High-current density alkaline electrolyzers: The role of Nafion binder content in the catalyst coatings and techno-economic analysis

We report high-current density operating alkaline (water) electrolyzers (AELs) based on platinum on Vulcan (Pt/C) cathodes and stainless-steel anodes. By optimizing the binder (Nafion ionomer) and Pt mass loading (mPt) content in the catalysts coating at the cathode side, the AEL can operate at the following (current density, voltage, energy efficiency -based on the hydrogen higher heating value-) conditions (1.0 A cm−2, 1.68 V, 87.8%) (2.0 A cm−2, 1.85 V, 79.9%) (7.0 A cm−2, 2.38 V, 62.3%). The optimal amount of binder content (25 wt%) also ensures stable AEL performances, as proved through dedicated intermittent (ON-OFF) accelerated stress tests and continuous operation at 1 A cm−2, for which a nearly zero average voltage increase rate was measured over 335 h. The designed AELs can therefore reach proton-exchange membrane electrolyzer-like performance, without relying on the use of scarce anode catalysts, namely, iridium. Contrary to common opinions, our preliminary techno-economic analysis shows that the Pt/C cathode-enabled high-current density operation of single cell AELs can also reduce substantially the impact of capital expenditures (CAPEX) on the overall cost of the green hydrogen, leading CAPEX to operating expenses (OPEX) cost ratio <10% for single cell current densities ≥0.8 A cm−2. Thus, we estimate a hydrogen production cost as low as $2.06 kgH2 −1 for a 30 years-lifetime 1 MW-scale AEL plant using Pt/C cathodes with mPt of 150 μg cm−2 and operating at single cell current densities of 0.6–0.8 A cm−2. Thus, Pt/C cathodes enable the realization of AELs that can efficiently operate at high current densities, leading to low OPEX while even benefiting the CAPEX due to their superior plant compactness compared to traditional AELs.

"CAPEX -Upscale to 1 MW": reports the calculations of the capital expenses (CAPEX) of the ideal 1 MW-scale AEL plant based on the DEP configurations studied in the present work. At first, the CAPEX of a generic 1 MW-scale AEL plant was estimated from literature data. 1 , (Lee et al., 2021) Then, following the cost breakdown of a generic MW-scale AEL plant reported by IRENA 1 and assuming AEL performances to meet those reported by the Korean Institute of Energy Research (Lee et al., 2021), the CAPEX was split into stack, DEP and Balance of Plant (BoP) costs and relative subcomponents.
Finally, upscaling the size of the DEP (cathode, anode and diaphragm) to 700 cm 2 and assuming the final ideal AEL to be composed by 5 stacks of 200 unit cells, (Lee et al., 2021) the total CAPEX of an ideal 1 MW-scale AEL plant for each DEP configuration tested in the present work was calculated. Such CAPEX estimation was made under the assumption that cathode and anode costs scale linearly with plant size.
The annual CAPEX was retrieved from overall CAPEX considering its depreciation through a capital recovery factor (CRF), calculated according to the following equation:

CRF =
i Rate × (1 + i Rate ) n (1 + i rate ) n − 1 3 where i rate is the discount rate and n is the AEL plant lifetime.
"OPEX -Upscale to 1 MW": reports the electrochemical data (i.e., current-voltage relationships) collected with different DEP configurations on a 5 cm 2 AEL single cell operating under industrially relevant conditions. Also, energy efficiency based on hydrogen higher heating value (energy efficiency HHV ) and gross system power of the ideal 1 MW-scale AEL are shown for each configuration.
Major contributors of operating expenses (OPEX), i.e., the electricity fed to the electrolyzer, the process water consumed, labor, maintenance and ancillary costs, were calculated. The OPEX related to electricity and process water, which are dependent on the electrolyzer performance, were calculated according to the gross power of the system and the water annual consumption, respectively.
In details, the cost of the electricity required for the actual electrolysis process was calculated according to the following equations: where m stands for mass.
On the other hand, labor, maintenance and other ancillary OPEX contributions were calculated as percentages of the total CAPEX of the whole system.
"Annual H 2 productivity at 1 MW": reports the amount of yearly produced hydrogen for each DEP configuration, all up-scaled to a 1 MW scale. Refer to the dedicated paragraph in the Experimental Section for details.
"Sensitivity to electricity cost": reports the variation of the overall H 2 production cost, CAPEX and OPEX contribution to the latter and CAPEX/OPEX ratio for different costs of the electricity used to power the AEL. Further details are available in the main text and in the Excel Spreadsheet.
"H 2 production cost vs Pt loading": reports the final calculation of the production cost of H 2 , starting from CAPEX and OPEX values and H 2 annual productivity for each DEP configuration.
According to reports on currently operative AELs, 1 the energy consumption for the actual electrolytic process accounts only for the 50% of the overall energy fed to the whole system, with BoP auxiliaries (gas and liquid circulation, gas compression…) requiring a similar energy fed. Therefore, the annual OPEX has been calculated doubling the OPEX Electricity . Refer to the dedicated paragraph in the Experimental Section for details.
"H 2 cost vs I, best Pt loading 10 -20 -30": reports an exploration of the effect of both operative current density and plant lifetime on the cost of H 2 production. All data reported in this sheet can be calculated simply replacing I, E and plant lifetime values in the dedicated entries in the "OPEX -MW ideal comparison". Note that an important assumption was made for exploring the effect of current voltage on the hydrogen production cost: as different operative current densities would result in different system power (when keeping a constant number of cells), the size of the AEL was modified according to the operative conditions themselves as to reach a 1 MW-scale in all cases. The size was modified increasing (ideally) the number of stacks, and therefore the total number of cells in the system. It was assumed that changing the number of stacks/cells does not imply differences in the CAPEX calculation.
Further comments can be found in the Excel Spreadsheet itself, highlighted in yellow.

Fixed parameters and assumptions made throughout the TEA
In the following, all the parameters that were fixed and assumptions that were made throughout the TEA are gathered in Tables S1-S4. Also, the same data are reported in the annexed Excel Spreadsheet. LHV H2 (hydrogen lower heating value) 120.0 kJ g -1 H2 Supplementary Figure 1. a) RDE CV curve of Pt/C catalysts in 1 M KOH (50 th CV scan, potential scan rate = 50 mV s -1 , m Pt =5.3 µg). These data were used to estimate the ECSA of our Pt/C through the H UPD method.  Figure 2. a,b) SEM image (panels i) together with corresponding EDS maps for C (panels ii) and Pt (panels iii) and c,d) EDS spectra measured fo r the binder-free Pt/C cathode before (panels a and c) and after the AST of the corresponding AEL (panels b and d). m Pt = 300 µg cm -2 . Figure 3. a,b) SEM image (panels i) together with corresponding EDS maps for C (panels ii) and Pt (panels iii) and c,d) EDS spectra measured for the as-produced Pt/C cathode with Nafion content = 10 wt% before (panels a and c) and after the AST of the corresponding AEL (panels b and d). m Pt = 300 µg cm -2 . Figure 4. a, b) SEM image (panels i) together with corresponding EDS maps for C (panels ii) and Pt (panels iii) and c,d) EDS spectra measured for the as-produced Pt/C cathode with Nafion content = 25 wt% before (panels a and c) and after the AST of the corresponding AEL (panels b and d). m Pt = 300 µg cm -2 . Figure 5. a, b) SEM image (panels i) together with corresponding EDS maps for C (panels ii) and Pt (panels iii) and c,d) EDS spectra measured for the as-produced Pt/C cathode with Nafion content = 50 wt% before (panels a and c) and after the AST of the corresponding AEL (panels b and d). m Pt = 300 µg cm -2 . Figure 6. a, b) SEM image (panels i) together with corresponding EDS maps for C (panels ii) and Pt (panels iii) and c,d) EDS spectra measured for the as-produced Pt/C cathode with Nafion content = 80 wt% before (panels a and c) and after the AST of the corresponding AEL (panels b and d). m Pt = 300 µg cm -2 . Figure 7. a) Photograph of a commercially available AEL stack (5 single cells, electrode area = 16 cm 2 ) (Fuel cell store). Membrane: proprietary porous polymer; electrode material: proprietary nickel based electrode. b) Cathodic galvanostatic polarization curve measured for the commercial AEL stack. c) Comparison between the galvanostatic polarization curves measured for the commercial AEL single cell and our Pt/C || SSM with a Nafion content of 25 wt%.