SnO2 Catalyzed Simultaneous Reinforcement of Carbon Dioxide Adsorption and Activation Towards Electrochemical Conversion of CO2 to HCOOH
Authors: Chen Zhou , Fan Tingting , Yi Xiaodong 
Published: May 2020

The increasing CO2 content in the earth's atmosphere has provoked the concerns about global warming. Electrochemical CO2 reduction reaction (CO2RR) has been identified as an attractive route meeting both economic and environmental requirements. HCOOH with advantages of easy storage and transportation is an important chemical intermediate and renewable energy carrier. Therefore, transferring CO2 into HCOOH is a promising and desirable option to reduce the CO2 emissions and generate renewable energy simultaneously. Tin based materials are promising candidates for CO2RR to HCOOH. The unsaturated O=O bonds in metallic oxide can store CO2 by bonding the unshared pair of electrons of CO2. Simultaneously, the activation energy barrier of CO2RR is lower because those electrons can transfer back from defects to the adsorbates and alter the d-band center of catalysts interacting with the adsorbed CO2. Herein, a highly efficient wavy SnO2 (Denoted as NW-SnO2) electrocatalyst having abundant low coordinated sites, oxygen vacancies and gain boundaries was synthesized by a facile and cost-effective method, which is advantageous in terms of its large scale practical applications.

Results

Firstly, we highlight the synthesis of the wavy SnO2 with abundant oxygen vacancies, low-coordinated active sites and gain boundaries via one pot urea assisted hydrothermal method, as shown in Fig. 1a. In a typical synthesis of NW-SnO2, 0.08 g SnCl4·5H2O and 0.8 g urea were firstly dissolved in 32 mL deionized water. Subsequently, 1.6 mL of hydrochloric acid was added under ultrasonic treatment. The whole solution was then transferred into a 50 mL Teflon-lined stainless-steel autoclave to react at 90 °C for 15 h.
The composition and structure crystallinity of the as-prepared wavy SnO2 and commercial nanoparticle SnO2 were initially characterized by performing X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). As shown in Fig. 1b, those marked diffraction peaks can be indexed to the rutile tetragonal phase of SnO2 (JCPDS card No. 41-1445). Low-resolution TEM image (Fig. 1c) reveals that the NW-SnO2 mainly possesses a well-dispersed worm-like wavy morphology. From the HRTEM image shown in Fig. 1e and 1f, we can clearly see the wavy network of NW-SnO2. It should be noted that each nanowire is composed of a number of interconnected crystal building blocks, as shown in the model. The lattice spacing marked in Fig. 1g is 0.34 nm and 0.23 nm, corresponding to the (110) and (101) planes of SnO2, respectively, which indicates the crystal purity of NW-SnO2 and is highly consistent with the results of XRD.

Electrocatalytic performance

To evaluate the electrocatalytic CO2RR performance of the NW-SnO2 and NP-SnO2, the linear sweep voltammetry (LSV) polarization curves of the NW-SnO2 and NP-SnO2 electrocatalysts in CO2 saturated 0.5 M KHCO3 solution were obtained (Fig. 2a). Fig. 2b shows potential-dependent FE of all products from CO2RR recorded by NP-SnO2 and NW-SnO2 electrocatalyst, respectively. Specifically, the formation of HCOOH starts at -0.4 vs VRHE (Fig. 2b), and the FE of HCOOH is up to maximum (87.4%) at -1.0 vs VRHE. In contrast, the maximum selectivity of 61.7% at -1.1 vs VRHE for HCOOH on commercial NP-SnO2 is much lower than NW-SnO2. Notably, the onset potential of HCOOH on NW-SnO2 is only around 190 mV, 0.2 V lower than that of NP-SnO2. The maximum energy efficiency (EE) is 57.5% and 38.0% for NP-SnO2 and NW-SnO2, respectively (Fig. 2e). At -1.0 vs VRHE, the NW-SnO2 reaches the maximum partial current density of HCOOH (22 mA/cm2), which is 2.3 times higher than the value of the NP-SnO2 (9.7 mA/cm2). More importantly, the 18 hours long-term stability of NW-SnO2 was also measured using chronoamperometry at -0.8 vs VRHE. Therefore, our NW-SnO2 electrocatalyst not only shows good selectivity and lower onset potential under lower loading (0.576 mg/cm2), but also enjoys the advantage of its easier synthetic method, which is an important factor for commercial applications.

Conclusion

To summarize, we synthesized wavy SnO2 with higher ratio of low-coordinated active sites and abundant gain boundaries and oxygen vacancies by an economical and simple method. The NW-SnO2 shows an onset potential of 190 mV, 0.2 V lower than that of the commercial NP-SnO2. In addition, NW-SnO2 electrocatalyst could electroreduce CO2 into HCOOH with a partial current density up to 22 mA/cm2 and a maximum FEHCOOH of 87.4% at -1.0 vs VRHE. The abundant low-coordinated sites, gain boundaries and oxygen vacancies all play an essential role for the increased production of HCOOH and the suppressed yield of H2. Importantly, this study paves a new path to design other metal oxide based electrocatalysts for CO2RR via tuning their surface structures.