Capacitive deionization (CDI) removes ions from water by applying a voltage across porous carbon electrodes that adsorb charged species, while membrane capacitive deionization (MCDI) adds ion-exchange membranes in front of those electrodes to improve selectivity and reduce co-ion leakage. In this dataset, MCDI-specific claims typically classify under both C02F water-treatment codes and ion-exchange membrane subject matter, whereas plain CDI claims lean more heavily on electrode and capacitor classifications like H01G. Drafting distinguishes the two carefully because membrane placement, material and configuration are frequently the point of novelty once basic electrode-based CDI is considered crowded prior art.

The tracked filing trend peaked at 26 in 2019, fell to 14 by the 2022 midpoint, and has continued downward toward the edge of the window. This pattern is consistent with a technology area where core electrode, membrane and system-architecture claims have already been staked out, leaving later filers to compete over narrower improvements. Some of the apparent drop in the most recent one to two years is likely a publication-lag artefact, since patent filings typically publish around eighteen months after they are filed, but the multi-year decline starting well before that lag window suggests a genuine slowdown in new claim activity.

China is the largest single receiving office in this dataset with 84 filings, more than the United States (47), Europe (26), South Korea (17), the WIPO PCT route (15) and the United Kingdom (7) combined would suggest if evenly spread. That concentration means any competitive or clearance analysis focused only on US or European filings will miss the majority of documented activity. Companies planning to manufacture or sell CDI/MCDI systems into or through China in particular should treat Chinese-language prior art as a primary rather than secondary search target.

Not extensively yet. Battery-related classifications (H01M) appear in only 10 of 216 families and power-grid classifications (H02J) in just 5, against 212 records in core water-treatment classifications (C02F). The representative recent filing, US12202748B2, is one of the clearer examples of this bridge, pairing a photovoltaic direct-drive power group with a membrane capacitive deionization water-purification unit and a grid-connected fallback. The relative scarcity of this combination in the broader dataset suggests it remains an open area rather than a settled one.

The assignee ranking spans the full 216-family dataset with activity spread across corporate, university and government-linked filers rather than concentrated in one dominant player, and co-filing between assignees is rare, with only five identified co-assignee pairs. Notably, every assignee tracked for recent-year momentum — including Doosan Heavy Industries, DWI Leibniz, Jiangsu Xinyi Sino-Australia Environmental Technology, Newsouth Innovations, Samsung Electronics and VWS UK — recorded zero filings in the most recent tracked year. That does not necessarily mean these organisations have exited the space; given publication lag, some may simply have unpublished applications still in the pipeline.