**1. Introduction**

Noble metals have been widely used as electrocatalysts for energy conversion devices, including fuel cells, water electrolysis, and metal–air batteries [1]. Fuel cells supply energy in a similar way as batteries, although they do not require charging and operate as long as fuel is provided [2,3]. Fuel cells fed with formic acid supply electricity and heat, based on the electrochemical oxidation of fuels in the anode and the reduction of oxygen in the cathode, where H2 is formed [4,5]. Pd nanostructures have been used specially in direct formic acid fuel cells that are considered green energy sources for portable electronics and hybrid vehicles due to their high open-circuit voltage, safety and reliability, and low fuel crossover effect, including Pd nanoparticles supported on graphene [6], since nanostructures have simple morphology [7] until hybrid variation of Pd–Cu [8] and Pd– Co [9]. Although these supported materials have great performance towards formic acid oxidation (FAO), the carbonaceous support is still a problem when working with devices that work at high voltages.

Conventional formic acid fuel cells usually employ a physical barrier for the separation of electrodes, which presents many limitations such as membrane fouling and clogging [10]. Therefore, fuel cells working with microfluids would take advantage of laminar flow as a fluid separator and avoid the use of membranes and their drawbacks [11]. Microfluidic fuel cells have the advantage of being portable and of carrying out small-scale processes

**Citation:** Martínez-Lázaro, A.; Ramírez-Montoya, L.A.; Ledesma-García, J.; Montes-Morán, M.A.; Gurrola, M.P.; Menéndez, J.A.; Arenillas, A.; Arriaga, L.G. Facile Synthesis of Unsupported Pd Aerogel for High Performance Formic Acid Microfluidic Fuel Cell. *Materials* **2022**, *15*, 1422. https:// doi.org/10.3390/ma15041422

Academic Editor: Alessandro Dell'Era

Received: 6 January 2022 Accepted: 10 February 2022 Published: 15 February 2022

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offering high efficiency of energy conversion. The development of this type of device allows to incorporate the electro-oxidation of formic acid for diverse technological applications on a small scale [12]. On the other hand, miniaturizing the cell may reduce fabrication costs. A study has recently presented a novel microfluidic fuel cell (MFC) that incorporates the innovation of using a laminar flow instead of the conventional solid membrane to separate the fuel and oxidant [13]. Hence, the membrane-related issues are eliminated in this new MFC, which also offers savings in the manufacturing costs. However, it is still necessary to develop effective materials for these types of devices.

The use of catalysts for generating hydrogen from formic acid could minimize the dependence on lithium batteries in a large number of mobile devices [7]. Formic acid can be decomposed catalytically according to the following reactions:

$$\text{HCOOH}\_{\text{(l)}} \rightarrow \text{H}\_{2}\text{(g)} + \text{CO}\_{2}\text{(g)}\tag{1}$$

$$\text{HCOOH}\_{\text{(l)}} \rightarrow \text{H}\_2\text{O}\_{\text{(l)}} + \text{CO}\_{\text{(g)}} \tag{2}$$

Noble metals have been extensively studied as catalysts due to their high efficiency, non-toxicity, and stability. Particularly, Pd is widely used in anodes for FAO [9,10]. The synergistic effect of the metallic phase and oxyphilic properties of the Pd surface provides active sites for adsorption and dissociation of formic acid besides providing promoters of oxygen-containing species at low potentials [14,15]. There are a great number of Pd-based catalysts in the bibliography for acid formic oxidation, however, improving their activity is still a requirement in order to be implemented in fuel cells.

Mesoporous materials with low density and a greater number of active sites such as aerogels would allow the use of less mass of the catalyst and, at the same time, to provide a high catalytic activity [12–14]. Noble metal aerogels have approximately 90% air and very low contents of the active metal which reduces the cost of the catalysts [15,16]. These materials are commonly obtained by a sol-gel process and supercritical drying [16,17], however, other techniques such as lyophilization allow promising aerogel qualities [18]. Lyophilization, like supercritical drying, shows high efficiency in the formation of metallic aerogels [19–21]; this has been demonstrated in works such as Cu(II) cryogels [22] and Pd/CeO2-ZrO2 alloy aerogels that have been used in the reduction of CO [23] poly(3 sulfopropylmethacrylate) (p(SPM)) cold gels for H2 production and mostly organic aerogels as supports for other catalytic materials [24–26].

The aerogel synthesis in this work was implemented under microwave radiation. This heating technology allowed, not only the saving of processing time that is usually associated with this type of heating but also to obtain materials with homogeneous and controlled low particles. This fact is quite advantageous, as the particles are usually obtained with more heterogeneous and big particle sizes when the materials are prepared under long conventional heating. The microwave heating allows heating of the bulk precursor solution without gradients, which favors a very good dispersion of nucleation points for the reaction occurring and therefore a better control of the final particle size. Furthermore, the use of microwaves may also influence some chemical reactions and the final products may present some chemical differences in comparison with the ones obtained by conventional heating. In this work, the use of microwave heating provides many benefits such as facile synthesis of homogeneous and low particle size synthesis of Pd aerogels, with high activity towards the oxidation of formic acid in an MFC due to their unique physicochemical characteristics.
