An Unprecedented CeO2/C Non-Noble Metal Electrocatalyst for Direct Ascorbic Acid Fuel Cells

Direct ascorbic acid fuel cells (DAAFCs) employ biocompatible ascorbic acid (AA) as fuel, allowing convenient storage, transportation, and fueling as well as avoiding fuel crossover. The AA oxidation reaction (AAOR) largely governs the performance of DAAFCs. However, AAOR electrocatalysts currently have low activity, and state-of-the-art ones are limited to carbon black. Herein, we report the synthesis of an unprecedented AAOR electrocatalyst comprising 3.9 ± 1.1 nm CeO2 nanoparticles evenly distributed on carbon black simply by the wet chemical precipitation of Ce(OH)3 and a subsequent heat treatment. The resultant CeO2/C shows a remarkable AAOR activity with a peak current density of 13.1 mA cm−2, which is 1.7 times of that of carbon black (7.67 mA cm−2). According to X-ray photoelectron spectroscopy (XPS), the surface Ce3+ of CeO2 appears to contribute to the AAOR activity. Furthermore, our density functional theory (DFT) calculation reveals that that the proton of the hydroxyl group of AA can easily migrate to the bridging O sites of CeO2, resulting in a faster AAOR with respect to the pristine carbon, -COOH, and -C=O sites of carbon. After an i-t test, CeO2/C loses 17.8% of its initial current density, which is much superior to that of carbon black. CeO2 can capture the electrons generated by the AAOR to protect the -COOH and -C=O sites from being reduced. Finally, DAAFCs fabricated with CeO2/C exhibit a remarkable power density of 41.3 mW cm−2, which is the highest among proton-exchange-membrane-based DAAFCs in the literature.


Introduction
Ascorbic acid (AA, vitamin C) can be either directly extracted from green plants or biochemically synthesized [1,2]. Relative to hydrogen and alcohol fuel, AA is much safer for use without fuel crossover and can be easily transported, stored, and refueled [3]. As an advantageous supplementation to hydrogen fuel cells, low-temperature direct AA fuel cells (DAAFCs) have many potential applications in portable chargers, unmanned aerial vehicles, cochlear implants, and pacemakers [4][5][6][7][8]. Since Fujiwara et al. first studied lowtemperature DAAFCs [9], many have contributed to this emerging field [10]. However, the development of DAAFCs has been seriously jeopardized by the slow kinetics of the AA oxidation reaction (AAOR) at the anode side. Additionally, proton-exchange-membranebased DAAFCs only have a record power density of 31 mW cm −2 [11]. It is crucial to investigate advanced AAOR electrocatalysts to improve the single's cell performance.
In recent years, several AAOR electrocatalysts have been investigated, including noble metals [12,13], TiO 2 [14], polymers [2,15,16], porphyrins [17,18], and high-surface-area carbon black [11,19,20]. Our group previously reported that carbon black (BP2000) is the most effective AAOR electrocatalyst [11]. However, state-of-the-art AAOR electrocatalysts remain as carbon black and deep un-derstanding on the nature of AAOR active sites are still missing. Inspired by the fact that CeO 2 can electrochemically oxidize methanol [21][22][23], MEAs with carbon black as the anodic electrocatalyst were also fabricated under the same conditions. Fuel cell tests were carried out on an in-house fuel cell system. Via a peristaltic pump, 0.5 M AA aq. was continuously delivered to the anode side at a flow rate of 15 mL min −1 , while humidified oxygen (100% RH) was supplied to the cathode side (400 mL min −1 ). The temperature of liquid fuel, O 2 , and single cell hardware was maintained at 80 • C. Polarization plots were collected using an electronic load (PLZ1004WH, Kikusui, Yokohama, Japan).

DFT Calculation
A pristine 4 × 4 graphene (GRA) layer model was functionalized by hydroxyl, carboxyl, or carbonyl group as well as by the introduction of a Stone-Wales defect. Models of cerium oxide cluster on the GRA were also built.
All models were fully optimized with the B3LYP hybrid functional using Gaussian 09 program [28]. The cerium atoms (Ce) in these models were described by large-core quasirelativistic pseudopotentials (ECP47MWB for tetravalent Ce and ECP48MWB for trivalent Ce) together with the corresponding basis sets for the valence shells [29,30]. For C, H, and O in the models, the 6-31G(d,p) basis set{#4} was employed [31]. All calculations were conducted in aqueous phase with the solvation effect of water considered through the solvation model based on density (SMD) [32]. The influence of dispersion was considered by Grimme's DFT-D3 correction in all calculations [33].
Vibrational frequency analysis was carried out to confirm all of the stationary points are minima and to abstract thermal corrections. Gibbs free energies were evaluated at 298.15 K and 1 atm.
The adsorption free energies of AA molecule on GRA (∆E ads ) were calculated as follows: where G GRA+AA , G GRA , and G AA are the total energies of the GRA with the adsorbate adsorbed, intact GRA, and intact AA molecule, respectively. The deformation energy of the AA molecule (E def,AA ) is defined as follows: where E dis,AA and E eq,AA are energies of AA molecule with distorted geometry in adsorbed models and in its intact form, respectively.

Results and Discussion
This section is divided into subsections. It provides a concise and precise description of the experimental results and their interpretation, as well as the experimental conclusions that can be drawn.

Synthesis and Structural Characterizations of CeO 2 /C
In a typical synthesis, carbon black was first mixed with a Ce(NO 3 ) 3 aqueous solution ( Figure 1a). Next, KOH aq. was added in to have Ce 3+ precipitate out on carbon as Ce(OH) 3 . Lastly, the sample was heat-treated at 250 • C in the air for 1 h, during which the Ce(OH) 3 was oxidized to form CeO 2 on carbon.
Transmission electron microscope (TEM) images reveal that the CeO 2 nanoparticles have an average size of 3.9 ± 1.1 nm on carbon without any noticeable agglomerations (Figure 1b). High-resolution TEM (HRTEM) images clearly show that the CeO 2 nanoparticles are crystalline with a typical lattice spacing of 0.31 nm (Figure 1c-e), which corresponds to the (111) crystal plane of fluorite CeO 2 [34]. The elemental mapping shows that sparse Ce is uniformly dispersed on carbon in the whole selected area (Figure 1f-i). According to the TGA and ICP results ( Figure S1 and Table S1), the loading of CeO 2 on carbon is 2.15 wt%. The X-ray diffraction pattern (XRD) shows that CeO 2 /C possesses characteristic diffraction peaks of a fluorite structure ( Figure S2), consistent with the HRTEM images  Figure 1d,e). According to the Raman spectra, the loading of CeO 2 did not change the ratio of the D peak to G peak (I D /I G ) of the carbon support ( Figure S3), in good agreement with the low-CeO 2 loading. Similarly, the N 2 adsorption/desorption measurements in Figure S4 show that the surface area of 2 wt% CeO 2 /C is similar to that of carbon. This implies that the low loading of CeO 2 barely affects the specific surface area of carbon. Transmission electron microscope (TEM) images reveal that the CeO2 nanoparticles have an average size of 3.9 ± 1.1 nm on carbon without any noticeable agglomerations (Figure 1b). High-resolution TEM (HRTEM) images clearly show that the CeO2 nanoparticles are crystalline with a typical lattice spacing of 0.31 nm (Figure 1c-e), which corresponds to the (111) crystal plane of fluorite CeO2 [34]. The elemental mapping shows that sparse Ce is uniformly dispersed on carbon in the whole selected area (Figure 1f-i). According to the TGA and ICP results ( Figure S1 and Table S1), the loading of CeO2 on carbon is 2.15 wt%. The X-ray diffraction pattern (XRD) shows that CeO2/C possesses characteristic diffraction peaks of a fluorite structure ( Figure S2), consistent with the HRTEM images (Figure 1d,e). According to the Raman spectra, the loading of CeO2 did not change the ratio of the D peak to G peak (ID/IG) of the carbon support ( Figure S3), in good agreement with the low-CeO2 loading. Similarly, the N2 adsorption/desorption measurements in Figure S4 show that the surface area of 2 wt% CeO2/C is similar to that of carbon. This implies that the low loading of CeO2 barely affects the specific surface area of carbon.

Electrochemical Activity of CeO2/C
The cyclic voltammetry (CV) curve of 2 wt% CeO2/C exhibits an obvious anodic peak residing at around 0.54 V (vs. RHE) in 0.5 M H2SO4 + 1 mM AA aq., corresponding to the AAOR (Figure 2a). The AAOR peak current density of 2 wt% CeO2/C reaches 13.1 mA cm −2 , which is about 1.7 times of that of carbon black (7.67 mA cm −2 , Table S2), which is the previously identified best AAOR electrocatalyst [11,19,20]. Moreover, 2 wt% CeO2/C has a lower onset potential of 0.441 V (vs. RHE) compared with that of carbon black. fur-

Electrochemical Activity of CeO 2 /C
The cyclic voltammetry (CV) curve of 2 wt% CeO 2 /C exhibits an obvious anodic peak residing at around 0.54 V (vs. RHE) in 0.5 M H 2 SO 4 + 1 mM AA aq., corresponding to the AAOR (Figure 2a). The AAOR peak current density of 2 wt% CeO 2 /C reaches 13.1 mA cm −2 , which is about 1.7 times of that of carbon black (7.67 mA cm −2 , Table S2), which is the previously identified best AAOR electrocatalyst [11,19,20]. Moreover, 2 wt% CeO 2 /C has a lower onset potential of 0.441 V (vs. RHE) compared with that of carbon black. furthermore, we found that the loading of 100 µg cm −2 2 wt% CeO 2 /C on a rotating disk electrode (RDE) resulted in the best AAOR activity ( Figure 2b). It is possible that there is a sufficient amount of electrocatalyst and efficient mass transfer at a loading of 100 µg cm −2 . Higher or lower than this loading, either the amount of electrocatalyst or the mass transfer is not optimal. In addition, the loading of CeO 2 on carbon apparently influences the peak current density of the AAOR (Figures 2c and S5). It turns out that all of CeO 2 /C electrocatalysts are superior to carbon and the 2 wt% CeO 2 /C has the highest AAOR peak current density. Nyquist plots of the carbon black and CeO 2 /C with different CeO 2 loadings on carbon show that the 2 wt% CeO 2 /C exhibits the smallest diameter of the half arc, which suggests it has the smallest charge transfer resistance and the fastest reaction kinetics (Figure 2d). This partly explains why the 2 wt% CeO 2 /C has the highest AAOR activity. sufficient amount of electrocatalyst and efficient mass transfer at a loading of 100 µg cm −2 . Higher or lower than this loading, either the amount of electrocatalyst or the mass transfer is not optimal. In addition, the loading of CeO2 on carbon apparently influences the peak current density of the AAOR (Figures 2c and S5). It turns out that all of CeO2/C electrocatalysts are superior to carbon and the 2 wt% CeO2/C has the highest AAOR peak current density. Nyquist plots of the carbon black and CeO2/C with different CeO2 loadings on carbon show that the 2 wt% CeO2/C exhibits the smallest diameter of the half arc, which suggests it has the smallest charge transfer resistance and the fastest reaction kinetics (Figure 2d). This partly explains why the 2 wt% CeO2/C has the highest AAOR activity.

Nature of Active Sites and AAOR Mechanism
It is necessary to further explore the nature of active sites and the AAOR mechanism. Our X-ray photoelectron spectroscopy (XPS) results show that the percentage of surface Ce 3+ residing at about 886.2 and 904.8 eV relative to the total amount of Ce species gradually decreases from 34.8% to 20.1% in the series of CeO2/C samples ( Figure 3 and Table S3) [34][35][36][37][38]. Ce 3+ and oxygen vacancies co-exist, so Ce 3+ is usually used to describe oxygen vacancies [39]. As shown in Figures 2c and 3b, it seems that the total Ce 3+ surface content correlates well to the AAOR activity. Moreover, the 2 wt% CeO2/C has both the highest Ce 3+ surface content of 0.68 wt% and the highest AAOR activity. We propose that the high oxygen vacancy content would be conducive for this improvement in the AAOR activity.

Nature of Active Sites and AAOR Mechanism
It is necessary to further explore the nature of active sites and the AAOR mechanism. Our X-ray photoelectron spectroscopy (XPS) results show that the percentage of surface Ce 3+ residing at about 886.2 and 904.8 eV relative to the total amount of Ce species gradually decreases from 34.8% to 20.1% in the series of CeO 2 /C samples ( Figure 3 and Table S3) [34][35][36][37][38]. Ce 3+ and oxygen vacancies co-exist, so Ce 3+ is usually used to describe oxygen vacancies [39]. As shown in Figures 2c and 3b, it seems that the total Ce 3+ surface content correlates well to the AAOR activity. Moreover, the 2 wt% CeO 2 /C has both the highest Ce 3+ surface content of 0.68 wt% and the highest AAOR activity. We propose that the high oxygen vacancy content would be conducive for this improvement in the AAOR activity. A DFT calculation was conducted to gain more insight into the role of ceria. In the models (Figure 4a,b), a ceria cluster is anchored on the GRA via four O-bridged bonds. We have also investigated the binding of ceria on GRA via a non-bonded interaction and found it thermodynamically unstable. Hence, the model of the physical adsorption of the ceria on GRA was not considered for the AAOR. The adsorption free energy of AA was calculated to be −40.61 kcal mol −1 with the aliphatic hydroxyl group (Figure 4a) and −53.53 kcal mol −1 with the hydroxyl group on the five-membered ring connecting to the ceria, respectively (Figure 4b). This strongly suggests that the ceria can provide adsorption sites for AA and thus functions as an efficient AAOR electrocatalyst. Interestingly, when the hydroxyl group of the five-membered ring interacts with the ceria, the proton of the hydroxyl group can easily migrate to the bridging O sites of the ceria. This further implies that the presence of the ceria would accelerate the AAOR process and explains why the 2 wt% CeO2/C has an excellent AAOR activity. (b) AA adsorbed on ceria cluster/GRA with two hydroxyl groups of the five-membered ring as bridges. Oxygen, carbon, hydrogen, trivalent cerium, and tetravalent cerium atoms are in red, black, white, light yellow, and bright yellow, respectively. The distances between neighboring atoms are in angstrom.

Drability of CeO2/C and its Origins
For potential applications, the durability of the 2 wt% CeO2/C was also investigated by the chronoamperometric method at 0.5 V (vs. RHE) with carbon black used as a comparison. After 4 h of an i-t test, the current density of the 2 wt% CeO2/C degrades by 18.8% A DFT calculation was conducted to gain more insight into the role of ceria. In the models (Figure 4a,b), a ceria cluster is anchored on the GRA via four O-bridged bonds. We have also investigated the binding of ceria on GRA via a non-bonded interaction and found it thermodynamically unstable. Hence, the model of the physical adsorption of the ceria on GRA was not considered for the AAOR. The adsorption free energy of AA was calculated to be −40.61 kcal mol −1 with the aliphatic hydroxyl group (Figure 4a) and −53.53 kcal mol −1 with the hydroxyl group on the five-membered ring connecting to the ceria, respectively (Figure 4b). This strongly suggests that the ceria can provide adsorption sites for AA and thus functions as an efficient AAOR electrocatalyst. Interestingly, when the hydroxyl group of the five-membered ring interacts with the ceria, the proton of the hydroxyl group can easily migrate to the bridging O sites of the ceria. This further implies that the presence of the ceria would accelerate the AAOR process and explains why the 2 wt% CeO 2 /C has an excellent AAOR activity.  A DFT calculation was conducted to gain more insight into the role of ceria. In the models (Figure 4a,b), a ceria cluster is anchored on the GRA via four O-bridged bonds. We have also investigated the binding of ceria on GRA via a non-bonded interaction and found it thermodynamically unstable. Hence, the model of the physical adsorption of the ceria on GRA was not considered for the AAOR. The adsorption free energy of AA was calculated to be −40.61 kcal mol −1 with the aliphatic hydroxyl group (Figure 4a) and −53.53 kcal mol −1 with the hydroxyl group on the five-membered ring connecting to the ceria, respectively (Figure 4b). This strongly suggests that the ceria can provide adsorption sites for AA and thus functions as an efficient AAOR electrocatalyst. Interestingly, when the hydroxyl group of the five-membered ring interacts with the ceria, the proton of the hydroxyl group can easily migrate to the bridging O sites of the ceria. This further implies that the presence of the ceria would accelerate the AAOR process and explains why the 2 wt% CeO2/C has an excellent AAOR activity. (b) AA adsorbed on ceria cluster/GRA with two hydroxyl groups of the five-membered ring as bridges. Oxygen, carbon, hydrogen, trivalent cerium, and tetravalent cerium atoms are in red, black, white, light yellow, and bright yellow, respectively. The distances between neighboring atoms are in angstrom.

Drability of CeO2/C and its Origins
For potential applications, the durability of the 2 wt% CeO2/C was also investigated by the chronoamperometric method at 0.5 V (vs. RHE) with carbon black used as a comparison. After 4 h of an i-t test, the current density of the 2 wt% CeO2/C degrades by 18.8% (b) AA adsorbed on ceria cluster/GRA with two hydroxyl groups of the five-membered ring as bridges. Oxygen, carbon, hydrogen, trivalent cerium, and tetravalent cerium atoms are in red, black, white, light yellow, and bright yellow, respectively. The distances between neighboring atoms are in angstrom.

Durability of CeO 2 /C and Its Origins
For potential applications, the durability of the 2 wt% CeO 2 /C was also investigated by the chronoamperometric method at 0.5 V (vs. RHE) with carbon black used as a comparison. After 4 h of an i-t test, the current density of the 2 wt% CeO 2 /C degrades by 18.8% (Figure 5a) and is much superior to that of carbon black with a current density degradation of 33.1%. This clearly shows that the 2 wt% CeO 2 /C has an apparently improved durability compared to that of the carbon black. After the i-t test, the surfaceoxygen-containing groups of the 2 wt% CeO 2 /C decrease from 33.7% to 24.5%, while those of carbon decrease from 34.4% to 17.6%, as shown in Figure 5b-d and Tables S4 and S5 [11]. It seems that the AAOR activity is closely related to the carbon surface oxygen species. We further employed a DFT to study the nature of the active sites on carbon.
Nanomaterials 2023, 13, x FOR PEER REVIEW 7 of 12 ( Figure 5a) and is much superior to that of carbon black with a current density degradation of 33.1%. This clearly shows that the 2 wt% CeO2/C has an apparently improved durability compared to that of the carbon black. After the i-t test, the surface-oxygen-containing groups of the 2 wt% CeO2/C decrease from 33.7% to 24.5%, while those of carbon decrease from 34.4% to 17.6%, as shown in Figure 5b-d and Tables S4 and S5 [11]. It seems that the AAOR activity is closely related to the carbon surface oxygen species. We further employed a DFT to study the nature of the active sites on carbon.   (Table S6). In contrast, the adsorption at the -C=O and -COOH sites are much stronger with an adsorption energy of −4.48 kcal mol −1 and −5.27 kcal mol −1 , respectively. In addition, the hydrogen bond distance is 1.76 Å between the carboxyl H of GRA and the carbonyl O of the AA's five-membered ring, while the hydrogen bond distance is 2.82 Å between the hydroxyl H of GRA and the carbonyl O of the AA's five-membered ring. This is consistent with the corresponding adsorption energy. It is certain that the AA molecule has a higher propensity to be adsorbed and oxidized at the carboxyl site than at the hydroxyl site. Furthermore, the -C=O site and GRA have an AA adsorption energy similar to that of the -COOH site and would also function as AAOR active sites.  Figure 6 shows the AA adsorption geometries of GRA with -OH, -C=O, and -COOH. The adsorption of AA at the -OH site is the weakest with an adsorption energy of −1.77 kcal mol −1 (Table S6). In contrast, the adsorption at the -C=O and -COOH sites are much stronger with an adsorption energy of −4.48 kcal mol −1 and −5.27 kcal mol −1 , respectively. In addition, the hydrogen bond distance is 1.76 Å between the carboxyl H of GRA and the carbonyl O of the AA's five-membered ring, while the hydrogen bond distance is 2.82 Å between the hydroxyl H of GRA and the carbonyl O of the AA's five-membered ring. This is consistent with the corresponding adsorption energy. It is certain that the AA molecule has a higher propensity to be adsorbed and oxidized at the carboxyl site than at the hydroxyl site. Furthermore, the -C=O site and GRA have an AA adsorption energy similar to that of the -COOH site and would also function as AAOR active sites. Nanomaterials 2023, 13, x FOR PEER REVIEW 8 of 12 During the i-t test, the total content of the -C=O and -COOH of the carbon black decreases from 10.6% to 6.7%. The -COOH has been completely lost with a possible partial conversion to -C=O, which increases from 5.0% to 6.7% after the i-t test. It is more than likely that the -COOH of the carbon black has been gradually reduced by electrons from the AAOR, resulting in the decrease in active sites and AAOR activity. In contrast, the total content of the -C=O and -COOH of the 2 wt% CeO2/C slightly decreases from 13.1% to 10.0% (Figure 5d and Table S5), suggesting that the CeO2 may have protected the -C=O and -COOH by capturing electrons and thus reducing the loss of active sites. Moreover, our HRTEM images of the 2 wt% CeO2/C after the durability test ( Figure S6) show that the diameter of the CeO2 nanoparticles remains almost unchanged. It is clear that the CeO2 nanoparticles are able to survive the i-t test, and the activity loss mainly comes from the degradation of the carbon support.

DAAFC Performance
In order to verify the performance of the CeO2/C electrocatalyst, 2 wt% CeO2/C was fabricated into the anode of a 4 cm 2 DAAFC single cell with commercial Pt/C (60 wt%) as the cathode. The single cell peak power density reaches 41.3 mW cm −2 at 80 °C after temperature optimization (Figure 7a,b). Compared with the single cell fabricated with carbon black (33.8 mW cm −2 ), the power density increases by 22%. This proves that the 2 wt% CeO2/C has an excellent activity in single cells. To the best of our knowledge, the peak power density of the 2 wt% CeO2/C is the highest among documented DAAFCs using a proton exchange membrane (Figure 7c and Table S7) [2,12,[18][19][20][40][41][42]. The peak power density of our DAAFCs with the non-noble metal anodic electrocatalyst is close to that of direct methanol fuel cells typically with PtRu/C as the anodic electrocatalyst [43][44][45]. We suspect that the relatively sluggish AAOR cannot generate enough protons for the cathodic oxygen reduction reaction, thus jeopardizing their single cell performance. Consequently, when an optimal concentration of 0.2 M H2SO4 and 0.5 M AA aq. was fueled to the anode, the maximum power maximum power density reached 52 mW cm −2 ( Figure  7d), which is 26% higher than that of the anode fueled without H2SO4. During the i-t test, the total content of the -C=O and -COOH of the carbon black decreases from 10.6% to 6.7%. The -COOH has been completely lost with a possible partial conversion to -C=O, which increases from 5.0% to 6.7% after the i-t test. It is more than likely that the -COOH of the carbon black has been gradually reduced by electrons from the AAOR, resulting in the decrease in active sites and AAOR activity. In contrast, the total content of the -C=O and -COOH of the 2 wt% CeO 2 /C slightly decreases from 13.1% to 10.0% (Figure 5d and Table S5), suggesting that the CeO 2 may have protected the -C=O and -COOH by capturing electrons and thus reducing the loss of active sites. Moreover, our HRTEM images of the 2 wt% CeO 2 /C after the durability test ( Figure S6) show that the diameter of the CeO 2 nanoparticles remains almost unchanged. It is clear that the CeO 2 nanoparticles are able to survive the i-t test, and the activity loss mainly comes from the degradation of the carbon support.

DAAFC Performance
In order to verify the performance of the CeO 2 /C electrocatalyst, 2 wt% CeO 2 /C was fabricated into the anode of a 4 cm 2 DAAFC single cell with commercial Pt/C (60 wt%) as the cathode. The single cell peak power density reaches 41.3 mW cm −2 at 80 • C after temperature optimization (Figure 7a,b). Compared with the single cell fabricated with carbon black (33.8 mW cm −2 ), the power density increases by 22%. This proves that the 2 wt% CeO 2 /C has an excellent activity in single cells. To the best of our knowledge, the peak power density of the 2 wt% CeO 2 /C is the highest among documented DAAFCs using a proton exchange membrane (Figure 7c and Table S7) [2,12,[18][19][20][40][41][42]. The peak power density of our DAAFCs with the non-noble metal anodic electrocatalyst is close to that of direct methanol fuel cells typically with PtRu/C as the anodic electrocatalyst [43][44][45]. We suspect that the relatively sluggish AAOR cannot generate enough protons for the cathodic oxygen reduction reaction, thus jeopardizing their single cell performance. Consequently, when an optimal concentration of 0.2 M H 2 SO 4 and 0.5 M AA aq. was fueled to the anode, the maximum power maximum power density reached 52 mW cm −2 (Figure 7d), which is 26% higher than that of the anode fueled without H 2 SO 4 .

Conclusions
In summary, a series of CeO2/C AAOR electrocatalysts were designed and simply synthesized by the wet chemical precipitation of Ce(OH)3 and a subsequent heat treatment. Among all documented electrocatalysts, 2 wt% CeO2/C exhibits the highest activity with a peak current density of 13.1 mA cm −2 , which is 1.7 times of that of carbon black (7.67 mA cm −2 ). According to our DFT calculations and XPS experiments, its remarkable activity may stem from its strong AA adsorption on the surface of the ceria and the easy migration of the proton of the hydroxyl group of AA to the bridging O sites of the ceria. After 4 h of an i-t test, 2 wt% CeO2/C retains 81.2% of its initial current density, which is much better than carbon black (66.9%). CeO2/C merely loses 27.1% of the -COOH and -C=O sites of the carbon support after an i-t test. In contrast, carbon black loses 48.8% of its -COOH and -C=O sites. CeO2 may capture electrons to protect the -COOH and -C=O sites from being reduced, thus enhancing durability. Finally, DAAFCs with a 1 mg cm −2 2 wt% CeO2/C anode show a maximum power density of 41.3 mW cm −2 , which is 2.2 times that of the corresponding value in the literature. This study opens up opportunities for the design and synthesis of advanced non-noble metal electrocatalysts for AAORs and illuminates the nature of AAOR active sites for the first time.
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Experimental: Electrochemical Measurements and Materials Characterizations; Figure S1: TGA curves of carbon black and CeO2/C with different CeO2 loadings; Figure S2: XRD patterns of carbon black and CeO2/C with different CeO2 loadings; Figure S3: Raman spectra of carbon black and CeO2/C with different CeO2 loadings; Figure S4: (a) N2 adsorption-desorption isotherms and (b) pore-size distributions of carbon black and 2 wt% CeO2/C. Inset: enlarged poresize distributions; Figure S5: (a) CV curves of carbon black and CeO2/C with different CeO2 loadings

Conclusions
In summary, a series of CeO 2 /C AAOR electrocatalysts were designed and simply synthesized by the wet chemical precipitation of Ce(OH) 3 and a subsequent heat treatment. Among all documented electrocatalysts, 2 wt% CeO 2 /C exhibits the highest activity with a peak current density of 13.1 mA cm −2 , which is 1.7 times of that of carbon black (7.67 mA cm −2 ). According to our DFT calculations and XPS experiments, its remarkable activity may stem from its strong AA adsorption on the surface of the ceria and the easy migration of the proton of the hydroxyl group of AA to the bridging O sites of the ceria. After 4 h of an i-t test, 2 wt% CeO 2 /C retains 81.2% of its initial current density, which is much better than carbon black (66.9%). CeO 2 /C merely loses 27.1% of the -COOH and -C=O sites of the carbon support after an i-t test. In contrast, carbon black loses 48.8% of its -COOH and -C=O sites. CeO 2 may capture electrons to protect the -COOH and -C=O sites from being reduced, thus enhancing durability. Finally, DAAFCs with a 1 mg cm −2 2 wt% CeO 2 /C anode show a maximum power density of 41.3 mW cm −2 , which is 2.2 times that of the corresponding value in the literature. This study opens up opportunities for the design and synthesis of advanced non-noble metal electrocatalysts for AAORs and illuminates the nature of AAOR active sites for the first time.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano13192669/s1, Experimental: Electrochemical Measurements and Materials Characterizations; Figure S1: TGA curves of carbon black and CeO 2 /C with different CeO 2 loadings; Figure S2: XRD patterns of carbon black and CeO 2 /C with different CeO 2 loadings; Figure S3: Raman spectra of carbon black and CeO 2 /C with different CeO 2 loadings; Figure S4: (a) N 2 adsorption-desorption isotherms and (b) pore-size distributions of carbon black and 2 wt% CeO 2 /C.