Single-step hydrogen production from NH3, CH4, and biogas in stacked proton ceramic reactors

SCIENCE 21 April 2022 Volume 376,Issue 6591 pp.390-393  

Electrifying membranes to deliver hydrogen

Sourcing hydrogen with ceramic reactors

An alternative to directly transporting hydrogen produced at large scales through steam reforming for applica­tions such as vehicular fueling is smaller scale, on-site production from methane or carriers such as ammonia. The hydrogen produced must be separated from co-produced carbon dioxide or nitrogen. Proton ceramic elec­trochemical reactors can extract pure hydrogen from gas mixtures by electrolytically pumping protons across the membrane at 800°C, but as the extraction proceeds, temperature gradients and entropic effects lead to effi­ciency drops. Clark et al. developed a nickel-based glass-ceramic composite interconnect that allowed for the design of a more complex reactor pathway (see the Perspective by Shih and Haile). Counterflowing streams balanced heat flows and maintained stable operating conditions that enabled 99% efficiency of hydrogen recovery. -PDS

Abstract

Proton ceramic reactors offer efficient extraction of hydrogen from ammonia, methane, and biogas by coupling endothermic reforming reactions with heat from electrochemical gas separation and compression. Preserving this efficiency in scale-up from cell to stack level poses challenges to the distribution of heat and gas flows and electric current throughout a robust functional design. Here, we demonstrate a 36-cell well-balanced reactor stack enabled by a new interconnect that achieves complete conversion of methane with more than 99% recov­ery to pressurized hydrogen, leaving a concentrated stream of carbon dioxide. Comparable cell performance was also achieved with ammonia, and the operation was confirmed at pressures exceeding 140 bars. The stack­ing of proton ceramic reactors into practical thermo-electrochemical devices demonstrates their potential in efficient hydrogen production.

Hydrogen can be produced from CHi-rich streams through steam reforming and water-gas shift (SMR+WGS, CH4 + 2H2O =CO2+ 4H2, llrxnH° = 164.7 kJ/mol) or from the emerging C-free H-carrier NH3 through ammonia dehydro-press1on occur downstream by pressure-swing absorption and mechanical compressors. Efficiencies typically improve with scalXXXXXX - XXXXX process over distributed H2 production for energy-carrier applications. (4).

H2 can also be !as Y-doped Ba2tures (300° to 8pure H2 from g;􀃅 !d electrochemically with proton (H+)-conducting ceramic membranes such XXX

ons (BZCY), which are functional and stable over a wide range of temperaironments (S.-lfl). Proton ceramic electrochemical reactors (PCERs) extract XXX

XXX  ically pumping protons across the membrane (Ejg..J.A). These offer process intensification (2) by integi - g reactions such as SMR+WGS or ADH with H2 separation and compression, high energy efficiencies by supp· lfil ; heat electrically (J), and reduced CO2 emissions when that electricity is renewable. (11).

(A) Schematic of a proton ceramic electrochemical continuous-flow reactor illustrating H2 separation from an H2 + N2 mixture. The local H2 compression ratio (top axis) increases as Pk,decreases upon H2 ex1raction along the reactor length (assuming a constant P}t), leading to a corresponding temperature increase. (B) Compression work for isothermal and non-isothermal H2 separation from an N2 + H2 mixture. The local compression ratio for H2 and the associated entropy difference L1S( x) = R In (Pll,/Pk, (x)) increase along the reactor coordinate, which leads to an increase in compression work and heat expelled from the compression process lw,,(x) = O(x) = T.O.S(x)]. If left unbalanced, this heat increases the temperature throughout the reactor, particularly in the latter parts, resulting in higher compression work than would be ideal for isothermal operation. (C) Energy balance and correlated voltages for thermally balanced operating modes, which include reaction (ER = liHR and UR= liHRlnF), charge transport (Ee= iRnF and Uohmic = iR), and compression !Eco = UNernstnF and UNemst = RT/nF In (Pll,/Pk,)1 for SMR+WGS at 750"C with an H2 compression ratio of 7.4 and NH3 cracking at 650"C with an H2 compression ratio of 5.4. See also fig. S2 for a decompression mode of operation. (D) Compression ratio as a function of Uohmlc + UNernst at 750"C for a maximum pH2 from 8 to 141 bars measured using representative PCER single cells (fig. S3) at ; = 50 mA/cm2, illustrating the different operation modes. The compression ratio range was covered by adjusting the minimum pH2 as well as gas flows to ensure a low degree of H2 extraction/dilution. The blue region consumes and the red region evolves heat.

As for any compression process, the work associated with electrochemical H2 compression is minimized by operating
isothermally (12). In a continuous-flow type of PCER, the compression ratio and associated entropy difference
of H2 across the membrane increase with the extent of separation along the reactor (Fig..1B. and fig. S1). This entropy
difference is expelled as heat (Q = TvS) during the compression process, which, if left unbalanced, leads to gradually increasing temperature along the reactor and in tum larger electric energy consumption per kilogram of compressed H2 (Fig..1.B.).

We present an XXXXX aided by multiphysics simulations and a new expansion-matched metal/glass-ceramic XXXX (IC) enabling deployable modular PCER stacks that retain the energy efficiencies and H2 recovering XXXX single cells (9) while achieving a 36-fold increased H2 production capacity. The reactor is designed with gas XXXX that allow internal heat exchange from exothermic to endothermic processes to minimize auxiliary heat input.

Our PCER can separate H2 by decompression while recovering electric energy or by compression through supply of electric energy (Fig..l.D.) at pressures up to 141 bars, illustrating the range of compression ratios and associated cell voltages that can be achieved throughout the PCER stack length. The PCER stack is a series of six barrels, each with six single cells connected electrically in parallel (Fig,2.A and fig. S4A), that uses a newly developed Ni-based glass-ceramic composite !Cs (13). A conductive washer of the same material is placed between the end of each membrane segment and the IC plate (Fig.2, E and F). Pure metals with higher thermal expansions, such as Ni or Cu, are prone to loss of electric contact during thermal cycling (fig. S5A, Ni washer) because of an expansion mismatch with the IC, glass-ceramic, and membrane segment (fig. S58). Our expansion-matched Ni/glass-ceramic composite washer is applied in a partially heat-treated condition as a sintered Ni/glass composite rather than a fully heat-treated Ni/glass-ceramic composite. This means that the washer can deform under the load applied during the heating phase of the sealing cycle by virtue of viscous flow in the glassy matrix phase in the washer and maintain intimate surface contact with both components.

XXX through the membrane and recovered as compressed H2 in the outer chamber. (D) Thermal expansion upon cooling of the BZCY /Ni support and the IC, and IC XXXXX of temperature. (E) Scanning electron micrograph cross section of the interface between the BZCY/Ni XXX a conductive glass-ceramic washer. (F) Schematics of IC and washer assembly.

The glassy XXXX phases in both the tubular cell support and the IC and produces a mechanically XXXXX sealing cycle, the glassy matrix phase in the washer crystallizes to produce a XXXX composite bridge, which retains excellent electrical continuity between the cell XXXX
thermal cycling. The adopted IC material exhibits conductivities 2500 S/cm at 750°C and XXXX expansion coefficients in a close-to-perfect match with the thermal expansion of the membrane support XXXX and fig. S5B), ensuring efficient current distribution throughout the stack and mechanical robustness. The XXXX stable under reducing and CO2-rich atmospheres, but can also be fitted with more oxidation
resistant metallic components such as Ag for operation under oxidizing conditions (fig. S5, C to E). The absence of Cr furthermore eliminates degradation issues related to formation of resistive Cr203 scales or evaporation of volatile Cr species during long-term operation at high temperatures.

During operation, individual cells will be net endothermic or exothermic depending on the degree of reaction and H2 separation and compression throughout the stack length (Figs.1D. and 2B), necessitating internal heat exchange. To guide the optimal design of the stack, we adopted a three-dimensional multiphysics model integrating coupled gas flows, heat transfer, current distribution, and reaction kinetics for SMR+WGS and ADH that captures the behavior of the stack from single cell to stack level (1.S). Our stack is designed with a U-bend type of gas flow pattern achieved by a manifold that distributes the incoming gas to three of the six gas channels in the stack while combining the three corresponding exhaust streams. For the axial type design, the fast kinetics of SMR and ADH concentrates the heat consumed by the endothermic reactions to the initial segments of the stack, whereas the heat caused by compression primarily evolves in the latter segments. This leads to temperature increase along the reactor length (Fig. 1A and .3, B and C) which in turn increases the cell Nernst voltage and compression work (Fig, 3D). Our U-bend design, however, mitigates this mismatch by spatially balancing the heat production from compression with the heat consumption of the reactions enabling a more uniform temperature profile (Fig.3A and Fig.S6, A and B). This in turn lowers cell Nemst voltages and thus the required compression work Fig.3F and Fig. S6C). Coupled with the high performing IC, the U-bend design allows currents (i.e., hydrogen fluxes) to self-regulate ac­cording to the local Nemst voltage (fig􀂄 S4). For anhydrous ADH, SMR+WGS and biogas this is particularly evident in the initial segment where the reaction is concentrated because of fast kinetics (figs. S7 to S9). The slower reac­tion kinetics of aqueous ADH on the other hand distributes the reaction over a larger portion of the stack (fig. S 10) .

Fig. 3. Multiphyslcs simulations of PCER stack thermally balanced operation.
Multiphysics simulations for a stack operating at 7so•c external temperature with 20 bars of total pressure on both sides of the membrane
and a mean current density of 0.60 A/cm2. Feed: 28.6% CH4 (0.597 NL/min), 71.4% H2O. Sweep: H2O (0.18 g/min). (A) Simulated temperature fields in a U-bend PCER stack architecture, also showing the gas inlet and outlet flow distribution. (B) Simulated temperature fields in an axial PCER stack architecture. (C) Temperature profiles on the reforming side in the axial and U-bend architecture along the reactor length. The thermal balancing by heat transfer between first cell (net endothermic) and last cell (net exothermic) for
U-bend PCER is illustrated by vertical arrows. (D) Mean compression work and Nernst voltage for each segment along the reactor length for the U-bend and axial PCERs. The values were obtained by integrating the compression work over each segment divided by the corresponding total flux or current. (E) Effect of Hz re­covery on the temperature distribution in U-bend and axial PCERs. (F) Effect of Hz recovery on the mean stack compression work and Nernst voltage for U-bend and axial PCERs.

To experimentally demonstrate integration of reactions beyond SMR+WGS in our PCER stack (9), single cells were
operated with NH3 in both anhydrous and aqueous form (.U). The cells achieve >97% conversion of NH3 even at open-circuit conditions (Fig.4.A) and near 100% conversions at high H2 recoveries thus leaving an effluent stream virtually free of residual NH3. The cells demonstrate comparable performance with anhydrous and aqueous NH3, CH4, and biogas, retaining near faradaic behavior to above 0. 7 Ncm2 (fig. S11), reflecting the catalytic versatility of the porous Ni-BZCY support. With CH4, the single cells even achieved >90% faradaic efficiency up to 7.4 Ncm2 (corresponding to a H2 flux of 47 normal milliliters-per-minute square centimeter) (fig. S12) thus doubling the H2 production capacity to-date with these materials (9).

(A) NH3 conversion as a function of H2 recovery, measured on a representative single cell (fig.S3) at 650•c and 10 bars (pNH3 = 7.25 bars;
pH20 = 2.75 bars), and aqueous NH3 at 750·c and 10 bars (pNH3 = 3.1 bars; pH20 = 5.8 bars; and Piner1 = 1.1 bars). Purple and green lines show the equilibrium conversion for NH3 and aqueous NH3, respectively. (B) and (C) CH4 conversion and yield of CO2 versus H2 recovery, re­spectively, of PCER stack at 750'C. (D) H2 production rate as a function of applied current density for the stack with N2/H2 mixture simulating complete NH3 decomposition (75o•c. 1 0 bars), methane (8oo•c, 15 bars, S/C = 2.5), and biog as (75o•c. 20 bars, S/C = 2.5). Effective current is calculated as current density x PCER stack area (36 x 15 cm2) and applied current as effective current/6 due to the series and parallel electric architecture. (E and F) H2 purity (dry basis) versus H2 delivery pressure and differential pressure across the membranes (E) and CO2 purity versus H2 recovery (F) for SMR+WGS in the stack at 75o•c. Reforming side pressure= 25 bars, H2 side pressure= 25 to 31 bars. Current den­sity= 0.69 A/cm2.

The 36-cell PCER stack achieves nearly full CH4 conversion and high H2 recoveries (>99%; Fig.4 and figs. S13 and
Sl4) for CH4 and biogas, enabling complete equilibrium shift and a CO2 rich effluent stream for facile carbon capture. The series and parallel design of the stack facilitates an effective aggregated current of up to 400 A (i > 0.73
A/cm2) with a H2 production rate up to 0.34 kg/day from CH4, 0.31 kg/day from biogas, and 0.34 kg/day from simulated fully decomposed NH3 streams (Fig.4D). We furthermore demonstrate H2 compression to 31 bar with a purity of 99.995% (Fig.4.E) facilitated for additional compression and use. The PCER stack shows promising stability, retaining a H2 production rate of 2 normal liters per minute after 1400 hours of operation (fig. S148). Both the H2 production rate [0.34 versus 0.025 kg/day (9)] and active area [584 versus 81 cm2 (14)] greatly surpass those of any reported for proton ceramic applications. Moreover, the PCER stacks have demonstrated that it is possible to deliver high-pressure H2 at high purity and a CO2 rich effluent at a hydrogen recovery and methane conversion >99%, which is highly competitive with Pd-based membrane reformers (table S2). These key performance indicators build the foundation for highly energy-efficient hydrogen production at system level.

System modeling (13) of a 1 ton/day distributed H2 production plant adopting our PCER stack (figs. S15 to S21l)  reveals that efficiencies of 91 % for CH4 and as high as 95% for anhydrous NH3 can be achieved by virtue of microthermal integration and downstream heat recovery. Furthermore, the PCER delivers a concentrated and pres­surized stream of CO2 when operated on methane or biogas Fig.4.E) that can be purified and liquefied by cryogenic distillation, eliminating the need for complex downstream absorption-based CO2 capture.

The high degree of process intensification achieved by our PCER stacks enables a fuel-flexible energy-efficient alternative to established technologies for distributed H2 production. Using a California 2020 electric grid carbon intensity scenario (82.92 gco2/MJeltec; see table S3 for references), H2 production with PCERs using CH4 as fuel would operate at lower emissions (75.7 vs. 124. l gco2/MJH2) than water electrolysis powered by grid electricity, even with­out CO2 sequestration. With decarbonization of the electric grid, CO2 sequestration is required for methane re XXXXPCERs operated on 010gas even orrer tt2 proaucuon w1tn net-negative caroon em1ss1on, as ctt4 from a XXXX process is XXXX scenarios have used the CA GREET model (15) which includes fugitive XXXX gas production that can be important (16).

To illustrate XXXX the PCER technology, comparable well-to-wheel emissions for battery electric vehicle XXXX engines (ICEs) with diesel fuel, and H2 fuel-cell electric vehicles (FCEVs) are XXXX  with sensitivity to electric grid carbon intensity. In the California 2050 scenario, the
XXXX using H2 produced from CH4 with PCERs including CO2 sequestration are 90% lower than XXXX with diesel fuel (145.4 gco2/km) and 26% lower than FCEVs using H2 from grid-powered water XXXX (19.8 gco2/km). NH3-based H2 can offer reduced emissions compared with on-site grid-powered water XXXX (19.8 gc02/km). H3 based H2 can offer reduced emissions compared with on-site electrolysis for a wide XXXX electric grid carbon intensities, making FCEVs fueled with NH3 -based H2 directly comparable to BEVs in terms of CO2 emissions (6.3 gc02/km, a reduction of 21 % compared with BEV in the California 2050 scenario). Here, NH3 is assumed produced at off-site locations with favorable renewable energy re­sources and transported as a liquid to the fueling station where efficient ADH and separation to H2 takes place using the PCER technology.

The growth of a new energy technology can be limited by access to raw materials. A detailed examination of raw materials' usage of the PCER stack (fig. S24) shows it is composed of non-precious, earth-abundant materials, suggesting no material availability setbacks for scaling.

Acknowledgments
Funding: This work was supported by Norway's Ministry of Petroleum and Energy through the Gassnova project CLIMIT grant 618191 in partnership with Engie SA, Equinor, ExxonMobil, Saudi Aramco, Shell, and TotalEnergies and the Research Council of Norway NANO2021 project DynaPro grant 296548. 

Author contributions: Conceptualization: D.C., H.M.-F., T.P., P.K.V., T.S.B., J.M.S., C.K.; Investigation: D.C., H.M.­F., M.B., I.Y.-T., D.8., K.N., L.A., T.P., D.K.P., M.I.V., S.R.-B., T.S.B., C.K.; Methodology: D.C., H.M.-F., M.8., I.Y.-T., S. A., L.A., T.P., P.K.V., D.K.P., S.R.-8., C.K.; Resources: M.B., D.B., K.N., M.I.V.; Software: I.Y.-T., S.A.; Supervision:
T. N., T.P., f .M.S., C.K.; Writing - original draft: D.C., H.M.-F., M.B., l.Y.-T., S.A., P.K.V., D.K.P., T.N., T.S.B., J.M.S.,C.K.;
Writing- review and editing: D.C., H.M.-F., M.B., I.Y.-T., T.S.B., D.B., S.A., L.A., T.P., P.K.V., D.K.P., S.R.-B., T.N., T.S.8., J.M.S., C.K. Competing interests: D.C., H.M.-F., M.B., I.Y.-T., D.B., S.A., K.N., D.K.P., T.S.B., and C.K. are employed by CoorsTek Membrane Sciences (CTMS). CTMS has filed relevant patent application PCT/EP2017/076340. T.N. is a member of the CTMS board. l.Y.-T.'s doctoral studies at the University of Oslo (UiO) are partially funded by CTMS. The remaining authors declare no competing interests.
Data and materials availability: All data are available in the main text or the supplementary materials. Experimental data are available online at http://hdl.handie.net/10251/181917.

Supplementary Materials
This PDF file includes:
Materials and Methods
Figs. S1 to S30
Tables S1 to S7
References (17-50)

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