Decoupled supercapacitive electrolyzer for membrane-free water splitting

Green hydrogen production via water splitting is vital for decarbonization of hard-to-abate industries. Its integration with renewable energy sources remains to be a challenge, due to the susceptibility to hazardous gas mixture during electrolysis. Here, we report a hybrid membrane-free cell based on earth-abundant materials for decoupled hydrogen production in either acidic or alkaline medium. The design combines the electrocatalytic reactions of an electrolyzer with a capacitive storage mechanism, leading to spatial/temporal separation of hydrogen and oxygen gases. An energy efficiency of 69% lower heating value (48 kWh/kg) at 10 mA/cm2 (5 cm–by–5 cm cell) was achieved using cobalt-iron phosphide bifunctional catalyst with 99% faradaic efficiency at 100 mA/cm2. Stable operation over 20 hours in alkaline medium shows no apparent electrode degradation. Moreover, the cell voltage breakdown reveals that substantial improvements can be achieved by tunning the activity of the bifunctional catalyst and improving the electrodes conductivity. The cell design offers increased flexibility and robustness for hydrogen production.

oxidation states, respectively.Similarly, the 4f5/2 peak was also resolved into two signals centered at 74.7 eV and 75.6 eV for Pt 0 and Pt 2+ oxidation states, respectively.The relative atomic concentration of Pt 0 and Pt 2+ was estimated using the peak areas, which were found to be 67.7 % and 32.3 %, respectively.The XPS analysis of the ACC sample is presented below.The survey scan of the sample (Figure S14a) is noteworthy, as commercial carbon cloth often contain significant impurities.The primary orbitals detected in the survey scan are C 1s, O 1s, F 1s, and Zn 2p, with several additional peaks that are difficult to assign without further information from the manufacturer.The predominant impurity present in the sample is Zn, but it is found at low atomic percentages of 4.0%.

Fig. S1 .
Fig. S1.Schematic representation of cell assembly and balance of plant.(A) Detailed construction of the cells in the supercapacitive electrolyzer.Schematic representation of the setup used for electrolysis experiments including hydrogen production quantification during (B) charging and (C) discharging steps.

Fig. S2 .
Fig. S2.Morphology of Pt/C electrodes.Scanning electron microscopy image of pristine graphite substrate and Pt nanoparticle deposited on graphite.

Figure
Figure S3c displays X-ray photoelectron spectroscopy (XPS) deconvoluted spectra of carbon.The C 1s spectrum was resolved into three distinct signals at binding energies of 284.6 eV, 285.4 eV, and 286.6 eV, corresponding to C-C, C=C, and C-OH functional groups, respectively.The O 1s spectrum (Figure S3b) exhibited two signals at 531.4 eV and 533.0 eV, attributed to C=O and C-OH groups, respectively(57,58).The Pt 4f spectrum (FigureS3d) revealed two peaks, representing the spin-orbital splitting of 4f7/2 and 4f5/2 photoemission lines.The 4f7/2 signal was deconvoluted into two peaks centered at 71.4 eV and 72.4 eV, corresponding to Pt 0 and Pt 2+

Fig. S11 .
Fig. S11.Comparison of double layer capacitance of electrodeposited phosphide electrodes.(A) Cyclic voltammetry of cobalt iron phosphide (CoFeP) coated Ni Foam in a non-faradaic region at different scan rate in 1.0 M KOH (B) Cdl calculated from the Idl obtained from CV measurements.

Figure
Figure S14b presents the deconvolution of the C 1s spectrum of ACC, displaying four distinct peaks corresponding to C-C (284.6 eV), C=C (285.5 eV), C-OH (286.8 eV), and C=O (289.0 eV) functional groups.The impurities present in the sample do not exhibit appreciable contributions, as their relative ratio is significantly lower than that of the bonds observed in Figure S14b.The

Fig. S15 .
Fig. S15.Cyclic voltammogram of activated carbon cloth.Cyclic voltammogram of activated carbon cloth (ACC) at different scan rates in the potential windows for water splitting.

Fig. S16 .
Fig. S16.Gas separation during decoupled electrolysis.Dissolved hydrogen and oxygen concentration changes in the liquid phase over consecutive charging and discharging cycles.

Fig. S17 .
Fig. S17.Energy consumption profile of a supercapacitive electrolyzer.Energy calculated from cell voltage profile of an electrolyzer assembly with cobalt iron phosphide (CoFeP) and activated carbon cloth (ACC) operated at current densities from 10 -100 mA/cm 2 .