Chemical programming for micro- and nanoarchitectonics of 3D/4D-printed thermoelectric materials

The adoption of renewable energy generation devices has become widespread as a “green” and sustainable approach to overcome the adverse effects of fossil fuels on the environment. With this increased uptake in mobile gadgets and integrated wearable electronics, portable power generators or self-powered internet-of-thing units become necessary. Hence, TE devices that convert thermal energy into electricity are emerging as a decarbonizing and environmentally viable heat/cold harvesting technology [1], [2], [3]. TE devices hold great promise pertaining to a broad range of applications in different device arrangements such as thermoelectric generators (TEGs) [4], thermoelectric coolers (TECs) [5], [6], [7], thermoelectric storage [8], [9], thermal sensing devices [10], [11], and human–machine interface platforms [12]. TE technology is fundamentally mature due to a well-established understanding of TE device working mechanisms and materials properties. To understand better TE materials properties and TE devices, simplified schematics (i–iii) are presented in Fig. 1(a) that summarizes the various TE effect phenomena responsible for TE power generation. The core concept of thermoelectricity is the “thermoelectric effect,” a broad term that includes phenomena where temperature differences within a material or between materials junctions can create an electric voltage or, alternatively, an applied voltage that can cause a temperature difference.

Based on different phenomena, the TE effect is further classified in more specific terms. Fig. 1a–i describe the Seebeck effect [13]. This effect occurs when two different conductors or semiconductors are joined together to form a system with different temperatures applied across a junction. This causes a voltage, known as the electromotive force (EMF), which moves the electrons in the circuit. The Seebeck coefficient (S) is defined as the ratio of the voltage developed to the temperature difference across a junction. This coefficient is commonly used to represent the TE performance of materials and devices. Conversely to the Seebeck effect is the Peltier effect [14], where an EMF is applied across the junction of two different materials, the heat is either absorbed (heating) or released (cooling) at the junction (Fig. 1a(ii)). Unlike the first two effects, where holes/electrons flow through a temperature gradient, in the Soret effect, the diffusion of positive/negative ions happens in a temperature gradient within the material or composite [15]. This can lead to a concentration gradient and generate an EMF (Fig. 1a(iii)).

The TE performance of materials with any of the above effects is measured by ZT, called the “Figure-of-merit” [16] a dimensionless entity described in Eq. (1):where S represents the Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, T is absolute temperature, κ_e is electronic thermal conductivity, and κ_l is lattice thermal conductivity. The power factor (S²σ) evaluates thermopower. A high S²σ and low κ_l are necessary for achieving a high ZT. The key to success in TE materials and device design is to maximize ZT performance. The detailed evolution of these TE effect derivations and evolution is well summarized and documented by Shi et al. [17]. Accordingly, various breakthroughs in TE material discovery and device design have been achieved to enhance ZT values.

So far, TE materials research has evolved widely in different generations of inorganic [18], [19], organic [20], [21], and ionic materials [22], [23] from molecular to structural levels. TE materials are also evaluated to understand fundamentals, synthesis, structural engineering, and material design perspectives [17]. However, TE device design plays

a significant role in successive electron/hole transport, leading to high efficiency and controlled thermal transport across the device to minimize thermal energy loss. Device design has advanced as a powerful tool to bring TE devices into practical use, whether for portable applications or industrial purposes. However, despite significant breakthroughs in high-performance TE materials, challenges persist in device design and the development of suitable fabrication methods. The key challenges in device fabrication are materials processing with highly accurate module design, precise positive/negative legs arrangements, ease of procedures, and cost-effectiveness. There are a variety of materials processing methods are utilized in search of effective TE output, including mechanochemical synthesis [24], melt-solidification as with arc melting [25] or zone melting [26], powder processing as with hot pressing [27] or spark plasma sintering [28], thin film deposition by physical [29] or chemical vapor deposition [30], sol-gel chemistry [31], electrochemical deposition [32], and additive manufacturing (AM) [33].

Among these methods, AM (also known as 3D printing) is the latest generation and holds great interest due to the non-subtractive operations that help to restrict valuable materials loss [34], [35]. Here, computer-aided object designs can be printed layer-by-layer additively on the substrate as described in Fig. 1b. This method allows miniature and precise prototyping, at low cost, effortless operation, and freedom of design [36]. Further, 4D printing is an emerging technique that offers smart functions for devices and a futuristic way to develop TE devices [37], [38], [39], [40]. Fig. 1c explains the 4D printing methodology, where 3D-printed TE materials turn into a dynamic fourth-dimension of time under the influence of stimuli like light, heat, humidity, pressure, electric/magnetic field, and pH change in the material. By these stimuli changes, the printed object can change shape or ordering to make it smart-functioned. The most important aspect of 3D/4D printing of TE materials is developing the desired printable feeds/ink and overcoming the interlayer energy loss from the printed TE devices. Hence, the “chemical programming” of TE materials is necessary. Fig. 1d summarizes the most popular strategies used to implement chemical programming, including sol-gel chemistry by integrating self-assembly of polymer hosts, suitable composite formation of TE material with polymer or metal host, doping, ionic decoration around the TE materials, alloying of metals, and multi-material printing. To succeed in the 3D/4D printing of TE materials, it is necessary to look at the progress made with various approaches in recent years.

In this review, we describe the revolution of 3D-printed materials, and the 3D printing methods developed for TE materials printing. All 3D/4D printing methods are correlated based on chemical programming approaches. We also elaborate on the 3D-printing of materials according to the different generations, for example, bulk inorganic materials [41], carbon-based materials [42], and organic–ionic materials [43]. Further, the rise of 4D printing in TEGs has been summarized and discussed. The future perspectives are also correlated to the knowledge gap with available TE materials. The key focus on progress spans the last three years (2022–2024). Several notable review articles published in the past five years provide detailed overviews of the properties and mechanistic insights into 3D printing strategies for TE materials [33], [34], [44], [45]. In the present case, the focus is specifically on the 3D/4D fabrication perspective of TE materials rather than their TE properties. The discussion theme targets how chemical programming is important and its effect on the performance of 3D-printed TE materials. Finally, the current challenges in chemical programming are discussed and possible solutions are proposed for the future development of 3D/4D printing in TE devices.