonation reaction occurring close to the boundary between the two aqueous streams, provided that the mobility of the protonated species was lower than the mobility of the co-cation in the background electrolyte (alkali metal cation in this case). The magnitude of the decrease in the potential was greater for protonated molecules with lower mobility or if the mobility of the background electrolyte cation was increased. Experimental results are presented for imidazole and D-histidine as proton acceptors present in 10 mM KCl, 10 mM NaCl, or 10 mM CsCl solution and co-flowing with a stream of 10 mM hydrochloric acid, which served as the proton source. Decreases in measured potential, in line with the predicted diminished potential, were obtained.

Harvesting waste thermal energy using a carbon-nanotube-based thermo-electrochemical cell

Low efficiencies and costly electrode materials have limited harvesting of thermal energy as electrical energy using thermo-electrochemical cells (or "thermocells"). We demonstrate thermocells, in practical configurations (from coin cells to cells that can be wrapped around exhaust pipes), that harvest low-grade thermal energy using relatively inexpensive carbon multiwalled nanotube (MWNT) electrodes. These electrodes provide high electrochemically accessible surface areas and fast redox-mediated electron transfer, which significantly enhances thermocell current generation capacity and overall efficiency. Thermocell efficiency is further improved by directly synthesizing MWNTs as vertical forests that reduce electrical and thermal resistance at electrode/substrate junctions. The efficiency of thermocells with MWNT electrodes is shown to be as high as 1.4% of Carnot efficiency, which is 3-fold higher than for previously demonstrated thermocells. With the cost of MWNTs decreasing, MWNT-based thermocells may become commercially viable for harvesting low-grade thermal energy.

Thermodiffusional transport of electrolytes in compact clays

The macroscopic Soret coefficient S(T) was measured for three porous media, namely mica, glass powder, and natural compact clay. At a mean temperature of T = 25 degrees C and with NaCl, S(T) for mica and glass powder was found to be equal to (3.1+/-0.7) x 10(-3) K(-1) and close to values for a free medium in agreement with theoretical predictions which are obtained under the assumption that the pressure gradient and the electric field are negligible on the pore scale. The main result is that for clay S(T) was found five times larger, presumably because of extra couplings with electrical phenomena. This latter measurement was confirmed by an independent technique based on the membrane potential.