## 3 Results
### Issues Remaining after MOXIE  
We cite six issues remaining after completion of the MOXIE Project. 1. Lack of measurement of degradation due to long-term steady state operation. It is expected that a full-scale Mars ISRU would operate for about 10,000 h in a steady state without interruption, except for rare shutdowns. MOXIE did not generate data on the degradation of the stack or the compressor for such long-term operation. MOXIE did acquire considerable data on stack degradation of efficiency due to thermal cycling aftershort duration runs, but the connection between degradation from long-termoperation and thermal cycling remains unclear. OxEon Energy LLC recently carried out some studies of degradation due to steady operation, but a significant gap remains regarding degradation from 10,000 h of operation. The compressor for MOXIE was not designed for long life and no data are available on long duration testing. 2. Uncertain quality control from stack to stack. MOXIE produced eleven approximately equivalent stacks. Stacks were characterized by (1) the absolute stack intrinsic area-specific resistance (iASR), (2) the variation in individual cell voltages after manufacture, and (3) leakage between the anode and cathode. The stack with the lowest iASR and lowest range of cell voltages was chosen as the stack that was sent to Mars. The eleven stacks varied widely in the magnitude of the three basic attributes. After MOXIE, OxEon Energy LLC made a point of assembling a large stack of 65 cells, but it is not clear what the variation was from cell-to-cell voltages within that large stack, or how reproducible such stacks can be manufactured. 3. Role of individual cells in creating the properties of stacks. While measurements on individual cells were made on the eleven MOXIE stacks as manufactured, only very limited measurements were made on stacks after cycling, and we lack data on how the cells contribute to the observed changes in stacks due to cycling (or long-term operation). Of particular interest is how the worst cell (with highest resistance) affects susceptibility to carbon formation. Within a stack, it is possible that non-uniformity in manifolding might contribute significantly to stack variability. 4. Low cathode pressure operation. The side reaction that limits how high a voltage can be applied to cells for oxygen production involves reduction of CO to carbon. The voltage limit is determined by the Nernst voltage for carbon formation, which increases at lower cathode pressures. It is therefore desirable to operate at lower cathode pressures, permitting higher cell voltage, and therefore higher current density. MOXIE was able to lower the cathode pressure to about 0.22 bar, but we have no data below that pressure. Lower cathode pressure also reduces the power required for compression of Mars gas. This parameter will be important in larger scale system designs. 5. Acquisition and compression of Mars atmosphere gas. Any Mars ISRU system requires acquisition and compression of Mars atmosphere gas. The simplest, most appealing approach is to use a mechanical compressor which runs continuously. While a prototype mechanical compressor was used successfully on MOXIE, it is not clear what the characteristics of a next generation compressor would be (mass, efficiency, lifetime). Two batch-type systems for acquisition and compression of Mars atmosphere gas based on sorption and cryogenic freezing were studied briefly. In the sorption process, a sorbent at low temperature is open to the atmosphere where it absorbs CO\({}_{2}\). Then it is closed off and heated to release CO\({}_{2}\) at comparatively very high pressure. In the cryogenic method, CO\({}_{2}\) is frozen out using a cryocooler while exposed to the atmosphere and then warmed after closing off, to release CO\({}_{2}\) at comparatively very high pressure [19; 23; 25; 26; 27; 48; 49]. These cannot yet be ruled out, pending further development. 6. How will NASA deal with the legacy of MOXIE?