To explore the potency of our projected assumption: human urine to carbon, we carbonized the dried yellowish deposit of the urine to 700 through 1100°C for 6 h in inert atmosphere to systematically analyze the effect of carbonization temperature on characteristics of resulting carbon (see Method Section). The detailed schematic depiction for the synthesis of the porous URC is shown in Figure 1. The water was fully removed from the collected urine samples by heating at 80°C for 48 h during the step-1 to obtain dried yellowish deposit from urine prior to the carbonization as shown in schematic depiction in the Figure 1. The obtained carbon powder is grayish black and from scanning electron microscopy (SEM) images, appears as a mixture of micro-size particles and nanofibrous structure as shown in Figure 2A–E. X-ray diffraction (XRD) measurements of these materials reveal rather complex signals of carbonaceous material and mixture of rock salts such as Sylvite and Halite, which are mineral forms of KCl and NaCl, respectively (Figure S1A in Supplementary Information (SI)). Interestingly, the XRD signal intensity of Sylvite gradually decreases compare to that of Halite as carbonization temperature increases and almost disappear after 1000°C. As the carbonization temperature increases beyond 900°C, the salts start to gasify by sublimation process and deposit as white powder on the inner wall of quartz tube inserted into the tube furnace, which is determined as mainly the evaporated Sylvite by XRD analysis. Since human urine contains as major constituents urea, chloride, sodium, potassium, creatinine and other inorganic and organic compounds and ions28, the urine is expected to result in a composite consisting of carbon and mixture of salts upon carbonization. To obtain pure carbon material, the mixture of rock salts (Sylvite and Halite) and other minor inorganic compounds present were removed by treatment with 0.1 M HCl solution. The rock salts as particles dispersed throughout the carbon composite unwittingly leave behind small pores, i.e. meso/macropores as well as micropores on and within the carbon structure. Thus the inherently present rock salts serve as natural porogen for the formation of beneficial pores in the carbon framework.

Figure 1 Schematic depiction of porous URC from human urine. Step-1: a dried yellowish brown deposit of urine obtained at 80°C for 48 h. Step-2: carbonization of the deposit under N 2 flow for 6 h. Step-3: etching and washing by 0.1 M HCl to remove the rock salts mixture that generates various pores within the carbon structure. Full size image

Figure 2 Effect of carbonization temperature on the formation of porous carbon from human urine. SEM images of mixture of carbon and rock salts obtained at URC-X-BW, where X is (A) 700, (B) 800, (C) 900, (D) 1000 and (E) 1100 before washing with diluted HCl. Whereas, (A′) SEM and (A″) TEM images of URC-700, (B′) SEM and (B″) TEM images of URC-800, (C′) SEM and (C″) TEM images of URC-900, (D′) SEM and (D″) TEM images of URC-1000 and (E′) SEM and (E″) TEM images of URC-1100 after HCl washing, demonstrating the porous morphology of carbon obtained after the HCl treatment. Full size image

To understand the mechanism of URC formation, it is worth pointing out that the composite is composed of a mixture of micro-sized particles and fibrous materials prior to HCl washing as seen in SEM images of Figure 2A–E. Upon increasing the carbonization temperature, the micro-sized particles break down into smaller structures and once-predominant fibers almost disappear (Figure 2D–E). The micro-sized particles mainly belong to carbon and the fibrous structure to inorganic salts. During carbonization at temperatures over 900°C, the majority of the volatile inorganic salts present in the dried urine starts to gasify by sublimation and results in a weight loss, which is consistent with decay of fibrous materials. Thus, the yield of resulting carbon varies with temperature of carbonization due to the change in the content of volatile salt materials. As an example, the weight for URC-1100-BW sample is 60% less compared with that of URC-700-BW before washing by HCl. This sublimation process is prominent and eventually the gasified salts cool down and are deposited on the inner wall of the tube furnace as white powder, which is found to be the mixture of rock salts by a range of analysis (Figure S2 in SI). The salts separated from carbon is an extra bonus of present work, which can be recovered from human waste and finds valuable applications such as de-icing salts. Thus, due to the presence of the salts in urine, numbers of pores are developed in the URC.

The SEM (Figure 2A′–E′) and TEM (Figure 2A″–E″) images of the URC clearly give an idea of the carbon having highly porous structure created by removal of salts. Consequently, we were able to obtain porous carbon material with higher surface area by carbonization and by just removing the inherent inorganic salts using dil. HCl. These results imply that the proposed synthesis route is very effective for developing pores and eventually increasing the surface area in URC materials without use of common troublesome pore-generating inorganic templates or post-activation33,48. In general, the urine deposit carbonized and separated from salts by acid treatment produces 300–400 mg of porous URC using 1 L of human urine in current simple template-free process; however, the final carbon yield is mainly dependent on the organic contents of the urine and carbonization temperature. Normally human adult excretes about 1 to 2 L of urine/day depending on health and age. Thus, from every healthy human's body, sterile liquid wastes can produce approximately 300–800 mg of URC per day. Moreover, urine can be freely and easily collected in large quantity form public toilet/bathrooms for the carbon synthesis and drying or evaporating larger volume of urine in an open air or sunlight involves comparatively much lower cost, but needs sufficient space.

The as-obtained salt-free carbon was further characterized by XRD, thermogravimetric analysis (TGA) and Raman measurement, which gives more insight on carbon formation using human urine waste (See Figure S1 to S4 in SI). The XRD patterns (Figure S1B in SI) of the carbon materials clearly revealed the changes after washing by 0.1 M HCl to remove the inorganic salts and other impurities. The XRD patterns of all the URC samples reveal the characteristic broad maxima peaks around 2θ = 25 (002) and 44° (100) of typical turbostratic carbon structure, while almost no characteristic signals for the salts or other impurities were observed. The (100) reflection corresponds to the honeycomb structure which is formed by sp2 hybridized carbons and tends to be more intense as carbonization temperature increases. On the other hand, the (002) reflections between 20 and 30°, corresponding to coherent and parallel stacking of graphene like-sheets, become broader, indicating the increase of amorphous nature in the URC samples as the temperature increases from 700 to 1100°C. The shifts towards the smaller angles for the (002) peak at higher temperature also suggest an increase in the interlayer spacing due to increased amorphous nature. This increase in the d-spacing is related to the doping of heteroatoms, which are located or incorporated into the graphite layers of the carbon materials. Moreover, the presence of heteroatoms, especially N and S within the carbon structure increases the defects and disorder49. These results are in well agreement with the Raman measurement shown in Figure S4 of SI.

Thermal stability of the carbon materials is also important in determining their end use for numerous applications. These URC materials show a good thermal stability as shown in Figure S3 in SI. Raman spectroscopy was used to investigate the structural changes induced by the presence of the heteroatoms in the URC material as shown in Figure S4 in SI. Raman spectrum of each of URC samples displays two broad bands at 1340 and 1575 cm−1, which are assigned to the D band and G band, respectively. The positions of these bands are similar to each other for all five URC materials, suggesting that the structures of the all carbon are similar, showing turbostratic feature as observed in XRD spectra of Figure S1B. Interestingly, however, the differences in the I D /I G ratio are observed and the increase in the intensity ratio suggests that the carbon structure becomes more disordered with increasing carbonization temperature.

Surface properties of the as-obtained carbon materials are very important for the electrochemical performance when used as an electrode material for ORR. Figure 3 reveals nitrogen isotherms and pore size distribution curves of the URC samples. Table 1 summarizes the total BET surface area, pore volume and pore diameter of the URC structures prepared at different temperatures. Porous structure can be generated by two main processes; first, the pores created by the evaporation of the salts present in the urine during carbonization process and secondly, the pores generated by dilute acid washing to remove the remaining salts particles from the carbon structures after the carbonization. N 2 isotherms for all the URC carbons exhibit more or less similar type I isotherm typical of microporous carbon materials. The presence of a H4 hysteresis loop is indicative of a solid containing both micropores and mesopores. URC-700 shows a steady adsorption curve as the pressure increases with limited hysteresis loop, suggesting the presence of parallel or slit-like pores. While, as the carbonization temperature increases, clear hysteresis loop profile along with increase in the adsorption curve at higher pressure was observed. This could be due to the changes in the pores types from parallel or slit-like to cage-like pores. Additionally, in general, the micropore volume decreases from URC-700 to URC-1100. BET surface area is 1080.8 m2/g for URC-700 and increases to 1436.8 m2/g for URC-800, but then decreases to 1064.9 and 811.4 m2/g for URC-900 and URC-1000, respectively. Further increase in the carbonization temperature beyond 1000°C decreases the surface area greatly probably as a large portion of the salts has already evaporated and this reduces the second step pore formation chance by acid washing. In particular, at 1000°C and higher, large portion of the inorganic salts present in initial dried urine deposit are evaporated and found on the inner wall out of heating zone of quartz tube due to the gasified sublimation (Figure S2B and D in SI), which is evidenced by greatly reduced weight of the carbon-salts composites and weak signal intensity of salt particles as shown in XRD results of URC-1000-BW and URC-1100-BW (Figure S1A in SI). The pore-size distribution calculated by density functional theory (DFT) method indicates that the carbonization temperature has some interesting effect on the average mesopore diameter of the URCs50. As the temperature increases from 700 to 1100°C, the mesopore diameter also increases from 2.1 nm (URC-700) to 3.2 nm (URC-1100) as presented in Table 1. Interestingly, URC-1000 and URC-1100 show similar mesopore diameter, which is found to be ca. 3.2 nm as shown in Figure 3B. This evidently supports that these mesopores are mainly created by the evaporation of salts during the carbonization at higher temperature rather than HCl washing. As shown in the inset of Figure 3B, it is clearly seen that URC materials contain number of micropores with diameters in the range of 0.4 to 1.6 nm formed mainly due to the removal of salts inherently present in urine by HCl washing. URC-700, -800 and -900 samples reveal high micropore volume percentage amounting to 82, 86 and 81% of respective total pore volume, whereas both URC-1000 and -1100 show decreased micropore volume percentage of 21% of respective total pore volume as shown in Table 1.

Table 1 Atomic composition obtained from XPS spectra and physical characteristics by nitrogen sorption data for various URCs Full size table

Figure 3 (A) Nitrogen adsorption–desorption isotherms and (B) the corresponding DFT pore-size distribution curves of the URC materials (Inset: magnified pore-size distribution). Full size image

X-ray photoelectron spectroscopy (XPS) survey scan (Figure 4A) shows the presence of C, O, N, P, Si and S in the URC. In particular, elements such as N, P, S, B and Si are widely believed to be responsible for the improvement in activity of carbon sample toward various electrochemical reactions15,16,17,18,19,20,24,49,51,52,53,54,55,56,57,58. Heteroatoms form covalent bond with the adjacent carbon in the carbon lattice and therefore their catalytic degradation is much less as compared to that of Pt-based catalysts, which are usually generated through their physical attachment over the carbon support. Among them, nitrogen is by far the most investigated heteroatom because as a neighbor of carbon with different physicochemical properties, it is fairly easy to have N-doped carbon. Electronegativity of the nitrogen (3.04) and carbon (2.55) in the carbon matrix can destroy the electro-neutrality of the adjacent carbon. Moreover, the C-C bond length of sp2 hybridized carbon changes with the introduction of N. Because of the changes in bond length and electronegativity of carbon framework due to the N doping, carbon surface becomes asymmetric in nature and gets more active towards ORR59. Furthermore, N atoms doped at various active sites in the carbon framework play an important role in ORR due to their free lone pair electron available for the interaction with oxygen.

Figure 4 (A) XPS survey scan of URC prepared at various carbonization temperature. Inset: magnified XPS survey scan for Si, P and S. and deconvoluted XPS spectra of (B) C 1s, (C) N 1s, (D) S 2p and (E) Si 2p for URC-1000. Full size image

The elemental compositions are monitored to evaluate the changes in the chemical composition as a function of carbonization temperature and summarized in Table 1. The total N-content in the carbon decreased from 9.8 to 2.0% with increase in carbonization temperature. However, no significant temperature dependence was found on S content with its scanty content of 1%. Furthermore, Si and P were also present in trace amounts (less than 1%). Figure 4B–E shows the deconvoluted C 1s, N 1s, S 2p and Si 2p photoelectron envelopes of URC-1000. C 1s spectrum indicates that the major fraction of carbon species is sp2 C at 284.6 eV, followed by sp3 (285.3 eV) hybridized C along with minor contribution (~286.5 eV) from different bonding configurations of carbon with oxygen or nitrogen (See Figure S5 of SI for details). Table S1 of SI summarizes the full width at half maximum (FWHM) values of sp2 and sp3 constituent peaks, which decrease by only 0.1 eV from URC-700 to URC-1100. This could be due to the effect of carbonization temperature and the presence of various heteroatoms in the URC. On the other hand, it is clearly seen that N 1s signal is split into three major peaks as pyridinic-N, pyrrolic-N and quaternary-N (Figure S6 and S7 of SI). The FWHM values decrease distinctly for the deconvoluted constituent peaks for N1-pyridinic, N2-pyrrolic and N3-quaternary configurations as shown in the Table S2 of SI. This may be attributed to the relatively higher nitrogen content (9.8%) for URC-700 compared to 2.0% in case of URC-1100. Sulfur is found to be present predominantly as two species such as aliphatic thiols or thioethers (~163.6 eV) and aromatic thiophenic sulfur (~164.6 eV). The Si 2p state of silicon is deconvoluted into two species such as Si-C-O (101.6 eV) and SiO 2 (102.8 eV). The phosphorous content for URC-1000 was detected negligible (0.2%, see Table 1) in the survey scan and thus excluded from the deconvolution of P 2p peak.

The cyclic voltammograms (CVs) at different carbon electrodes clearly show the oxygen reduction reaction (ORR) peaks near at −0.2 V vs Ag/AgCl in the O 2 -saturated 0.1 M KOH solution (Figure S8 in SI). The electrocatalytic activity and kinetic information of the various carbon materials for ORR were evaluated by rotating ring-disk electrode (RRDE) measurements and compared with commercial 20 wt% Pt/C in Figure 5. In Figure 5A, ring current decreases significantly from URC-700 to URC-1000, showing that the intermediate product of H 2 O 2 was greatly decreased. The peroxide yield is less than 15% for all five URC materials at all potentials as seen in Figure 5B. The onset potential also shifts towards the positive direction and the activity improves to greater extent from URC-700 to URC-1000 in alkaline solution as shown at the bottom of Figure 5A. The URC-1000 shows its onset potential (−0.03 V), almost identical to that of commercial 20 wt% Pt/C. The current densities are more or less −3.5 mA/cm2 (±0.01) for URC-800, -900 and -1000 electrodes, which are still less than −4.8 mA/cm2 for the 20 wt% Pt/C.

Figure 5 (A) Steady state ORR polarization plots of ring (top) and disk (bottom), (B) peroxide (HO 2 −) formation yield, (C) the number of electron exchanged during oxygen reduction at different potential for URC materials. The data of 20 wt% Pt/C (E-TEK) is also presented for comparison. (RDE/RRDE experiments were carried out at rotating speed 1600 rpm and 10 mV s−1 potential scan rate in O 2 -saturated 0.1 M KOH). Full size image

The n values for all five URC electrodes and Pt/C are shown in Figure 5C, which is about 3.7 ± 0.05 at different potentials for URC-800 to URC-1100. These results imply that oxygen reduction follows mainly an efficient four electron process similar to that of state of the art Pt/C electrode. The comparable catalytic activity of URC materials with commercial Pt/C has significantly important practical implications. Thus, the results confirm that the ORR activities of URC materials increased with the carbonization temperature from 700 to 1000°C. In particular, the URC-800, -900 and -1000 reveal excellent ORR performances with the best for URC-1000, whereas URC-700 and URC-1100 show comparatively poor ORR activity compared to the other three electrodes in accordance with CV results (Figure S8 of SI). This is attributed to insufficient carbonization and resulting high resistivity at low temperature of 700°C and the comparatively low surface area and weak heteroatom content for URC-1100.

One of the major challenges in fuel cell applications is durability of electrocatalysts in the electrode52. In order to determine the stability of the URC materials, ORR forward peak maximum currents were recorded for URC-1000 and commercial 20 wt% Pt/C catalysts during the repeated potential cycling up to 5000 (Figure S9A of SI). After 5000 cycles, about 35% fading in activity was observed for URC-1000, whereas the Pt/C catalyst lost activity by more than 72%, indicating better long term stability for URC-1000. Pt is the best single electrocatalyst for oxygen reduction in a fuel cell. However, in an alcohol fuel cell, alcohol as fuel added in anode side can crossover the membrane toward the cathode electrode, where the alcohol can be oxidized, leading to loss of considerable activity. Therefore, methanol tolerance for the cathode catalysts is highly required to ensure a good performance. The CV scans for the commercial Pt catalyst in an electrolyte containing O 2 -saturated methanol (3.0 M) illustrate that the cathodic peaks for oxygen reduction disappear due to the more prominent methanol oxidation (Figure S9B in SI). However, in the same O 2 -saturated methanol solution (Figure S9C in SI), the URC-1000 maintains strong ORR selectivity with no specific activity to methanol oxidation, indicating that the URC-1000 catalyst is completely insensitive toward methanol. Obviously, the methanol oxidation on Pt/C caused by crossover of a high-concentration methanol fuel becomes very pronounced, which severely deteriorates the cell performance. In this study, the excellent methanol tolerance of the URC-1000 plays a key role, which maintains selective reduction of oxygen. This can give rise to a better cell performance in DMFCs and will be in fact potentially suitable for application to the passive-type DMFCs, which are designed to directly use high-concentration methanol as fuel.

In the investigation of electrode materials of electrochemical cells, the conductivity is also one of the essential properties of materials. The resistivity was measured as a function of applied pressure, using a resistance measuring device and a potentiometric circuit (See Figure S10 in SI for details). As shown in Figure 6, as the pressure increases, the resistivity decreases for all the samples including graphite. The URC carbon obtained at lower temperature (700°C) exhibits high resistivity, indicating the carbonization at higher than 800°C is favourable for electrical conductivity. This can be understood by the fact that the increase in temperature certainly improves the graphitization in agreement with the increase in sp2 hybridization (Figure S5 in SI) and eventually increases the overall conductivity. The URC-1000 and URC-1100 obtained in this work display conductivity similar to that of graphite.

Figure 6 Resistivity vs pressure curves of the URC materials in comparison to graphite (Inset: magnified curves for URC-900, URC-1000, URC-1100 and Graphite). Full size image