This doctoral work develops strategies to recover low-grade waste heat (<100°C) and convert it into useful work. Such heat is abundant and widely available, making it an attractive target for energy harvesting. Thermoelectrochemical cells (TECs) exploit temperature gradients across electrolytes to drive redox reactions, generating thermopower that surpasses conventional solid-state thermoelectrics by orders of magnitude. This dissertation investigates TECs in a rarely studied passive cooling configuration, where only the hot side is fixed. Unlike active cooling, this setup reveals strong dependencies on electrode spacing, heat transport, environmental conditions, and fluid dynamics. Understanding these interactions yields design rules to optimize TEC performance. The study further integrates photothermal electrode materials and hydrogels to create scalable, flexible, and low-cost devices for harvesting both heat and light. Oil-based plasmonic nanofluids were explored to localize sunlight and generate steep thermal gradients, but their incompatibility with standard redox couples limited applicability. In contrast, poly(vinyl alcohol) hydrogels effectively suppressed heat transport, overcame solubility limits, and sustained large passive temperature differences while minimizing leakage. Performance was enhanced by incorporating chaotropic salts such as guanidine chloride, achieving thermopower values up to 4.5mVK^{-1} and normalized power densities of 5mWm^{-2}K^{-2}. Multiphysics modeling and experiments showed that low-conductivity hydrogels stabilize convective ion transport, maximizing power without active cooling. Modular device prototypes further demonstrated the potential of TEC harvesters for real-world applications. Finally, the thesis addresses anomalously high evaporation rates recently reported in PVA hydrogels. Through gravimetric analysis, spectroscopy, thermal imaging, and pore characterization, we identify evaporation enhancement as strongly dependent on PEDOT:PSS loading, peaking at 5%vol. Results indicate that apparent optical absorption is largely due to scattering, contrarily with what generally reported in literature. Moreover, structural changes, such as pore shrinkage and reduced conductivity, govern charge and mass transport. Dynamic illumination experiments show minute long dynamics suggesting that evaporation is driven by thermal and osmotic effects rather than direct photonic influence. These findings highlight the coupled roles of photothermal activity, hydrogel microstructure, and interfacial charge dynamics, advancing understanding of hydrogel-assisted solar evaporation.